IMEP

IMEP stands for Indicated Mean Effective Pressure. It is the average pressure inside a cylinder over the portion of the engine cycle during which power is produced. In other words, IMEP is the indicated work per cycle divided by the engine’s swept (displacement) volume. Because it is based on indicated pressure data, IMEP reflects the engine’s capacity to perform work independent of mechanical losses such as friction and pumping losses.

Calculation and interpretation: IMEP is derived from the in-cylinder pressure trace obtained with a pressure transducer during a controlled test. The instantaneous pressure is integrated with respect to volume over a cycle to yield the indicated work, W_i. IMEP = W_i / V_d, where V_d is the displacement per cycle. For a multi-cylinder engine, IMEP may be reported per cylinder or as an engine-wide value, and indicated power IP = IMEP × V_d_total × (number of power strokes per second). Units are commonly expressed in bar or MPa.

Relation to other measures: IMEP is used to compare engine performance independent of size, efficiency, and losses. Brake mean effective pressure (BMEP) uses brake power and includes frictional losses, so BMEP is typically lower than IMEP for the same operating point.

Applications and limitations: IMEP is widely used in engine research, development, and performance mapping to evaluate combustion quality and thermal efficiency. It requires precise in-cylinder pressure measurements and controlled test conditions; it varies with rpm, operating conditions, and fuel/air strategy, so it is not a fixed property of an engine.