A Computational Study on the Effects of Pre-Ignition Processes on Diesel Engine Combustion and Emissions

Abstract

There has been significant progress in reducing NOx and particulate emissions from diesel engines. However, many challenges remain particularly in view of the global energy issues and increasingly stringent emission regulations. Several recent efforts have focused on achieving low-temperature, premixed combustion for simultaneously reducing NOx and PM emissions, but without any detrimental effect on fuel consumption and energy density. Various strategies being explored include homogeneously charged compression ignition (HCCI), reducing flame temperature through excessive EGR, enhancing premixed combustion by controlling injection parameters, and promoting premixing by using early injection and low cetane number fuels. The present study is aimed at examining the effects of injection timing, initial gas temperature, and cylinder and piston wall temperatures on the spray processes, and thereby on the ignition, combustion and emission characteristics in a diesel engine. The reacting two-phase flow field in a 1.9L, 4-cylinder GM diesel engine is simulated using a CFD code ‘CONVERGE’, which employs an innovative cut-cell Cartesian method for grid generation, and a semi-detailed reaction mechanism for n-heptane combustion. A 51.430 sector with a single hole is considered to simulate the 7-hole common-rail injector. Results indicate that while the initial gas temperature does not affect the spray and combustion behavior qualitatively, it modifies combustion temperatures and thus NOx emissions noticeably. On the other hand, the piston and cylinder wall temperatures qualitatively influence the spray behavior and thereby the combustion and emission behavior. The injection timing has a strong influence on the spray and mixture formation processes, and thus on the combustion and emission characteristics. Delaying the start of injection (SOI) can lead to a significant reduction in NOx formation with only a moderate increase in soot formation. A detailed analysis of the spray and combustion processes indicated two main fuel consumption regions, one near the piston bowl wall and the other in the main spray near the injector. Fuel consumption in the first region mainly follows the conventional diesel combustion model involving rich premixed burning and diffusion burning, while that in the second region involves premixed combustion. As the SOI is delayed, the spray impingement on the piston bowl wall increases, causing more fuel consumption in the first region, which leads to reduction in NOx but increase in soot formation, indicating a tradeoff between NOx and soot emissions. However, with further delay in the SOI, the amount of fuel consumption in the first region increases significantly, while that in the main spray region involves lean premixed combustion. The net effect is a significant reduction in NOx with only a moderate increase in soot emission. Future studies will focus on the effects of modifying the level of premixing and the ignition delay on diesel engine combustion and emission.