Brake Mean Effective Pressure (BMEP) - EngineKnowHow

Whilst torque provides a measure of the work capable by engine it can be difficult to compare how effective different engines are at generating this work when they have a range of different characteristics, eg. displacement, number of cylinders, ignition system etc.

For example, when comparing engines by their torque curves then displacement is a dominant factor as a higher displacement equals more air in the cylinder for combustion.  Alternatively, when comparing engines by their power then engine speed is a dominant factor.

A method to compare engines, regardless of displacement or ignition system etc. is to consider the average pressure acting on the top of the piston during the power stroke, this is known as the Brake Mean Effective Pressure or BMEP.  BMEP is an indication of how effectively the cylinder displacement has been used to generate torque.

BMEP is calculated from:

 

 

 

or

 

 

 

Where  nr  is  the number of crank revolutions per power stroke; 2 for a four stroke engine, 1 for a two stroke engine.

Below are examples of several engines with their BMEP indicating how effective each engine is in developing torque relative to its displacement.