A Phenomenological Unburned Hydrocarbon Model for Diesel Engines

Intensified emission regulations as well as consumption demands lead to an increasing significance of unburned hydrocarbon (UHC) emissions for diesel engines. On the one hand, the quantity of hydrocarbon (HC) raw emissions is important for emission predictions as well as for the exhaust after treatment. On the other hand, HC emissions are also important for predicting combustion efficiency and thus fuel consumption, since a part of unreleased chemical energy of the fuel is still bound in the HC molecules. Due to these reasons, a simulation model for predicting HC raw emissions was developed for diesel engines based on a phenomenological two-zone model. The HC model takes three main sources of HC emissions of diesel engines into account: Firstly, it contains a sub-model that describes the fuel dribble out of the injector after the end of injection. Secondly, HC emissions from cold peripheral zones near cylinder walls are determined in another sub-model. This peripheral zone sub-model also considers the influence of the cylinder wall temperatures on HC emissions, which makes it also suitable for the evaluation of cold start conditions. To simplify the calibration process, the peripheral zone sub-model is coupled to an existing nitrogen oxide (NO) peripheral zone model. Thirdly, HC emissions from overmixed regions are considered. In these regions, the air-fuel mixture is too lean and consequently the temperatures are too low for HC to fully oxidize. For the peripheral zone and the overmixing sub-models, suitable empirical equations are used to calculate the HC oxidation. The simulation results of the HC model have been validated against experimental data from a direct injection diesel engine. The simulation results show a high degree of accuracy over the entire engine map with regard to HC emissions as well as the fuel energy bound in the UHC.