Insights into Conventional and Low Temperature Diesel Combustion Using Combustion Trajectory Prediction Model

Abstract

Attempting to bridge the gap between typical off-line engine simulations and online real-time control strategies a computationally efficient model has been created that predicts the combustion trajectory (path through the ϕ-T plane). To give an indication of time progression in the combustion event the results are first shown as a function of crank angle, but most discussion is focused on the behavior of the combustion trajectory on the ϕ-T plane. The conditions investigated here include injection timing sweeps between a conventional and late timing (8° and 0° before top dead center, or bTDC, respectively) as well as full exhaust gas recirculation (EGR) sweeps at both timings.

These test conditions highlight how EGR influences the combustion trajectory at different timings – i.e., showing the typical soot-NOx trade-off and the defeat of this trade-off when low temperature combustion (LTC) is obtained. The major insight gained from this modeling approach is how LTC trajectory is different from conventional case in the ϕ-T plane. Attempting to understand the differences and hypothesizing about the causes suggests that there is no specific region that is defined as LTC. In fact the LTC trajectory looks very similar to a conventional one with just subtle differences that keep it from moving into the soot formation region. Additionally, the traditional conceptual explanations for diesel combustion are explored relative to how they are illustrated in the combustion trajectory, especially the transition from pre-mixed to mixing controlled combustion. Understanding this behavior in this context aids in explaining the different observations for the LTC modes.