The maximum compression pressure position relative to top dead centre as an indication of engine cylinder condition and blowby

Abstract

The compression curve of a DI (Direct Injection) diesel engine is affected by various parameters, such as heat loss, initial temperature and pressure, speed, compression ratio and blowby. Usually the engineer makes use of the measured motored pressure diagram and especially of the maximum compression pressure value to determine the engine condition (wear). However, in order to have an accurate indication of the engine condition, one must consider accurately all the previous mechanisms. This poses a very difficult task since, for example, not even the compression ratio is usually accurately known for an operating engine. Thus, a low maximum compression pressure value may not be caused by increased blowby rate, and so this may lead to a wrong indication of cylinder condition. The present work introduces a new method for determining cylinder condition (wear) and especially the blowby rate. The method is based on the determination of the position of the maximum compression pressure value relative to the TDC (Top Dead Centre) position. The maximum pressure position (before TDC) varies greatly with blowby rate, but is practically unaffected by heat losses, initial temperature and pressure and compression ratio. Experiments have been conducted on a Ricardo E-6, DI diesel engine for various compression ratios, engine speeds and operating temperatures (cold and warm operation) under motoring conditions. Pressure has been measured using a time step equivalent to 0.1°CA, while the TDC position has been determined using a magnetic pickup device. A mathematical simulation accounting for heat losses and blowby has been used to examine the effect of various parameters on maximum compression pressure and its position relative to TDC. The results from both experiments and simulation are found to be in good agreement concerning pressure curves, maximum pressure position and their variation with the parameters examined. The results are promising, showing a high potential for determining any cylinder condition by processing the corresponding measured pressure diagram.