One other aspect that affects drivetrain smoothness is the variation in rotational speed arising from the sharing of piston kinetic energy amongst the different phases. There are only three piston phases in a four stroke inline six and, whilst the nature of the piston motion can never result in a 3rd-order imbalance (the pistons have no oscillation at that rate), it does appear in the kinetic energy exchange between the pistons and crankshaft (mathematically because of the squaring of velocity resulting in a specific intermodulation of frequencies). This means that a constant kinetic energy level for the rotating and reciprocating masses will result in the flywheel rotation speeding up and slowing down three times per revolution. This will in turn result in a cyclic so-called inertial torque being overlaid onto the combustion torque output from the crankshaft, which also has three pulses per revolution.