Second-Order Forces Acting Vertically

Fig 1: [Identifying Second-Order Forces Acting Pattern]

Inertial forces are generated because both of the descending pistons in a 4-cylinder engine at a certain crankshaft angle move a longer distance than the ascending pistons (the lateral movement of the crankshaft accelerates the descending but delays the ascending piston movement). Therefore, the common centre of gravity of the ascending and descending masses varies, which leads to ascending and descending forces that vary periodically twice per crankshaft revolution and make the engine vibrate vertically.