Mean Effective Pressures - EngineKnowHow

The mean effective pressures of an engine are measures of the work required and outputted by an engine.  By looking at the pressure acting across the top of the piston it’s possible to determine the transfer of work from the gas to and from the piston.  Mean effective pressures are also independent of engine displacement and are therefore very important for engine development.

 

Below is a plot of Cylinder Pressure vs. Cylinder Volume (P-V diagram) of an actual spark ignition engine operating under part load conditions.  Note that the pressure is a log scale to show increased detail of the bottom section of the loop (where pumping losses occur).  Work can be calculated from a P-V diagram as work is equal to the area within the two highlighted areas.

 

Gross Mean Effective Pressure, or GMEP is the work delivered to the piston from the compression and expansion strokes of the four stroke engine cycle.  GMEP is the positive work extracted by the piston.

 

Pumping Mean Effective Pressure, or PMEP is the work required by the piston during the intake and exhaust strokes of the four stroke engine cycle.  PMEP is the negative work required by the piston to draw the fresh charge into the cylinder and to push the exhaust from the cylinder.

 

The Indicated Mean Effective Pressure, or IMEP is the net work extracted by the piston and is the summation of work over all four strokes of the four stroke engine cycle; the compression, expansion, exhaust and intake strokes.  IMEP is calculated as:

 

 

Friction Mean Effective Pressure, or FMEP is work which is lost due to mechanical friction and parasitic losses.  Mechanical friction is for example, the friction between the piston rings and the cylinder walls whilst parasitic losses are for example, the driving of the oil pump, water pump and alternator.  Engine friction is largely a function of engine speed and you feel the friction of the engine as engine braking.

 

Brake Mean Effective Pressure, or BMEP is the work available at the crankshaft and can be calculated by:

 

 

BMEP is also a function of engine torque and displacement and can be calculated by:

 

 

 

Where n is the number of crank revolutions per power stroke, 2 for a four stroke engine.

 

Mechanical Efficiency

The mechanical efficiency of an engine is a measure of how efficiently work available at the piston is transferred to the crankshaft.  Mechanical efficiency is calculated as:

 

 

 

Boosting

When an engine is highly boosted, either by a turbocharger or a supercharger the intake pressure will be greater than the atmospheric pressure and at times higher than the exhaust pressure.  As a result, the PMEP will be positive as the turbocharger or the supercharger is delivering the fresh charge and the piston mustn’t work to draw the fresh charge into the cylinder.  IMEP is therefore:

 

 

Turbochargers and superchargers can both increase the IMEP of an engine, however, as a supercharger is driven by the crankshaft it is a parasitic loss and will increase the FMEP of the engine, reducing its mechanical efficiency. As a turbocharger is driven by the further expansion of the exhaust gases in a turbine, possible as the temperature of the exhaust exiting the cylinder is higher than the atmospheric temperature, a turbocharger offers higher efficiency gains then a supercharger.  It will however increase the work required by the engine to expel the exhaust from the cylinders.