Partially Pre-Mixed Auto-Ignition of Gasoline to Attain Low Smoke and Low NOx at High Load in a Compression Ignition Engine and Comparison with a Diesel Fuel

A Swedish MK1 diesel fuel and a European gasoline of ∼95 RON have been compared in a single cylinder CI engine operating at 1200 RPM with an intake pressure of 2 bar abs., intake temperature of 40°C and 25% stoichiometric EGR at different fuelling rates and using different injection strategies. For the same operating conditions, gasoline always gives much lower smoke compared to the diesel fuel because of its higher ignition delay; this usually allows the heat release to be separate in time from the injection event. NOx can be controlled by EGR. With dual injection, for diesel fuel, there can be significant heat release during the compression stroke because of the pilot injection earlier in the compression stroke. For a fixed total fuelling rate, compared to single injection, this reduces fuel efficiency and increases the lowest achievable level of smoke. With gasoline, pilot injection helps reduce the maximum heat release rate for a given IMEP and enables heat release to occur later with low cyclic variation compared to single injection. This enables higher mean IMEP to be reached with lower smoke, NOx and maximum heat release rate compared to single injection. One of the operating points reached with gasoline with double injection had mean IMEP of 15.95 bar (stdev. 0.112 bar), AVL smoke opacity of 0.33% (FSN < 0.07), ISNOx of 0.58 g/kWh, ISFC of 179 g/kWh, ISHC of 2.9 g/kWh, ISCO of 6.8 g/kWh and peak pressure of ∼ 120 bar. At the same operating conditions, to get such low level of smoke with Swedish MK1 diesel fuel, IMEP has to be below 6.5 bar. There is scope for further improvements by increasing intake pressure and the EGR level and through optimisation of the injection and mixture preparation strategy e.g. more injection pulses and injector design e.g. more holes.