Why you should never buy a car without a tachometer
Ted B.
fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
or lower. My experience with my current vehicle is similar to several other
vehicles I've owned. In every case, I've found that the best fuel economy
is achieved somewhere above 70MPH. In the case of my current vehicle,
3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
the obvious, which is how do I know that my tachometer and speedometer are
accurate? I don't. But the bottom line is, in top gear, I can achieve
BETTER fuel economy at slightly higher than normal highway speeds.
Actually, the better fuel economy is due to slightly higher engine RPM, and
the higher ground speed is COINCIDENTAL.
I've always known that every engine has a "sweet spot", but didn't know the
exact scientific explanation for my personal observations until just
yesterday. During research on various engine types, I happened to discover
that Otto Cycle engines are most fuel-efficient at RPMs near (not
necessarily at, but near) 40% of redline. I had no reason to disbelieve
that particular scientific fact. I did the math for my own car, and found
that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT
3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my
own observations that 3500RPM (43% of redline) is very fuel efficient, and
3000RPM or less (37% of redline or lower) is not as fuel efficient.
Obviously, my engine is very fuel-efficient near 40% of redline, AS IT
SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I
had no idea that my engine SHOULD be most efficient at about 40% of redline
before I observed that it WAS most efficient at 3500RPM. So the results I
observed were not results that I "expected" to observe. The facts are what
the facts are, and now I have the scientific explanation stating YES, it
should be that way.
From what I could gather, the reason for the lower fuel efficiency below
~40% of redline in an Otto Cycle engine has to do with valve timing and
other aspects of engine performance. Most Otto Cycle engines have fixed
valve timing, so the maximum fuel economy is achieved at a certain RPM, and
if you run the engine higher OR LOWER than that RPM, your engine is not
achieving maximum fuel efficiency. Some engine manufacturers have come out
with variable valve timing Otto Cycle engine designs, with limited success
in improving fuel efficiency at lower RPMs. I gather that the success is
"limited" because fixed valve timing isn't the only factor making an Otto
Cycle engine most efficient at a certain RPM.
Anyway, now that I know WHY I'm seeing fuel economy peak at certain RPMs, I
know exactly what to look for in my NEXT vehicle. IF it has an Otto Cycle
engine, it WILL have a tachometer, or I won't buy it. During the test
drive, I'm taking it out on the highway to see how fast the car is
travelling at 40% of the engine redline. The way I USUALLY drive, a car
would be most efficient for me if it hit 40% of the engine's redline between
70 and 75MPH. Ironically, my current car is designed for a less
conservative driver than its current owner is. :) While I do get close to
80 sometimes, the vast majority of my highway driving is in the range of
70-75MPH, so that is where I need a car to be most efficient. SIMPLE,
right?
So if you want to find a car that will be very fuel-efficient for the way
you drive, simply look at the tachometer. Wherever it ends, muliply the
highest number by (.4). This will give you a good estimate of the most
fuel-efficient speed (RPM) of the engine. Then take it out on the highway
and see if the engine will run at that speed (RPM) at the speed you normally
drive on the highway. If the engine RPM is too high OR too LOW at your
normal highway speed, you might want to find a car with an engine that is
better designed for the way YOU drive. That is, if your goal is to find a
fuel-efficient vehicle. -Dave
On a side note, it's easy to understand why manual tranny vehicles are more
fuel-efficient for use in City driving. You have to get the engine RPMs UP
to maximize fuel economy, and you just can't do that at low speeds in a
slush-box.
N8N
Ted B. wrote:
> Over the years I've owned my current ride, I have observed that it is most
> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> or lower. My experience with my current vehicle is similar to several other
> vehicles I've owned. In every case, I've found that the best fuel economy
> is achieved somewhere above 70MPH. In the case of my current vehicle,
> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
> the obvious, which is how do I know that my tachometer and speedometer are
> accurate? I don't. But the bottom line is, in top gear, I can achieve
> BETTER fuel economy at slightly higher than normal highway speeds.
> Actually, the better fuel economy is due to slightly higher engine RPM, and
> the higher ground speed is COINCIDENTAL.
<snip>
just curious Ted, what do the power/torque curves look like for your
particular engine, and/or what engine is it? Do you have access to a
full BSFC map for your engine? Would be interesting to see.
thanks,
nate
Ted B.
"N8N" <njn...@hotmail.com> wrote in message
news:1126620409.9...@g14g2000cwa.googlegroups.com...
Y'know, I've searched online to find exactly that information on my engine,
or any similar engine. I've had no luck finding it, so far. I know I've
got the brochure for my car in my house somewhere. I'm going to pull it out
and see if I can get a hardcopy of the power/torque curves. IF so, I should
be able to post specific information on what the curves look like for my
particular engine. I didn't mean to focus on my engine in particular. I
just thought it was REALLY INTERESTING to learn that Otto Cycle engines
achieve maximum fuel efficiency near 40% of redline. (especially as that
agrees EXACTLY with the performance I have observed over many years of
driving vehicles with Otto Cycle engines) Knowing THAT particular data
point makes it a little easier to pick a new car when you are car shopping.
But only if the car has a tachometer. If it doesn't have a tachometer, then
you have no idea what ground speed the engine was made to be most efficient
at. That is, without running many hundreds of miles of tests first. :)
Best I can come up with at the moment is that the engine in my current car
is a Mitsubishi "4g64" 2.4L inline 4. -Dave
N8N
Really I was just curious if there was anything magical about 40% or
redline, or if that happened to coincide with the torque peak, which
I've always heard was a good approximation of where the BSFC map is
most favorable even under part throttle conditions. Having a nasty
Real Job(tm) I can't be looking for that info just at the moment :(
nate
todd_...@performancesimulations.com
Ted B.
>> car
>> is a Mitsubishi "4g64" 2.4L inline 4. -Dave
>
> Really I was just curious if there was anything magical about 40% or
> redline, or if that happened to coincide with the torque peak, which
> I've always heard was a good approximation of where the BSFC map is
> most favorable even under part throttle conditions. Having a nasty
> Real Job(tm) I can't be looking for that info just at the moment :(
>
> nate
>
Well, I found some numbers for the Nissan 3.5L V6 (aka Maxima) engine. This
engine has been one of the best automotive engines in the world for several
years, so I think it's a good "example" to use.
Redline is 8000, so maximum fuel efficiency should be about 3200RPM, for
that particular Otto Cycle engine (8000 X .4). Peak horsepower is 265 at
5800RPM and peak torque is 255 at 4400RPM. Nissan does use variable valve
timing, but this would tend to favor lower RPMs, as higher RPMs don't need
the valve timing trick to produce more power or torque.
SO, peak fuel efficiency RPM for that particular Otto Cycle engine seems to
be below -both- the horsepower and torque peaks, which makes sense. You
don't need MAXIMUM horsepower or torque to keep a car moving, but more
torque will be helpful for acceleration.
Now I want to hear from Nissan Maxima 3.5L owners . . . what speed (MPH) is
equivalent to 3200RPM engine speed in your Nissan Maxima in top gear (O/D
enabled, if it's an automatic)? If you pay attention to RPM or MPH, have
you noticed an increase in fuel efficiency at that speed??? Since it's an
Otto Cycle engine with an 8000RPM redline, it should be most fuel efficient
around 3200RPM (8000 X .4). IS IT most fuel-efficient near 3200 RPM? -Dave
Ted B.
<todd_...@performancesimulations.com> wrote in message
news:1126622947.6...@z14g2000cwz.googlegroups.com...
> Interesting. How did you calculate that?
>
How did who calculate what? -Dave
223rem
It is well over 80 mph, and, with my 2 recent speeding tickets, I never
cruise at that speed.
If you pay attention to RPM or MPH, have
> you noticed an increase in fuel efficiency at that speed??? Since it's an
> Otto Cycle engine with an 8000RPM redline, it should be most fuel efficient
> around 3200RPM (8000 X .4). IS IT most fuel-efficient near 3200 RPM? -Dave
I dont have hard data, but no way it is most efficient at that kind of cruising
speed.
Ted B.
>> Now I want to hear from Nissan Maxima 3.5L owners . . . what speed (MPH)
>> is equivalent to 3200RPM engine speed in your Nissan Maxima in top gear
>> (O/D enabled, if it's an automatic)?
>
> It is well over 80 mph, and, with my 2 recent speeding tickets, I never
> cruise at that speed.
>
>
> If you pay attention to RPM or MPH, have
>> you noticed an increase in fuel efficiency at that speed??? Since it's
>> an Otto Cycle engine with an 8000RPM redline, it should be most fuel
>> efficient around 3200RPM (8000 X .4). IS IT most fuel-efficient near
>> 3200 RPM? -Dave
>
> I dont have hard data, but no way it is most efficient at that kind of
> cruising
> speed.
>
Interesting! Is it really well over 80MPH at the most fuel-efficient range
of 3200RPM for the 3.5L V6 Otto Cycle engine? I guess that would make it
kind of tough to gather hard data. Who would keep the engine running at a
speed that equates to well over 80MPH for hundreds of miles? That would be
pretty gutsy in most areas of the U.S. Not unsafe at all, but not a good
idea, either, unfortunately.
From what you just told me though, it would seem that the Maxima was
designed for high-speed cruising, well over 80MPH. So it wouldn't be an
appropriate choice for a conservative driver. I don't doubt many people
think it's a real BLAST to drive, though. :) -Dave
John S.
Ted B. wrote:
> Over the years I've owned my current ride, I have observed that it is most
> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> or lower. My experience with my current vehicle is similar to several other
> vehicles I've owned. In every case, I've found that the best fuel economy
> is achieved somewhere above 70MPH. In the case of my current vehicle,
> 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state
> the obvious, which is how do I know that my tachometer and speedometer are
> accurate? I don't. But the bottom line is, in top gear, I can achieve
> BETTER fuel economy at slightly higher than normal highway speeds.
> Actually, the better fuel economy is due to slightly higher engine RPM, and
> the higher ground speed is COINCIDENTAL.
Your experience seems to be the opposite of most advice given to
maximize milage. I would like to find out more about how you conducted
your test.
If I understand you correctly you are saying that your car gets better
milage at 78mph (3500rpm) in high gear than it got at lower increments
say: 70mph, 60mph and 50mph?
Were you able to hold all the variables constant for a long enough time
to get a reasonably accurate reading for each increment?
Did you use the car computer to measure the miles per gallon?
My experience is exactly the opposite of yours. The lower the rpms are
kept the better the milage. Many manual transmission drivers don't
know when to shift and usually blow any opportunity to maximize milage.
Ted B.
YES. And that is EXACTLY how my particular Otto Cycle internal combustion
engine SHOULD perform. All cars are different obviously, but if they use an
Otto Cycle internal combustion engine, maximum fuel economy will be achieved
at close to 40% of the engine's redline. The only "difference" from one car
to the next (assuming they both use an Otto Cycle engine) will be what
ground speed that most fuel-efficient RPM equates to. In many cars, that
most fuel-efficient ground speed happens to be above 70MPH. My car is one
of them. IF the engine RPM drops below 3000, the engine itself is less
fuel-efficient. This means it takes more fuel to travel a certain distance,
and COINCIDENTALLY, this means that it is less fuel-efficient at a lower
ground speed. In your terms, this means that at 60MPH (for example), fuel
efficiency is LOWER.
For an automobile to be most fuel-efficient at say, 50MPH, the Otto Cycle
engine would need to be running near 40% of redline at 50MPH. IF that
theoretical Otto Cycle engine was running at 40% of redline at a ground
speed of 50MPH, then you would see a DECREASE in fuel economy at either
60MPH or 40MPH. Not because your ground speed changed up or down, but
because the Otto Cycle engine was operating outside of it's most
fuel-efficient RPM. (The ground speed change is coincidental)
I don't understand why you are questioning my test methods, when my results
exactly agree with how Otto Cycle engines are supposed to work. If I said
that I've found that water freezes at Zero Celsius, would you question how I
measured that? Measuring MPG at certain speeds is so simple the average
2nd-grader could do the math accurately. Some of them could even do it in
their head. I've performed the same measurements many times, with similar
results. All of this was done before I even knew what results I should
EXPECT to see. The fact is, my Otto Cycle engine should be most
fuel-efficient at around 3200RPM. BEFORE I KNEW THAT, I'd noticed, through
several years of driving experience, that the most fuel-efficient speed for
the engine was 3500RPM. That means my particular Otto Cycle engine is
performing EXACTLY as it should. If my tachometer is dead-on balls
accurate, my Otto Cycle engine is most fuel efficient at about 40% of
redline, EXACTLY AS IT SHOULD BE.
Yes, this means 78MPH is most fuel-efficient, and 70, 60 and 50 are less
fuel-efficient. -Dave
todd_...@performancesimulations.com
using the Otto cycle? I'm not disagreeing, just curious how you went
about it :-)
John S.
Interesting...apparently Dave is signing for Ted B. or vice versa.
Anyway...
Why don't you tell us in detail how you measured the mileage at various
increments.
A meaningful test would be one that does the following for each speed
increment:
1. Uses the same car.
2. Drive the same route.
3. The car starts with a full tank of gas.
4. Covers a meaningful distance, at least 100 miles but preferably a
full tank.
5. The car is held at a constant speed as much as possible.
6. The milage is computed the same way each time, either with the
car's averaging computer or by a complete fillup at the end.
The reason I'm questioning your statement is that the effect of wind
resistance on milage increases faster than speed. (It isn't arithmetic)
I can see this effect on my cars during a 375 mile trip that is
repeated several times a year to North Carolina. Driving at 60mph
results in 33mpg while driving at 80mph results in 26mpg for the same
distance. I'm puzzled about that you can have such different results.
Dave C.
<todd_...@performancesimulations.com> wrote in message
news:1126643187....@g43g2000cwa.googlegroups.com...
> How did you calculate the 40% bit and so forth in your original post
> using the Otto cycle? I'm not disagreeing, just curious how you went
> about it :-)
>
I didn't calculate it. Through research, I learned that the Otto Cycle
engine is most fuel-efficient at about 40% of redline RPM. That's not my
calculation, it's just how the Otto Cycle engine works. -Dave
todd_...@performancesimulations.com
>From your original post:
"During research on various engine types, I happened to discover
that Otto Cycle engines are most fuel-efficient at RPMs near (not
necessarily at, but near) 40% of redline. I had no reason to
disbelieve
that particular scientific fact. I did the math for my own car, and
found
that (surprise, surprise) I SHOULD achieve maximum fuel economy at
ABOUT
3200RPM, with my own Otto Cycle engine. "
Was curious what math you said you did there for your own car. No big
deal really, was just curious. I've written a couple of engine
simulations and it hasn't yet dawned on me how one would use the Otto
Cycle to derive any conclusions like this. I'm not arguing or saying
it's wrong, after all, the best fuel efficiency isn't going to be at 1
mph, nor 300 mph, so it'd have to be somewhere in between :P
Todd Wasson
Performance Simulations
http://www.PerformanceSimulations.com
http://www.PerformanceSimulations.com/files/ToddSim9a.wmv
Dave C.
> resistance on milage increases faster than speed. (It isn't arithmetic)
> I can see this effect on my cars during a 375 mile trip that is
> repeated several times a year to North Carolina. Driving at 60mph
> results in 33mpg while driving at 80mph results in 26mpg for the same
> distance. I'm puzzled about that you can have such different results.
>
You shouldn't be puzzled at all. You are missing certain facts that would
clear things up quickly for you. Does your car have a tachometer? If so,
what is the highest number that the tachometer reads? Multiply that times
.4. For example, if your tach goes up to 9000, multiply by .4 to get 3600.
NOW, assuming your tach goes up to 9 grand, and your 40% level is 3600, you
will probably find that 3600RPM is somewhere below 80MPH in your car. 33MPG
at 60MPH is not bad, as far as fuel economy goes, and I don't even KNOW what
kind of car you drive. BUT, based on the fact that you get 33MPG at 60 but
only 26MPG at 80, it's easy to (LOGICALLY) conclude that, YOUR engine is
running closer to 40% at 60MPH than it is at 80MPH. This is just a total
guess, but 80MPH for your engine might be 50% or higher, but definitely
higher than the optimum 40% RPM speed. -Dave
Ad absurdum per aspera
can be limited by a variety of factors, such as valve float, piston
speed, vibrations, or a business manager thinking about warranty claims
on those two-sigma outliers on the left side of the quality
distribution. Only some of these factors would directly relate to
efficiency aspects of engine theory. I've got a further hunch that the
lowest limiting factor gets rounded down to the nearest couple hundred
rpm.
Thoughts?
--Joe
todd_...@performancesimulations.com
want as light a spring as you can, but that lowers the red line.
Piston speed relates more to connecting rod strength, but they can take
considerable rpm, well above the redline or the typical spot where
you'll want to put the horsepower peak. Basically the power game is to
get the horsepower peak where you want it, then use as light a valve
spring as you can. The result is you usually end up with the redline
not too far over the horsepower peak. Indeed, there's bound to be a
safety factor built in. They'll lower the redline a bit from what will
probably be fine. I ran my engine a good 500-600 rpm past the redline
and the whole engine sound changed. Most likely valve float there.
:-) I don't recommend trying that though of course..
N8N
Sometimes, sometimes not. I once had to over-rev an old Rabbit GTI,
the valves did indeed float about 700 RPM over redline (was turning
left, oncoming driver ran a red light and was headed right toward me,
didn't feel like taking the time to shift.) I've repeatedly
over-revved my GTI 1.8T however, as the only indication that you've
exceeded the redline is the actual tachometer. Power doesn't fall off,
valves don't float, it just pulls like a freight train. I miss that
car (sold it to my mom, if you can believe that. She loves it.) I
have no idea what consideration the engineers used to set the redline;
the earlier 16V NA engines (same basic block) had a much higher redline
so it couldn't have been piston speed or bearing issues. Maybe just a
little CYA?
Now my old cars - all Studebakers - yeah, the valvetrain is the
limiting factor. 5200 RPM - float city, it's all over. I'm working on
rectifying that situation :)
Haven't blown up an engine yet (knock on wood) save for an old Bimmer
that I didn't abuse but I suspect that the PO did...
nate
Dave C.
> >
> > I didn't calculate it. Through research, I learned that the Otto Cycle
> > engine is most fuel-efficient at about 40% of redline RPM. That's not
my
> > calculation, it's just how the Otto Cycle engine works. -Dave
>
>
> >From your original post:
>
> "During research on various engine types, I happened to discover
> that Otto Cycle engines are most fuel-efficient at RPMs near (not
> necessarily at, but near) 40% of redline. I had no reason to
> disbelieve
> that particular scientific fact. I did the math for my own car, and
> found
> that (surprise, surprise) I SHOULD achieve maximum fuel economy at
> ABOUT
> 3200RPM, with my own Otto Cycle engine. "
>
> Was curious what math you said you did there for your own car. No big
> deal really, was just curious. I've written a couple of engine
> simulations and it hasn't yet dawned on me how one would use the Otto
> Cycle to derive any conclusions like this. I'm not arguing or saying
> it's wrong, after all, the best fuel efficiency isn't going to be at 1
> mph, nor 300 mph, so it'd have to be somewhere in between :P
>
> Todd Wasson
> Performance Simulations
OH! I guess I misunderstood the question. My own car tops out at 8000 RPM,
so if the maximum fuel efficiency RPM is ABOUT 40% (for any Otto Cycle
engine, not just mine) of that, then 8000 X .4 equals 3200. So, 3200RPM is
ABOUT where my own engine should be most fuel efficient. I already knew
that 3500RPM was the most fuel-efficient speed for my engine, so when I
learned that it should be about 3200RPM, I wasn't surprised, at all. Yes,
that computes. :) -Dave
223rem
The Maxima engine has a redline of 6600, and a max of 8000.
40% of redline makes more sense to me.
John S.
Wish the world of fuel efficiency were that simple as applying a
constant to rpms.
At 60mph the engine is turning 2000 rpm. What's the next guess.....
Chuck Tomlinson
>Over the years I've owned my current ride, I have observed that it is most
>fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
>observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
>or lower. My experience with my current vehicle is similar to several other
>vehicles I've owned. In every case, I've found that the best fuel economy
>is achieved somewhere above 70MPH.
This discussion pops up every so often on r.a.d., and once
again, I feel the need to chime in... I don't understand
why a modern vehicle should behave like yours does, but
what do I know. FWIW, I have never observed similar
behavior in any vehicle I've driven for extended periods.
[...]
>I've always known that every engine has a "sweet spot", but didn't know the
>exact scientific explanation for my personal observations until just
>yesterday. During research on various engine types, I happened to discover
>that Otto Cycle engines are most fuel-efficient at RPMs near (not
>necessarily at, but near) 40% of redline. I had no reason to disbelieve
>that particular scientific fact.
I suspect it is far from a scientific fact. At best, it's a
very rough correlation. Even then, rough correlations can
be useful, but I think this one has little relevance to
maximizing cruising fuel economy in a street car or truck.
Did your research discuss part-throttle fuel efficiency or
only wide-open throttle (WOT) fuel efficiency? Street
vehicles don't cruise at WOT, so without access to a map of
BSFC plotted against engine load *and* rpm, the only way to
plot fuel consumption vs speed is by accurate measurement.
FWIW, cruising around for a while at roughly a certain speed
and then measuring fuel usage some time later is *not* an
accurate measurement technique. Without *instrumentation*,
either built-in or added-on, any attempt at correlating mpg
to speed is (at best) a guess.
>I did the math for my own car, and found
>that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT
>3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my
>own observations that 3500RPM (43% of redline) is very fuel efficient, and
>3000RPM or less (37% of redline or lower) is not as fuel efficient.
>Obviously, my engine is very fuel-efficient near 40% of redline, AS IT
>SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I
>had no idea that my engine SHOULD be most efficient at about 40% of redline
>before I observed that it WAS most efficient at 3500RPM. So the results I
>observed were not results that I "expected" to observe. The facts are what
>the facts are, and now I have the scientific explanation stating YES, it
>should be that way.
If _I_ use the 40% "rule", my car (4th-gen Corvette 6-speed)
should deliver best fuel economy at about *100 mph*. It
does not. In fact, it delivers best mpg at about 40 mph,
and mpg decreases continuously with higher speeds. At 100
mph/40% redline, mpg is roughly half of the maximum.
[...]
>On a side note, it's easy to understand why manual tranny vehicles are more
>fuel-efficient for use in City driving. You have to get the engine RPMs UP
>to maximize fuel economy, and you just can't do that at low speeds in a
>slush-box.
No way. Increasing rpm (at a given speed) is an excellent
way to *reduce* fuel economy. The engine output does not
increase (constant speed), but the internal friction power
required by the engine increases. IOW, the engine requires
more power consumption to do the same work.
Furthermore, increasing cruising rpm at constant road speed
will force a reduction in throttle angle (more rpm, same
power output), which increases pumping losses. All in all,
running higher rpm than required to maintain cruising speed
is *bad* for fuel economy.
FWIW, if I drop from 6th to 5th in my car at 65-75 mph, mpg
(observed on the mpg readout) will drop by ~20%, even though
5th gear puts the engine at the magic 40% at 67 mph.
I have no way of knowing whether your vehicle does what you
think it does, but I *know* that not all vehicles behave
similarly.
--
Chuck Tomlinson
John S.
> Ted B. wrote:
> clipped it - no need to copy all that again.
So "Dave" or "Ted" or "Alice" what ever name you are posting under now.
When are you going to tell us the details of your milage test. We're
waiting for the specifics on how you determined in the real world that
a car going at 80 mph get's better milage than one going 50 or 60 mph.
Anything is possible if you ignore enough of the real world, like wind
resistance and inefficiency of the motor at high speeds. Yeah, just
take the high RPM marker on the tach and multiply by .40 to get the
most efficient engine and therefore car speed. Why don't you send that
in for the Car Guys next show...that'll be great material for saturday
morning laughs!
I'm sitting here thinking about my brother-in-law and his 1995 vette.
As I remember it is loafing along at about 1500 rpm or maybe a little
slower at 60mph. By your formula he should be getting great milage
somewhere above 100mph right? Wonder why it doesn't work that way.
Carl Taylor
> Over the years I've owned my current ride, I have observed that it is most
> fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> or lower. My experience with my current vehicle is similar to several other
> vehicles I've owned. In every case, I've found that the best fuel economy
> is achieved somewhere above 70MPH.
Several of your OTHER vehicles have also had the mysterious ability to
get better MPG while producing more power at higher RPMs?
NOW we know you're really making this stuff up. Some people always seem
to be abducted by aliens while others never see them.
C.T.
Carl Taylor
> If I understand you correctly you are saying that your car gets better
> milage at 78mph (3500rpm) in high gear than it got at lower increments
> say: 70mph, 60mph and 50mph?
>
> Were you able to hold all the variables constant for a long enough time
> to get a reasonably accurate reading for each increment?
>
> Did you use the car computer to measure the miles per gallon?
Ted/Dave has stayed consistently vague about those details. His car is
a 1995 Mitsubish Galant, and here are its published MPG figures:
http://www.fueleconomy.gov/feg/noframes/11897.shtml
MPG (city) 23
MPG (highway) 30
MPG (combined) 26
I think this all got started when someone created a thread suggesting
3,000 RPM mandate would save fuel. Dave/Ted said no way, I get better
MPG at 3,500 RPM, and he offered no legit reasons for it. He even made
a crazy analogy to a human arm lifting a weight so slowly that it
fatigued from static stress (engines suffer no such weakness).
What we have here is a chronic speeder inventing more ways to justify
speeding. He's really just full of it and all these polite responses
amuse me.
C.T.
Carl Taylor
> "Ted B." <no...@nohow.not.ever> wrote:
> >Over the years I've owned my current ride, I have observed that it is most
> >fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also
> >observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG
> >or lower. My experience with my current vehicle is similar to several other
> >vehicles I've owned. In every case, I've found that the best fuel economy
> >is achieved somewhere above 70MPH.
>
> This discussion pops up every so often on r.a.d., and once
> again, I feel the need to chime in... I don't understand
> why a modern vehicle should behave like yours does, but
> what do I know. FWIW, I have never observed similar
> behavior in any vehicle I've driven for extended periods.
No need to be that tactful. His claims are mostly a ruse to justify
speeding. I don't know if it's deliberate or the happy coincidence of
faulty measurements, since he never quite cites his methods. Here's the
official word on his car:
http://www.fueleconomy.gov/feg/noframes/11897.shtml
> If _I_ use the 40% "rule", my car (4th-gen Corvette 6-speed)
> should deliver best fuel economy at about *100 mph*. It
> does not. In fact, it delivers best mpg at about 40 mph,
> and mpg decreases continuously with higher speeds. At 100
> mph/40% redline, mpg is roughly half of the maximum.
That agrees perfectly with the findings of this and other links on the
topic.
http://auto.howstuffworks.com/question477.htm
There's no free "power lunch" and fuel economy is the price to be paid.
There will always be people who can't handle the truth when it
conflicts with a lifestyle. My whole crusade on r.a.d. was/is about
exposing that selfish aspect of human nature. I'll never win, I know.
I'm fighting millions of years of mind-game evolution!
> Furthermore, increasing cruising rpm at constant road speed
> will force a reduction in throttle angle (more rpm, same
> power output), which increases pumping losses. All in all,
> running higher rpm than required to maintain cruising speed
> is *bad* for fuel economy.
Put more simply, once you're in top gear you burn more fuel the faster
you go. Except in a gravity-free vacuum, keeping a mass moving at a
higher speed requires more power, and more power means more air and
fuel passing through the cylinders. Good old physics, right Chuck?
We're in a spot where we need to conserve oil more than ever but some
people won't give up their fast ways. The lengths they'll go to don't
surprise me, given the history of this newsgroup. How about you, old
sage? You might even be able to squeeze 30 MPG out of that Corvette if
you hold down your highway speed in 6th with a tailwind.
C.T.
todd_...@performancesimulations.com
I was referring more to the mechanical limitations that dictate the
real red line; i.e., the point where you can be pretty sure something
is going to break. Valve float in the 'real redline' context is
generally going to be the limiting factor in a production car. Of
course, the red line that's stuck on your tach is going to be set lower
than that to give some safety margin. Who knows, maybe there are cars
out there where manufacturers got a good deal on tachs that had a
redline that was well under the limit, but still over the power peak
and just decided to throw them in ;-)
todd_...@performancesimulations.com
Oh, ok. Actually, after I'd written that I reread your earlier post
and realized that's what you'd done.
I understand you're talking in approximate terms here, but that rule is
really not quite right. If you're indeed getting the best fuel economy
at that speed and rpm, I'd take the 40% bit as a mere coincidence :-)
Ted B.
> I understand you're talking in approximate terms here, but that rule is
> really not quite right. If you're indeed getting the best fuel economy
> at that speed and rpm, I'd take the 40% bit as a mere coincidence :-)
>
But it's not coincidence at all. An Otto Cycle engine (any Otto Cycle
engine, not just my own) will be most fuel-efficient near 40% of redline
RPM. So the fact that my own Otto Cycle engine is indeed most
fuel-efficient near 40% of redline is hardly a coincidence. -Dave
Ted B.
"223rem" <223...@sbcglobal.com> wrote in message
news:iQLVe.328704$_o.170543@attbi_s71...
> Is it 40% of the red line, or of the max RPM?
>
> The Maxima engine has a redline of 6600, and a max of 8000.
>
> 40% of redline makes more sense to me.
That's a good question. I've seen some tachs marked with yellow above about
6000, and red closer to 8000. Then some tachs will be marked red in a range
from 6000-8000. I'm betting the actual redline is 8000, but some
tachometers are marked red at the lower RPM range to encourage drivers to
try to stay below that. That is what makes most sense, logically. But it's
a great question! -Dave
Ted B.
> Wish the world of fuel efficiency were that simple as applying a
> constant to rpms.
>
> At 60mph the engine is turning 2000 rpm. What's the next guess.....
>
But it is indeed just that simple!!! Again, you are guessing, and you need
to collect more facts before any conclusion can be drawn. If your engine is
really turning 2000RPM at 60MPH, and seems to be relatively fuel-efficient
at that speed, my best guess would be that it would be even MORE
fuel-efficient at a slightly higher speed. That's my guess, as I really
doubt any engine would have a 40% speed of 2000RPM. For that to be true,
the engine would need to have a max speed of 5000RPM, and the tachometer
would likely be marked to discourage you from going beyond 4000RPM. That's
really slow.
As I posted before, in your car, it's a good bet that 60MPH is closer to 40%
than 80MPH. But unless you can post the max RPM for your car, there is no
way to determine where the 40% range is. 60MPH/2000RPM might be 30% and
80MPH might be 60%, and BOTH might be outside of optimum fuel-efficiency
range.
This is exactly why I said you should never buy a car without a tachometer.
With a tach, you can do the math to get a damn good estimate of where the
engine's most fuel-efficient RPM might be. WITHOUT a tach, you will have to
find the most fuel-efficient point by lots of trial and error. In your
case, 60MPH is fairly fuel-efficient and 80MPH is somewhat less so. Where's
the point of greatest fuel efficiency? Neither one of us knows. You just
have two data points out of dozens of possible data points. -Dave
Ted B.
> speeding. I don't know if it's deliberate or the happy coincidence of
> faulty measurements, since he never quite cites his methods. Here's the
> official word on his car:
>
> http://www.fueleconomy.gov/feg/noframes/11897.shtml
>
Uhhhhh . . . to put it tactfully, I am most likely a more conservative
driver than YOU are. When I learned how the Otto Cycle engine is supposed
to perform, I realized that my own vehicle was made for high-speed cruising.
And thus, my own vehicle is not the most appropriate choice of vehicle for
the way I -usually- drive. If you look at my driving record over the past
thirty years, it is spotless. I mean, spotless as in no accidents, no
speeding tickets and no parking tickets. That's not bad considering that
I've averaged 100K per year of driving for many of those years.
I know what the EPA estimates are for my car. I also know that EVERY
automobile I have ever driven has EXCEEDED EPA estimates for fuel economy .
. . in some cases significantly exceeded EPA estimates for fuel economy.
The Otto Cycle internal combustion engine is most fuel-efficient near 40% of
redline. That's not a calculation, and certainly not my calculation. If
you own an Otto Cycle engine that is NOT most fuel-efficient near 40% of
redline, then you have an Otto Cycle engine that is not acting like an Otto
Cycle engine. I won't presume to speculate on why your Otto Cycle engine
does not act like an Otto Cycle engine. But every car I've ever owned has
had an Otto Cycle engine that performs like (DUH!!!) an Otto Cycle engine.
My current car with an Otto Cycle engine is no exception. But now that I
know why it behaves like it does, I'm seriously thinking of trading it for
something that is more fuel-efficient at lower speeds. -Dave
Ted B.
> I'm sitting here thinking about my brother-in-law and his 1995 vette.
> As I remember it is loafing along at about 1500 rpm or maybe a little
> slower at 60mph. By your formula he should be getting great milage
> somewhere above 100mph right? Wonder why it doesn't work that way.
>
Good question . . . does it or does it NOT have an Otto Cycle engine? Does
Chevy use variable valve timing? There are certain modifications that can
be made to an Otto Cycle engine to make it waste less heat energy at RPMs
outside of the optimum 40%. But I did not design the Corvette engine, so I
have no idea why it is not acting like an Otto Cycle engine SHOULD act. I
would ask this though . . . has anybody ever run a Corvette at a 40% speed
"above 100MPH" for a few hundred miles to measure the fuel economy? That's
a serious question, BTW. -Dave
Ted B.
> get better MPG while producing more power at higher RPMs?
>
> NOW we know you're really making this stuff up. Some people always seem
> to be abducted by aliens while others never see them.
>
> C.T.
Yeah, they all had Otto Cycle engines. Some coincidence, eh?
This is what you fail to understand . . . even at the optimum 40% RPM rate,
the internal combustion engine is TERRIBLY inefficient. MOST of the
potential energy is wasted as heat. If you run your engine at the optimum
RPM (40%), LESS energy is wasted. So running the engine at a slightly
higher RPM in some cases doesn't really "gain" more energy, it just WASTES
LESS OF IT.
I don't know why I even bother. You are so hell-bent on believing that
speed kills that you will always believe that simply slowing down will solve
all of the world's problems, even though the science, in some cases,
disagrees with you.
I'd really like to hear you explain how you own the only Otto Cycle engine
in the world that does not perform like an Otto Cycle engine, though. Seems
like I've asked you that before. -Dave
Harry K
I have seen you blow a lot of smoke and useing mirrors but have seen
absolutely zip from you to show actual data. Your claim that 'every
car I ever had..." is countered by government warnings that the
estimated mileage on new cars (actually old ones too) never reach that
level. They never had and until the formulas are changed, they never
will.
I and others are waiting for you to post your methodology for measuring
your mileage. It would also help for you to post some engineering
reports supporting your 40% assertion.
Harry K
Ted B.
>
> http://www.fueleconomy.gov/feg/noframes/11897.shtml
>
> MPG (city) 23
> MPG (highway) 30
> MPG (combined) 26
>
> I think this all got started when someone created a thread suggesting
> 3,000 RPM mandate would save fuel. Dave/Ted said no way, I get better
> MPG at 3,500 RPM, and he offered no legit reasons for it. He even made
> a crazy analogy to a human arm lifting a weight so slowly that it
> fatigued from static stress (engines suffer no such weakness).
>
> What we have here is a chronic speeder inventing more ways to justify
> speeding. He's really just full of it and all these polite responses
> amuse me.
>
> C.T.
Earlier, I posted that an Otto Cycle engine is most fuel-efficient at about
40% of redline. This means my car should be most fuel-efficient at ABOUT
3200 RPM. I had earlier (before I knew that) observed that my car is most
fuel-efficient at 3500RPM. Since so many people seem to (incredibly) want
to disbelieve that the Otto Cycle engine works like an Otto Cycle engine, I
did more research on the matter. I found *another* source that states that
the Otto Cycle engine is most efficient at 40-45% of redline. This agrees
with the source I found earlier, which stated the most fuel-efficient range
was about 40% of redline. My own engine is running at 43% at 3500 RPM, so
it falls RIGHT IN THE MIDDLE of it's most fuel-efficient operating speed at
3500RPM, according to the second source I just found:
http://home.earthlink.net/~graham1/MyToyotaPrius/Understanding/InternalCombustion.htmQuote from above:" A modern Otto cycle engine tends to be most efficient at 40% to 45% of its"red-line" r.p.m. and 70% to 80% of its peak torque. At higher r.p.m.,friction losses in fast-moving engine parts increases. Higher torque isachieved using "fuel enrichment", which reduces efficiency. At lowertorque, the engine suffers most from what is termed "pumping loss"(discussed below). At the efficiency "sweet spot", the engine producesaround 40% of its rated peak power. For the Echo engine, for example, thepeak power is 108 hp, so it will be most efficient in the general area of 35to 50 hp. Ideally, then, we would like to size the engine in a car so thatin the most common driving situations, we use about 40% of the maximum powerthe engine can deliver."Later in the same article, it is stated:" The fact that automobile engines run for most of the time at a smallfraction of peak power and hence well below the efficiency "sweet spot" iscalled the partial power problem. "So, the EPA estimates for my car might be influenced somehow by the "partialpower problem" as detailled above. While I'm a very conservative driverwith a SPOTLESS driving record going back decades, I'm also not afraid toDRIVE a car, like so many drivers (or should I call them "riders") seem tobe. In other words, a loud engine doesn't intimidate me, and neither doG-forces. Take my current car, for example . . . "partial power" is not asmuch of a problem in my car, as it is a 5-speed manual, and I tend toupshift it at relatively high RPM. The only time "partial power" worksagainst me is on the highway, where I tend to stay BELOW the efficiencysweet spot, most of the time. -Dave
Ted B.
> absolutely zip from you to show actual data. Your claim that 'every
> car I ever had..." is countered by government warnings that the
> estimated mileage on new cars (actually old ones too) never reach that
> level. They never had and until the formulas are changed, they never
> will.
>
> I and others are waiting for you to post your methodology for measuring
> your mileage. It would also help for you to post some engineering
> reports supporting your 40% assertion.
>
> Harry K
>
http://home.earthlink.net/~graham1/MyToyotaPrius/Understanding/InternalCombustion.htmQuote from above:" A modern Otto cycle engine tends to be most efficient at40% to 45% of its"red-line" r.p.m. and 70% to 80% of its peak torque. Athigher r.p.m.,friction losses in fast-moving engine parts increases. Highertorque isachieved using "fuel enrichment", which reduces efficiency. Atlowertorque, the engine suffers most from what is termed "pumpingloss"(discussed below). At the efficiency "sweet spot", the engineproducesaround 40% of its rated peak power."
John S.
What nonsense. I know you want to impress yourself and others with
your knowlege of automobile terminology by salting your messages with
"Otto Cycle Engine". Lets stick with the basics here. You are
asserting that a car that runs at 40% of the RPM indicated the tach
will get the best gas milage. For many cars on the road today that
would would be well in excess of 90mph. Most automotive authorities
I'm aware of believe that it takes more energy to keep a car moving at
a constant high rate of speed than a constant lower rate of speed.
50mph vs 90 mph for example. Not surprisingly that has been my
experience too.
So when are you going to give us the details of your exhaustive test
the results of which seem to contradict the real world experience of so
many others.
Ad absurdum per aspera
with people's minds? Make all cars display instantaneous cost, based
on computed gas mileage (from fuel flow and speed) with the data
automatically updated by having the computer talk by Bluetooth to
transponders on gas-station signs.
Somewhat less flippantly, a mileage display could maintain historical
data for the car and show a curve of mileage vs. speed, so you could
decide for yourself what's a good tradeoff among schedule, cost, and
the need to keep up with traffic. We should be able to *know* rather
than guess when a particular car is past its engine's optimum and/or
running into an aerodynamic wall (and seeing how this varies with
elevation). The historicals could even be used to some extent as a
maintenance indicator and/or an alert to changes in driving habits or a
guide to selecting the most cost-effective brand of gas for your car
or...
Actually such a display is so useful from a variety of standpoints that
I'm surprised more cars aren't capable of it. Knowing your best
economy cruise (through measurement, rather than with the SWAG that
there's a broad optimum somewhere in the 40-55 mph range) can be a big
deal when trying to save money, help collectively cope with an energy
supply problem... or deciding whether to turn back or nurse the remains
of your tank to the next town.
--Joe
Chuck Tomlinson
>>
>> I'm sitting here thinking about my brother-in-law and his 1995 vette.
>> As I remember it is loafing along at about 1500 rpm or maybe a little
>> slower at 60mph. By your formula he should be getting great milage
>> somewhere above 100mph right? Wonder why it doesn't work that way.
>
>Good question . . . does it or does it NOT have an Otto Cycle engine?
It does.
>Does Chevy use variable valve timing?
Not in that engine (which is identical to my car's).
>There are certain modifications that can
>be made to an Otto Cycle engine to make it waste less heat energy at RPMs
>outside of the optimum 40%. But I did not design the Corvette engine, so I
>have no idea why it is not acting like an Otto Cycle engine SHOULD act.
IMHO, the Corvette engine acts just the way an Otto Cyle
engine should act (see below). The key issue here, I think,
is the difference between _engine efficiency_ and _vehicle
fuel economy_.
>I would ask this though... has anybody ever run a Corvette at a 40% speed
>"above 100MPH" for a few hundred miles to measure the fuel economy? That's
>a serious question, BTW.
There's no need. As I mentioned before, '92-up Corvettes
can display "instantaneous" and distance-averaged mpg, which
have proven quite accurate in my car. On several road
trips, I've set the cruise at 100 mph while driving across a
desert (passing dry & salt lake beds that are very close to
level grade), zero the average mpg, and check it when I
finally have to drop out of 100 mph cruise (in those
conditions, it converges to an average in well under a
minute). My car consistently returns 20-21 mpg at 100 mph.
Using the same technique at other speeds, I've found that my
car gets 28-29 mpg at 70 mph, and ~40 mpg at 40 mph (all in
top gear). With this information (and approximations to my
car's mass, aero properties, and rolling resistance), it's
easy enough to estimate engine efficiency at each of these
cruising speeds. The results:
40 mph, 9 hp, 40 mpg, 1.0 gal/hr, 0.66 (lb/hr)/hp
70 mph, 27 hp, 28 mpg, 2.5 gal/hr, 0.56 (lb/hr)/hp
100 mph, 62 hp, 20 mpg, 5.0 gal/hr, 0.48 (lb/hr)/hp
The last number in each row is a common measure of engine
fuel efficiency, the brake specific fuel consumption (BSFC),
expressed as fuel mass flow rate divided by output power.
Lower is better, for BSFC. For gasoline engines, minimum
part-throttle BSFC tends to be about 0.38 lb/hr/hp (more or
slightly less).
Anyway, you can see that the engine efficiency is increasing
(lower BSFC) even as the fuel economy is decreasing.
That's how Otto-cycle engines typically behave in passenger
vehicles, and that's why you *cannot* predict the rpm for
maximum _vehicle fuel economy_ based on the rpm (and load)
of maximum _engine efficiency_.
--
Chuck Tomlinson
223rem
> The key issue here, I think,
> is the difference between _engine efficiency_ and _vehicle
> fuel economy_.
Very good point!
Ted B.
> your knowlege of automobile terminology by salting your messages with
> "Otto Cycle Engine". Lets stick with the basics here. You are
> asserting that a car that runs at 40% of the RPM indicated the tach
> will get the best gas milage.
No, I'm not asserting it. I'm simply repeating facts as stated by people
who know a lot more about the Otto Cycle engine than either one of us do.
There's a HUGE difference, there.
>For many cars on the road today that
> would would be well in excess of 90mph.
In some cases, YES, that's possible. I haven't experienced it myself, but
have no reason to doubt that it could happen. For my current car, the most
fuel-efficient speed is 3500RPM, 43% of redline, and 78MPH (take your pick,
all are correct). Several cars I've owned have been most efficient
somewhere above 70MPH. To think that a car MIGHT be most fuel-efficient
above 90MPH is not too hard to imagine. That's only about a 13MPH
difference from my own car, and it's not exactly a sports car.
Most automotive authorities
> I'm aware of believe that it takes more energy to keep a car moving at
> a constant high rate of speed than a constant lower rate of speed.
> 50mph vs 90 mph for example. Not surprisingly that has been my
> experience too.
Yes, but do those "automotive authorities" realize that at optimum RPM of an
Otto Cycle engine, less energy is wasted as heat, leaving MORE ENERGY
available to keep the car moving? Again, it's not really that you are
pulling extra energy out of the engine, it's just that the engine is
converting more of the available energy to kinetic energy. Thus, the
greater fuel efficiency.
ON THE OTHER HAND, it's easy to believe that many cars would indeed be more
fuel efficient at 50MPH than 90MPH, because (NOT coincidentally) some Otto
Cycle engines are running closer to 40% or so at 50MPH than 90MPH.
But a blanket statement like slow down to save fuel isn't necessarily going
to be helpful for all circumstances, unless slowing down brings your Otto
Cycle engine closer to the 40-45% range where it is most
uel-efficient. -Dave
Ted B.
Not really. At best, it's a red herring. Outside of the engine, no other
component of the car is adding kinetic energy to the car. Unless it's a
hybrid with electric motor(s), but that's a whole other topic.
The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
There is no way around that, as that's just how the Otto Cycle engine works.
Change any other component of the car, and you might lower or raise gas
mileage. But if the Otto Cycle engine is still allowed to operate at about
40% of redline RPM, that is STILL going to be it's most fuel-efficient
speed. -Dave
Arif Khokar
> The Otto Cycle internal combustion engine is most fuel-efficient near 40% of
> redline.
It isn't. My car will probably get around 40 mpg at 40 to 45 mph. At
60 to 65 mph it gets around 26 to 27 mpg. At 75 to 80 mph it gets
around 23 to 24 mpg.
My car has both instantaneous and average mpg readings with which to
verify these figures.
todd_...@performancesimulations.com
Ted B. wrote:
> > What nonsense. I know you want to impress yourself and others with
> > your knowlege of automobile terminology by salting your messages with
> > "Otto Cycle Engine". Lets stick with the basics here. You are
> > asserting that a car that runs at 40% of the RPM indicated the tach
> > will get the best gas milage.
>
> No, I'm not asserting it. I'm simply repeating facts as stated by people
> who know a lot more about the Otto Cycle engine than either one of us do.
> There's a HUGE difference, there.
>
I think you missed the part about me writing engine simulation software
in our earlier exchange. If you'd like to discuss the Otto Cycle and
what that precisely means and entails, and how it might or might not
have any bearing on what you currently believe about engines and fuel
economoy, feel free to ask.
> >For many cars on the road today that
> > would would be well in excess of 90mph.
>
> In some cases, YES, that's possible. I haven't experienced it myself, but
> have no reason to doubt that it could happen. For my current car, the most
> fuel-efficient speed is 3500RPM, 43% of redline, and 78MPH (take your pick,
> all are correct). Several cars I've owned have been most efficient
> somewhere above 70MPH. To think that a car MIGHT be most fuel-efficient
> above 90MPH is not too hard to imagine. That's only about a 13MPH
> difference from my own car, and it's not exactly a sports car.
>
If I'm not mistaken, what you're stating is that one can multiply the
redline by around 40% and take a look at the speedometer when
travelling at that engine speed in top gear. Whatever speed you're
running at that point is going to give the optimum fuel mileage,
correct? And the reason for this is because of the scientific fact
that an Otto Cycle engine attains peak fuel economy at that engine
speed (40% of redline). (Note that that figure is most likely
referring to engine speed and fuel conversion efficiency, not that in
combination with vehicle speed and fuel mileage.)
Consider the consequences of that if it is or were indeed true. This
literally means that if you swap out your transmission for a direct
drive unit (1:1 ratio), and assuming your car came stock with a 3:1
differential ratio, your "40% of redline" speed has now tripled. Your
optimum speed was 78mph before, which I still maintain is rather
coincidentally tied to your 43% value.
With the new driveline unit your optimum fuel mileage now occurs at
234mph if what you're saying is correct. Congratulations, the fuel
crisis has now been put off for quite some time. ;-) All we needed
were higher gears all along, go figure.
This reminds me of a time in high school when a buddy wanted to put a
2.32 rear end ratio in his Camaro so he could cruise along at 2000rpm
in excess of 100mph. Unfortunately he did not realize that the engine
did not produce enough power at 2000 rpm to allow that speed to be
reached, even at full throttle. But I digress.
Anyway, this should continue to be a fun debate so long as nobody here
is the sort of person who would insist his MD was wrong about his
"broken leg" diagnosis. ;-)
By the way, the link you posted for your source of information did not
work (probably too long and it got abbreviated when looking here
through Google). Could you please post it on two lines or with a space
half way in between so I can copy/paste it together and view the site?
Thanks,
Daniel J. Stern
> The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
> There is no way around that, as that's just how the Otto Cycle engine works.
> Change any other component of the car, and you might lower or raise gas
> mileage. But if the Otto Cycle engine is still allowed to operate at about
> 40% of redline RPM, that is STILL going to be it's most fuel-efficient
> speed.
Horseapples. There are so many design, implementation and operational
factors that go into fuel efficiency that picking a random percentage of
"redline RPM" (for which there is absolutely zero standard definition
other than "wherever the carmaker prints the red line on the tach, if so
equipped") and calling it the most efficient speed is grossly untenable.
todd_...@performancesimulations.com
Ted B. wrote:
> "223rem" <223...@sbcglobal.com> wrote in message
> news:SiXVe.329298$x96.300841@attbi_s72...
> > Chuck Tomlinson wrote:
> >> The key issue here, I think,
> >> is the difference between _engine efficiency_ and _vehicle fuel economy_.
> >
> > Very good point!
>
> Not really. At best, it's a red herring. Outside of the engine, no other
> component of the car is adding kinetic energy to the car.
At a constant 78mph nothing is adding any kinetic energy to the car at
all, INCLUDING the engine. :-)
>
> The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
> There is no way around that, as that's just how the Otto Cycle engine works.
> Change any other component of the car, and you might lower or raise gas
> mileage. But if the Otto Cycle engine is still allowed to operate at about
> 40% of redline RPM, that is STILL going to be it's most fuel-efficient
> speed. -Dave
Dave, this 40% of redline RPM efficiency value you read about refers to
"fuel conversion efficiency," not "fuel mileage." They are two vastly
different things and are not equal or directly representative of each
other by any stretch of the imagination. In fact they have different
units.
All that the "fuel conversion efficiency" value means is that for every
gallon of fuel you're burning, you're getting the most possible power
out of the engine at that particular RPM. It does not give any
indication of fuel mileage at all.
Daniel J. Stern
> > What nonsense. I know you want to impress yourself and others with
> > your knowlege of automobile terminology by salting your messages with
> > "Otto Cycle Engine". Lets stick with the basics here. You are
> > asserting that a car that runs at 40% of the RPM indicated the tach
> > will get the best gas milage.
>
> No, I'm not asserting it. I'm simply repeating facts as stated by people
> who know a lot more about the Otto Cycle engine than either one of us do.
You're asserting it, calling it a "fact" without a shred of evidentiary
support, and telling us we should believe you because you heard it from
really knowledgeable people.
You will have to do a great deal better than that if you wish to be taken
seriously. Just saying "Otto cycle! Otto cycle! Otto cycle! Otto cycle!"
isn't going to cut it.
Daniel J. Stern
> > I and others are waiting for you to post your methodology for
> > measuring your mileage. It would also help for you to post some
> > engineering reports supporting your 40% assertion.
>
> http://home.earthlink.net/~graham1/MyToyotaPrius/Understanding/InternalCombustion.htmQuote
> from above:" A modern Otto cycle engine tends to be most efficient at40%
> to 45% of its"red-line" r.p.m.
Ah. You read it on the interweb, therefore it's true.
Just so we're clear on the source you're citing as "authoritative".
Daniel J. Stern
> Otto Cycle engines.
> Otto Cycle.
> 40-45%.
> the Otto Cycle,
> Otto Cycle
> I don't know why I even bother.
> 40%.
> Otto Cycle Otto Cycle.
> Otto Otto Otto, Cycle Cycle Cycle.
> 40%.
That makes all of us. Polly wanna cracker?
todd_...@performancesimulations.com
> work (probably too long and it got abbreviated when looking here
> through Google). Could you please post it on two lines or with a space
> half way in between so I can copy/paste it together and view the site?
>
>
Nevermind on the link, I got it now. Good site too.
John S.
Ted B. wrote:
> > What nonsense. I know you want to impress yourself and others with
> > your knowlege of automobile terminology by salting your messages with
> > "Otto Cycle Engine". Lets stick with the basics here. You are
> > asserting that a car that runs at 40% of the RPM indicated the tach
> > will get the best gas milage.
>
> No, I'm not asserting it. I'm simply repeating facts as stated by people
> who know a lot more about the Otto Cycle engine than either one of us do.
> There's a HUGE difference, there.
Unfortunately that is about all you are bringing to this discussion
You are quite adept the copy-and-paste of partial bits of information
but you seem to have little understanding of what they mean.
>
> >For many cars on the road today that
> > would would be well in excess of 90mph.
>
> In some cases, YES, that's possible. I haven't experienced it myself, but
> have no reason to doubt that it could happen. For my current car, the most
> fuel-efficient speed is 3500RPM, 43% of redline, and 78MPH (take your pick,
> all are correct). Several cars I've owned have been most efficient
> somewhere above 70MPH. To think that a car MIGHT be most fuel-efficient
> above 90MPH is not too hard to imagine. That's only about a 13MPH
> difference from my own car, and it's not exactly a sports car.
Good grief man...when was the last time you drive something beyond a
motorized rollerskate!
Taking your impossible example to the extreme if we geared up a car to
a 1:1 ratio just imagine the distance we could cover on a gallon of gas
and the speed...wheee!!! My goodness that would demonstrate how
efficient a passenger car with a Otto Cycle (4 stroke) engine could be.
But why stop at 1:1, lets go further... a .5:1, .05:1, why there is
just no limit to how efficient and fast we can make our cars.
>
> Most automotive authorities
> > I'm aware of believe that it takes more energy to keep a car moving at
> > a constant high rate of speed than a constant lower rate of speed.
> > 50mph vs 90 mph for example. Not surprisingly that has been my
> > experience too.
>
> Yes, but do those "automotive authorities" realize that at optimum RPM of an
> Otto Cycle engine, less energy is wasted as heat, leaving MORE ENERGY
> available to keep the car moving? Again, it's not really that you are
> pulling extra energy out of the engine, it's just that the engine is
> converting more of the available energy to kinetic energy. Thus, the
> greater fuel efficiency.
>
> ON THE OTHER HAND, it's easy to believe that many cars would indeed be more
> fuel efficient at 50MPH than 90MPH, because (NOT coincidentally) some Otto
> Cycle engines are running closer to 40% or so at 50MPH than 90MPH.
For the umpteenth time no it is not.
The very simple reason of why this is so is apparently beyond you.
Just take you car, fill it up with cas, reset the average mpg to zero
and embark of a 350 mile trip at 55mph. Repeat the same drive with a
full tank of gas this time at your favorite 78mph. Use the speed
control to be sure.
And report back the results.
Daniel J. Stern
> By the way, the link you posted for your source of information did not
> work (probably too long and it got abbreviated when looking here through
> Google). Could you please post it on two lines or with a space half way
> in between so I can copy/paste it together and view the site?
Or, much better and less cumbersome, make the long URL into a short one
before posting it, by visiting www.tinyurl.com .
Harry K
Now, now Todd. Yau are trying to confuse him with facts, something
that he apparently has very little association with. He thinks that
repeating Otto cycle and 40% repeatedly somehow proves his assertions
(and that is all they are).
Harry K
Chuck Tomlinson
>"223rem" <223...@sbcglobal.com> wrote...
>> Chuck Tomlinson wrote:
>>> The key issue here, I think,
>>> is the difference between _engine efficiency_ and _vehicle fuel economy_.
>>
>> Very good point!
>
>Not really. At best, it's a red herring.
Interesting. I offered you information (actual data that I
gathered from vehicle instrumentation) that shows how fuel
economy can decrease while engine efficiency is increasing.
Do you need clarification of the data? Assumptions? Math?
My data shows that your premise is wrong. Furthermore, it
also shows (quite clearly IMHO) that engine efficiency
(BSFC) and fuel economy (e.g., mpg) are *completely*
different.
[...]
>The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
>There is no way around that, as that's just how the Otto Cycle engine works.
You don't seem to understand that engine efficiency (i.e.,
BSFC) is a function of both load *and* rpm. In fact, over
an engine's normal operating speed range, BSFC varies at
least as much with load as with rpm. But you choose to
ignore varying loads and base your conclusions solely on
rpm. That's not valid!
Here's a simple question: Do you think that your engine has
the same efficiency at 40% of redline in 1st gear as it does
at 40% of redline in top gear? If not, why not?
>Change any other component of the car, and you might lower or raise gas
>mileage. But if the Otto Cycle engine is still allowed to operate at about
>40% of redline RPM, that is STILL going to be it's most fuel-efficient
>speed. -Dave
IMHO, you need to understand two things:
1. Engine efficiency (BSFC) is a *strong* function of engine
load, as well as engine rpm.
2. Vehicle road load (i.e., the power required to hold a
steady speed) increases quickly at highway speeds. For
typical passenger cars and highway speeds, required power is
roughly proportional to speed squared (the exponent is
usually slightly larger than 2, actually...).
Items 1 & 2 can be used to show item 3:
Maximum engine efficiency is *very different* from maximum
vehicle fuel economy.
--
Chuck Tomlinson
Dave C.
OK, again, yet someone else who is posting data points based on MPH. Where
is the 40% RPM speed, and what does it actually DO there (as opposed to what
a car computer estimates) -Dave
Dave C.
http://home.earthlink.net/~graham1/MyToyotaPrius/Understanding/InternalCombustion.htmQuote
> > from above:" A modern Otto cycle engine tends to be most efficient at40%
> > to 45% of its"red-line" r.p.m.
>
> Ah. You read it on the interweb, therefore it's true.
>
> Just so we're clear on the source you're citing as "authoritative".
>
Well I've read it several places. I have no reason to doubt that it's true,
when it perfectly agrees with what I have observed over the past few
decades. It seems to be the speed kills crowd that is most determined to
spin this into something *I* claim is true, as opposed to facts as
documented by people more knowledgeable than me. -Dave
Dave C.
> literally means that if you swap out your transmission for a direct
> drive unit (1:1 ratio), and assuming your car came stock with a 3:1
> differential ratio, your "40% of redline" speed has now tripled. Your
> optimum speed was 78mph before, which I still maintain is rather
> coincidentally tied to your 43% value.
>
> With the new driveline unit your optimum fuel mileage now occurs at
> 234mph if what you're saying is correct. Congratulations, the fuel
> crisis has now been put off for quite some time. ;-) All we needed
> were higher gears all along, go figure.
Funny. Your example is rather extreme, but in an odd way, it does make
perfect sense. Take my car for example. If the gear ratio of the various
manual tranny gears was changed a bit, I could move the most fuel-efficient
speed from 78 down to 70, which would be more useful. I doubt if I could
move it up to 234, though. At some point, lack of horsepower would be a
HUGE problem. If the car isn't moving at all, it's obviously not going to
be fuel-efficient. But if you triple the size of the engine at the same
time? Yes, maybe you could indeed push the most fuel-efficient speed up to
234. I'm not gonna drive that beast, but I'm sure there will be some
takers. :) -Dave
Brent P
> Interesting. I offered you information (actual data that I
> gathered from vehicle instrumentation) that shows how fuel
> economy can decrease while engine efficiency is increasing.
> Do you need clarification of the data? Assumptions? Math?
Just to back this up.
This should be expected because engine efficiency is measured as useful work
out over energy in and fuel economy is measured in distance traveled over
volume of consumed.
For instance, an engine can be at peak efficiency, accelerating a vehicle,
but mpg will will suffer because the distance traveled isn't as much had
the vehicle used the same volume of fuel at a constant speed. Even though
in the later case, the engine may not be operating as efficently given
load, rpm, etc.
Dave C.
>
> You're asserting it, calling it a "fact" without a shred of evidentiary
> support, and telling us we should believe you because you heard it from
> really knowledgeable people.
>
> You will have to do a great deal better than that if you wish to be taken
> seriously. Just saying "Otto cycle! Otto cycle! Otto cycle! Otto cycle!"
> isn't going to cut it.
Hey, I'm accepting it as a fact, as nobody has posted any evidence to
dispute it, just more mantras of "slow down to save fuel" and useless
information such as "BUT MY CAR GETS 55MPG at 40MPH". (uhhh . . . yes, it
might . . . now what's your point?) -Dave
Dave C.
> factors that go into fuel efficiency that picking a random percentage of
> "redline RPM" (for which there is absolutely zero standard definition
> other than "wherever the carmaker prints the red line on the tach, if so
> equipped") and calling it the most efficient speed is grossly untenable.
>
So you think that other aspects of car design can change the physics of the
Otto Cycle engine? Do tell . . . -Dave
Dave C.
> "fuel conversion efficiency," not "fuel mileage." They are two vastly
> different things and are not equal or directly representative of each
> other by any stretch of the imagination. In fact they have different
> units.
>
> All that the "fuel conversion efficiency" value means is that for every
> gallon of fuel you're burning, you're getting the most possible power
> out of the engine at that particular RPM.
And meanwhile, the car is? . . . (class, help him out a little here)
Nobody?
Moooooooooooooving. Eating up miles. -Dave
Chuck Tomlinson
I posted my car's fuel economy at 40% of redline: 20-21 mpg.
My engine computer's estimate is *excellent*. It is based
on measurement of engine airflow, exhaust mixture, and on
known (and learned) injector flow behavior. I have
correlated my trip computer's fuel usage estimate to pump
fills on long road trips, and found it to remain within 1%
of cumulative pump readings over thousands of miles.
What's *your* measurement methodology, Dave?
--
Chuck Tomlinson
Arif Khokar
> > > The Otto Cycle internal combustion engine is most fuel-efficient near
> > > 40% of redline.
> Arif Khokar wrote:
> > It isn't. My car will probably get around 40 mpg at 40 to 45 mph. At
> > 60 to 65 mph it gets around 26 to 27 mpg. At 75 to 80 mph it gets
> > around 23 to 24 mpg.
> >
> > My car has both instantaneous and average mpg readings with which to
> > verify these figures.
> OK, again, yet someone else who is posting data points based on MPH. Where
> is the 40% RPM speed, and what does it actually DO there (as opposed to what
> a car computer estimates)
The car's engine's redline is 7000 rpm. Forty percent of that is 2800
rpm. At 2800 rpm in top gear, the car's speed is about 70 mph. The
car's fuel efficiency is clearly not as high at that speed as it would
be when the engine is running at around 1800 rpm which corresponds to
around 45 mph.
Bernard Farquart
"Dave C." <no...@nohow.not> wrote in message
news:Nh1We.10412$Wd7....@newsread1.news.pas.earthlink.net...
Only one poster replying to you really belongs in the
"speed kills" crowd, so that's not it.
Perhaps it is because people who have MPG calculators
in the dash see different data, they disbelieve you.
Bernard
>
>
N8N
This whole thread is making me wish that there were a small,
inexpensive yet accurate flow meter that could be spliced into a 5/16"
or 3/8" fuel line, for those of us who have vehicles without computers
from which to pick injector duty cycle, RPM, and road speed info.
c'mon aftermarket peep holes, get crackin'!
nate
Matthew Russotto
Ted B. <no...@nohow.not.ever> wrote:
>
>The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
>There is no way around that, as that's just how the Otto Cycle engine works.
Do you really think repeating that assertion over and over again makes
it any more credible?
--
There's no such thing as a free lunch, but certain accounting practices can
result in a fully-depreciated one.
Brent P
> This whole thread is making me wish that there were a small,
> inexpensive yet accurate flow meter that could be spliced into a 5/16"
> or 3/8" fuel line, for those of us who have vehicles without computers
> from which to pick injector duty cycle, RPM, and road speed info.
Hmm.... I wonder if the flow meters in the product I work on can handle
gasoline... they aren't cheap... but I can probably get my hands on some
that are no longer fit for duty in the machine.
Garth Almgren
>>Wish the world of fuel efficiency were that simple as applying a
>>constant to rpms.
>>
>>At 60mph the engine is turning 2000 rpm. What's the next guess.....
>
> But it is indeed just that simple!!! Again, you are guessing, and you need
> to collect more facts before any conclusion can be drawn. If your engine is
> really turning 2000RPM at 60MPH, and seems to be relatively fuel-efficient
> at that speed, my best guess would be that it would be even MORE
> fuel-efficient at a slightly higher speed. That's my guess, as I really
> doubt any engine would have a 40% speed of 2000RPM. For that to be true,
> the engine would need to have a max speed of 5000RPM, and the tachometer
> would likely be marked to discourage you from going beyond 4000RPM. That's
> really slow.
<raises hand>
As far as I have been able to tell from research on and offline, my
Mustang has an approximate redline of 4100-4200 RPMs (though the tach
reads up to 6000 and has no marked redline; they used the same tach for
all the various engines), but I can't remember a time when I took it
above 3700 RPMs (that's ~40 MPH in first). My best mileage comes at
around 2400 RPMs and 64 MPH, about where the engine has it's peak
torque. That's 57-58% of redline, if my redline estimate isn't far off.
Bottom line, there's no single number you can point to and say "You'll
get the best mileage at X!!" (where X is speed, RPMs, or what have you)
for every car.
(FWIW, I track my gas mileage at every fillup with a program on my Zire.)
--
~/Garth |"I believe that it is better to tell the truth than a lie.
Almgren | I believe it is better to be free than to be a slave.
******* | And I believe it is better to know than to be ignorant."
(p...@v6stang.com for secure mail info) --H.L. Mencken (1880-1956)
Dave C.
> rpm. At 2800 rpm in top gear, the car's speed is about 70 mph. The
> car's fuel efficiency is clearly not as high at that speed as it would
> be when the engine is running at around 1800 rpm which corresponds to
> around 45 mph.
>
Damn, I wish I had more time. I'd be willing to bet you money that you are
wrong. The only condition I would attach is that I'm allowed to inspect the
car (mainly for obvious stuff like proper tire pressure, fluid levels, just
to make sure that it will be safe to test) and allowed to drive it. I'm a
very good driver with a perfect driving record, and yes, I do have a license
(before some smartass asks). Your car would be in very good hands. I'd
baby it, unlike I treat my own car. :) I'd be shocked if your car actually
got better mileage at 45 than it did at 70. I don't think it will get bad
mileage at 45, but I'm sure it will do better at 70. Most cars would.
Besides which, 1800 is a really LOW RPM for that engine. That's only ~25%
range. It's going to be wasting too much heat energy at that range to get
anywhere near to optimum fuel economy. -Dave
Dave C.
> "speed kills" crowd, so that's not it.
>
> Perhaps it is because people who have MPG calculators
> in the dash see different data, they disbelieve you.
>
> Bernard
As they should, if they are only looking at their own vehicle. From what I
read, not all vehicles will get better mileage at higher speeds/RPM. A lot
of them will. But seeing someone say that their own car doesn't really
doesn't prove anything, one way or the other. -Dave
Dave C.
> >There is no way around that, as that's just how the Otto Cycle engine
works.
>
> Do you really think repeating that assertion over and over again makes
> it any more credible?
Do you think denying physics will make it go away? Don't bother answering
that.
Geez, as I wrote before, keep believing I'm wrong. I'll use my newfound
knowledge to send less of my money to the big oil companies. You all can
feel happy knowing that you're right and that you're going to waste a lot of
fuel proving it, DAMMIT. -Dave
223rem
>
> Do you think denying physics will make it go away? Don't bother answering
> that.
>
> Geez, as I wrote before, keep believing I'm wrong. I'll use my newfound
> knowledge to send less of my money to the big oil companies. You all can
> feel happy knowing that you're right and that you're going to waste a lot of
> fuel proving it, DAMMIT. -Dave
>
Assuming that you're not trolling (starting to look like a big assumption),
and that you're right, dont you think that one would find your fuel saving
rule in all owner's manuals?
Bernard Farquart
"Dave C." <no...@nohow.not> wrote in message
I thought I heard somewhere that "all otto cycle engines"
something something or something.
Maybe I didn't hear that?
Bernard
Bernd Felsche
>"Ted B." <no...@nohow.not.ever> wrote:
>>Not really. At best, it's a red herring.
[snip]
>Maximum engine efficiency is *very different* from maximum
>vehicle fuel economy.
Chuck; haven't you worked out that the only way to change his
immutable opinion is by percussion? :-)
--
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
\ / ASCII ribbon campaign | I'm a .signature virus!
X against HTML mail | Copy me into your ~/.signature
/ \ and postings | to help me spread!
Bernd Felsche
>In article <43284ff7$0$38737$892e...@authen.white.readfreenews.net>,
>Ted B. <no...@nohow.not.ever> wrote:
>>
>>The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
>>There is no way around that, as that's just how the Otto Cycle engine works.
>Do you really think repeating that assertion over and over again makes
>it any more credible?
Maybe if he puts a GIF anim on a web page. :-)
todd_...@performancesimulations.com
Dave, I am flabbergasted that you're arguing with me about this. For
the past several years I've made my living writing vehicle dynamics
software, including a couple of engine simulation packages, one of
which models pressure wave dynamics and their full interaction between
cylinders throughout all intake and exhaust channel junctions. In
other words, I do all the mathematics that predicts exactly the sort of
things we're discussing in this thread and put it into software form
for people to use. You're thigh is snapped in two places and sticking
out of your skin, yet you're telling the MD your leg isn't broken.
todd_...@performancesimulations.com
Dave, "fuel efficiency" is not the same thing as "fuel mileage."
"Fuel mileage" is expressed in miles per gallon. That, as you know,
means "how many gallons of fuel are consumed per mile travelled." This
is really the more important thing to consumers and saves them money
when maximized.
"Fuel EFFICIENCY" is expressed as a PERCENTAGE. From your own source:
"Argonne Labs measured the efficiency of the Japanese Prius engine to
be 34% (good for any engine at its peak) at only 13.5 hp.
"
So, Dave, how many miles per gallon is 34% fuel efficiency equal to?
If you're getting 34% fuel efficiency, how many gallons of fuel are you
burning every hour?
todd_...@performancesimulations.com
Chuck Tomlinson wrote:
> "Ted B." <no...@nohow.not.ever> wrote:
> >"223rem" <223...@sbcglobal.com> wrote...
> >> Chuck Tomlinson wrote:
> >>> The key issue here, I think,
> >>> is the difference between _engine efficiency_ and _vehicle fuel economy_.
> >>
> >> Very good point!
> >
> >Not really. At best, it's a red herring.
>
> Interesting. I offered you information (actual data that I
> gathered from vehicle instrumentation) that shows how fuel
> economy can decrease while engine efficiency is increasing.
> Do you need clarification of the data? Assumptions? Math?
>
> My data shows that your premise is wrong. Furthermore, it
> also shows (quite clearly IMHO) that engine efficiency
> (BSFC) and fuel economy (e.g., mpg) are *completely*
> different.
>
<snip excellent stuff>
> Chuck Tomlinson
Dave, Chuck is 100% correct here. You're confusing "fuel economy"
(mileage) with "fuel efficiency" (percentage of fuel's heating value
that is converted to mechanical power).
One is "miles per gallon," the other is a percentage. They mean
completely different things. If they were the same thing, they would
not have different definitions and be expressed in different units!
todd_...@performancesimulations.com
Precisely.
todd_...@performancesimulations.com
todd_...@performancesimulations.com
If that makes perfect sense to you, something is seriously wrong not
only with you ability to discern the difference between fuel efficiency
(percentage) and fuel economy (mileage), but your common sense is
seriously lacking as well. Do you have any idea how much power would
be required to run your car at 234mph? You do understand that if the
fuel efficiency is fixed, and you require twice as much horsepower to
travel at some speed above 78mph, the amount of fuel consumed per
second doubles? If your speed does not also AT LEAST double along with
that doubling in fuel consumption, you're mileage will DECREASE even
though your fuel efficiency has remained the same.
As Chuck and others pointed out, the fuel efficiency is also tied to
the load (the amount of power/torque you're demanding from the engine),
and will increase as you open the throttle provided you're gearing the
car to remain at the same engine speed.
Let's take a practical example here to analyze this further. Hopefully
you will begin to see the difference between fuel efficiency
(percentage) and fuel economy (mileage) and get a glimpse into how they
are really related.
First of all, let's define "fuel conversion efficiency," which is
exactly the same thing as "engine efficiency," which is what is being
talked about at the web site you referred to on the Otto Cycle.
>From Heywood's book, "Internal Combustion Engine Fundamentals," page 52
(of about 900, no kidding, this is covered at the beginning of chapter
2):
"The fuel energy supplied which can be released by combustion is given
by the mass of fuel supplied to the engine per cycle times the heating
value of the fuel. The heating value of a fuel, Qhv, defines its
energy content. Is is determined in a standardized test procedure in
which a known mass of fuel is fully burned with air, and the thermal
energy released by the combustion process is absorbed by a calorimeter
as the combustion products cool down to their original temperature.
This measure of an engine's "efficiency," which will be called the fuel
conversion efficiency nf, is given by:
nf = Wc/mf Qhv = (Pnr/N)/(mdto nr/N)Qhv = P / mdot Qhv))
Where mf is the mass of fuel inducted per cycle. Substitution for
P/mdotf from [the equation] gives:
nf = 1 / (sfc*Qhv)"
----
Ok, that's the end of the quote. It goes on to show that a typical
heating value for gasoline is 44MJ/kg or 19,000 Btu/lbm. In fact,
later on in the book he actually calculates the heating value of
several fuels from the chemical equations for them, but we don't need
to go there.
Here's what all this means, Dave: If you burn 1kg of gasoline, you
will release 44MJ of energy. The faster you burn that fuel, the more
power you can get. I.e., if you burn two kg of gasoline every second
you can get twice as much power as if you only burned one. And of
course, like most other things in science, there is an equation that
calculates this:
Horsepower = MJ/sec * 1341.02209
This equation is derived from the very definitions of power and energy,
so it is as true as 2 + 2 = 4.
Using the equation, how much potential power could be harnessed if you
burned 1kg of gasoline per second?
Horsepower = MJ/sec * 1341.02209
Horsepower = 44 * 1341.02209
Horsepower = 59,004
Wow, that's a lot of horsepower, isn't it? Gasoline is some pretty
powerful stuff. 1Kg of gasoline is a lot of gas though. 1 gallon of
gas is 2.64kg, so to get 59,000 horsepower in a 100% efficient engine
you'd need to burn a gallon of gasoline about every three seconds .
These numbers are pretty huge so let's look at a bit more practical
example that applies more to typical cars under freeway cruising
conditions. Another equation from Heywood's book is:
HP = ((Cr*Wv + 0.0025*Cd*Av*MPH^2) * MPH) / 375
Where:
Cr = Coefficient of rolling resistance (between 0.012 and 0.015
typically)
Wv = Weight of the vehicle
Cd = Coefficient of drag
Av = Frontal area of the vehicle
MPH = miles per hour.
HP = the amount of power required to maintain whatever MPH you pop into
that equation given the other stuff above.
Assuming we have a 3500lb car with a 24 sq ft frontal area, a
coefficient of drag of 0.38, and a rolling resistance coefficient of
0.15, here's the amount of power required to travel at speeds from 20
to 100 mph. I'm assuming the drivetrain is 85% efficient, so if you do
the calculations yourself, divide the above HP number by 0.85. This
way, we're seeing the amount of power at the flywheel instead of the
driving wheels so later on our fuel calculations are more accurate.
Also, I threw in values for 200mph and 300mph just for fun:
20 mph 3.9 hp
40 mph 11.2 hp
60 mph 25.3 hp
80 mph 49.8 hp
100 mph 88.0 hp
200 mph 605.2 hp
300 mph 1980.7 hp
We'll refer to these values later. Just for the sake of interest, here
is the amount of power required for the rolling resistance due to tire
deformation (the 0.015 coefficient of rolling resistance) as well as
the amount required to overcome pure aerodynamic drag ("wind" below):
20 mph 3.3 hp rolling 0.6 hp wind
40 mph 6.6 hp rolling 4.6 hp wind
60 mph 9.9 hp rolling 15.5 hp wind
80 mph 13.2 hp rolling 36.6 hp wind
100 mph 16.5 hp rolling 71.5 hp wind
200 mph 32.9 hp rolling 572.2 hp wind
300 mph 49.4 hp rolling 1931.3 hp wind
Ever wonder what percentage of drag is due to tires versus rolling
resistance? Here are the values for this car:
20 mph 85.2% rolling 14.8% wind
40 mph 59.0% rolling 41.0% wind
60 mph 39.0% rolling 61.0% wind
80 mph 26.5% rolling 73.5% wind
100 mph 18.7% rolling 81.3% wind
200 mph 5.4% rolling 94.6% wind
300 mph 2.5% rolling 97.5% wind
Just for interests sake, note that at 60mph 39% of the drag is due to
the tires. That's pretty significant. If you changed tire pressure or
tires to lower the coefficient of rolling resistance to 0.012, here's
what happens at 60-100mph:
60 mph 23.4 hp
80 mph 47.2 hp
100 mph 84.7 hp
60 mph 33.8% rolling 66.2% wind
80 mph 22.3% rolling 77.7% wind
100 mph 15.6% rolling 84.4% wind
How much of an improvement is this?
60 mph 8.1%
80 mph 5.5%
100 mph 3.9%
Those percentages very roughly indicate the improvement in fuel economy
(mileage, not "fuel efficiency", which will actually decrease!) with
the lower drag tires. But not quite... As Chuck pointed out, these
values would be at different throttle settings and influence the fuel
efficiency. Here, you'd be closing the throttles slightly to maintain
the slightly lower power output, and most likely in doing so you would
would LOWER "fuel efficiency" and simulataneously RAISE FUEL ECONOMY.
Did you catch that? You're getting less horsepower per gallon of gas
(less fuel efficiency), but at the same time you require less power to
maintain these speeds so are most likely getting an increase in MPG.
Fuel efficiency went down, and fuel economy went up.
Anyway, because of the throttling effect Chuck talked about, the real
percentage in fuel economy (miles per gallon, not "fuel efficiency")
gained is going to be quite a lot lower than the numbers above, but at
least it gives a ballpark figure. And again, I can't stress enough
that fuel efficiency went one direction while fuel economy went the
other. You and I while driving our cars around don't really care about
fuel efficiency, which is a percentage, at all. We care how much it
costs to get from point A to point B. That's fuel economy, which is
miles per gallon. As I and several others said before, they are not
the same thing. They can move in opposite directions.
Ok, moving right along to the subject of the current debate. Let's see
if we can get some rough estimations of fuel mileage. To really do
this accurately we would need BSFC numbers at different loads (which
accounts for the change in throttle position) and different engine
speeds (not vehicle speeds), as Chuck and others have correctly stated
several times already. Just for the heck of it, we'll go ahead and try
some estimations and see if anything interesting pops up.
Ok, Dave, what we're going to do is assume that we're following your
rules here. We will gear the car so that the engine is always
operating at peak "fuel conversion efficiency" (same as the "engine
efficiency" you refer to, per the Heywood definition). Then we will
attempt to analyze what effect this might roughly have on fuel economy,
to see if indeed fuel economy is maximized when the engine is running
at the speed at which maximum fuel conversion efficiency exists at part
throttle (the 40% of redline bit).
In other words, are fuel efficiency and fuel economy the same thing in
the end? Let's see if we can find out.
Let's stick to the 60mph, 80mph, and 100mph speeds because that's a hot
spot with the the speed limit debate. Personally, I like high speed
limits and like to drive fast on the freeway, but at the same time I
think a valid argument for raising speed limits can not be found in an
analysis on fuel economy.
>From before, this is the amount of power required to run at 60, 80, and
100mph in our example vehicle:
60 mph 25.3 hp
80 mph 49.8 hp
100 mph 88.0 hp
Again, we will assume the car is geared so the engine is operating at
40-45% of the redline where our imaginary engine is producing its best
"fuel efficiency" (not economy).
For now, let's pick a fuel efficiency for our imaginary engine and see
what happens if it does not change with throttle position. Later, we
will use an actual example from a real engine where efficiency changes
with throttle position to examine what happens more closely. Dave,
from the web site you provided a link to:
"Argonne Labs measured the efficiency of the Japanese Prius engine to
be 34% (good for any engine at its peak) at only 13.5 hp."
That's extremely good compared to anything anyone here is probably
driving. I'd prefer to use something that is more indicative of what
we're used to, so I'll refer to page 723 of "Internal Combustion Engine
Fundamentals" where a graph of mechanical efficiency versus percent
load can be found for one particular engine. Mechanical efficiency is
the ratio of actual power output to what the power output would be
without the losses due to friction, i.e., throttle position. In other
words, this shows the pumping loss due to throttling. The fuel
efficiency (not economy) should scale up and down right along with the
graph as the throttle position changes pretty closely. So, we'll
assume that the full throttle fuel efficiency is 34% (rather than part
throttle like the Prius is), and scale that down by the mechanical
efficiency in the graph to get our "part throttle fuel efficiency."
And again, we're running at 40% of redline here so we are getting the
maximum fuel efficiency at this load that we can possibly get, per your
insistance.
Before we get the fuel efficiencies at part throttle, we need to find
the percent load (the graph shows mechanical efficiency as a function
of percent load). Percent load is the percentage of the total
available engine power at whatever RPM we're looking at. Let's assume
your engine produces 200 hp at its peak, up near the redline. At the
same time, we gear the car to keep the engine speed at 40% of the
redline where we'll have the peak part throttle efficiency. We'll
assume our engine has a pretty flat torque curve at part throttle in
this mid range and produces a maximum of only 130 hp at these speeds.
This is slightly above 1/2 maximum power output at peak rpm and is a
reasonable approximation for the purposes of this discussion.
Actually, that might be a bit high, which will result in lower MPG
calculations later on, but to heck it with it, let's do it anyway.
To summarize, our engine at full throttle at 40% of redline can produce
130 hp. We now need to know the percent loads throughout our speed
range. To find this we can simply divide the following horsepower
numbers into 130 hp:
60 mph 25.3 hp
80 mph 49.8 hp
100 mph 88.0 hp
The resulting "percent loads" are:
60mph 19.5%
80mph 38.3%
100mph 67.7%
That means that at 80mph, you are asking the engine to provide 38.3% of
the total power it is capable of producing in the mid rpm range
(130hp).
Looking at the graph, mechanical efficiency (the ratio of actual power
output to what the power output would be without the losses due to
friction, i.e., throttling losses primarily) varies from 45% to 80%
within the range we're interested in. For starters we'll assume the
mechanical efficiency is constant, right smack in the middle of that at
60%. Our best fuel efficiency is 34%, and thanks to all of our
throttling, we've reduced that by 60% to 20.4%.
Our "fuel efficiency" at part load in these conditions is 20.4%.
Now, how can we find our "fuel economy"? We are not getting 20.4
"percent" miles per gallon. That makes as much sense as saying I
stepped on a scale today and it told me I weighed 150 seconds or
degrees.
The heating value of our fuel is 44MJ/kg. Since there's 2.63kg of gas
in a gallon, we can say our heating value is 115.72MJ/gallon. We
converted MJ per second into horsepower, so let's write an equation
that goes the other way and converts horsepower into MJ per second
instead.
Horsepower = MJ/sec * 1341.02209
Therefore:
MJ/sec = Horsepower / 1341.02209
Now, taking our horsepower numbers from before and using that last
equation, let's calculate how many MJ/sec we need our fuel to produce
to keep us running along at these speeds:
60 mph 25.3 hp = 0.01887 MJ/sec
80 mph 49.8 hp = 0.03714 MJ/sec
100 mph 88.0 hp = 0.06562 MJ/sec
How many gallons of fuel must we burn per second to produce this?
Easy. Just divide the above MJ/sec numbers into 115.72, which is the
heating value of our fuel in MJ/gallon.
60 mph 25.3 hp = 0.000163 gallons per second
80 mph 49.8 hp = 0.000321 gallons per second
100 mph 88.0 hp = 0.000567 gallons per second
Now, Dave, remember how you said a lot of that heat goes straight out
the exhaust because even the Otto Cycle engines are horribly
inefficient? You're right. If we had 100% efficient engines we would
only need to burn the amount of fuel shown above to maintain those
speeds in our example car. However, as we determined before, our fuel
efficiency is 20.4% in these conditions. Therefore, we need to scale
these values up by a factor of 1 / 0.204 to find the real fuel
consumption rates.
Results:
60 mph 25.3 hp = 0.000799 gallons per second
80 mph 49.8 hp = 0.001574 gallons per second
100 mph 88.0 hp = 0.002779 gallons per second
Or (multiplying by 3600 seconds in an hour):
60 mph 25.3 hp = 2.8764 gallons per hour
80 mph 49.8 hp = 5.6664 gallons per hour
100 mph 88.0 hp = 10.0044 gallons per hour
That is how much fuel we must burn every hour in order to maintain
those speeds, including the effects of "fuel efficiency," which is at
our peak in ALL OF THE ABOVE SITUATIONS. I.e., we have geared the car
to keep the engine running at 40% of redline for 60, 80, and 100mph.
>From above it can be seen that at 100mph we will burn 3.47 times more
fuel every hour than we will at 60mph. However, obviously we are not
going 3.47 times further every hour (100mph is not 3.47 times 60mph).
Therefore, "fuel economy" (miles per gallon) has decreased
substantially, even though "fuel efficiency" remained constant. The
only way you are going to get more fuel economy (miles per gallon) at
100 mph than 60 mph is if your fuel efficiency increased, due to a
change in throttle position which produces a lower pumping loss (less
energy wasted rather than something gained as you mentioned earlier),
by a whole bunch. We'll get to that in a minute using the part
throttle data from Heywood's book to see if that's what indeed happens
as you say.
Now... BURN THIS INTO YOUR SKULL... The "fuel efficiency" is exactly
the same at all of the above speeds. It's 20.4%. The "fuel economy"
is "gallons per mile," which is different. To get the fuel economy we
can just divide our speed by the fuel consumption rate:
60 mph 60/2.8764 = 20.9 MPG (miles per gallon)
80 mph 80/5.6664 = 14.1 MPG (miles per gallon)
100 mph 100/10.0044 = 10.0 MPG (miles per gallon)
See that, Dave? Even though "fuel efficiency" stayed the same, the
"fuel economy" changed dramatically as speed was increased.
Now, the astute reader will notice that the mileage in these
calculations dropped substantially more with increasing speed than it
does in reality. The reason for this was covered already by Chuck.
Regardless of whether or not we regear the car to operate at 40% of the
redline where the peak fuel efficiency is maximized for each of the
above speeds, the fact is you must open the throttle more. Obviously
at any given engine rpm you must open the throttle more to produce 88hp
versus 49.8hp or 25.3hp. This is where we get back to page 723 of the
book.
In reality, as Chuck and the web site you referred to pointed out, the
efficiency increases as the throttle is opened. The mechanical
efficiency is a function of percent load. We calculated the load
percentages already. Here they are again:
60mph 19.5%
80mph 38.3%
100mph 67.7%
Here are the mechanical efficiencies for an engine at all of those load
percentages. I'm not making these numbers up, they are measurements
from a real engine that was tested throughout the entire throttle range
at mid range engine speed; the speeds in question:
60mph 50%
80mph 70%
100mph 85%
Now, our fuel efficiency scales up and down right along with these
mechanical efficiences. Our peak, full throttle fuel efficiency is 34%
at this "ideal 40% of redline." Therefore, here are our fuel
efficiencies with throttling effects included at the different engines
speeds, including the effects of changes in gear ratio:
60mph 17.0%
80mph 23.8%
100mph 28.9%
The fuel efficiency increased with speed here because we are opening
the throttle more, which is reducing engine friction (primarily
throttling work loss). Dave, this equates to the comment you made
about how when you open the throttle, you are "wasting less energy"
rather than "gaining more." The end result is you're getting more
power per unit fuel being burned, so whether you're "wasting less" or
"gaining more" is irrelevant. The end result is the same. You get
more POWER per gallon. But do you get more MILES per gallon? That's a
different question.
To find out, let's go back to our 100% efficient engine where we found
out how many gallons of fuel we burned per hour (not per mile):
60 mph 25.3 hp = 0.000163 gallons per second
80 mph 49.8 hp = 0.000321 gallons per second
100 mph 88.0 hp = 0.000567 gallons per second
Scaling this up to gallons per hour (multiply by 3600 seconds in an
hour):
60 mph 25.3 hp = 0.5868 gallons per hour
80 mph 49.8 hp = 1.1556 gallons per hour
100 mph 88.0 hp = 2.0412 gallons per hour
Ok, now, to compensate for the fact that our fuel efficiency is
changing due to changing throttle position, we will use the data we
derived from the real data on the part throttle engine. All of those
fuel efficiency values (the percentages) can now be divided into the
above fuel consumption rates, which are assuming a 100% efficient
engine. Here we're doing the same calculation before where we divided
everything by 20.4% by assuming the fuel efficiency didn't change with
throttle position, but instead using the different fuel efficiencies at
different throttle positions that now compensate for throttle position.
Fuel efficiences (again, these are percentages and not miles per
gallon):
60mph 17.0%
80mph 23.8%
100mph 28.9%
Actual fuel consumption rates including variations in throttle
position:
60 mph 3.4518 gallons per hour
80 mph 4.8554 gallons per hour
100 mph 7.0630 gallons per hour
Now, again, "fuel economy" is something entirely different, and we can
now calculate what the trend in fuel economy variation with speed will
be. As before, all we need to do is divide the speed by the
consumption rate. Here are the results:
60 mph 17.38 MPG (miles per gallon)
80 mph 16.47 MPG (miles per gallon)
100 mph 14.16 MPG (miles per gallon)
The mileages here are lower than you're used to seeing, probably
because of a mismatch between the engine data graph and the
hypothetical engine in this example, as well as the rather large 24 sq
foot frontal area, fairly high drag coefficient, powerful engine, and
rather inefficient drivetrain (85% at part throttle), but what's
important here is that even when you take into account all of these
effects, the "fuel economy" is decreasing with increasing speed, while
the "fuel efficiency" is increasing very sharply because of the
throttling changes. The variation in "fuel efficiency" with ENGINE
SPEED is very different from the variation in "fuel economy" with
VEHICLE SPEED.
FUEL ECONOMY is DECREASING while FUEL EFFICIENCY is INCREASING as
vehicle speed increases and engine speed is held constant, due
primarily to the throttling effect both you and Chuck have referred to.
--------------
In summary:
Dave, as you've been told by virtually everyone in this thread, "fuel
efficiency" and "fuel economy" are not the same thing. Through the use
of gearing, placing the ENGINE rpm at "peak fuel efficiency" (40-50% of
redline in the Otto Cycle, or any percentage of redline in any engine
or cycle at all) does not also place the VEHICLE and engine in
combination at "peak fuel economy." In fact, as can be seen above, the
two variables move in opposite directions at part throttle at constant
speed as that speed is altered.
If you continue to argue this point, you deserve to be horribly abused
as a blatantly ignorant moron like Laura Bush Killed whatever. I shall
be forced to taunt you incessantly as a raging idiot from this day
forwards if you do so. ;-)
Furthermore, if you bother to continue to argue without reading and
giving an honest attempt to understand my analysis, which I've now
spent a significant amount of time on, I will be personally offended to
a level I care not to describe. In addition, your failure or success
will be a public exhibition of your level of intelligence. Please
don't disappoint us. If there is something you don't understand, ask.
But it would be idiotic in the extreme for you to continue to tell me
I'm wrong at this point because of your damn 40% scientific fact, which
does not refer to fuel economy, but rather fuel efficiency, which both
move in opposite directions anyway in the case we're debating as just
illustrated.
"Peak FUEL EFFICIENCY is at 40% of redline?" Yes, that's probably
about right. I don't argue with that.
"Peak FUEL ECONOMY (mileage) is at 40% of redline?" Utter nonsense.
If it is in your case, it's a coincidence.
I must note on the side that it is refreshing to see that nobody else
required such a lengthy explanation. Their faculties of common sense
and unbiased experience were all that were required to come to the same
conclusions I have through a reasonably detailed analysis. Hats off to
you guys. Good to see some smart folks around this group.
todd_...@performancesimulations.com
Harry K wrote:
> todd_...@performancesimulations.com wrote:
> > Ted B. wrote:
> > > "223rem" <223...@sbcglobal.com> wrote in message
> > > news:SiXVe.329298$x96.300841@attbi_s72...
> > > > Chuck Tomlinson wrote:
> > > >> The key issue here, I think,
> > > >> is the difference between _engine efficiency_ and _vehicle fuel economy_.
> > > >
> > > > Very good point!
> > >
> > > component of the car is adding kinetic energy to the car.
> >
> > At a constant 78mph nothing is adding any kinetic energy to the car at
> > all, INCLUDING the engine. :-)
> >
> > >
> > > The Otto Cycle engine is most fuel efficient at 40-45% of redline RPM.
> > > There is no way around that, as that's just how the Otto Cycle engine works.
> > > mileage. But if the Otto Cycle engine is still allowed to operate at about
> > > 40% of redline RPM, that is STILL going to be it's most fuel-efficient
> > > speed. -Dave
> >
> > Dave, this 40% of redline RPM efficiency value you read about refers to
> > "fuel conversion efficiency," not "fuel mileage." They are two vastly
> > different things and are not equal or directly representative of each
> > other by any stretch of the imagination. In fact they have different
> > units.
> >
> > All that the "fuel conversion efficiency" value means is that for every
> > gallon of fuel you're burning, you're getting the most possible power
> > indication of fuel mileage at all.> Now, now Todd. Yau are trying to confuse him with facts, something
> that he apparently has very little association with. He thinks that
> repeating Otto cycle and 40% repeatedly somehow proves his assertions
> (and that is all they are).
>
> Harry K
Indeed.
Ted B.
>
> "Fuel mileage" is expressed in miles per gallon. That, as you know,
> means "how many gallons of fuel are consumed per mile travelled." This
> is really the more important thing to consumers and saves them money
> when maximized.
>
>
> "Fuel EFFICIENCY" is expressed as a PERCENTAGE. From your own source:
> "Argonne Labs measured the efficiency of the Japanese Prius engine to
> be 34% (good for any engine at its peak) at only 13.5 hp.
> "
I don't know why this concept is so hard to grasp for some people. If the
engine is most efficient at a certain RPM, that is exactly the RPM that will
get the greatest fuel economy. I'm not talking about max horsepower or max
torque, I'm talking about the RPM at which the greatest -percentage- of heat
energy is converted to KINETIC ENERGY. This is going to be the most
fuel-efficient RPM for the engine, because the engine is producing the
greatest ratio of output kinetic energy in relation to rate of fuel
consumption. Now, what some people are failing to grasp is that the engines
that these facts apply to are (duh) used to provide kinetic energy to
accelerate an automobile and keep it moving. Nothing could be simpler,
IMHO. Yet it seems to go right over the heads of many people.
Simple question: If you know the RPM that your engine is most
fuel-efficient at, are you only going to run your engine at that speed while
the car is in Neutral? Because that is the only way you can avoid
maximizing the fuel efficiency of your automobile if you are running it's
engine at the most fuel-efficient RPM. -Dave
todd_...@performancesimulations.com
My friend, you are sorely mistaken.
Ted B.
>> And meanwhile, the car is? . . . (class, help him out a little here)
>>
>> Nobody?
>>
>> Moooooooooooooving. Eating up miles. -Dave
>
> Dave, I am flabbergasted that you're arguing with me about this. For
> the past several years I've made my living writing vehicle dynamics
> software, including a couple of engine simulation packages, one of
> which models pressure wave dynamics and their full interaction between
> cylinders throughout all intake and exhaust channel junctions. In
> other words, I do all the mathematics that predicts exactly the sort of
> things we're discussing in this thread
And I'm just as surprised that you're arguing against me if you know as much
as you claim to know. We both know that at a certain RPM, an engine is
wasting the least amount of heat energy, and thus outputting the greatest
ratio of kinetic energy in relation to the rate of fuel consumption. YOU
YOURSELF wrote just that, although you used more technical terms to describe
it. How is it that an engine can be most fuel-efficient at a certain RPM
and yet you expect the vehicle that the engine is installed in to NOT be
most fuel-efficient when the engine is running at that RPM? The only way
that might happen would be if some other component of the engine (or
possibly the entire rest of the vehicle) was grossly mis-matched with the
engine. For an extreme example, a 100HP engine would not be too
fuel-efficient if installed in a cement mixer, regardless of what RPM it ran
at. (in fact, the cement mixer might not move at all) -Dave
Ted B.
>>> in the dash see different data, they disbelieve you.
>>>
>>> Bernard
>>
>> As they should, if they are only looking at their own vehicle. From what
>> I
>> read, not all vehicles will get better mileage at higher speeds/RPM. A
>> lot
>> of them will. But seeing someone say that their own car doesn't really
>> doesn't prove anything, one way or the other. -Dave
>
>
> I thought I heard somewhere that "all otto cycle engines"
> something something or something.
>
> Maybe I didn't hear that?
>
> Bernard
No, that much is true. What so many people are forgetting is that outside
of optimum RPM, fuel efficiency will decrease. This means that if 50MPH is
most efficient, 40MPH will be less efficient and 60MPH will be less
efficient. I see people saying things like, "but my car is more efficient
at 50MPH than 60MPH". Well, duh. But the people posting comments like that
are trying to prove that ALL cars with Otto Cycle engines will be most
fuel-efficient when driven at low speeds like (for example) 50MPH. -Dave
Ted B.
>> Geez, as I wrote before, keep believing I'm wrong. I'll use my newfound
>> knowledge to send less of my money to the big oil companies. You all can
>> feel happy knowing that you're right and that you're going to waste a lot
>> of
>> fuel proving it, DAMMIT. -Dave
>>
>
> Assuming that you're not trolling (starting to look like a big
> assumption),
> and that you're right, dont you think that one would find your fuel saving
> rule in all owner's manuals?
>
Yes, I would THINK so, but that doesn't seem to be the case. My owner's
manual says (from memory) that my car should be most fuel-efficient around
50MPH. I have no idea how that was calculated, unless whoever wrote it
assumed that the car would be in drive gear at the time. Silly me, I keep
throwing it into OVERdrive and messing up their calculations. :)
On this topic, I did discover something VERY INTERESTING about my own car,
and there is no way in HELL this is a coincidence:
I'd previously found that my most fuel-efficient speed was
3500RPM/43%/78MPH. At first, I jumped to the conclusion that my car was
therefore designed for high-speed cruising. Then I realized HEY, that's in
OVERdrive . . . I wonder what the most fuel-efficient speed is in DRIVE???
So I downshifted and adjusted the engine to 3500RPM/43%. I'll be DAMNED if
the speedometer wasn't glued to . . . (you see this coming, don't you?)
********* 55MPH!!! *********
As I wrote before, there is no way in HELL that that is a coincidence. I
guess my car is just a family sedan, after all. :) Designed for maximum
fuel economy at 55MPH in Drive. That makes perfect sense, considering the
market that the car was targeted at.
How this relates back to your question? Well most car owners
(unfortunately) wouldn't use a tachometer, even if they had a good
understanding of how engine performance changes at certain RPMs. (the
readers of this ng are obviously the exceptions, as a greater percentage of
readers of this ng probably WOULD use a tach) But, just about all car
owners are familiar with the speedometer, and use it regularly. So for the
manufacturer of my car to suggest 50MPH as a speed to maximize fuel economy
does make sense. It SHOULD be pretty fuel-efficient at 50MPH in Drive, as
that's awful darn close to 40%. The KISS principle obviously applies here.
It's easiest to tell a car owner to drive 50MPH. To try to explain that
they can also achieve good fuel economy at 78MPH (by upshifting) would be
confusing for a lot of people. Plus, it's not good to advise people to
break the law (in most areas, 78MPH would be illegal). So, 50MPH it
s. -Dave
todd_...@performancesimulations.com
Dave, you are now officially branded a raging, fucking jackass. Take
your discussion and shove it straight up your ass if you're not going
to bother reading my post. I spelled it out plain as day, and spent
several hours writing that in a way a high school student could
understand. I firmly believe that everyone here that bothers to read
it will understand and learn from it.
Everyone except you.
I urge everyone here not to bother discussing this matter any further
with this fucking ignorant idiot.
Goodbye, Dave. Try not to drown in the shower tomorrow morning before
work. The hot knob is on the left.
todd_...@performancesimulations.com
Ted B.
> <raises hand>
>
> As far as I have been able to tell from research on and offline, my
> Mustang has an approximate redline of 4100-4200 RPMs (though the tach
> reads up to 6000 and has no marked redline; they used the same tach for
> all the various engines), but I can't remember a time when I took it
> above 3700 RPMs (that's ~40 MPH in first). My best mileage comes at
> around 2400 RPMs and 64 MPH, about where the engine has it's peak
> torque. That's 57-58% of redline, if my redline estimate isn't far off.
Are you sure that ~4200 RPM isn't where the engine hits the REV limiter? If
I used the REV limiter as redline, my own car would be most fuel efficient
at 55%, which wouldn't make sense. But if we look at your car, 2400/6000 is
exactly 40%. According to facts I've gathered from my research, THAT IS
EXACTLY RIGHT!!! That is, I'm not surprised at all to hear your engine
getting maximum fuel economy right at 40%, when it should achieve maximum
fuel economy somewhere between 40% and 45%.
Another interesting thing you wrote . . .
According to you, your car gets its best mileage at 64MPH. Many people have
posted that their car gets maximum fuel economy somewhere between 40 and
50MPH (and many of them are probably right, as their engine could be running
close to optimum RPM at those speeds). Some have gone so far as to claim
that such low speeds (regardless of engine RPM) should achieve maximum fuel
economy in any car. They essentially say to slow down to save fuel. I'm
sure that they would tell you that 64MPH is unnecessarily wasteful, because
it is not fuel efficient. WHAT WOULD YOU SAY TO THOSE PEOPLE?
Me, I'd say that I'm not going to slow down to waste fuel just because
someone else has their head up their ass. But I'd like to hear your view on
the matter. -Dave
Dave
todd_was...@performancesimulations.com wrote:
> Todd Wasson
> Performance Simulations
> http://www.PerformanceSimulations.com
I only snipped it cause it was so long, but great post. Very
informative and interesting.
Another Dave
todd_...@performancesimulations.com
Thanks, Dave. Unfortunately the bonehead still thinks he knows more
about this subject than anyone else here, myself included. Some people
are like that. I'll never understand them, but they're out there :-)
223rem
Chuck Tomlinson
>>
>Are you sure that ~4200 RPM isn't where the engine hits the REV limiter? If
>I used the REV limiter as redline, my own car would be most fuel efficient
>at 55%, which wouldn't make sense.
LOL! We'll get back to that... AFAIK, the rev limiter is
always set at or beyond the redline. A redline that
couldn't be reached (except with a heinous downshifting
error) would be purely academic, wouldn't it?.
[...]
>Me, I'd say that I'm not going to slow down to waste fuel just because
>someone else has their head up their ass.
Which reminds me: how _did_ you determine that your engine's
redline is 8000+ rpm? The manufacturer of your 140 hp
Galant seems to think that the redline is ~6000 rpm, and has
set the rev limiter accordingly. Do the numbers on your
tach go all the way to "8", Dave? Is _that_ why you think
your redline is 8000 rpm?
--
Chuck Tomlinson
223rem
> Dave, you are now officially branded a raging, fucking jackass.
More likely, a troll.
Chuck Tomlinson
>
>Chuck; haven't you worked out that the only way to change his
>immutable opinion is by percussion? :-)
Dave has actually stunned me. Maybe I haven't been paying
attention, but I thought he was a pretty rational guy.
It's like a flashback to the bad old days of Lloyd Llogic
and Carlian physics: an apparently coherent person has
latched on to an untenable idea and cannot (or will not)
allow reason, analysis, and data to influence him.
I was surprised when he blew off Ed White's well-reasoned
arguments and analyses, but when Todd Wasson laid the whole
story out for him, and he remained absolutely uneducated,
well... I didn't see that coming.
A smart friend of mine (and fellow engineer) recently told
me: "Many people *prefer* to believe in magic". Sadly, he
might be right.
--
Chuck Tomlinson
Garth Almgren
>><raises hand>
>>
>>As far as I have been able to tell from research on and offline, my
>>Mustang has an approximate redline of 4100-4200 RPMs (though the tach
>>reads up to 6000 and has no marked redline; they used the same tach for
>>all the various engines), but I can't remember a time when I took it
>>above 3700 RPMs (that's ~40 MPH in first). My best mileage comes at
>>around 2400 RPMs and 64 MPH, about where the engine has it's peak
>>torque. That's 57-58% of redline, if my redline estimate isn't far off.
>
>
> Are you sure that ~4200 RPM isn't where the engine hits the REV limiter?
Yep, I'm fairly sure; the car is 23 years old, and barely has an
electronic ignition. :) Though some Duraspark II cars had rev limiters
in the later years, this isn't one of them.
> If
> I used the REV limiter as redline, my own car would be most fuel efficient
> at 55%, which wouldn't make sense. But if we look at your car, 2400/6000 is
> exactly 40%.
But, as far as I can tell, the documented redline is either 4100 or 4200
RPMs. I'm pretty sure that the engine, even when new, would grenade
itself before it got to 5000, let alone 6000. I don't think they took
much care in balancing it...
> According to facts I've gathered from my research, THAT IS
> EXACTLY RIGHT!!! That is, I'm not surprised at all to hear your engine
> getting maximum fuel economy right at 40%, when it should achieve maximum
> fuel economy somewhere between 40% and 45%.
Heck if I know; I'm just reporting my experience and what I know about
my particular car.
> Another interesting thing you wrote . . .
>
> According to you, your car gets its best mileage at 64MPH.
Give or take a couple MPH, yeah.
>
> Many people have
> posted that their car gets maximum fuel economy somewhere between 40 and
> 50MPH (and many of them are probably right, as their engine could be running
> close to optimum RPM at those speeds). Some have gone so far as to claim
> that such low speeds (regardless of engine RPM) should achieve maximum fuel
> economy in any car. They essentially say to slow down to save fuel. I'm
> sure that they would tell you that 64MPH is unnecessarily wasteful, because
> it is not fuel efficient. WHAT WOULD YOU SAY TO THOSE PEOPLE?
Just this: according to my records, cruising in my car at 60-65 nets me
as much as 5 MPG more than cruising (*not* stop-and-go) at 45-50 MPH. 5
MPG is *not* an insignificant difference; it's the difference between
210 and 275 miles per tank for me.
> Me, I'd say that I'm not going to slow down to waste fuel just because
> someone else has their head up their ass.
Me, the added mileage is just a perk; I'm simply not going to slow down
below the average speed and statistically increase my risk of collision
"just because someone else has their head up their ass."
--
~/Garth |"I believe that it is better to tell the truth than a lie.
Almgren | I believe it is better to be free than to be a slave.
******* | And I believe it is better to know than to be ignorant."
(p...@v6stang.com for secure mail info) --H.L. Mencken (1880-1956)
Arif Khokar
> Damn, I wish I had more time. I'd be willing to bet you money that you are
> wrong. The only condition I would attach is that I'm allowed to inspect the
> car (mainly for obvious stuff like proper tire pressure, fluid levels, just
> to make sure that it will be safe to test) and allowed to drive it. I'm a
> very good driver with a perfect driving record, and yes, I do have a license
> (before some smartass asks). Your car would be in very good hands. I'd
> baby it, unlike I treat my own car. :) I'd be shocked if your car actually
> got better mileage at 45 than it did at 70.
There's a 35 mile stretch of 4 lane highway that's posted at 55 mph.
Enforcement is on the heavy side most of the time and I usually don't go
faster than 65 mph (engine rpm is around 2400 to 2600).
If I start on that trip after resetting the stored average mileage value
in the trip computer, at the end, I have an average of 27 and
occasionally as high as 29.5 mpg.
On the interstate (85 mile drive), I drive 75 to 80 (engine rpm around
3000 to 3200), I get around 23 to 24 mpg.
If I recalculate the 40% figure based on the highest value on the tach
(8000 rpm), then it would be 3200 rpm.
I suppose the next time I drive on a 2 lane road and get stuck behind
traffic going 45 mph and travel at that speed for about half an hour,
I'm sure my mileage rating will exceed 30 mpg.