Feasibility and limitations of premixed diesel combustion in multi-cylinder engines.

Abstract

This thesis identifies and assesses strategies for achieving Low Soot Combustion (LSC) in automotive diesel applications. The interaction of fundamental parameters such as fuel chemistry, mixture formation and temperature-control was investigated and the implications thereof on LSC were explored. For the purpose of this work, LSC is defined to have an emissions index for particulate matter (EI-PM) of equal or less than 0.4 g/kg of fuel and an emission index of oxides of nitrogen (EI-NOx) of equal or less than 1 g/kg of fuel. Several distinct strategies were investigated using a single-cylinder engine including retarded LSC, premixed LSC and fuel with different chemical properties to identify the most promising approach to achieve LSC for diesel engines. A premixed strategy with diesel fuel was identified as a promising approach to achieve LSC for passenger car applications. Subsequently, this concept was refined and implemented into a 1.7L, 4-cylinder, production type diesel engine. A combination of narrow spray cone angles along with injection of a substantial amount of fuel at very advanced injection timings and EGR rates of more than 50% created combustion patterns that no longer followed the traditional soot/NOx trade off, and NOx and PM emissions could be reduced simultaneously. Consequently, the developed combustion strategy established predominantly premixed diesel combustion and exhibited reductions in NOx emissions of 82% and in PM emissions of 39% with a 4.5% penalty in net thermal efficiency compared to the base engine configuration.