FUEL EFFICIENCY PART 3

The piston engine is a heat engine and as such, its efficiency is forever limited by the laws of thermodynamics. In practice, the efficiency those laws allow is never reached for various reasons.

When fuel burns in a cylinder, some of the resultant heat raises the pressure, which then drives the piston. The power delivered to the piston is the product of the pressure, the piston area and its speed. On the induction stroke, the pressure is low and power is taken from the piston. On the compression and exhaust strokes, the pressure is positive, but the piston is moving against the pressure and once again power is lost. Averaging the pressure out over two revolutions gives the mean effective pressure (MEP).