RE: [T3] Wacky Mileage

[Jim Adney <jadney@vwtype3.org>]

On 18 May 2004 at 21:14, Greg Merritt wrote:

> At 9:57 PM -0400 5/18/04, Kevin Guarnotta wrote:
> >
> >I try to pay attention to where the gas pedal is when driving, the closer to
> >the floor, the worse your mileage.
> 
> 	I was surprised to learn that this is not true.
> 
> 	I recall that brake specific fuel consumption -- the rate of 
> fuel consumption per horsepower delivered -- is lowest when rpms are 
> low (low internal friction) AND the throttle is wide open (high 
> intake throughput / low resistance).

This may be true, but I don't think it's really useful. The reason is that 
there's just no way to keep the engine here for more than very brief periods. 
Plus, real world engines have features (throttle pumps, throttle valve 
switches) which make full bore acceleration less economic.

This approach becomes feasible when they take a single cylinder Briggs & 
Stratton engine and build a special fuel economy car around it. Such vechiles 
are used in fuel economy competitions where they get more than 1000 mi/gal 
(really!) But they get it thru low weight, low speed, low frontal area, bicycle 
wheels/tires, and an operation scheme where they run the car up to 15 mph with 
the B&S at full bore, then cut the engine and let the "car" coast back down to 
about 5 mph, and repeat. If you even let the engine idle during the coast-down 
time you lose much of your advantage.

Running the carb at WOT reduces the "pumping losses" which amount to the effort 
required to pull air thru the carb. Obviously this effort is minimized if the 
throttle is wide open (WOT.) In the situation above, I suspect that they remove 
the throttle valve completely, and there is certainly no throttle pump.

Keeping speed low is a VERY big factor, because air friction tends to go 
approximately proportional to speed squared. This may not seem like such a big 
deal, but it means that the air friction on a car traveling at 70 mph is about 
150 times greater than that for the same car at 10 mph, which is about the 
average speed in those fuel economy contests.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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