Differences between automotive & airplane engines

Chris Wells wrote:
How are "normal" airplane engines tuned to run at a lower rpm? What
changes would have to be made to an automotive engine to shift the
power band down accordingly?

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Dear Chris,

They are not 'tuned' to run at lower rpms, they are DESIGNED to do so.
Indeed, although both are engines, in purely engineering terms they
have surprisingly little in common.

In mechanical terms the two main differences between variable-speed
engines, such as found in cars, and 'normal' aircraft engines are in
the cam timing and the sizing of the bearings. But you're really
looking at an entirely different engineering philosophy, in that with
an aircraft engine reliability is given a higher priority than any
other factor.

Another major difference is the ratio between nominal and peak power.
The service life of a car's engine is based on a nominal output equal
to about 25% (or less) of the engine's peak output. That is the level
of output the engine is expected to produce for approximately 98% of
its service-life. The only time it will be asked to produce more is
when accellerating or climbing a grade. By comparison, the nominal
output of an aircraft engine is about 70% of its peak (or take-off)
rating, dropping to about 55% if the objective is to achieve maximum
time between overhauls.

A major problem in the on-going contraversy about converting car
engines for use in airplanes is that most Americans are not well versed
in automotive engineering and the best example of that may be seen in
the comments produced any time an engineer uses the term.
'Automotive,' of course, means ANYTHING that moves under its own
power and 'automotive engineering' covers everything from the space
shuttle to motorbikes... unless you happen to be one of the millions of
superbly ill-educated Americans who use 'automotive' when they mean
'automobile.' This is far more than grammatical nit-picking, in that
it is impossible to carry on a meaningful dialogue without properly
defined terms.

A by-product of that lack of education is how Americans view
'horsepower,' typically insisting that 50hp (at 5000rpm) is EXACTLY THE
SAME as 50hp (at 1000rpm). Indeed, most will whip out their calculator
and 'prove' they are identical :-) But as the Wright brothers
discovered more than a hundred years ago, horsepower is not a factor in
the equation of flight. With powered flight, the factor we must
concern ourselves with most is thrust. Working back through the
equation, for a given propeller efficiency & rpm we will eventually
arrive at a given quanta of torque which then may be converted into
units of 'horsepower,' should we wish to do so, although it serves no
useful purpose. But when we DO use 'horsepower' we must be careful to
never use it in isolation, always identifing the rotational speed at
which that 'horsepower' is being produced.

And along about here someone will discover the simple solution of
putting a gear-train between the thing producing all that lovely
'horsepower' and the propeller producing all that necessary thrust.
Indeed, the more astute will point out that the Wright brothers did
exactly that, using an arrangement of bicycle chains as a a
torque-multiplier :-)

-R.S.Hoover