On Zero-dimensional Modelling of Combustion and NOx Formation in Diesel Engines

Abstract: Diesel engine developers are facing major challenges when meeting future demands on NOx and particulate emissions. The exhausts after treatment systems are rapidly improving and the technologies available in a few years will be capable of cutting emission levels close to the demanded levels. However, in order to reach the low emissions required in a long-lasting, cost effective way, the potential of reducing engine-out emissions must be fully utilized. Fortunately, engine management systems and hardware are now available to optimize fuel injection and heat release for low emissions while maintaining the trademarks of the diesel engine, i.e. low fuel consumption and dependable performance. The possibilities offered by the new technologies are so wide-ranging that, in order to use them optimally, models must be developed to guide the engine developer. The model developed by the author is a zero-dimensional multizone model. Input to the model could be either a measured pressure trace or an arbitrary heat release rate. One of the cornerstones of the model is the assumption that NO is formed in zones that are burning close to stoichiometric conditions. The actual lambda value in these zones is found by comparing calculated and measured concentrations in the exhaust. According to the results, some of the effects observed on NO formation when varying the parameters of injection are due to the differences in local lambda. It has been found that the local lambda determined with the model for a given injection system stays constant, even if the settings of the injection system are changed. Thus, there are reasons to expect that predicted trends of limited variations of injection parameters will be close to reality. The results obtained with the model agree with results from more advanced models and with laser based measurements in optically accessible engines.