"Artemus" wrote in
:


"chaniarts" wrote in message
...


Well... That's really what the overdrive is for. Not below 30mph
(I think most cars kick them in around 50-55mph), but to reduce
engine RPM at high speed. And actually many engines are supposed
to be most efficient running around 3/4 throttle.

Puckdropper

3/4 throttle without specifying the gear doesn't mean anything. 3/4
throttle in overdrive gear in my car is probably 130mph, which
probably isn't it's most fuel efficient.

The transmission gear is irrelevant when specifying engine efficiency.
The engine is more efficient at a throttle plate opening of 1/2 to
3/4. Below this opening the pumping losses around the plate go way
up. Above this and the mixture gets richer on carbureted engines, FI
may be different.

Back in the 70's one of the car mags ran an article on a BMW study of
the most efficient acceleration method. The end result was to
accelerate at 1/2 throttle opening up to 2,000 rpm and upshift. This
is repeated until the desired speed is reached then use the highest
gear, without lugging the engine, and the appropriate throttle
opening. Their data showed that small throttle openings caused low
efficiency, and mpg, due to the high pumping losses. Long sloooow
acceleration times were almost as bad a jack rabbit start.
Art



This has generated bit of research for me. The Wikipedia article on
Engine Efficiency says 3/4 horsepower rating (and not 3/4 throttle) would
be most efficient, while the Prius article says that the Prius runs the
engine with the throttle fully open as much as possible to minimize
pumping losses.

In a simple system like a hydraulic powered mower, 3/4 throttle could
equal 3/4 horsepower output. In something more complex, 3/4 throttle may
not be able to produce the 3/4 horsepower output.

However, the Wikipedia article on Engine Efficiency could be wrong about
3/4 rated horspower output. That particilar article was rather poorly
written and doesn't cite its sources.

Puckdropper
--
Never teach your apprentice everything you know.