OT characteristics of diesel engines

[Murmansk]

I've just bought a VW Caddy with a 1.6 TDI diesel engine, having not had a diesel engine before I'm wondering how they differ from petrol.

One thing I've noticed is that if I'm driving slowly for example through a car park and I put it into second gear, even before I've applied any "gas" it'll be pulling quite strongly whereas a petrol car would be coming close to stalling in my experience.

Can anyone comment on what kind of revs a diesel engine like this will be producing maximum power - I've found it'll cruise happily in 5th at 60mph on just less than 2000 RPM

[charles]

In article <f67abf8d-9464-4c6d...@googlegroups.com>,

The other thing you'll find different is engine braking, It's much more
effective than on a petrol car. I drove various diesel cars for about 30
years, so have forgotten how a petrol car went. I'm now using an EV - which
I've had for 2 years.

--
from KT24 in Surrey, England - sent from my RISC OS 4té
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

[NY]

Yes, when I first got a diesel car (Peugeot 306 with 1.9 non-HDI engine)
in 1995, it was a revelation that it would pull on the level in 1st or
2nd with no throttle input and the engine at idling. Makes it very easy
to crawl along in a queue of traffic: foot off throttle, lift clutch
until car is going at speed of car in front, drop the clutch and let the
car roll.


Not sure what the max-torque and max-power engine speeds are for a
diesel, but they will be a lot less than for a petrol. At a guess, maybe
3000 rpm.

When I took my second diesel car (Pug 306 with 2.0 HDi engine) into the
garage, I was loaned a 1.8 petrol. Its 0-60 and indeed 0-20 times were
superb, and took some getting used to, but on the motorway the engine
was screaming away at about 4000 rpm in top gear (5th? 6th?) and the car
had bugger-all 50-70 acceleration in any gear when I came to overtake a
lorry that I'd got stuck behind. I was glad to get my own car back:
50-70 acceleration and quieter engine at 70 are far more important to me
than 0-60 time.

My present car, Pug 308 with 1.6 HDi, is now 15 years old and has done
190,000 miles. And its acceleration and low-speed torque are still damn
good, and its fuel economy still averages about 50-55 mpg (average over
650 miles / 60 litre tankful) as it did when the car was new.


The low-speed torque of a diesel make it a doddle to do a hill start on
a steep hill. I was going up a 1:3 / 33% hill near me and the car in
front ground to a halt. When he eventually got going, after a lot of
engine racing and rolling back towards me several times, I set off. In
first, the car set off with the engine going not much above idling, so I
didn't have to slip the clutch as much as I would in a petrol where you
might need 1500-2000 rpm to avoid stalling when letting the clutch up on
a 33% hill.

The only diesel that defeated me was a Mark IV VW Golf which had a very
odd engine whose engine-management system seemed to be programmed to cut
the fuel completely (so the car stalled) if you let the clutch in too
far or didn't have high enough revs, where most cars would just labour a
bit, giving you chance to dip the clutch or apply more engine revs. I
was test-driving it with a view to buying it, and the salesman said that
every one did it until they got used to it. Not a well-programmed ECU if
it sulked and threw its toys out of the pram because I got things
slightly wrong, rather than failing gracefully. The later model, Mark V,
had a much nicer engine that behaved like a proper diesel.

[Tim Lamb]

In message <f67abf8d-9464-4c6d...@googlegroups.com>,
Murmansk <stai...@gmail.com> writes

>I've just bought a VW Caddy with a 1.6 TDI diesel engine, having not
>had a diesel engine before I'm wondering how they differ from petrol.

Have a dip through the VW forums. I use voltswagen Forum.co.uk but there
is a choice.

The 1.6 Peugeot diesel is an excellent engine now hampered by CO2 and
NOX emission boltons:-(

We have one fitted in a 2009 Fiesta and one in a 2012 VW Passat Estate.
Ample torque for such a big vehicle.

The early versions have issues with MOT smoke failures due to soot
collecting in the DPF. (diesel particulate filter) The cure is to use
low Sulphur fuel (expensive) and do a long motorway run prior to the
test.
Ours have both had recent necessary trips to the Suffolk coast:-)

>
>One thing I've noticed is that if I'm driving slowly for example
>through a car park and I put it into second gear, even before I've
>applied any "gas" it'll be pulling quite strongly whereas a petrol car
>would be coming close to stalling in my experience.
>
>Can anyone comment on what kind of revs a diesel engine like this will
>be producing maximum power - I've found it'll cruise happily in 5th at
>60mph on just less than 2000 RPM

--
Tim Lamb

[Tim+]

NY <m...@privacy.net> wrote:

> The only diesel that defeated me was a Mark IV VW Golf which had a very
> odd engine whose engine-management system seemed to be programmed to cut
> the fuel completely (so the car stalled) if you let the clutch in too
> far or didn't have high enough revs, where most cars would just labour a
> bit, giving you chance to dip the clutch or apply more engine revs.

Modern dual mass flywheels don’t like slogging at very low revs. The do a
great job of damping vibrations but can be damaged if the revs drop too
much, hence some manufacturers fit a low rev engine cut out to protect the
flywheel.

You soon get used to it.

Tim

--
Please don't feed the trolls

[The Natural Philosopher]

On 10/09/2023 17:45, charles wrote:
> In article <f67abf8d-9464-4c6d...@googlegroups.com>,
> Murmansk <stai...@gmail.com> wrote:
>> I've just bought a VW Caddy with a 1.6 TDI diesel engine, having not had
>> a diesel engine before I'm wondering how they differ from petrol.
>
>> One thing I've noticed is that if I'm driving slowly for example through
>> a car park and I put it into second gear, even before I've applied any
>> "gas" it'll be pulling quite strongly whereas a petrol car would be
>> coming close to stalling in my experience.
>
>> Can anyone comment on what kind of revs a diesel engine like this will be
>> producing maximum power - I've found it'll cruise happily in 5th at 60mph
>> on just less than 2000 RPM
>
> The other thing you'll find different is engine braking, It's much more
> effective than on a petrol car. I drove various diesel cars for about 30
> years, so have forgotten how a petrol car went. I'm now using an EV - which
> I've had for 2 years.
>

To OP

A turbodiesel is generally optimised for peak torque and efficiency in
the 2000 - 4000 RPM range.

Once you have a 'blown' engine it is generally easier to pump more air
in to get power than to push the rev limits.

That also leads to less issues with inlet and exhaust shaping, valves
and general longevity issues with rods and bearings.

My big 3.0 v6 TDi likes to chunter along at 1500 RPM.

--
"What do you think about Gay Marriage?"
"I don't."
"Don't what?"
"Think about Gay Marriage."

[Alan J. Wylie]

Tim+ <tim.d...@gmail.com> writes:

> Modern dual mass flywheels don’t like slogging at very low revs. The do a
> great job of damping vibrations but can be damaged if the revs drop too
> much, hence some manufacturers fit a low rev engine cut out to protect the
> flywheel.

Driving through town, I missed a gear change and instead of going into
2nd, went into 4th. There was a lot of clattering for a short
time. After that, there was a lot of vibration at certain revs. It took
a lot of thought at the local garage before they diagnosed a broken
dual mass flywheel.

--
Alan J. Wylie https://www.wylie.me.uk/

Dance like no-one's watching. / Encrypt like everyone is.
Security is inversely proportional to convenience

[ajh]

On 10/09/2023 21:21, Tim+ wrote:
> hence some manufacturers fit a low rev engine cut out to protect the
> flywheel.

I wonder if that's why my 2010 1.6 fiesta cuts out if I keep it in too
high a gear. Unlike Tim Lamb's mine had no dpf from new and that's why I
am keeping it. It still manages 71mpg.

[The Natural Philosopher]

Indeed.

Low revving diesels should be able to do 200k miles easily. Rev limited
turbo petrol at least 150k.

Help save the planet buy not scrapping perfectly good cars



--
"The great thing about Glasgow is that if there's a nuclear attack it'll
look exactly the same afterwards."

Billy Connolly

[Animal]

The main difference is they love low rpm. Lower rpm for the same power out is more fuel efficient. Treat over 2k rpm as unnecessarily fast if you want economy.
Some can run on free alternative fuels, but do your reading to avoid problems.

[Andrew]

On 11/09/2023 09:12, Alan J. Wylie wrote:
> Tim+ <tim.d...@gmail.com> writes:
>
>> Modern dual mass flywheels don’t like slogging at very low revs. The do a
>> great job of damping vibrations but can be damaged if the revs drop too
>> much, hence some manufacturers fit a low rev engine cut out to protect the
>> flywheel.
>
> Driving through town, I missed a gear change and instead of going into
> 2nd, went into 4th. There was a lot of clattering for a short
> time. After that, there was a lot of vibration at certain revs. It took
> a lot of thought at the local garage before they diagnosed a broken
> dual mass flywheel.
>

And a 2000 pound bill to replace it :-(.

Some garages even replace with a nom-dual mass clutch
and flywheel, but the dual mass jobbie is part of the
overall emissions calculations for that model.

[Andrew]

On 11/09/2023 18:11, ajh wrote:

If you do enough miles to justify a diesel engine then
a dpf should not be an issue.

[NY]

"Andrew" <Andr...@btinternet.com> wrote in message
news:uds5sp$23l3l$2...@dont-email.me...

I had to have my DPF replaced some time ago (my car has done 190,000 miles
now) and the bill was nasty. They had to replace the catalytic converter at
the same time because a thread stripped on a connection between the two,
though the garage consoled me with the thought that the cat probably didn't
have much more life in it so would have needed replacing fairly soon. The
total bill was £1000. That's by far the biggest single bill I've had to pay
for the car. It's still on its original clutch and as far as I know the
dual-mass flywheel is OK, though I'm not sure what the symptoms of a
failing/failed DMF are.

I didn't know that DMFs don't like very low engine revs under heavy load -
rather makes them a silly choice for a diesel-engined car ;-)

71 mpg is pretty damn good. My car, a Peugeot 308 1.6 HDi, has averaged
about 55 mpg over the 180,000 miles that I've had the car. That's with very
little around-town driving, mostly motorways, A roads and country lanes, so
not much constant 30 mph driving and stop/start.

The car still goes like a bomb - plenty of acceleration out of bends and
pulling up hills when I need it. The only fault it has now is rather lumpy
running at low speeds - eg approach a hazard with my foot off the
accelerator, then apply a bit of power to maintain constant speed to to
accelerate very slightly - this sometimes causes a lag of about a second
before the engine responds. It improved after the garage replaced the air
filter (due to a misunderstanding, this had not been replaced for several
services) but seems to have come back again.

I'm keeping the car "forever" because it is worth a lot more to me as a
working car than for its negligible second-hand value. When I come to
replace it, I imagine I'll be looking for the newest second-hand diesel that
I can buy once sales of new diesel- and petrol-mechanical cars are banned
and you can only buy petrol-electric hybrids and pure-electric cars. I'm
scared stiff with a pure-electric than I might find myself stranded
somewhere or have to break a long journey for longer than a normal
pee-and-coffee break while the car is recharged. There is also the
interesting social problem of how you pay to use a friend's electricity if
you go to stay with them: it's not just a case of stop at any
reasonably-priced garage somewhere on the journey, fill up and be on my way
again in five minutes.

[The Natural Philosopher]

On 14/09/2023 10:02, NY wrote:
> I'm keeping the car "forever" because it is worth a lot more to me as a
> working car than for its negligible second-hand value.

Amen to that.

I pour a lot of maintenance money into mine, to keep it reliable and
smart, but it's still way less than the HP on a new one would be, or the
contract hire, and besides, the newer ones aren't so fast :-)

There is a sweet spot with old luxury cars where the resale value is
practically nothing even on a low mileage example, but you still have
100k+ in the engine and gearbox.

And you get to drive a £40k+ car for less than £4k...

--
Socialism is the philosophy of failure, the creed of ignorance and the
gospel of envy.

Its inherent virtue is the equal sharing of misery.

Winston Churchill

[Tim Lamb]

In message <udutrv$2k7no$2...@dont-email.me>, The Natural Philosopher
<t...@invalid.invalid> writes

>On 14/09/2023 10:02, NY wrote:
>> I'm keeping the car "forever" because it is worth a lot more to me as
>>a working car than for its negligible second-hand value.
>
>Amen to that.
>
>I pour a lot of maintenance money into mine, to keep it reliable and
>smart, but it's still way less than the HP on a new one would be, or
>the contract hire, and besides, the newer ones aren't so fast :-)
>
>There is a sweet spot with old luxury cars where the resale value is
>practically nothing even on a low mileage example, but you still have
>100k+ in the engine and gearbox.
>
>And you get to drive a £40k+ car for less than £4k...

Horses and courses.

Both our diesel cars are used mostly for short, local trips. Bad news
for exhaust filter blockage.
When new, 12 or so years back, the Fiesta engine management would
trigger a *burn* cycle where the coolant fan ran at full speed and
excess fuel was put through the injectors. Performance would be way down
until the next engine switch off so I learned to do this on the move:-)
Not noticed this happen for some years and followed up your annual
engine oil/filter change with the MOT mechanic. He agreed and included
engine air intake filter followed by a generous high speed run pre-test.

My intent has been to replace the Fiesta with an electric *shopping
trolley* but new prices and user preferences make this a struggle:-)
>

--
Tim Lamb

[The Natural Philosopher]

On 14/09/2023 14:16, Tim Lamb wrote:
> My intent has been to replace the Fiesta with an electric *shopping
> trolley* but new prices and user preferences make this a struggle:-)

I keep thinking about it, but frankly they are so dull ...and
expensive...and depreciate even faster than a Jaguar.,..and they still
need insuring and charging and and and they wont do the really long
trips I do once or twice a year..

And are currently no cheaper than diesel even WITH fuel tax and VAT on top

--
Karl Marx said religion is the opium of the people.
But Marxism is the crack cocaine.

[ajh]

On 14/09/2023 14:16, Tim Lamb wrote:

> My intent has been to replace the Fiesta with an electric *shopping
> trolley* but new prices and user preferences make this a struggle:-)

Even though I can buy a pug/corsa EV new for £20k and by adding
another 4kWp of PV panels I could run it for 9 months of the year
without buying electricity it makes no sense until one of my other cars
bites the dust. Also I do a fraction of the mileage I did before I
retired. As it is I only use the diesel for trips exceeding 50 miles, it
costs next to nothing in devaluation now, I service it every 8k miles
and tax goes from free to a small amount in April. At 70mpg it cost only
10p/mile to run even at current prices and I like it.

I still don't understand how your 2009 fiesta has a dpf and my 2010 one
doesn't. Same 1.6 pug engine.

[ajh]

On 14/09/2023 10:02, NY wrote:
>
>

> 71 mpg is pretty damn good. My car, a Peugeot 308 1.6 HDi, has averaged
> about 55 mpg over the 180,000 miles that I've had the car. That's with
> very little around-town driving, mostly motorways, A roads and country
> lanes, so not much constant 30 mph driving and stop/start.

Same basic engine but my car is an "eco" model, a bit lower power and
low ground clearance. I seldom use motorways ( reminds me of commuting
to work too much) and am generally not in a hurry but don't exceed an
indicated 70mph or 60mph on single carriage ways. I switch off at
traffic lights manually and try and anticipate the conditions ahead to
avoid braking, as my mate says " driving uses fuel, braking wastes it".

I got slightly better economy from my work 206 van with the 1.4 hdi and
that retired with me at 305k miles as no one else at the firm would take
it on.

>
> I'm keeping the car "forever" because it is worth a lot more to me as a
> working car than for its negligible second-hand value.

Exactly but I am completely sanguine about an all electric EV, I have
been tempted to treat myself to a Tesla Y extended range dual motor for
£60k, it accelerates faster than my bike and out paces it by 15mph but
my grand-kids may enjoy the money more.

[Tim Lamb]

In message <kmh05i...@mid.individual.net>, ajh
<ne...@loampitsfarm.co.uk> writes

Me neither! It is a 1.6 duratorq + DPF TDCI 90kW registered Sept. 2019
and built in Czechoslovakia
Still has road tax of £20!

--
Tim Lamb

[ajh]

On 15/09/2023 09:29, Tim Lamb wrote:
>
> Me neither! It is a 1.6 duratorq + DPF TDCI  90kW registered Sept. 2019
> and built in Czechoslovakia

Ah I must have misread, I thought it was 2009

[NY]

On 14/09/2023 20:00, ajh wrote:
> On 14/09/2023 10:02, NY wrote:
>>
>>
>> 71 mpg is pretty damn good. My car, a Peugeot 308 1.6 HDi, has
>> averaged about 55 mpg over the 180,000 miles that I've had the car.
>> That's with very little around-town driving, mostly motorways, A roads
>> and country lanes, so not much constant 30 mph driving and stop/start.
>
> Same basic engine but my car is an "eco" model, a bit lower power and
> low ground clearance. I seldom use motorways ( reminds me of commuting
> to work too much) and am generally not in a hurry but don't exceed an
> indicated 70mph  or 60mph on single carriage ways. I switch off at
> traffic lights manually and try and anticipate the conditions ahead to
> avoid braking, as my mate says " driving uses fuel, braking wastes it".

I thought I was pretty good at driving economically: I accelerate
moderately rather than using maximum power - unless I have to pull out
into the only available gap in a long stream of traffic - and I
anticipate when I'll need to slow down, lifting off the power early and
letting friction and air resistance do the job, rather than staying on
the power till the last moment and then braking hard.

I remember that a couple were in then news in 2005 or so, when they
drove a diesel Pug 306 (not sure which engine) around the country and
achieved an average of 90 mpg. But details of the run were very scarce:
were they generally keeping close to speed limits so as to keep up with
the general flow of traffic, or were they driving a lot slower, maybe at
some optimum speed for the gearing and engine so as to reduce air
resistance and run the engine at its most efficient.

I never know with a diesel whether it it more efficient to drive in a
higher gear with the engine straining a bit more or a lower gear with
less engine torque being needed. There are often conditions where I
could choose either 5th or 6th. I tend to use the highest gear that
works, but I wonder whether sometimes it would be better to change down
and let the engine rev slightly faster but straining less hard.

[The Natural Philosopher]

On 15/09/2023 10:59, NY wrote:
> I remember that a couple were in then news in 2005 or so, when they
> drove a diesel Pug 306 (not sure which engine) around the country and
> achieved an average of 90 mpg. But details of the run were very scarce:
> were they generally keeping close to speed limits so as to keep up with
> the general flow of traffic, or were they driving a lot slower, maybe at
> some optimum speed for the gearing and engine so as to reduce air
> resistance and run the engine at its most efficient.
>
> I never know with a diesel whether it it more efficient to drive in a
> higher gear with the engine straining a bit more or a lower gear with
> less engine torque being needed. There are often conditions where I
> could choose either 5th or 6th. I tend to use the highest gear that
> works, but I wonder whether sometimes it would be better to change down
> and let the engine rev slightly faster but straining less hard.

I am relatively confused by my diesel. It *seems* to be most efficient
at 70mph. But things have been muddied by the fact the garage identified
a sticking slide pin in a rear brake (or brakes) (despite the fact I
specifically asked them about that a year ago, and they said it was
'fine' and its barely done any miles since). So any time I don't use
the brakes at all gives better economy than when I do....

But for a normally aspirated diesel the best efficiency is generally
light throttle low RPM.

It's more complicated when you strap turbos on. But the manufacturers
will generally optimise economy for steady state at the test speed,
which is a constant 56mph IIRC. In most IC engines you can trade power
for efficiency so it tends to be that any rapid acceleration will
increase fuel consumption.

I don't seem to suffer much degradation up to around 85ph or down to
below 50mph, but its hard to say because 50mph generally means a twisty
road or villages with stop-starts.

And its stop starts that destroy my economy. I try and coast to a halt
whenever possible, but any application of brakes is a loss of energy I
wont get back.


--
Canada is all right really, though not for the whole weekend.

"Saki"

[Tim Lamb]

In message <kminkp...@mid.individual.net>, ajh
<ne...@loampitsfarm.co.uk> writes

No I mistyped ! Purchased Sept. 2009.

--
Tim Lamb

[ajh]

On 15/09/2023 10:59, NY wrote:

Firstly I don't give a lot of conscious thought to driving economically
and even driven fast I return 66MPG. When the car was new (daughter's
company lease then) I am sure I got 80mpg on one occasion. At the time I
dealt with the firm I worked for's fuel cards and we had two pug 206s
the other chap commuted 80 miles in to work and back and averaged 44mpg
and I did 45 miles in and back all but 5 miles motorway and consistently
managed over 70.

>
> I never know with a diesel whether it it more efficient to drive in a
> higher gear with the engine straining a bit more or a lower gear with
> less engine torque being needed. There are often conditions where I
> could choose either 5th or 6th. I tend to use the highest gear that
> works, but I wonder whether sometimes it would be better to change down
> and let the engine rev slightly faster but straining less hard.

A long time ago I used to pull things with tractors and would allow them
to really bog down before de clutching and it didn't seem to be bad,
this fiesta won't allow lugging like that and cannot pull 5th at below
35mph and even then I must change down to accelerate whereas the petrol
car I drive will pull away from 20mph in 5th.

[ajh]

Curiouser and curiouser.

Mine is a 1.6 econetic (the greenest ford at the time :-)) and only 89hp
which is about 65kW. It's definitely not something you overtake with
unless on a long straight road.

[Vir Campestris]

On 15/09/2023 11:32, The Natural Philosopher wrote:
>
> I am relatively confused by my diesel. It *seems* to be most efficient
> at 70mph. But things have been muddied by the fact the garage identified
> a sticking slide pin in a rear brake (or brakes) (despite the fact I
> specifically asked them about that a year ago, and they said it was
> 'fine' and its barely done any miles since).  So any time I don't use
> the brakes at all gives better economy than when I do....
>
> But for a normally aspirated diesel the best efficiency is generally
> light throttle low RPM.
>
> It's more complicated when you strap turbos on. But the manufacturers
> will generally optimise economy for steady state at the test speed,
> which is a constant 56mph IIRC. In most IC engines you can trade power
> for efficiency so it tends to be that any rapid acceleration will
> increase fuel consumption.
>
> I don't seem to suffer much degradation up to around 85ph or down to
> below 50mph, but its hard to say because 50mph generally means a twisty
> road or villages with stop-starts.
>
> And its stop starts that destroy my economy. I try and coast to a halt
> whenever possible, but any application of brakes is a loss of energy I
> wont get back.

Thanks to single lane roadworks, and one incident with a tractor, I know
my wife's car gives better economy the slower I go. Down to 32MPH. (I
pressed reset on the "average fuel consumption" setting on the trip
computer)

I'm not prepared to drive that slowly on a regular basis.

Why do you think yours is best at 70?

Andy

[Tim+]

He’s an embittered art student.

[The Natural Philosopher]

Because that seems to be what the MPG meter says.

I dont do enough steady runs, but endless cruising at 70 seems to get me
about 44mph, and 40 is the best I got in a long 40mph roadwork section.
Being stuck in traffic through Cambridge the other day was around 15mpg

And 20mph doesn't work either.

As I said, it is very dependent on a lot of variables like engine size,
aerodynamics. how many fixed losses through steering pumps, alternators
compressors and fans there are... Dumping a viscous fan for electric
normally is a HUGE improvement

And what the engines best efficiency point is. Your 2000rpm in top gear
may net you 40mph. Mine nets me over 70...

The one thing I do know is that stop start in a car that doesn't either
switch off or store kinetic energy is the very worst consumption of all,
which is why car hating councils put in traffic lights, speed bumps and
chicanes.

--
Labour - a bunch of rich people convincing poor people to vote for rich
people by telling poor people that "other" rich people are the reason
they are poor.

Peter Thompson

[David Paste]

On Saturday, 23 September 2023 at 12:57:54 UTC+1, The Natural Philosopher wrote:

> I dont do enough steady runs, but endless cruising at 70 seems to get me
> about 44mph, and 40 is the best I got in a long 40mph roadwork section.
> Being stuck in traffic through Cambridge the other day was around 15mpg

What engine is it, JOOI?

[Andy Burns]

The Natural Philosopher wrote:

> Vir Campestris wrote:
>
>> Why do you think yours is best at 70?
>>
> Because that seems to be what the MPG meter says.

How does that compare with what the petrol pump and odoometer say?

[Animal]

Nearly always better to use lower revs & more load. Diesel efficiency suffers a lot at low load

[Animal]

On Friday, 15 September 2023 at 11:32:44 UTC+1, The Natural Philosopher wrote:
> On 15/09/2023 10:59, NY wrote:
> > I remember that a couple were in then news in 2005 or so, when they
> > drove a diesel Pug 306 (not sure which engine) around the country and
> > achieved an average of 90 mpg. But details of the run were very scarce:
> > were they generally keeping close to speed limits so as to keep up with
> > the general flow of traffic, or were they driving a lot slower, maybe at
> > some optimum speed for the gearing and engine so as to reduce air
> > resistance and run the engine at its most efficient.
> >
> > I never know with a diesel whether it it more efficient to drive in a
> > higher gear with the engine straining a bit more or a lower gear with
> > less engine torque being needed. There are often conditions where I
> > could choose either 5th or 6th. I tend to use the highest gear that
> > works, but I wonder whether sometimes it would be better to change down
> > and let the engine rev slightly faster but straining less hard.
> I am relatively confused by my diesel. It *seems* to be most efficient
> at 70mph. But things have been muddied by the fact the garage identified
> a sticking slide pin in a rear brake (or brakes) (despite the fact I
> specifically asked them about that a year ago, and they said it was
> 'fine' and its barely done any miles since). So any time I don't use
> the brakes at all gives better economy than when I do....
>
> But for a normally aspirated diesel the best efficiency is generally
> light throttle low RPM.
>
> It's more complicated when you strap turbos on.

Yes, but not much. A turbo increases power output & reduces efficiency a bit. For max mpg stay below the rpm where the turbo kicks in noticeably.

> But the manufacturers
> will generally optimise economy for steady state at the test speed,
> which is a constant 56mph IIRC.

At 56 in 5th a diesel is typically too high rpm for best efficiency. The gearing is more chosen for acceptable acelleration at speed. Higher gears would improve mpg, but more gears costs. This is partly why the 60s saw trucks with a 12 speed box.

> In most IC engines you can trade power
> for efficiency so it tends to be that any rapid acceleration will
> increase fuel consumption.
>
> I don't seem to suffer much degradation up to around 85ph or down to
> below 50mph, but its hard to say because 50mph generally means a twisty
> road or villages with stop-starts.
>
> And its stop starts that destroy my economy. I try and coast to a halt
> whenever possible, but any application of brakes is a loss of energy I
> wont get back.

Add engine braking when you don't have all day to coast to stops. Fuel use zeroes during, for an electronically controlled engine.

[The Natural Philosopher]

The petrol pump always reads zero. because it is a diesel

I dunno. I am not interested in the absolute mpg, only the relative mpg
for different driving styles.

According to the Interwebby thing, I have a 15 gallon tank, which
generally nets me about 400-480 miles, but I never let it run out. Once
the engine nearly cut going downhill. That's as close as I wanted to
take it.

450 mile range would be around 30mpg, which is my worst case figure for
short trips and town traffic.

But I don't fill it up when the engine cuts. In general I put in about
60 litres after about 440 miles, which agrees fairly well with the sort
of MPG displayed in my normal driving - about 32-34.

So I think the onboard mpg meter is not too bad.

Its a BIG engine. It will have lots of fixed losses - friction, water
pump, alternator, power steering, air con - and it wont normally shift
into top gear much below 60mph.

What seems to be best is top gear around 65-80mph. I think that's around
2000 - 2200 rpm.




--
"Corbyn talks about equality, justice, opportunity, health care, peace,
community, compassion, investment, security, housing...."
"What kind of person is not interested in those things?"

"Jeremy Corbyn?"

[Vir Campestris]

One day when you have time on your hands try a steady 60. Or ideally 56
- that's one of the speeds they publish the numbers for.

Roadworks mean I've tried 60, 50 and 40. Speed limits mean I've tried
70, and that tractor got me 32. The slower the better.

OK, it's a small petrol, not a big diesel...

Andy

Andy

[The Natural Philosopher]

TBH I am not that bothered. I am just glad that at highway speeds a
280bhp twin turbodiesel uses less diesel than my old Freelander did. All
about the aero..

> Roadworks mean I've tried 60, 50 and 40. Speed limits mean I've tried
> 70, and that tractor got me 32. The slower the better.
>
> OK, it's a small petrol, not a big diesel...
>

And that does indeed make a big difference.
All I wanted to say in this thread was that apples are not oranges, and
the best mpg speed for one car is not the best mpg speed for another.
Back in the day the aerodynamic bricks we all used to drive were
probably best at a steady 45mph, which curiously used to be a fairly
common speed limit in those days. Not seen it for years..


--
In theory, there is no difference between theory and practice.
In practice, there is.
-- Yogi Berra

[Fredxx]

Petrol engines suffer a great loss of efficiency at part load.

[The Natural Philosopher]

prior to turbos diesels worked best at ultra low revs and power.
Commercial diesels were tuned for the cruise and going up hills they
'rolled coal' .
Turbo diesels can run at very high boost pressures and generate a lot
more power without smoke. But still its not normal or needed to make
them revvers.

I think mine is redlined at 4500, and it never goes over 4000 because
its designed not to .

I actually looked at the rev counter today. 1500rpm was around 60mph in
6th gear.


--
“It is not the truth of Marxism that explains the willingness of
intellectuals to believe it, but the power that it confers on
intellectuals, in their attempts to control the world. And since...it is
futile to reason someone out of a thing that he was not reasoned into,
we can conclude that Marxism owes its remarkable power to survive every
criticism to the fact that it is not a truth-directed but a
power-directed system of thought.”
Sir Roger Scruton

[NY]

"The Natural Philosopher" <t...@invalid.invalid> wrote in message
news:uesh95$21f6m$3...@dont-email.me...

> On 25/09/2023 14:32, Fredxx wrote:
>> On 23/09/2023 22:48, Animal wrote:
>>> On Friday, 15 September 2023 at 10:59:30 UTC+1, NY wrote:
>>>> On 14/09/2023 20:00, ajh wrote:
>>>>> On 14/09/2023 10:02, NY wrote:

>>>> I never know with a diesel whether it it more efficient to drive in a
>>>> higher gear with the engine straining a bit more or a lower gear with
>>>> less engine torque being needed. There are often conditions where I
>>>> could choose either 5th or 6th. I tend to use the highest gear that
>>>> works, but I wonder whether sometimes it would be better to change down
>>>> and let the engine rev slightly faster but straining less hard.
>>>
>>> Nearly always better to use lower revs & more load. Diesel efficiency
>>> suffers a lot at low load

Good. That's how I tend to drive: better in 3rd on a roundabout with foot to
the floor rather than having to rev-match down to 2nd gear with less
throttle. Shame that my wife's 1.6 turbo-diesel Honda CR-V hasn't got the
torque to do that (heavier car, maybe more reliance on turbo boost) and
needs one or even two gears lower than my Pug would be happy with. I wonder
if it's running out of turbo boost: approach hazard in 6th with no throttle
(and so no exhaust beyond the inlet air going out of the exhaust) and so no
turbo boost when I call for power. Drop a couple of gears, and the engine
burns fuel, produces exhaust and thus spins the turbo and starts
accelerating the car.

>> Petrol engines suffer a great loss of efficiency at part load.
>
> prior to turbos diesels worked best at ultra low revs and power.
> Commercial diesels were tuned for the cruise and going up hills they
> 'rolled coal' .
> Turbo diesels can run at very high boost pressures and generate a lot more
> power without smoke. But still its not normal or needed to make them
> revvers.
>
> I think mine is redlined at 4500, and it never goes over 4000 because its
> designed not to .

It's rare that I've had mine above 4000, and only briefly when accelerating
from rest or near-rest to pull out of a junction into heavy traffic and
haven't had chance to change up.

> I actually looked at the rev counter today. 1500rpm was around 60mph in
> 6th gear.

That's very high geared. My Pug 308 (1.6 HDi) does about 2000 at 60 mph. But
that is still a *lot* less than the 1.8 petrol 306 that I was loaned by a
garage when my car went for servicing. That did about 4500 rpm in top (6th?)
gear, and at that engine speed the car had bugger-all acceleration. If I got
behind a lorry at 50 on the motorway and then went to overtake, the car was
utterly gutless in getting up to 70, no matter whether I tried 4th, 5th or
6th gear. My diesel Pug manages it moderately well in 6th and with a real
kick up the backside in 5th. For me, 40-60 or 50-70 acceleration is far more
important than the much-touted 0-60 time ;-)




One question. What does the "panel" think about this symptom of my Pug,
which has only developed in the last thousand miles or so: slow down in (for
example) 5th gear from 40 to 30. Change to 4th as I level out at 30. Press
the accelerator slightly to increase speed a smidgen (eg 25 to 30 mph) and
the engine falters and stutters, before starting to accelerate. It's never
done this before in the 190,000 miles I've had the car. Otherwise, the
engine still has shit-hot acceleration: it's only very gentle around-town
acceleration where it happens. The car had been running for some time with
an air filter than needed changing (due to a misunderstanding with another
garage as to what was being replaced at each service). When that was
replaced by a different garage who were investigating the 30 mph stutter,
the 30 mph symptom was not fixed, but the general acceleration 40-60 etc on
country lanes was dramatically improved. So "raw acceleration" was being a
bit starved of air, but curing that hasn't helped with the stutter. It's the
1.6 110 hp engine, running on normal diesel (as opposed to premium) - often
supermarket brand but sometimes Jet or Esso fuel. It seems to happen equally
often with a cold engine or after driving for a long time when the temp
gauge is reading normal.

Is it likely to be a sign of a turbo that needs to be changed? The DPF and
cat were changed at about 155,000 miles.

I know *exactly* what happens if the turbo hose comes off and the engine
runs normally-aspirated, because a garage once forgot to retighten the
jubilee clip on the turbo-output/engine-inlet hose (after doing work which
required its removal) and the hose came off, and then the same thing
happened again a few thousand miles ago when a grommet perished so the hose
would not grip onto the turbo even with the jubilee as tight as I could get
it. That was a total lack of acceleration and inability to go above about 40
mph: the stutter lasts a second or so, and is nowhere near as bad as the "no
turbo assistance" fault ;-)

[The Natural Philosopher]

On 25/09/2023 19:54, NY wrote:
> One question. What does the "panel" think about this symptom of my Pug,
> which has only developed in the last thousand miles or so: slow down in
> (for example) 5th gear from 40 to 30. Change to 4th as I level out at
> 30. Press the accelerator slightly to increase speed a smidgen (eg 25 to
> 30 mph) and the engine falters and stutters, before starting to
> accelerate.

No fault codes?

> It's never done this before in the 190,000 miles I've had
> the car. Otherwise, the engine still has shit-hot acceleration: it's
> only very gentle around-town acceleration where it happens. The car had
> been running for some time with an air filter than needed changing (due
> to a misunderstanding with another garage as to what was being replaced
> at each service). When that was replaced by a different garage who were
> investigating the 30 mph stutter, the 30 mph symptom was not fixed, but
> the general acceleration 40-60 etc on country lanes was dramatically
> improved. So "raw acceleration" was being a bit starved of air, but
> curing that hasn't helped with the stutter. It's the 1.6 110 hp engine,
> running on normal diesel (as opposed to premium) - often supermarket
> brand but sometimes Jet or Esso fuel. It seems to happen equally often
> with a cold engine or after driving for a long time when the temp gauge
> is reading normal.

Mmm. Dirty MAF sensor(s) or sticky injectors spring to mind. Or a small
vacuum leak. Faults like this if there are no fault codes are really
difficult to find. Do you have an ODBC code reader? there may be a code
stored that doesn't cause a 'check engine' light.

Tried an' italian' tune up?

I mean the fault sounds like low fuel condition when 'call for power'
which might be causes by a dirty MAF sensor, a vacuum leak misleading
the MAF sensor, or a sticky or dirty fuel injector. It could even be a
sticking and failing primary fuel pump... or maybe the turbo is
sticking...If it were petrol Id suspect the throttle flap but diesels
don't have them AFAICR. Might even be some odd sensor or solenoid
sticking somewhere else like variable valve timimg if it has that.

There are things *you* can tip in the tank that clean up the fuel
system, but what you need is that rarest of things - an honest decent
intelligent diesel tech who will e.g. monitor fuel pressure, look at the
OBDC data when the engine is running and then *think* about what it means.

https://www.amazon.co.uk/dp/B075NHH5GN

At £6 its a shot to nothing


I am afraid at 190,000 miles this might be something you just live with
till it gets so obvious even a thick mechanic can sort it, or a sign its
time to trade the car.
A new turbo for example could easily be a grand...fuel pump around 300
Dunno about injectors - never had one fail. MAF sensors similar money.

Oddly enough I had a similar fault when stuck behind a horse box at
12mph...must have been a VERY expensive horse....and the 'restricted
performance' light came on. I got codes on BOTH MAF air sensors, saying
'unrealistic numbers' or some such. The fault disappeared in switch off
and hasn't re-appeared since.

Again I think there is stuff you can spray into the intake that cleans
up that area and that might be worth doing if you can handle removing
the air box and filter.

https://www.amazon.co.uk/wynns-WY23381-Diesel-EGR-Cleaner/dp/B00BKC9LHA/ref=pd_bxgy_sccl_2/259-1921750-6062316

Or at more money

https://www.amazon.co.uk/Liqui-Moly-Cleaner-diesel-valves/dp/B01IBXML4K

The fact that it runs well above idle suggests that all the sensor
wiring is intact, and all the sensors are working, but it doesn't mean
they are working properly. Wear or dirt or stickiness anywhere in the
fuel system will do this...



--
How fortunate for governments that the people they administer don't think.

Adolf Hitler

[Fredxx]

On 25/09/2023 19:54, NY wrote:

> "The Natural Philosopher" <t...@invalid.invalid> wrote in message
> news:uesh95$21f6m$3...@dont-email.me...
>> On 25/09/2023 14:32, Fredxx wrote:
>>> On 23/09/2023 22:48, Animal wrote:
>>>> On Friday, 15 September 2023 at 10:59:30 UTC+1, NY wrote:
>>>>> On 14/09/2023 20:00, ajh wrote:
>>>>>> On 14/09/2023 10:02, NY wrote:
>
>>>>> I never know with a diesel whether it it more efficient to drive in a
>>>>> higher gear with the engine straining a bit more or a lower gear with
>>>>> less engine torque being needed. There are often conditions where I
>>>>> could choose either 5th or 6th. I tend to use the highest gear that
>>>>> works, but I wonder whether sometimes it would be better to change
>>>>> down
>>>>> and let the engine rev slightly faster but straining less hard.
>>>>
>>>> Nearly always better to use lower revs & more load. Diesel
>>>> efficiency suffers a lot at low load
>
> Good. That's how I tend to drive: better in 3rd on a roundabout with
> foot to the floor rather than having to rev-match down to 2nd gear with
> less throttle.

Diesels don't suffer the induction losses a petrol does at part
throttle. Therefore the gain is minimal and you may get a better
efficiency at higher revs. Plus your engine and dual mass flywheel wear
will be significantly higher when labouring your engine.

<snip>

> One question. What does the "panel" think about this symptom of my Pug,
> which has only developed in the last thousand miles or so: slow down in
> (for example) 5th gear from 40 to 30. Change to 4th as I level out at
> 30. Press the accelerator slightly to increase speed a smidgen (eg 25 to
> 30 mph) and the engine falters and stutters, before starting to
> accelerate. It's never done this before in the 190,000 miles I've had
> the car. Otherwise, the engine still has shit-hot acceleration: it's
> only very gentle around-town acceleration where it happens. The car had
> been running for some time with an air filter than needed changing (due
> to a misunderstanding with another garage as to what was being replaced
> at each service). When that was replaced by a different garage who were
> investigating the 30 mph stutter, the 30 mph symptom was not fixed, but
> the general acceleration 40-60 etc on country lanes was dramatically
> improved. So "raw acceleration" was being a bit starved of air, but
> curing that hasn't helped with the stutter. It's the 1.6 110 hp engine,
> running on normal diesel (as opposed to premium) - often supermarket
> brand but sometimes Jet or Esso fuel. It seems to happen equally often
> with a cold engine or after driving for a long time when the temp gauge
> is reading normal.

A stutter in an HDI engine implies no fuel. Do you have access to an OBD
tool where you can view pressures and demand as well as throttle position?

> Is it likely to be a sign of a turbo that needs to be changed? The DPF
> and cat were changed at about 155,000 miles.

If the oil is changed regularly then I see no impending reason why the
turbo should be changed.

[Vir Campestris]

On 25/09/2023 11:05, The Natural Philosopher wrote:
> On 24/09/2023 21:08, Vir Campestris wrote:
>>
>> One day when you have time on your hands try a steady 60. Or ideally
>> 56 - that's one of the speeds they publish the numbers for.
>>
> TBH I am not that bothered. I am just glad that at highway speeds a
> 280bhp twin turbodiesel uses less diesel than my old Freelander did. All
> about the aero..
>

Fair enough.

>> Roadworks mean I've tried 60, 50 and 40. Speed limits mean I've tried
>> 70, and that tractor got me 32. The slower the better.
>>
>> OK, it's a small petrol, not a big diesel...
>>
> And that does indeed make a big difference.
> All I wanted to say in this thread was that apples are not oranges, and
> the best mpg speed for one car is not the best mpg speed for another.
> Back in the day the aerodynamic bricks we all used to drive were
> probably best at a steady 45mph, which curiously used to be a fairly
> common speed limit in those days. Not seen it for years..
>
>

I'll still be surprised if the decrease in energy required to push the
mass through the air more slowly (remember, air drag is proportional to
the square of the speed) is really outweighed by a drop in engine
efficiency.

Your most economical speed may well be different to my wife's little car
- but I'll be surprised if it's as high as 70.

Andy

[The Natural Philosopher]

It's not just that, its all the fixed losses in the engine - friction
and all the belt driven gubbins attached to it.

Think: if you car burns a gallon an hour *not moving at all*, until the
aero losses are *also* a gallon an hour, you haven't reached optimum speed.

But as I said 70mph is about 1800rpm in top gear. I need to be rather
faster than 40mph to GET to top...the auto box seldom lets the RPM drop
below 1200...that is roughly where No 1 turbo kicks in..

> Your most economical speed may well be different to my wife's little car
> - but I'll be surprised if it's as high as 70.
>

There seems to be remarkably little difference between about 50 and 70mph.
A little car with a small engine has very small fixed losses. Ergo it
will have a much lower optimal speed.


> Andy

--
It is the folly of too many to mistake the echo of a London coffee-house
for the voice of the kingdom.

Jonathan Swift