[SGVLUG] [OT] Especially for Tom & his Prius.... [my rebuttal, then I'll shut up]

[David Lawyer]

> On Sat, Jul 15, 2006 at 08:22:15PM -0700, David Lawyer wrote:

On Sun, Jul 16, 2006 at 09:58:53AM -0700, Dustin Laurence wrote:
> If you want to discuss calculating the carbon emissions of particular
> cars, which is certainly a reasonable thing to do, at least provide
> warning that you're veering off onto a new subject.
Part of the reason was that I didn't read the thread from the start
but picked items in the middle.  But I still hope it wasn't too far
off topic.
> 
> > For comparing a diesel car to a gasoline car where both are the same
> > energy efficiency (per vehicle-mile):
> 
> Gosh.  You're comparing a poor diesel to a better gasoline engine,
> because ordinarily the diesel will have *better* energy efficiency.

You left out the context.  I did this only to illustrate the effects
of the higher energy content of diesel fuel and the effects of more
CO2 from diesel.  I didn't mean to imply that diesel and gasoline were
equally efficient.  They are not.  But I previously stated that a
diesel was not too much different in efficiency if you compare it to a
gasoline engine at full throttle.  I'll try to look more into this as
I could be wrong.  But I've got data that I just looked at that may
imply I'm right.  More about this later.

> It's inherent in the cycle.  (This is *in addition to* the greater
> energy content of the fuel you correctly point out.)  I'm not going to
> discuss biased comparisions--it's pointless.  A 6-71 diesel gets fewer
> MPG than a model airplane engine too.
> 
> If you want a fair comparison, you may compare a diesel with 40% better
> mpg than the gasoline engine.

Some say 30% better.  But this comparison is with a gasoline engine
which varies in thermal efficiency between 0% and 30% in actual
operation.  Diesel maintains it efficiency much better at varying
loads since it doesn't have the pumping losses that a gasoline engine
has at part throttle.  I've heard it estimated that autos average
about 15% efficiency.  If diesels are only 30% max and 20% average,
this would explain the increased efficiency.  But the maximum
efficiency would be the same in this case.  Right now I would like to
know for sure what the maximum thermal efficiency of an automotive
diesel is.  I know that railroad locomotive diesels approach 40%
thermal efficiency yet go significantly lower at off peak loads.
There is a book (in Russian) written about this reduced efficiency
problem for railroad diesels (the entire book is on this topic).

So I'm in agreement with you that in typical operation at varying
loads and speeds, the diesel is say 30% or so more efficient and thus
should get say 35% more miles per gallon.  But the info I have (and it
could be wrong) seems to imply that peak efficiencies aren't that much
different.  The peak efficiency of diesel should be higher due to both
it's higher compression ratio and faster burning.  But per tests in
1949, a gasoline engine (installed in an Oldsmobile with water pump,
fan, generator, etc.) was nearly 30% efficient (at best with a 12:1
compression ratio).  See SAE Journal, June 1949, pp.17-21 "High
compression engine performance" by Max M.  Roensch.  But at a 10:1
compression ration it was only about 27%.  At 8:1 compression it was
only about 25%.  

Then I've got a personal communication from Daimler Benz (Mercedes)
dated 1975 in Germany showing a peak diesel thermal efficiency of only
27 1/2 %.  This is with road load at 70 mi/hr.  It could be higher say
on a grade ??  Germany uses the lower heat of combustion so their
efficiencies should be about 9% higher than US efficiencies since the US
used to use (and often still does) the higher heat of combustion.  But
it looks like the US article in 1949 must have used the lower heat of
combustion to make the efficiencies look good.  I didn't realize this
until just a few minutes ago when I converted reported lb/hp-hr to
efficiency.

So I may have been wrong about this and I'll try to find some new
data.  Another troubling thing is the high efficiency reported in 1949
for an engine that must have had only 2 valves per cylinder and likely
wasn't valve-in-head.  Was it really this good?  If so, why are
today's engines not that much better?  One reason may be the use of
the lower heat of combustion in 1949 assuming that the higher heat of
combustion is still being used today.  The US Dept. of Energy is still
using the higher heat of combustion for transportation but I don't
know about the case for thermal efficiency.

			David Lawyer