Some Considerations on the Estimation of the Heat Release of DI Diesel Engines Using Modelling Techniques

Simulation models are widely used from research engineers to investigate the combustion mechanism of DI diesel engines. These models can be used, as tools to either comprehend information provided by experimental data or to perform predictions and assist the development process. As widely recognized a valuable source of information for engine performance and emissions studies is the cylinder pressure trace. It can provide after processing information concerning the combustion rate of fuel injected inside the combustion chamber. Often it is also used to calibrate simulation models or even to derive correlations to represent the combustion rate of fuel inside the combustion chamber. The present research team has during the development process of a simulation model for the description of DI diesel engine performance and emissions realized that there exists a serious problem. When comparing the heat release rate derived from the simulation through the combustion rate of fuel considering the heat exchange with the cylinder walls to the one determined from the processing measured cylinder pressure a serious error exists. This is observed even though a very good match is obtained between the cylinder pressure traces measured and calculated. After a thorough check of the thermodynamics to assure that no error exists in the simulation, it is revealed that the problem has to do with the method used for the estimation of the experimental heat release rate and mainly the source used, i.e. the cylinder pressure trace. As observed the initial heat release rates obtained from the cylinder pressure trace is overestimated while the late one is overestimated. The relevant error increases with the decrease of load and engine speed and has been observed considering various engine designs. The main purpose of the present work is to point out the problem and quantify the relevant error while in a future communication the actual source of this error will be analyzed by detail.