COMBUSTION OF HYDROTREATED VEGETABLE OIL IN A DIESEL ENGINE: SENSITIVITY TO SPLIT INJECTION STRATEGY AND EXHAUST GAS RECIRCULATION

This work explores the potential to optimize advanced common-rail engines for operation with hydrotreated vegetable oil (HVO). The single-cylinder engine research focuses on adjusting the injection strategy and external exhaust gas recirculation (EGR) to achieve the optimum performance-emissions trade-off using HVO. The engine is operated at a fixed rotational speed of 2000 rpm and under constant load (net indicated mean effective pressure of 0.45 MPa). Split fuel-injection strategy is used: main injection timing is fixed but pilot injection is varied both in terms of timing and quantity. The engine tests, without turbocharging, are conducted under non-EGR conditions and using approximately 27% EGR rate. Results with HVO are compared with results when using diesel fuel. Within the constraints of a single, representative operating point, the results highlight that when using the factory map-based injection strategy, HVO offers soot emissions below 0.015 g/kWh, a 50% reduction when compared to diesel fuel. Nitrogen oxides (NOX) emissions at the same conditions are, however, 10% higher than for diesel fuel. That correlates with higher peak in-cylinder pressures and temperatures. Advancing the pilot HVO injection reduced NOX emissions to the level of the diesel baseline, and although soot emissions increased, they remained 25% lower than with diesel. Interestingly, the two tested fuels exhibited very different responses to EGR. Generally, at 27% EGR, HVO produced twice as much soot as diesel. The heat release analysis indicates this sensitivity to EGR stems from HVO's higher cetane number causing faster auto-ignition, resulting in less premixed combustion and hence producing more soot. Generally, HVO offered more complete combustion than diesel fuel. Regardless of pilot fuel injection strategy, CO emission was reduced by approximately 50% with HVO for both EGR and non-EGR conditions. HVO also benefits emissions of unburned hydrocarbons, in terms of both total values and also unlegislated aldehydes and aromatics.