Analysis of Mixing and Thermal Effects on Low-Temperature Combustion in IC Engine Operation

Abstract

The effects of thermal and mixing conditions of the in-cylinder charge in internal combustion (IC) engines on emissions in low-temperature combustion (LTC) regimes are analyzed numerically. In the analysis, concepts of an equilibrium temperature (T_eq), peak temperature (T_peak), and anticipated emissions (AE) are introduced. Also, mixture condition representations in temperature (T) vs. temperature (T) space (called T − T plot) and anticipated emissions (AE) vs. temperature (T) space (called AE − T plot) are proposed to represent the in-cylinder mixture quality and emission characteristics of the combustion. Five combustion pathways are identified using a T_peak − T_eq plot, and it is applied to describe both HCCI and DI engine combustion. An optimal temperature window and an optimal mixing window are defined and demonstrated in LTC engine operation. The results show that stratification of mixture temperature due to evaporation cooling and wall heat transfer significantly affects UHC/CO emissions of LTC engine operation. The T − T and AE − T plots reveal information about mixture inhomogeneity, completeness of burning of local mixtures, anticipated levels of emissions, and optimal timing of mixing that can maximize improvements in emissions. The new method of analysis is useful to identify and understand the evolution of in-cylinder mixture conditions in engine combustion.

Keywords:

• Detailed chemistry
• Emissions
• Low-temperature combustion
• Mixing
• Thermal effect

The authors acknowledge the financial support from the General Motors-University of Wisconsin Collaborative Research Laboratory.

Notes

∗Conditions for the sweet spot case of Opat et al. (2007).

^†Conditions for the conventional diesel combustion simulation.