An experimental study of low-temperature combustion in a diesel engine

Increased efficiency and reduced emissions demands from users and legislative organisations have lead to the development of advanced combustion technologies for diesel engines. Exhaust gas recirculation (EGR) is a widely used technology to control diesel combustion and emissions, primarily to reduce emissions of oxides of nitrogen (NOx). Implementation of high levels of EGR (> 50%) is able to simultaneously reduce both emissions of NOx and particulate matter (PM) to ultra low levels. However, high EGR combustion is subject to reduced combustion efficiency and stability with increased total hydrocarbon (THC) and carbon monoxide (CO) emissions. This thesis presents research into low temperature diesel combustion (LTC) operation and the effects on combustion and emissions when the engine is operated under air, fuel and EGR rates encountered during transitions between LTC and conventional diesel operation modes. This has resulted in an improved understanding of the diesel combustion process and pollutant emissions with high rates of EGR, different fuel injection pressures and timings, post fuel injection and exhaust back pressures. The sensitivity of LTC to variations in engine speed, fuel injection quantity, and EGR rate and intake manifold temperature were investigated. Pseudo-transient operation of the engine was studied to interpret the transient performance of a diesel engine during transients within LTC and from LTC to conventional diesel combustion in a new European driving-cycle (NEDC) test. Experimental investigations were conducted on a single cylinder research diesel engine. Cylinder pressure, fuel consumption and gaseous and particulate emissions (filter smoke number, size distribution, and total number) were measured. The results showed that an increase in EGR rate can realise LTC on the research engine. Fuel injection parameters influenced the combustion phasing, and control of this was able to improve the combustion stability and to reduce the THC and CO emissions. The low smoke number for the LTC diesel combustion was a result of reduced mean particle size with possible changes in particulate composition. EGR is the most critical parameter influencing the LTC combustion and emissions. Transient simulation of an engine exhibits significant discrepancies in EGR rate and boost pressure. Pseudo-transient points at intermediate load condition showed significantly increased emissions, particularly smoke number. Retarded fuel injection timing and increased boost pressure were demonstrated to be an effective strategy to reduce smoke emissions for these pseudo-transient operating points.