Abstract
A new phenomenological combustion model has been built for small DI Diesel engines, based on the well-known Dec´s conceptual model for DI Diesel combustion, and tested on a 2.0L Diesel with Direct Injection engine from PSA Peugeot-Citroën. The model distinguishes 5 zones : a liquid fuel core, a vapor fuel/air gas zone, a zone of premixed combustion, a diffusion zone, and the surrounding air (diluted or not by EGR). Siebers and co-workers spray model is incorporated, that provides local informations in the cylinder : vapor-phase fuel penetration length, spray spreading angle, liquid length, fuel-air equivalent ratio in the different zones, and lift-off length ; the accuracy of the spray model when applied to transient DI Diesel jet is discussed. Premixed combustion is supposed to be controlled by flame propagation in a quasi homogenous mixture, whereas a frequencial approach is used for diffusion combustion. A wide comparison is made between heat release calculated with the model and experimental heat release from test bench. The model is not only able to calculate the heat release at all speeds and loads without any adjustment of the empirical coefficients, but allows to find the principal tendencies when modifying the fuel injection parameters (start of injection, dwell pilot-principal injection, pilot quantity, injection pressure...) or the air management strategies (EGR ratio, cylinder pressure and temperature at inlet valves closure, fresh air mass in the cylinder,...). Thus, this model constitutes a relatively simple and fast tool for the calculation of the heat release and for the comprehension of the whole combustion process, which is of particular importance when trying to reduce exhaust emissions. Finally, some new investigations are needed. Particularly, the interactions between the various jets (one jet with adjacent jets, and jets arising from different injections in the case of split injection) and between the jets and the combustion chamber walls have to be taken into account.
Keywords: Diesel, phenomenological combustion model, premixed / diffusion combustion, heat release, EGR