Decomposition of harmonic wavelets of torsional vibrations as basis for evaluation of combustion in compression-ignition engines

The aim of analysis presented in the article is results of measurements of torsional deflections of a crankshaft of three-cylinder internal combustion diesel engine. The wavelets of the run of torsional twists were processed by decomposition to harmonics using FFT. In order to exactly recognize all dynamic aspects of gas forces acting during working stroke, decomposition was done in two ways: first one relaying on records encompassing one working cycle (two revolutions), the second one based at separated periods encompassing angular an interval from TDC (Top Dead Centre) of one cylinder to TDC of subsequent in firing order. For three-cylinder engine, width of the interval is 240 degrees of crank angle. In order to obtain reliable results, torsional deflection signals acquisitions were recorded numerous times including changes of outer conditions (temperature, atmospheric pressure) and different values of the engine loads. The main goal of conducted experiments was looking for an answer of a question if exist any differences between frequency spectrums of harmonics of run of full cycle ( two revolutions) and the same cycle divided by three separate intervals. Analysis was conducted in order to state any possibility of diagnostic signals acquisition caused by abnormalities of combustion process. Results were presented in form of comparison of frequency spectra taken from full cycle and divided intervals.