Indicated Mean Effective Pressure

The area enclosed by the p-v diagram of an engine is the indicated work done by the gas on the piston. The IMEP is a measure of the indicated work output per unit swept volume, a parameter independent of the size and number of cylinders in the engine and engine speed.

IMEP is defined as¹:

 `imep=W_i/V_s` (Equation 1)

where:
 W_i is the indicated work in Newton metres
 V_s is the swept volume per cylinder in cubic metres

Brunt² notes that IMEP is highly sensitive to crank angle phasing errors and thermal shock, virtually insensitive to random noise and totally unaffected by absolute pressure referencing errors.

From Brunt³ et al:

• Errors in calculated IMEP are mainly caused by thermal shock, crank angle phasing errors and transducer sensitivity,
• Coarse crank angle resolution, incorrect con rod length, signal noise and integration period error should produce relatively small errors.

The combustion analysis software calculates indicated mean effective pressure using the following equation:

 `imep=1/V_s sum p*dV` (Equation 2)

This equation has been shown to be both computationally efficient and provide good robustness to coarse crank angle resolutions³.

Using the formula between -180 and +180 degrees provides the gross IMEP and, outside of this range, the pumping IMEP. The addition of these two parameters provides the net IMEP that includes the input of energy from combustion and the losses due to pumping.

Additionally, the pumping imep is split at atmospheric pressure between the upper and lower pumping loop IMEP. The upper pumping loop indicates the amount of energy required to propel the combustion products through the exhaust value and piping system. The lower loop indicates the losses due to induction, including the throttling losses across the intake valve⁴.

catool Implementation: See Return_IMEP() in analysis.c

References:
1. Stone, R., "Introduction to Internal Combustion Engines," Macmillan Press Limited, Basingstoke, Hampshire, 1999.
2. Brunt, M. F. J., Pond, C. R., "Evaluation of Techniques for Absolute Cylinder Pressure Correction," SAE Paper 970036, 1997
3. Brunt, M. F. J., Emtage, A. L., "Evaluation of IMEP Routines and Analysis Errors," SAE Paper 960609, 1996.
4. Young, M. B., Lienesch, J. H., "An Engine Diagnostic Package (EDPAC) - Software for Analyzing Cylinder Pressure-Time Data," SAE Paper 780967, 1978.