Date: Thu, 9 Aug 2001 04:15:59 EDT
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: On Fuel Economy - Some Teutonic Observations
Content-Type: text/plain; charset="US-ASCII"
I was just scanning through some of my Automobiltechnische Zeitschrift (ATZ)
paper copies and came upon the engineering study done by Audi engineers in
the development of the 2.3 liter 5 cylinder gasoline engine for the 1987 Audi
100. I thought I would share two very interesting data sets with the list. At
issue are the very detailed fuel consumption maps done for and submitted to
the German Government for fuel consumption certification.
Now measuring fuel consumption of and engine on an engine dyno is difficult
enough, valid numbers for a moving vehicle are particularly challenging
(euphemism). Along with the many driveline, road surface and ambient issues,
the car profile, drag coefficient, frontal surface area, etc., etc conspire
against meaningful numbers. So the factory engineers took lots of
measurements with two variables constant (car and road test cycle).
The first key result is a plot of urban fuel consumption (DIN cycle) vs. curb
weight. There 108 data points, all for the 1987 Audi 100 with 10 valve,
naturally aspirated with catalytic converter and 5 speed manual transmission.
The data shows a linear relationship for curb weights of 750 kg to 1600 kg.
The fuel economy went from 8 l/100 km (750 kg) to 16 l/100 km (1600 kg).
Linear. Linear! Twice the mass, half the fuel economy. So urban cycle fuel
economy of Joe Blow's Westy vs. Sara Etheria's Westy will depend linearly on
their respective masses!
Second data set. Part throttle (read cruise) fuel consumption numbers for the
Audi 100 in fifth gear. Up to 60 km/h data shows a near constant 6 l/100 km.
from 60 to 200 km/h the fuel consumption rises exponentially to 23.5 l/100 km
at 200 km/h. So at 70 mph vs 55, the consumption has increased by 30 % for 80
vs 55, the increase is 80 %. This number is a strong function of frontal
surface area, so the difference for the vanagon will be even more. So
comparing Hubert Schmuk in his fully loaded Westfalia with three kids, wife,
2 dogs and all the necessary accouterments averaging 80 miles per hour on his
gasoline and water powered 2.1 WBXer probably really gets half the mileage
that Lenny MilqueToast got on his Westfalia carrying wife, Perrier Water and
lots of open space for souvenirs running at 55 mph.
Frank Grunthaner
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