In a message dated 10/15/02 1:44:25 PM, gull@CYBERSPACE.ORG writes:

<< I'm not sure that would hold if you did the comparison between two gas
engines instead of between two diesels.

What's the difference?  The throttle plate.  Diesels don't have one.

If you have a small gas engine and a larger gas engine producing the same
horsepower output, the larger engine will be at a smaller throttle
opening.  That means a higher manifold vacuum, and more energy lost
pulling air through the smaller opening. >>

David, Nate,

I certainly agree about the difference in operational throttle
characteristics and the different load parameters for diesel vs. gas. In fact
I was trying to tease interested list members to check out the archives in
which I have pontificated on this very issue. In the brake specific fuel
consumption curves for the 2.0 L AC, the 1.9 L and 2.0 L WBxer, and 1.6, 1.8
and 2.0 L I4s as well as the Audi gasoline I5's (all published in the
technical papers I references) the peak efficiency is typically given at peak
load. Again, the efficiency of the engines can be compared, just given these
values. The variation in efficiency for different loads and different rpm
values is also contained in those curves.

But my point then and now remains that there are some non trivial differences
in fuel efficiency for different engine designs, but that the real issue was
not rpm, but rather load. As I tried to paraphrase before, you pay (fuel
consumption) for the work you demand. Now the key point here was in response
to the never dying wisdom on this list that if an engine spins at 3000 rpm
while a second engine must turn at 4000 rpm to hold 60 mph in a 5300 pound
Vanagon, the second engine will use 33% more fuel than the first. Nonsense.
As is analytically shown in the BSFC curves and the load/thrust analysis that
I shared in the archives. The issue is load or work demanded.

And frankly, the highly efficient TDi is probably the most effective engine
for the Vanagon. Turbo diesels are impressive, and the TDi technology even
more so.

Sorry for the soapbox,

Frank Grunthaner