Numerical Simulation of Diesel Spray Combustion in an Automotive-Size Diesel Engine

Abstract

Better knowledge of complex in-cylinder processes that affect emissions and limit the fuel efficiency in diesel engines would aid the development of low-emissions and more efficient engines. Two outstanding issues that are not properly addressed by current understanding are jet-wall and jet-jet interactions, which impact significantly the in-cylinder flow field, air-fuel ratio distribution, and temperature – and thus soot formation. Compared with older generations of engines, these interactions are much more important in modern and proposed future engines, due to trends of downsizing, higher injection pressures, and oxidiser dilution.

One way this knowledge may be gained is through numerical modelling. However, it is unclear the extent to which models can capture these interactions, due to few focussed studies which directly compared modelled jet-wall and jet-jet interactions with experimental in-cylinder optical measurements. This study aims to help bridge this gap. A pragmatic approach is taken to the modelling wherein standard and well understood spray and turbulence models are coupled with relatively detailed chemical kinetic models and, where possible, with a full cycle, full geometry model of the engine. The modelling is comprehensively compared with experimental data. This requires going beyond the typical comparisons of pressure traces and engine-out emissions to examine the phenomena in detail as they happen inside the engine, which is achieved by comparisons to a suite of measurements in two optically accessible engines (one small-bore engine at UNSW and one heavy-duty engine at Sandia National Laboratories). After the usual comparisons of the heat release rate, comparisons of fuel-PLIF with modelled fuel mass fraction are used to understand the transient mixture formation process. Early-stage chemiluminescence and formaldehyde PLIF are compared with modelled fuel formaldehyde mass fraction to evaluate whether the simulations can predict cool flame, first-stage ignition. Chemiluminescence from OH*and OH-PLIF is compared with the modelled OH* and OH mass fractions, respectively, to assess the ability to predict the high temperature combustion regions. Finally PAH PLIF is compared with modelled single-ring aromatic mass fraction to evaluate the ability of the model to predict soot precursors.

In the small-bore engine, comparisons of model and experiment are first performed with a full cycle engine model for a single fuel-jet interacting with the bowl wall for a range of different injection pressures. The comparisons are first performed for a baseline injection pressure of 70 MPa, and then extended to study effects of higher injection pressures. Numerical experiments are then performed to study jet-jet interactions at different injection pressures using fuel injection through two jets. In the heavy duty engine, a sector mesh model is adopted due to the intake geometry being unavailable. Comparisons of model and experiment are performed for two bowl geometries, with the focus on predictions of aromatic compounds. Overall, these comparisons are shown to be quite successful. In all the considered cases a good agreement is obtained for the heat release rate, in-cylinder fuel-air mixing, and the timing and location of cool flame and high temperature combustion. The location and timing of aromatic species also agrees quite well in the heavy duty engine, but leaves room for improvement. The numerical results are further analysed revealing features that are not obvious from the experiments alone. In particular they highlight that jet-wall and jet-jet interactions have a major and leading order influence on combustion in modern diesel engines. The flow-flame interactions involved during these phenomena are quite complex and quite geometry specific, highlighting the need for further development of conceptual models for diesel engine combustion affected by jet-jet and jet-wall interactions, which relative to older conceptual models of conventional diesel engine combustion are at a much less advanced stage