Two-stage combustion strategy for reducing NOx emissions in a compression ignition engine

Abstract

This paper describes the effects of two stage combustion strategy on the engine performance and the exhaust emission characteristics in a compression ignition engine. The two-stage combustion strategy targets reduction of NOx emissions by decreasing oxygen concentration for second stage combustion. Thus, the first injection was provided in order to consume in-cylinder oxygen, rather than generate power. A multi-dimensional CFD code was utilized to predict engine performance and emission characteristics. For the accurate and efficient computational calculation of ignition and combustion characteristics of diesel fuel, the reduced n-heptane mechanism was used in this study. The calculation for two-stage combustion was performed after validating against the experimental result. The KH-RT breakup model and gas-jet model was applied for the prediction of spray behavior and characteristics. To calculate the ignition and combustion process, CHEMKIN II [1] code was used. This mechanism was composed of 30 species and 65 reactions. It was revealed that the peak combustion pressure and the peak heat release rate in two-stage combustion strategy were reduced. It was due to the separate fuel injection and combustion. In the exhaust emission characteristics, two-stage combustion significantly reduced the nitrogen oxides (NOx) emission.