What is mean effective pressure?
Mean effective pressure is an imaginary pressure if it is acted on a piston during the entire power stroke then it would produce the same amount of net work as that produced during the actual cycle.
Why do we calculate mean effective pressure?
We know that the product of pressure and change in volume gives out to be work done in a closed system. But this relation is valid only when the system is under constant pressure. It is not the case with Internal combustion engines because pressure varies over the entire power stroke and it is not constant. Hence the concept of mean effective pressure is used in which we assume an average pressure to be acting over the entire power stroke and then multiply mean effective pressure with the change in volume between TDC and BDC to get the work done in the IC engine.
How do we calculate or plot MEP?
The mean effective pressure is measured using a pressure recording gauge called an 'indicator' which is attached to the cylinder of the reciprocating IC engine. The same gas pressure acts on both the engine piston and the indicator piston.
Calculating mean effective pressure from an indicator diagram
What do we do with mean effective pressure?
We calculate the indicated power produced by the IC engine once the mean effective pressure is known.
Here is a solved problem for better understanding.