Re: If you want to be invisible on a car lot...


Bill Baka wrote:

Mark,
You must be one of those who really flunked physics, BADLY.
I did this with a 1961 Rambler flathead 6 and took it from 25 MPG up to
38 MPG+ on the highway at 65 MPH.

From _just_ a change in gear ratio? Sorry, but that sounds _very_
implausible.

If you have a car, and a stopwatch,
and can make a fairly reliable measurement, then try something before
you go all self righteous on me. I took my 4,400 pound 1966 Chrysler
with the 2.73 rear end and 440 engine, and timed the coast down from 70
to 60. It was 15 seconds in neutral, but only 7 seconds in gear with a
727 automatic. The SIMPLE deduction is that half the energy was wasted
just rotating the engine.

It's SIMPLE, all right, but it's wrong. At least, if you imagine it's
just piston ring friction at work.

The great bulk of the coast-down difference you described comes from
fighting the compression and the pumping losses of the engine, not just
the friction of the piston rings and other moving components.

Big DUH, bicycle genius.
Do the math and see how much energy it takes to make each 2 pound piston
accelerate and decelerate up and down at 2,500 RPM.

Sure, it takes energy to accelerate a piston. That energy comes almost
entirely from the nearby piston which is decelerating. They are simply
trading kinetic energy back and forth.

Have you taken a thermo course? If so, imagine a control volume around
the engine. If your losses were just energy accelerating and
decelerating the piston, how would the energy leave the system?


You should be aware of variable displacement engine technology, in
which certain cylinder's valves are kept closed. The result is more
gas mileage, largely because of less pumping loss. If auto makers
could get the same result by just raising gear ratios (and programming
for rapid downshift when necessary) they'd certainly take that less
complicated approach.

- Frank Krygowski

.

Relevant Pages

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