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Old 03-14-2013, 05:40 PM
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Roelof's Avatar
Join Date: Aug 2007
Location: Holland
Posts: 7,439

I think you are wrong with your calculations... The max speed of the piston is the same as the rotating speed....

The stroke of current .21 engines is about 17mm, 1 round is 53.4mm in distance.
Lets say @46.000 rpm is 766rps. In 1 second it is running 766 x 0.0534 = 41 meter/sec which is 143km/u (no sonic sound)
I can remember a limit for lubrication has a magic number of 35m/s...

V=a x t , a = V/t

t is 1 round /4 (the moment from 0 to max speed) so that is 1/766 and again /4 = 0.326msec

a = 41/0.000326 = 125624 m/s^2

10m/s^2 = 1G so you are talking about 12562.4G

I know a piston+rod is about 7.5 gram so it will create 94kg force @46.000rpm

This is pure theoretic because a 90 degrees travel of the crakshaft is not giving a halfway of the piston travel. A 90 degree exhaust timing is somewhere arround 7mm so there is a 7mm and a 10mm acceleration and not exactly half. And these calculations are in a lineair movement but actually is is more like a sinus shape so actually the 94 kg is a bit higer.

As you can see with such forces manufacturers are searching the limits of the mechanical strength.

By the way, some of the noise of an engine is comming from the cooling head, the vibrating fins can produce some noise....
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