Message-ID: <3901A8B0.228B32D0@regandesigns.com>
Date: Sat, 22 Apr 2000 08:27:12 -0500
From: Brent Regan <brent@regandesigns.com>
To: Lancair List <lancair.list@olsusa.com>
Subject: Re: engines
Mime-Version: 1.0

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Rather than get into a long discussion about the physics of the internal
combustion engine I would strongly suggest that, if you are interested, you
read up on the subject. The most cited authors on the subject are Ricardo,
Heywood and Taylor. Check the SAE web site.

One very interesting example of engine comparison is given in "The Internal
Combustion Engine in Theory and Practice" by Charles Feyette Taylor where he
compares a large marine diesel engine (26,500 cuin/cylinder) to a Cox 049
(.10 cuin) model airplane engine. Both engines are loop scavenged two
stroke.

Of particular interest is that smaller engines make more horsepower per
cubic inch displacement (per cylinder) than larger engines. The ratio is
approximately proportional the cube root of displacement.

What I found remarkable is that several factors are almost identical between
the behemoth and the flea. They are: Brake Mean Pressure (average pressure
on the piston), Mean Piston Speed, Specific Output (horsepower per square
inch of piston area) and the ratio to their Weight to their Displacement.

If you want a lot of horsepower out of the lightest package you want a lot
of little pistons. Little pistons make for short strokes and short strokes
make for high crank RPM. Motorcycle engines make this point well. The bad
news in airplane engines is that high RPM and propellers don't get along so
now you need a gearbox and the gearbox must be counted into the engine
weight. Did I mention that the thermal efficiency of small cylinders is
considerably poorer that large cylinders?

After a little investigation you will find that engine comparison is a much
more complex matter than it would seem at first blush. Comparing horsepower
and displacement or weight only tells a fraction of the story.

Regards
Brent Regan

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