Date:         Tue, 15 Oct 2002 11:45:44 EDT
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      Turbo Diesel Power and Economy Comments/ Gasoline Too
Comments: To: nwall@opei.org
Content-Type: text/plain; charset="US-ASCII"

Recently, Nate Wall on the Audi-VW-Diesels list reposted the URL to the
excellent SAE paper on the design of the VW/Audi Turbo Diesels
(http://4crawler.cruiserpages.com/Diesel/SAE/vwtdsae.shtml). I have always
found it good reading! In any case, Nate quoted it during a discussion of the
power generated by the Audi 5-cylinder diesel.

I wanted to bring to the listee's attention, an interesting factoid readily
found in this paper (and in the many other technical papers to be found on
Alistair Bell's site). The paper cites the brake specific fuel consumption of
both the 4 and 5 cylinder turbodiesels (not TD1s). This key number gives the
actual measurements of the amount of fuel consumed by a given engine to
develop a given power at the flywheel. The minimum number gives the true
efficiency of the engine and could be used with drag and cross-section
calculations to determine the optimum operating speed for best fuel economy.

In any case, the numbers for the 1.6L 4-cyl TD and the 2.0L 5-cyl TD are 264
and 261 grams per kilowatt hour. Please note that the 5 cyl engine with 20%
larger displacement (swept volume) actually uses less fuel to produce the
same amount of power as the 4-cyl design. Both measurements were taken at 6
Bar boost pressure and 1800 rpm. Clearly the 5 cylinder engine is the more
efficient design. It produces more power in total than the 4 cylinder design
while consuming less fuel per unit power developed.

My point in raising this is just to point out that larger engines do not
consume more fuel ... the fuel used is related directly to the work done (or
requested as it were). You pay for what you use. With a modern engine
(compare gas to gas, TD to TD and TDi to TDi) if you call on the powertrain
to push a Vanagon at 50 mph weighing 5300 pounds on a level highway, your
fuel consumption could be the same weather you were drawing power from a 1.9L
waterboxer, a 2.0 L I4 or a 3.3 L Subie. The real difference in consumption
will be in engine design efficiency (see my Archival discussions on subject) a
nd proper gearing to match the load curve to engine characteristics. So a 1.3
L high efficiency engine could give poorer fuel economy than a 3.3 L SVX if
the power requirement pushed the smaller engine into a rich A/F ratio power
mode to drive the load.

To summarize, fuel economy is not determined to first order by engine
displacement and rpm, but rather by work demanded and engine design
efficiency.

Sorry, I just couldn't resist,

Frank Grunthaner