Accuracy Limits of IMEP Determination from Crankshaft Speed Mesurements

The paper presents a method of determining the Indicated Mean Effective Pressure (IMEP) and the gas pressure torque of a multi-cylinder engine using data obtained from the measurement of the crankshaft's speed variation. At steady state operating conditions a Fourier series describe the gas pressure torque of a cylinder and the resultant torque may be obtained by adding the harmonic components corresponding to all cylinders. Only the major harmonic orders, having the same phase for all cylinders add algebraically appearing with large contributions in the spectrum of the resultant torque. The lowest major component has a low frequency and, at this frequency, the crankshaft behaves dynamically like a rigid body. In this situation it is possible to correlate the amplitude of this harmonic order of the gas pressure torque to the same harmonic order of the crankshaft speed. The statistical nature of the cycle-to-cycle variation in the engine operation determines limits of the accuracy by which the IMEP and the gas pressure torque may be calculated by this method. The errors are fairly low at high and medium engine loads but increase at low loads.