Effects of EGR and intake pressure on PPC of conventional diesel, gasoline and ethanol in a heavy duty diesel engine

Partially Premixed Combustion (PPC) has the potential of simultaneously providing high engine efficiency and low emissions. Previous research has shown that with proper combination of Exhaust-Gas Recirculation (EGR) and Air-Fuel equivalence ratio, it is possible to reduce engine-out emissions while still keeping the engine efficiency high. In this paper, the effect of changes in intake pressure (boost) and EGR fraction on PPC engine performance (e.g. ignition delay, burn duration, maximum pressure rise rate) and emissions (carbon monoxide (CO), unburned hydrocarbon (UHC), soot and NOX) was investigated in a single-cylinder, heavy-duty diesel engine. Swedish diesel fuel (MK1), RON 69 gasoline fuel and 99.5 vol% ethanol were tested. Fixed fueling rate and single injection strategy were employed. For diesel and gasoline PPC, in those conditions with a predominantly mixing-controlled combustion process, increasing EGR or reducing intake boost could increase ignition delay and shorten burn duration. Combustion started later. Peak heat release rate found in premixed burn phasing was higher but occurred much later before TDC, which is beneficial to reduce maximum pressure rise rate level. Lowering boost pressure and intake oxygen further more resulted in more premixed combustion due to longer ignition delay. A large portion of the heat release occurred after TDC and the pressure rise rate was lower. For ethanol PPC, due to long ignition delay, there was mainly premixed combustion and it had much shorter burn duration compared to diesel and gasoline PPC. Less EGR was beneficial for reducing pressure rise rate as a result of lower peak heat release rate after TDC timing and slower burn. Ignition delay peaked at moderate intake boost pressure level and decreased with both lower and higher boost. Decreased EGR / increased intake boost pressure reduced emissions of CO, UHC and soot, with corresponding improvements in combustion efficiency and gross indicated efficiency, particularly at relatively low oxygen concentration. Generally, high EGR was necessary to inhibit NO _X emission.