Mean Effective Pressure and Compression Ratio Relationship

The mean effective pressure (MEP) is the average pressure acting on the piston during the power stroke of an engine cycle. It is an indication of the engine's ability to convert fuel energy into useful work. The compression ratio (CR) is the ratio of the volume of the combustion chamber at its largest to its smallest volume. It is a critical parameter that influences the engine's performance.

The MEP of an engine is affected by the compression ratio. A higher compression ratio results in a higher MEP, while a lower compression ratio results in a lower MEP. Therefore, the selection of the compression ratio is vital to optimize the engine's performance.

Air-Fuel Ratio and MEP Relationship

The air-fuel ratio (AFR) is the ratio of the mass of air to the mass of fuel in the combustion chamber. It is another critical parameter that influences the engine's performance. The AFR affects the combustion process in the engine and, therefore, affects the MEP.

The MEP is maximum when the AFR is lower than stoichiometric. Stoichiometric AFR is the air-fuel ratio required for complete combustion of the fuel. For gasoline, the stoichiometric AFR is approximately 14.7:1. At AFR values lower than stoichiometric, the combustion process is incomplete, resulting in a higher MEP. Incomplete combustion causes the unburnt fuel to exit the engine cylinder, but it releases energy into the cylinder walls. Therefore, the MEP is higher when the AFR is lower than stoichiometric.

Conclusion

The MEP is a critical parameter that indicates an engine's ability to convert fuel energy into useful work. The compression ratio and AFR are two critical parameters that affect the MEP. The MEP is maximum when the AFR is lower than stoichiometric. Therefore, it is essential to choose the compression ratio and AFR carefully to optimize the engine's performance.