Engine vibration
Through combustion, where chemical energy is converted into mechanical energy, gas forces are generated which act on the piston crown. The reciprocating movement of the pistons and connecting rods, combined with the rotation of the crankshaft, generate inertial forces that act on the engine block and cause it to vibrate in various ways. At low engine speeds these gas forces are greater than the inertial forces but at high engine speeds the converse is true. The most significant forces arise periodically once or twice per crankshaft revolution. They are known as first-order and second-order forces. The first-order inertial forces are completely cancelled out since the crankshaft is balanced and the two piston pairs, 1-4 and 2-3, reverse direction simultaneously when they reach top dead center and bottom dead center.