What is Mean Effective Pressure?
Mean effective pressure is the ratio of work done (W) during the working stroke(s) of a cycle to the stroke volume or swept volume (Vs) of the cylinder. It is denoted by ‘pm‘ and its unit is N/m2.
In order to derive an expression for mean effective pressure of Otto cycle, we have to find out an expression for work done and stroke volume of Otto cycle.
From my previous article, p-V and T-s diagrams of Otto cycle are as follows:
|p-V Diagram||T-s Diagram|
From my previous article,
In process 1-2 (isentropic compression),
In process 3-4 (isentropic expansion),
Also from the p-V diagram above,
V1 = V4 and V2 = V3
Let Vc = V2 = V3 = 1
Work done during Otto cycle,
W = Work done during isentropic expansion (process 3-4) – Work done during isentropic compression (process 1-2)
This is the expression for work done (W) in terms of r, k, γ and p1.
Now, let us derive an expression for stroke volume Vs in terms of r.
We know that in Otto cycle,
Vs = V1 – V2 (See p-V diagram above)
Now, Mean effective pressure,
The above expression can be written as
which is the required expression for mean effective pressure of otto cycle