Development of a Detailed Friction Model to Predict Mechanical Losses at Elevated Maximum Combustion Pressures

Engineers use phenomenological simulation models to determine engine performance. Using these models, we can predict with reasonable accuracy the heat release rate mechanism inside the engine cylinder, which enables us to obtain a prediction of the pressure history inside the engine cylinder. Using this value and the volume change rate of the combustion chamber, we can then estimate the indicated power output of the engine. However, in order to obtain the brake engine power output we must have an indication for the mechanical losses, a great part of which are friction losses. Up to now various correlations have been proposed that provide the frictional mean effective pressure as a function mainly of engine speed and load. These correlations have been obtained from the processing of experimental data, i.e. experimental values for the indicated and brake power output of engines. Using these correlations we can get an acceptable prediction of the friction mean effective pressure and thus, estimate the engine brake power output from the corresponding indicated value. The process is especially important when analyzing the performance of engines on the field where the brake power output cannot be measured directly or when developing new engines. A serious problem associated with the use of these correlations is that in the relevant expressions exist coefficients that are usually unknown. For this reason, these models have to be calibrated using known values, which is a disadvantage. Another problem is that these models predict in a very rough manner the effect of the maximum combustion pressure which nowadays has been increased considerably in production DI diesel engines; in some cases values around 170 -180 bar have been reported. To overcome the previous problems and mainly to examine the effect of maximum combustion pressure on frictional losses, in the current work a detailed model for the estimation of all frictional forces from the moving elements of the engine is developed. Using the developed model, it is made possible to predict the history of the various forces during an engine operating cycle and mainly friction mean effective pressure and mechanical efficiency. The model has been applied on a modern diesel engine with high maximum combustion pressures at three different engine-operating speeds and two different loads. As revealed, the model predicts with good accuracy the frictional losses at all engine operating conditions examined and the effect of the maximum combustion pressure. The contribution of the last one is revealed not to be very significant at high values, despite initial expectations.