Influence of TDC determination methods on mean indicated pressure errors in marine diesel engines

The paper presents the results of comparative analysis of influence of different methods of TDC determination for the calculation errors of mean indicated pressure (MP). Lemag method based on zero point of first order derivative, used in PREMET pressure analyzers, was compared to the method based on a polynomial model of the compression process. Comparisons were made on the operating measurement data obtained on the low speed marine engine RTA96C which forms the main propulsion of the vessel and the medium speed engine MAN L28/32A. Measurements were made with onboard combustion PREMET analyzer. It was found that the method based on zero point of first order derivative can give the deviations of position of TDC in excess of 2°CA which is caused by disturbances associated with the measurement of pressure and interference associated with the generation of the angular axis. These errors are related to the accuracy of the methods of determining of the first derivative and the availability of a sufficient range of pure compression after TDC. The best results of determining the TDC position was obtained from the original method based on a polynomial model of the compression process. It should be emphasized that the proposed method enables individual adjustment of TDC for each cylinder and can be used even when the combustion begins before TDC. The conducted research proved the existence of significant influence of indicator graph TDC location on the mean indicated pressure calculation errors.