Application and evaluation of a detailed friction model on a DI diesel engine with extremely high peak combustion pressures

摘要

During the past years, extensive research efforts have led to the development of diesel engines with significantly improved power concentration and fuel efficiency as compared to the past. But unfortunately, the increase of engine thermal efficiency is accompanied by a sharp increase of peak cylinder pressure. At the moment, peak pressures in the range of 230-240 bar have been reported. Naturally, a question remains as to whether such increased peak pressures could have an overall detrimental impact on mechanical efficiency. Initially, it was expected that these would have a negative impact and this was the motive for conducting the present work and developing a detailed friction model. Up to now, various correlations have been proposed that provide the friction mean effective pressure as a function of engine speed and load mainly, neglecting the effect of peak pressure or using data up to 130-140 bar. Thus, if we consider that peak cylinder pressures tend to increase as already stated, it seems that these relationships predict in a very rough manner the effect of peak combustion pressure on frictional losses. The use of a friction model is also important, since it is possible to convert indicated data into brake ones in cases where it is not possible to measure brake power output. In a previous study, the present research team has presented an analytical model used to estimate friction losses in a DI diesel engine with high peak pressures up to 190 bar. At that time, predictions were also made for cases up to 240 bar but it was not possible to validate the calculations since no experimental data existed. For this reason, in the present work are given results obtained from a prototype single- cylinder heavy-duty research engine capable of operating at 240 bar. Using this data, an effort is made to validate the developed friction model at a wider area of operating conditions as far as speed and load are concerned and mainly to examine if initial predictions at such high peak pressures are verified. Using the developed model, it was made possible to have values for the various friction torque components during an engine cycle avoiding cycle average and thus estimating friction mean effective pressure and mechanical efficiency. This also enables us to examine the effect of rotational speed, load and peak cylinder pressure on frictional losses occurring at various engine components in terms of instantaneous friction torque and mechanical loss percentages. The predictive capability of the model was assessed over various peak cylinder pressures, covering several cases of engine load and speed. It was found to be quite good even at extremely high pressures (230 bar).