ameyam

Basically, SI engines need to be set to fire (produce the spark) a few degrees after the piston has passed TDC. That way, the momentum of the piston (and hence the direstion of rotation) is maintained. This time gap between the piston reaching TDC and the firing of the plug is called ignition delay. The igition delay should be such as to maintain the rotation but not so large that the spark plug fires after there is significant loss in compression as that leads to loss of power and inefficient combustion (smoke). Also, the spark must be produced so that there is sufficient time for the flame propogation (in the fuel air mix) while the engine is within its compression range (which is around 5-7 in gas engines, if I remember correctly. This is the sweet spot for max power). However, as the engine revs up or heats up, the ignition delay has to reduce (this is called advancing ignition) as the crank speed gives a shorter time for compression to be in the sweet spot. On the other hand, flame propogation time is fairly constant (it can be reduced by additives such as TEL). Thus there is a minumum ignition delay. This is also the reason why gas engines are limited in their max RPMs to around 8500 rpm or so.

Reverse starting happens because of pre-ignition in spark ignition engines. Preignition occurs when the plug fires before the piston reaches TDC pushing the piston in the opposite direction to that in which it is supposed to rotate. It usually happens because the ignition delay is not properly set for a cold low speed start. You will not see this problem if using a starter because the starter turns the engine over faster. When the engine backfires like this, it usually dies or auto corrects on the next revolution as the ignition fires at the correct time after TDC. However, if the ignition only counts time after TDC, it is possible to sustain the reverse rotation. Usually, cutting the engine and then restarting it in the proper direction after a reverse run will get it running correctly. Its jsut a matter of knowing what you are doing.

So much for theory, I hope they get the reverse running problem sorted out, I intend to get this engine soon

Ameyam