Dear;

Engine is an oscillating system and as such it is to be enforced when piston stops and changes movement from upward to downward (TDC). Because fuel is supplied as mist and combusts slow only about 7% of fuel has released energy at that point, which maximized pressure acing on piston, without creating too much of it prior to TDC that would stall thye engine. Ther is a balance between paracitic torque generated prior to TDC and max pressure occured in TDC, that starts at that point propelling engine. This ballance relates to proper advancement of ignition.

Stan

http://www.youtube.com/watch?v=sEf8va1S7Sw

Among other things notice that what appears to be a large amount of advance is quite a small amount of piston travel.