Mean effective pressure

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The mean effective pressure is an abstraction of the pressure exerted into the combustion chamber of an internal combustion engine. Mean effective pressure is abbreviated MEP and often BMEP for "Brake" MEP for denoting dynamometer testing.

It is calculated by taking the torque exerted by the engine over a revolution for a two-stroke engine and two revolutions for a four-stroke, and dividing it by its displacement. In SI units, for MEP in pascals

MEP = 2 πT/V for a two-stroke,

MEP = 4 πT/V for a four-stroke.

where

T is torque (in newton metres)

V is volume (in cubic metres)

For example, a four-stroke motor producing 160 N·m (the rotating speed is irrelevant, but the torque is the power divided by it) from 2 litres of displacement is going to have (4π)(160 N·m)/(0.002 m³) = 1005000 N/m² = 1005000 Pa = 1.005 MPa of MEP (or 10.05 bars).

If the same engine produces 76 kW at 5400 rpm (90Hz), its torque is 76000 W (or N·m·s^-1) / 2π / 90 s^-1 = 134 N·m; its MEP will be (4π)(134 N·m)/(0.002 m³) = 0.842 MPa (8.42 bars). As piston engines mostly have their maximum torque at a lower rotating speed than the maximum output, the MEP is lower at full power, it's because the efficiency is lowered by more friction losses, air viscosisty, below optimum combustion, etc.

To gain a step, the formulas for coming directly from power are, because P/(2πf) = T

MEP = P/(f·V) for a two-stroke,

MEP = 2P/(f·V) for a four-stroke

where

P is power (in watts)

V is volume (in cubic metres)

f is rotating speed (in hertz)

MEP is a useful comparison tool between different engines, and is a good indicator of the level of performance achieved by its conceptor and manufacturer. It measures the efficiency of the conversion from the indicated mean effective pressure in the cylinder to the output shaft and the level of pressure attained in an engine. It’s important to remember that the values produced by the formula are for theoretical analysis only, and do not reflect the actual pressures inside an individual combustion chamber.