[SGVLUG] [OT] Especially for Tom & his Prius.... Update

[David Lawyer]

> > On Sat, Jul 15, 2006 at 08:22:15PM -0700, David Lawyer wrote:
> > If you want a fair comparison, you may compare a diesel with 40% better
> > mpg than the gasoline engine.
> 
On Wed, Jul 19, 2006 at 02:19:00AM -0700, David Lawyer wrote:
> 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.

I just checked on this and found this estimate
http://www.geocities.com/MotorCity/Shop/3589/efficiency.html which
claims 7% for city and 10% for highway for cruising autos.  This
doesn't include the negative efficiency during coast down or idling.
And it does include transmission and drive train losses (in other
words it fuel tank to wheels --but not to the road as there are high
losses in the tires).  So I was grossly over optimistic just like the
case for hybrid charge-discharge cycles.

Regarding my previous claim that the peak efficiencies of gasoline and diesel
are not too far from equal:
> 
> So I may have been wrong about this and I'll try to find some new
> data.

I haven't found it yet but I realize that I was wrong, although I was
almost right many years ago.  The reason is that while the diesel has
significantly improved in efficiency over the last 20 years or so,
apparently the gasoline engine hasn't improved that much.  An article
by the Union of Concerned Scientists states that about 10 years ago
diesels for automobiles didn't use direct fuel injection, but instead
injected the fuel into a precombustion chamber.  Result was about a
15% drop in efficiency.  So if diesels for autos were only 30%
efficient 30 years ago, this would make them 35% efficient today
(maximal efficiency).   There could be other improvements too and my
30% estimate could have been low.  Truck diesels today are often over
40% efficient.  So I expect automotive diesels to be close to 40%
today (but I need to check this out more) which is NOT roughly equal
to 30% as I thought.

However, one thing is puzzling.  If diesels are really 40% efficient
at best, and since the part-load diesel efficiency is much better than
the gasoline engine which gets 30% efficiency at best, why doesn't
diesel MPG equal well over double the gasoline auto's MPG?

			David Lawyer