The following definitions are helpful in discussing SI engine efficiency fundamentals (Heywood 1988):
Mean Effective Pressure (MEP) = Work per cycle/displaced volume
Indicated Mean Effective Pressure (IMEP) = Work delivered to the piston over the compression and expansion strokes, per cycle per unit displaced volume
Friction Mean Effective Pressure (FMEP) = Total friction work per cycle per unit displaced volume
BMEP can be calculated as follows:
|Brake Mean Effective Pressure (BMEP) = IMEP – FMEP||(1)|
FMEP consists of the following three components:
Pumping Mean Effective Pressure (PMEP) = Work per cycle done by the piston on the in-cylinder gases during the inlet and exhaust strokes. PMEP is positive for naturally aspirated engines and negative for supercharged and turbocharged engines at high loads.
Rubbing Friction Mean Effective Pressure (RFMEP) = Work per cycle dissipated per cycle in overcoming friction due to relative motion of adjacent components in the engine.
Accessory Mean Effective Pressure (AMEP) = Work per cycle required to drive engine accessories (pumps, fans, alternator, etc.) essential to engine operation.
Therefore, FMEP can be expressed as follows:
|FMEP = PMEP + RFMEP + AMEP||(2)|
Brake thermal efficiency (BTE) is subsequently defined as the ratio of work delivered divided by the heating value of the fuel (generally lower heating value since the water in the exhaust is in vapor form):
|BTE = BMEP × displaced volume / (mf × QLHV)||(3)|
Where: mf = mass flow rate of fuel
QLHV = Lower heating value of fuel
A similar expression is used to calculate indicated thermal efficiency (ITE).
The relationships discussed above are shown in Wade et al. (1984) for an engine operating condition representative of the FTP drive cycle.
Heywood, J.B. 1988. Internal Combustion Engine Fundamentals. New York: McGraw-Hill.
Wade, W.R., J. E. White, C. M. Jones, C. E. Hunter, and S. P. Hansen. 1984. Combustion, Friction and Fuel Tolerance Improvements for the IDI Diesel Engine. SAE Technical Paper 840515.