[Alan L]

Originally Posted by chris_seven

One of the issues of uneven piston inertia is that the individual cylinder will decelerate at different rates within the cycle and this will cause a slight unevenness in the way the engine runs.

If you take a 500cm^3 single cylinder engine at 8000rpm it will change speed by around 80 rpm within a single revolution and this will need nearly 1kgm^2 of inertia as a flywheel to keep it running smoothly. If the friction is consistent then the torque which this generates will change the speed change within a revolution.

I spent some years manufacturing single cylinder engine test rigs used by several F1 engine suppliers and have seen these effects quite clearly demonstrated. At 20,000rpm which was the maximum speed of the test systems we supplied it all became a bit tricky and speed changes increased to around 250rpm within a single revolution.

If you reduce the weight of two pistons then this effect will be increased and could set up some high order torsional vibrations which could result in a reduction on fatigue life.

It would be possible to set up a mathematical model to try to analyse this but it would be much faster to just match the weights.

What I was suggesting was to reduce weight of 1 piston by 9gm to match the other (6th) , then pair them up - at the light end of the crank (since they would be the lightest pair). Each piston in 1-4 position would be balanced as would 2-5 and 3-6. Alternatively, is there no way to add weight to get them all the same? If I have to I guess I'll just have to pull 9 gm off 5, but it seems a cumbersome way to solve the problem. I ended up with a 6th piston at the light end of the lightest group weight.
regards
Alan