Re: [alfa] Diesels and dollars

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 9:40 AM 5/26/04, Gwynne W. Spencer wrote:
>You missed my point about throttling - I meant to be talking about fuel
>efficiency (MPG) and it is not only the pumping losses that reduce it, but
>the fact that the cylinders are running at very low pressure effective
>ratios due to the high intake manifold vacuum. If my memory serves me OK,
>the formula for power at WOT includes the compression ratio to the 0.4
>power - at lower throttle settings this does not apply, since the cylinders
>are not filled to capacity at atmospheric pressure, but some much lower
>pressure. This also results in lower fuel efficiency, one of the reasons
>that the gas engine is so much less efficient than the diesel which
>operates WOT all the time.

NO.

The pumping losses and the difference in (practical, usable) compression
ratio ARE the only differences in efficiency between compression ignition
and spark ignition engines which work in favor of compression ignition. Two
that work in favor of spark ignition engines being more efficient are the
fact that the Otto (spark ignition) cycle is inherently more efficient than
the Diesel cycle (due to the addition of heat at constant volume rather
than at constant pressure) and the lower internal friction losses that are
a direct result of the usually lower compression ratio in spark engines.

The lower MEP in a throttled spark engine means lower output (which is the
raison d'etre of the throttle control), but it does NOT lower the
efficiency of the thermodynamic cycle. Any throttling process is
thermodynamically irreversible--and the 'pumping loss'--the fact that the
engine must exert WORK to pump the throttled working fluid it has used back
up to atmospheric pressure--is the ONLY penalty that spark engines pay for
being throttled.

In considering the above facts, one must understand that both the Otto and
Diesel cycles are theoretical idealizations of what really goes on inside
an engine---in the real world, a spark engine does not burn its fuel/air
charge at constant volume (instantaneously when the piston is at TDC)--and
compression ignition engines spend a LOT of money striving to make their
engines act more like an Otto cycle engine--the higher the injection
pressure, the FASTER they can get the fuel into the chamber, and the closer
they can approach a constant volume burn, and therefore the higher
efficiency of the idealized Otto cycle.

On the other hand--a diesel makes more torque than a spark engine--at low
engine speeds--precisely because of the fact that its fuel is burned at a
slower rate than in a spark engine--because of the slower burn, the
cylinder pressure stays higher later in the power stroke, when the con rod
has more leverage on the crank. Going the other way--compression ignition
engines are VERY limited in rpm--NOT so much because of the heft of their
internal parts, but because of the mechanical limitations on how fast their
fuel can be injected. Above a certain limiting rpm, it's simply not
possible to get the fuel injected into a diesel's cylinder and burned in
time for it to do any good !

Greg

>
>Just what is the 0 - 60 time for your Saab ?? and what are the gear ratios
>? (another major factor in acceleration performance) the data  I found
>(from Saab) was for the manual transmission car only @ 6.5 seconds (I
>believe the MB data is for an automatic tranny)
>
>But the real story is that we were comparing diesel and gasoline power and
>the MB cars give us the perfect opportunity to compare essentially
>identical cars, one with a gas engine, the other with diesel - how about
>that ??!!
>
>And lastly, I know that throttling losses do not apply at full throttle -
>and that is one of the reasons for diesel superiority in some of the regimens.
>
>Gwynne Spencer
>
>At 06:30 PM 5/25/2004 -0600, you wrote:
>>Yeah but, my 1997 SAAB Aero does zero to 60 in the same time as the Benz
>>turbo diesel, makes 256 lb ft of torque from only 2.3 liters, and is way
>>more fun to drive than any Benz in the price range. Of course the Aero
>>weighs only 3200lbs, but the engine is 2/3 the size of the Benz. And it
>>gets 35 mpg on the highway.
>>
>>BTW, full performance comes at wide open throttle where a gasoline engine
>>suffers no more pumping losses than the diesel.
>>
>>Cheers
>>
>>.At 10:04 AM 5/25/2004 -0400, you wrote:
>>>Turbo diesels make the engine even more efficient as the EFFECTIVE
>>>compression ratio is higher. This does not translate directly to
>>>throttled gasoline engines as they commonly operate at high intake vacuum
>>>levels resulting in an effective compression ratio way lower than the
>>>full throttle theoretical.  The performance numbers for the E class
>>>gasoline and diesel powered cars comes from  road tests - also
>>>see:  http://www.msnbc.msn.com/id/4863112/
>>>
>>>Gwynne Spencer
>>>
>>>At 09:22 PM 5/24/2004 -0600, you wrote:
>>>>While I have zero interest in driving any product from Daimler Chrysler,
>>>>with the possible exception of the Mitsubishi Evo and the extremely
>>>>improbable exception of the McLaren Mercedes, I ask whether the
>>>>supposedly superior diesel Mercedes is a turbo. If not, then I accept I
>>>>am mistaken about the superiority of gasoline power for passenger cars.
>>>>If, as I suspect, the Mercedes is supercharged then I maintain my
>>>>position that equivalent gasoline engines are more suitable for
>>>>passenger cars than any diesel. If the engine is the same sized,
>>>>turbo'd, etc then the gasoline powered car should be quicker.
>>>>
>>>>
>>>>Michael Smith
>>>>White 1991 164L
>>>>Original owner
>>>>--
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>>
>>Michael Smith
>>White 1991 164L
>>Original owner
>--
>to be removed from alfa, see /bin/digest-subs.cgi
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