MAF reading g/s

[Harry Bloomfield]

My Autel OBD diagnostics unit reports its MAF reading as nn.nn g/s.

Is anyone able to tell me what the g/s means please?

My best idea idea so far is gallons of air per second, but 17.23 seems
rather a lot for an engine at tickover. I know the MAF is working fine
BTW, from checking its voltage output.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

[Mrcheerful]

Harry Bloomfield wrote:
> My Autel OBD diagnostics unit reports its MAF reading as nn.nn g/s.
>
> Is anyone able to tell me what the g/s means please?
>
> My best idea idea so far is gallons of air per second, but 17.23 seems
> rather a lot for an engine at tickover. I know the MAF is working fine
> BTW, from checking its voltage output.

grams per second seems to be the answer, though why, rather than a volume is
unclear, perhaps because of the density of air changing with temperature?

[Tim+]

I guess the clue is in the name, Mass Air Flow detector. It's the "mass"
of air passing it that is measured by heat dissipation from the element.
Thinner air will remove less heat even though it has the same volume. Being
thinner, it requires less fuelling.

Tim

[Gareth Magennis]

"Harry Bloomfield" <harry...@NOSPAM.tiscali.co.uk> wrote in message
news:mn.c3bf7dc813...@NOSPAM.tiscali.co.uk...

The ECU would need to work out the ratio of air mass to fuel mass injected.
The fuel mass can be calculated from the volume and the ambient temperature,
the air mass read directly from the MAF, so everything should still work OK
from the Dead Sea to the Khardung La.

(I presume the above, but stand to be corrected)



Gareth.

[Dave Baker]

Here's the problem though. The lambda sensor will be mandating an air fuel
ratio of 14.7 which means 1.17 grams of fuel per second. That's 4.2 kg/hr or
9.3 lbs/hr which is nearly 1.25 gallons per hour. Now a car travelling at a
steady 50 mph and achieving 40 mpg is also burning 1.25 gallons of fuel per
hour and a car at tickover can't be burning anywhere near that much. From
people's readings from fuel consumption sensors in modern cars about 0.2 to
0.3 gallons per hour at tickover is more like it. So that air mass reading
is 4 or more times higher than possible if it's in grams/second.
--
Dave Baker

[Mrcheerful]

I found this while hunting around:
"for the 3RZ motor they are 2.6 - 3.7 gm/sec at idle and 9.2 - 13.3 gm/sec
at 2,500 rpm without load"
That is a fairly big engine and shows the logically right figure as Dave
described.

So the OP getting 17 odd, maybe a factor of ten out?

[Dave Baker]

Mrcheerful wrote:

> Dave Baker wrote:
>> Here's the problem though. The lambda sensor will be mandating an air
>> fuel ratio of 14.7 which means 1.17 grams of fuel per second. That's
>> 4.2 kg/hr or 9.3 lbs/hr which is nearly 1.25 gallons per hour. Now a
>> car travelling at a steady 50 mph and achieving 40 mpg is also
>> burning 1.25 gallons of fuel per hour and a car at tickover can't be
>> burning
>> anywhere near that much. From people's readings from fuel consumption
>> sensors in modern cars about 0.2 to 0.3 gallons per hour at tickover
>> is more like it. So that air mass
>> reading is 4 or more times higher than possible if it's in
>> grams/second.
>
> I found this while hunting around:
> "for the 3RZ motor they are 2.6 - 3.7 gm/sec at idle and 9.2 - 13.3
> gm/sec at 2,500 rpm without load"
> That is a fairly big engine and shows the logically right figure as
> Dave described.
>
> So the OP getting 17 odd, maybe a factor of ten out?

I had a hell of a job finding good data online about this but finally came
across a Canadian study of idling fuel consumption for gasoline car engines.

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CEcQFjAG&url=http%3A%2F%2Fs3.amazonaws.com%2Fzanran_storage%2Fidlefreevt.org%2FContentPages%2F15971038.pdf&ei=5RA5UM2FDaiy0QWsjYGYAQ&usg=AFQjCNGVt1XK6IPsQmggAdoppsdGJZVr2A&sig2=yHGwdwHk0YwIUNIKhgqDRQ&cad=rja

There was considerable variability in the data but larger engines generally
consumed less fuel per litre of engine size per hour than small ones and a
good average for small capacity post 1991 FI engines was about 0.6
litres/hr/litre of engine size so 1.2 litres/hr for my 2.0 Focus. However
this was 1999 data and I suspect modern thin oils and low friction piston
skirt design etc have reduced this somewhat so I'll settle on 0.5l/hr/litre.

1 litre per hour for my 2 litre engine equates to 0.21 imperial gallons per
hour or 0.26 US gallons per hour which ties in well with the 0.2-0.3 US
gallons per hour people are quoting on American forums for cars with
instantaneous fuel readout computers.

Running that through my fuel consumption analysis spreadsheet, 1 litre/hr of
fuel at 14.7 A/F ratio equates to 3 grams of air per second which also ties
in nicely with your figures above so clearly that's the ballpark idle air
consumption for engines of that sort of capacity. I'm sure you could double
that or more for a big V8. For reference that's about 2.5 litres of air per
second at STP.

So at least I've learned something about a topic I'd never considered
before.
--
Dave Baker

[RJH]

On 25/08/2012 19:49, Dave Baker wrote:
> Mrcheerful wrote:
>> Dave Baker wrote:
>>> Here's the problem though. The lambda sensor will be mandating an air
>>> fuel ratio of 14.7 which means 1.17 grams of fuel per second. That's
>>> 4.2 kg/hr or 9.3 lbs/hr which is nearly 1.25 gallons per hour. Now a
>>> car travelling at a steady 50 mph and achieving 40 mpg is also
>>> burning 1.25 gallons of fuel per hour and a car at tickover can't be
>>> burning
>>> anywhere near that much. From people's readings from fuel consumption
>>> sensors in modern cars about 0.2 to 0.3 gallons per hour at tickover
>>> is more like it. So that air mass
>>> reading is 4 or more times higher than possible if it's in
>>> grams/second.
>>
>> I found this while hunting around:
>> "for the 3RZ motor they are 2.6 - 3.7 gm/sec at idle and 9.2 - 13.3
>> gm/sec at 2,500 rpm without load"
>> That is a fairly big engine and shows the logically right figure as
>> Dave described.
>>
>> So the OP getting 17 odd, maybe a factor of ten out?
>
> I had a hell of a job finding good data online about this but finally came
> across a Canadian study of idling fuel consumption for gasoline car engines.
>
> http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CEcQFjAG&url=http%3A%2F%2Fs3.amazonaws.com%2Fzanran_storage%2Fidlefreevt.org%2FContentPages%2F15971038.pdf&ei=5RA5UM2FDaiy0QWsjYGYAQ&usg=AFQjCNGVt1XK6IPsQmggAdoppsdGJZVr2A&sig2=yHGwdwHk0YwIUNIKhgqDRQ&cad=rja
>

Just as an aside, interesting comments on stop-start systems - restrict
to 6-10 times a day and 45s and longer I think it says. Very different
to how it works on my Mini.

They don't seem to understand the meaning of the word 'cost' though -
only ever expressed in money.

Rob

[Harry Bloomfield]

Mrcheerful explained :

Thanks, yes it could well be the Autel firmware misinterpreting the
number.

What I do know is that I occasionally get a low MAF reading OBD fault
code generated and if I switch on my Synergy's MAF compensation, the
fault is never generated.

Switch off generates figures in the 14 g/s range
Switch on generates figures in the 17 g/s range

[DavidR]

"Gareth Magennis" <sound....@btconnect.com> wrote

>
> The ECU would need to work out the ratio of air mass to fuel mass
> injected.
> The fuel mass can be calculated from the volume and the ambient
> temperature, the air mass read directly from the MAF, so everything should
> still work OK from the Dead Sea to the Khardung La.
>
> (I presume the above, but stand to be corrected)

Fuel has a permitted specific gravity range from the pumps due to chemical
variation up as well as from temperature. But as it doesn't change in rapid
time, presumably the necessary conversion of mass to injector time is
adequately derived from lambda sensor history.

[Peter Hill]

On 25/08/2012 19:49, Dave Baker wrote:

If the ECU "knows" you will be stopped for more than 45secs you could
develop it to win the lottery. To have any hope predicting duration of
stop it would have to be manual control and most people wouldn't
(didn't) bother. If you knew the lights at a junction take over 45sec to
cycle and you just pulled a red you could switch off.

The Canadian study discussed the current vehicle stock with starters
that are only designed for a few starts per day. Most start/stop systems
the starter/generator/battery has been beefed up. Early Citroen
start/stop used a 20Kw motor/generator in the flywheel. The car actually
moves off on the starter - something that will kill a normal starter
motor quite quickly. (not that any Citroen is electrically trustworthy)

The report failed to discus "hot start enrichment". My car has a
solenoid that disconnects the fuel pressure regulator from the manifold
and sets it to atmospheric for 3 min after a hot start (water over
45�C). Others have enrichment tables in the ECU.

Low load (cruise) is also very inefficient. The bigger or more powerful
the engine is the worse it is as it's further from it best efficiency.
The BSFC (grams/Kw.h) is much higher (2-3x) at low loads than at peak
efficiency. The low mpg at lower speeds is due to low power needed and
not the engine efficiency. All emission driven efficiency improvements
like electronic ignition, fuel injection, DOHC, 4 valves per cylinder
have the biggest effect at high rpm and load - that's why they all
appeared first on racing engines. Variable valve timing is also used for
improved high rpm efficiency without reducing low rpm efficiency.
http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption
This is why so many car makers are downsizing and turbocharging (or in
VW case with 1.4L twin charging). the smaller engine at cruise rpm is
more highly loaded and will give better mpg. The old fix of ever taller
overdrive gears to give better mpg results in a vehicle that has lower
top gear acceleration over 60-100mph compared to one with an engine
1/3rd the size with small turbocharger and a true top gear.

Conclusion of fuel consumption studies is that the solution is a range
extender Hybrid electric, using all electric transmission. What most
Hybrid makers have failed to do so far is tune engine to run at the
fixed speed to obtain best BSFC instead of over a wide range. Then
design the generator for peak efficiency at that speed. No need for
complex valve control, miller cycles etc. So some of what it costs in
electrical generator, motor and battery is offset by the reduced engine
cost.

Almost all range extenders are too big. A range extender engine/gen only
needs to supply a little bit more than M-way cruise load - about
30-40bhp. GM think the Volt needs 73Kw of generator, it's a range
extender, not an excess load leveler. They usually specify an over large
1.2-1.5L running at about 3000rpm. It's the engine they make now with a
generator. It's the result of years of tuning for a wide load range
required for car engines with a gearbox. They have started thinking
about cutting a cylinder off to make the 4 a 3, to reduce friction and
increase loading. They need to take a much smaller 350-500cc 2 or 3
cylinder engine and tune it to give best BSFC at higher rpm. A Honda
CX500 would do for a development buck.

--
Peter Hill
replace nospam with domain host name to reply

[Dave Baker]

"Peter Hill" <peter....@nospam.demon.co.uk> wrote in message
news:46t0s.119208$0b5....@fx28.am4...

> Almost all range extenders are too big. A range extender engine/gen only
> needs to supply a little bit more than M-way cruise load - about 30-40bhp.
> GM think the Volt needs 73Kw of generator, it's a range extender, not an
> excess load leveler. They usually specify an over large 1.2-1.5L running
> at about 3000rpm. It's the engine they make now with a generator. It's the
> result of years of tuning for a wide load range required for car engines
> with a gearbox. They have started thinking about cutting a cylinder off to
> make the 4 a 3, to reduce friction and increase loading. They need to take
> a much smaller 350-500cc 2 or 3 cylinder engine and tune it to give best
> BSFC at higher rpm. A Honda CX500 would do for a development buck.

You have to be a bit careful with the assumptions about the optimum sized
engine for a range extender. If you look at the charts of BSFC for engines
most produce their lowest BSFC at about or just under half of peak rpm and
about 80% throttle. That works out to be about 35-40% of the peak bhp
output. For example.

http://www.saturnfans.com/forums/showthread.php?t=116769

This implies that to generate 30-40 bhp you want an 80 bhp engine to operate
it in its best BSFC range.

However, and it's a big but, a smaller engine operating at a higher
percentage of its peak output might not be in its best BSFC range but due to
lower frictional losses might still have a better BSFC than the larger
engine. Or it might not as smaller engines tend to be less thermally
efficient than larger ones. The only way to be sure is to pick the engine
with the best BSFC at the target output regardless of engine size or peak
output. Regardless of other considerations that pretty much mandates a
diesel rather than a petrol to start with.

For many years now I've been working on the problem of designing the most
fuel efficient vehicle with the help of various spreadsheets I've written to
calculate road load bhp from drag and vehicle mass data and turn that into
mpg using BSFC assumptions. I'd be happy to share the equations if you want
to write your own version.

When bringing electric vehicles or hybrids into the equation I see a number
of options. Pure electric vehicles have very limited range and long battery
recharge times. No use for long trips. So how best to design a hybrid?

If you design in a long range on electric power only the battery mass,
volume and cost has to be high. If you design an optimum fuel economy
vehicle with only a small efficient diesel engine the performance is low. A
solution might be a hybrid with a "shopping" range electric battery pack,
say 30 miles only and an 80 bhp motor plus a 20-25 bhp diesel engine for
long trips.

A vehicle with 16 sq ft frontal area, 0.26 Cd, 0.011 rolling resistance and
1800 lbs weight would only require 16 bhp to cruise at 70 mph. At a BSFC of
0.35 from a small diesel engine that would give over 100 mpg at 70 mph. For
acceleration the electric engine kicks in to give the full 100 bhp. A bit
like KERS in F1 cars. For short trips you use electric power only and
recharge from the mains overnight for the cheapest possible motoring. For
long trips the system cruises on diesel and keeps the batteries charged at
25% to kick in only when needed. You plug the batteries in to charge back to
100% more cheaply from the mains when the journey is over.

With regenerative braking to charge the batteries in stop start motoring you
could avoid diesel use for most short journeys but still have unlimited
range for long ones from the fuel tank and petrol stations.

For my own use which consists of mainly 20 mile shopping trips with the
occasional 500 mile holiday one it would be ideal. The same performance as
my Focus but three times the mpg on long trips and almost zero cost on short
ones.

With such a limited electric range you could manage with conventional and
cheap lead acid batteries - about 8 to 10 of them at 15 kg each and �500
instead of the many thousands of pounds for Ni hybrid batteries.

Thoughts welcome.
--
Dave Baker