F1, trickle-down, etc.

[Craig Dotson]

> Yes, and 5cc model airplane engines run piston speeds that are
> higher still.  When you reduce the size of something, you lose mass
> far faster than you lose strength.  An F1 cylinder is 300cc, not 100cc.
> The stroke may be the same, but the piston weighs quite a bit more than
> the kart piston, so the forces involved are considerably higher, even
> for a slower engine speed, and the specific strength of the kart pieces
> will be higher.  By any measure against production engines, or even most
> car racing engines, these piston speeds are very high indeed.  The fact
> that there are even higher speeds out there doesn't alter that fact.

RPM and piston speed are very different.  If you really want to compare the
performance of an engine, RPM means nothing unless you're talking about two
identically-configured engines.  If you want to compare engines vastly
different in design (such as a model airplane, race bike, or F1 engine) you
need to look at mean piston velocity and mean effective pressure, rather
than RPM and displacement power density (hp/L or hp/cui).

Vp, or mean piston velocity, is 2 times the stroke times the frequency (in
Hz, not RPM).  Mean effective pressure can be either Brake, Indicated or
some argue Frictional.  It is defined as the power times a constant, divided
by the quantity of the frequency (Hz) times the displacement.  For a
4-stroke engine, the constant=2, and for a 2-stroke engine the constant=1.

If you evaluate these numbers for a VAST array of engines, you'll see they
are all approximately of the same magnitude (within an order of magnitude
anyway).  Here, the small variations are what tell you about performance.

Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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