Mean effective pressure

The mean effective pressure (MEP) is a quantity relating to the operation of a reciprocating engine and is a measure of an engine's capacity to do work that is independent of engine displacement. Despite having the dimension of pressure, MEP cannot be measured. When quoted as an indicated mean effective pressure (IMEP), it may be thought of as the average pressure acting on a piston during the different portions of its cycle. When friction losses are subtracted from the IMEP, the result is the brake mean effective pressure (BMEP).

Let:

Then, BMEP may be used to determine an engine's power output as follows:

Since we know that power is:

We now see that, BMEP is a measure of expressing torque per displacement:

And thus, the equation for BMEP in terms of torque is:

Speed has dropped out of the equation, and the only variables are the torque and displacement volume. Since the range of maximum brake mean effective pressures for good engine designs is well established, we now have a displacement-independent measure of the torque-producing capacity of an engine design – a specific torque of sorts. This is useful for comparing engines of different displacements. Mean effective pressure is also useful for initial design calculations; that is, given a torque, standard MEP values can be used to estimate the required engine displacement. However, mean effective pressure does not reflect the actual pressures inside an individual combustion chamber – although the two are certainly related – and serves only as a convenient measure of performance.

Brake mean effective pressure (BMEP) is calculated from measured dynamometer torque. Net indicated mean effective pressure (IMEP) is calculated using the indicated power; i.e., the pressure volume integral in the work per cycle equation. Sometimes the term FMEP (friction mean effective pressure) is used as an indicator of the mean effective pressure lost to friction (or friction torque) and is just the difference between IMEP and BMEP.