Engine orders are simply the amplitudes of the frequency components which are the multiples of the rotating frequency. Engine orders, which are determined by an order analysis are extensively used in the vibration and noise work to identify the source of excitation (order) and, hence, its frequency of an engine induced problem. For example, a four cylinder in-line engine will always have its second order component as the dominant excitation.
- In four-cylinder four-stroke engines this notation is often used to denote an engine order where the frequency is two times the engine rotational speed.
- Basic firing frequency of a six-cylinder four-stroke engine.
- Two times engine firing frequency of a four-cylinder four-stroke engine. It is the basic firing frequency of an eight-cylinder four-stroke engine.