From: Peter Hill on
On Tue, 6 Oct 2009 01:04:02 +0100, "DavidR" <curedham(a)4bidden.org.uk>
wrote:

>"Pete M" <pete.murray(a)SPAMFREEblueyonder.co.uk> wrote
>> Mike Barnes wrote:
>>>
>>> 1 The diesel's power is available over a wider range of revs. You
>>> presumably quote peak power, which applies at only one speed. The
>>> petrol engine's power will fall off very quickly either side of the
>>> peak revs, whereas the diesel engine's won't.
>>
>> Diesel power bands are notoriously narrow, especially compared to that of
>> any good petrol engine. Turbo diesels have even narrower useable power
>> bands.
>
>Not necessarily. If a diesel has maximum power at 4000rpm and maximum torque
>at 2000rpm, which is exactly the same proportion as the 6000rpm and 3000rpm
>of a petrol engine.
>
>On top of that, a petrol engine typically bangs its head at 6500rpm whereas
>a modern diesel gracefully fades to 5000rpm. So, I suggest that a diesel
>overall, is
>actually wider.
>
>For me it is so. My present (diesel) and previous (petrol) have similar
>power to weight yet there is no doubt that between 30-80 the diesel gear
>ranges are both faster and wider. We're not talking nuances here, it's like
>going back 25 years to the time when cars were nearly half a ton lighter.
>Only from standing start (where absolute rev limit counts more than rev
>range), combined with the turbo lag does it lack petrol's finesse and so
>delivers a possibly inferior 0-40 time.

I think you are comparing Diesel turbo with a N/A petrol.

Try that comparison with any petrol turbo up to 21 years old and you
will find you need a Diesel with a Compound turbo to get anywhere
close to matching it. Do the comparison with a petrol turbo under 10
years old and the turbo compound Diesel loses.

This is a comparision of driven wheel torque, it shows how a selection
of engines will respond in a given chassis if all geared for the same
speed.
%rpm = 100 x rpm / redline rpm.
Normalised torque (Nm) = redline rpm x torque (Nm) / 1000. (I've used
/1000 to keep the figures sane)
http://i61.photobucket.com/albums/h76/skyshack/Diesel_vs_Petrol.png
The BMW with vanos wins for overall range - it's 1.5x bigger but a
2001 SR20VET petrol turbo with Vtec wins over the upper part of the
range and would be the winner if they raced. The 2008 1.9 TTiD turbo
compound diesel matches a 10 year old 1999 SR20DET 2L petrol turbo at
what could be on road cruise speed but over the rev range it loses.
Conventional single turbo 1.9L diesels lacking torque delivered to
wheels can't even match a 1989 1.8 petrol turbo. The more modern of
the 2, the JTD just manages right at the end of the redline - when
both have dropped dead. The TDCi peaks earlier but loses out on top
end so is better for towing.

Specific torque is torque divided by engine size. This removes the
"I've got a big one" effect and shows how well the engineers did / how
hard the engine is working.
%rpm = 100 x rpm / redline rpm
Specific torque (Nm) = redline rpm x torque (Nm) / capacity (l) x
1000. (I've used /1000 to keep the figures sane)
http://i61.photobucket.com/albums/h76/skyshack/Diesel_vs_Petrol_specific_torque.png
The smaller 1.9 diesels and 1.8 turbo petrol engine all move up while
the big N/A motor falls to the bottom - it's lazy and that's why you
need a big one.

The best Diesel turbo has 2 turbos. Engine makers have yet to deliver
a VVTi (Vtec/Vanos etc) sequential twin turbo petrol engine. If they
ever do it will look like the SR20VET curves but at 50% redline rpm it
will ramp up again to a peak 20-25% higher.

Are Diesels as good as petrol?
NO and never can be, development engines have hit a cylinder pressure
limit and the heads are fatigue cracking under test. There's no more
to be had unless you accept short engine life.
--
Peter Hill
Spamtrap reply domain as per NNTP-Posting-Host in header
Can of worms - what every fisherman wants.
Can of worms - what every PC owner gets!
From: DavidR on
"Peter Hill" <peter.usenet1(a)nospam.demon.co.uk> wrote
> On Tue, 6 Oct 2009 01:04:02 +0100, "DavidR" <curedham(a)4bidden.org.uk>
>>"Pete M" <pete.murray(a)SPAMFREEblueyonder.co.uk> wrote
>>>
>>> Diesel power bands are notoriously narrow, especially compared to that
>>> of
>>> any good petrol engine. Turbo diesels have even narrower useable power
>>> bands.
>>
>>Not necessarily.
>
> I think you are comparing Diesel turbo with a N/A petrol.

Yes, obviously. Nothing in the first paragraph suggested anything else.
Unless you're suggesting that the definition of "any good petrol engine"
inherently excludes all n/a engines.

I'm never going to trot out the diesel "torque" thing. It is obvious
nonsense. But petrol turbo has never become mainstream despite a few
examples knocking around for over 30 years. For all their impressive open
throttle delivery, fuel consumption and everyday driveability have never
offered a realistic selling point. Perhaps the new crop of direct injection
engines fixes that?