<snip> Quite simply, the acceleration of the piston is at its greatest at TDC and BDT. To accelerate anything, you have to supply a force. At TDC, <snip> Is not piston acceleration zero *at* TDC and *at* BDC as the piston is at a dead stop at that instant in time? Perhaps de/acceleration is highest immediately before/after TDC/BDC? -glen