Re: [alfa] Diesels and dollars

["Gwynne W. Spencer" <gwynnes1@xxxxxxxxxxx>]

NO - there is a 3rd factor, the fact that a gallon of diesel produces about 
27% more BTUs - just plain more energy!

And - the throttled gasoline engine running at lower compression pressures 
at part throttle does not lower efficiency - where do I find this 
information; it is new to me. The formula for "air standard efficiency" 
includes a term for compression ratio, but, of course CR is a meaningless 
term in a throttled normally aspirated engine.

Gwynne Spencer

At 08:16 AM 5/26/2004 -0600, you wrote:
>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
> >>>>--
> >>>>to be removed from alfa, see /bin/digest-subs.cgi
> >>>>or email "unsubscribe alfa" to majordomo@domain.elided
> >>
> >>Michael Smith
> >>White 1991 164L
> >>Original owner
> >--
> >to be removed from alfa, see /bin/digest-subs.cgi
> >or email "unsubscribe alfa" to majordomo@domain.elided
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