Partially premixed combustion of hydrotreated vegetable oil in a diesel engine : Sensitivity to boost and exhaust gas recirculation

Hydrotreated vegetable oil (HVO) has potential to emerge as an alternative fuel to mineral diesel due to its favourable properties. The present study investigated HVO under partially premixed compression ignition combustion mode with boost-exhaust gas recirculation (EGR) and also in conventional combustion mode. Single-cylinder engine tests focused on optimising a single representative operating point in the middle of the engine operating envelope. The optimisation focused on the trade-off between NOX and particulate matter, as the adopted multi-pulse strategy provides stable combustion onset independently of the cylinder mixture conditions. Sensitivity in emissions comes from large differences in the early, premixed combustion phase. Air-path optimized HVO combustion favours higher EGR rates (25 % vs. 20 %) and lower boost pressures than diesel (130 kPa vs 165 kPa). At such conditions HVO has 1.5 percentage points higher indicated thermal efficiency (43.5 %) than diesel. At the same time, HVO yields an ultra-low particulate level (0.055 g/kWh) and engine-out NOX emissions are 46 % better than optimised diesel combustion. Together with a 37 % reduction in total hydrocarbon emissions, the elimination of aromatics also provides an additional incentive for HVO.