A Calculation method for indicated mean effective pressure based on harmonic analysis of pressure waveform

Abstract

A new method for calculating indicated mean effective pressure (i.m.e.p.) was derived from harmonic analysis of pressure waveform in working space, instead of conventional numerical integration of the pressure-volume (P-V) diagram. The method is applicable to a number of displacement-type machines including engines. This paper first introduces a generalized i.m.e.p. equation, then derives the specific i.m.e.p. equations for various mechanisms such as the piston-crank, swash plate, etc. The proposed i.m.e.p. equations introduced show that i.m.e.p. is determined by very low frequency components, of less than several hundred Hz, included in the pressure waveform. The reason is that the amplitude of volumetric variation becomes rapidly smaller as the order of its harmonics increases. This means that the i.m.e.p. equations proposed are more accurate where the number n of sam-pling pressure data per cycle is decreased and allow highspeed calculation as compared with the conventional method. The error of the i.m.e.p. equation for internal combustion engines with a piston-crank driving mechanism is less than ±0.1 per cent. For a Stirling engine with a Ross yoke drving mechanism the erros is less than 0.4 per cent, even if n is decreased until four per cycle because the pressure waveform is close or similar to the sinusoidal wave. The digital i.m.e.p. meter developed as an application of the i.m.e.p. equation is also described, and its validation is discussed.