Advantages of Fuels with High Resistance to Auto-ignition in Late-injection, Low-temperature, Compression Ignition Combustion

Oxides of nitrogen (NOx) and smoke can be simultaneously reduced in compression ignition engines by getting combustion to occur at low temperatures and by delaying the heat release till after the fuel and air have been sufficiently mixed. One of the ways to obtain such combustion in modern engines using common-rail direct injection is to inject the fuel near top dead centre with high levels of exhaust gas recirculation (EGR) - Nissan MK style combustion. In this work we study the effect of fuel auto-ignition quality, using four fuels ranging from diesel to gasoline, on such combustion at two inlet pressures and different EGR levels. The experiments are done in a 2 litre single-cylinder engine with a compression ratio of 14 at an engine speed of 1200 RPM. The engine can be easily run on gasoline with a single injection near TDC, even though it cannot be run with very early injection, in the HCCI mode. Moreover for any given condition, gasoline has a significantly higher ignition delay for the same combustion phasing and hence results in very much lower NOx and smoke for a given load compared to diesel fuels. Using gasoline, an IMEP of 14.86 bar could be reached with ISFC of 178 g/kWh, smoke < 0.4 FSN, peak pressure of 133 bar, ISNOx of 1.21 g/kWh, and ISHC and ISCO < 4 g/kWh. It was not possible to get comparable performance with diesel fuels with low smoke at the same operating conditions. Further improvements in all these parameters using gasoline should be possible by optimising injector and engine design and operating conditions and bringing in other strategies such as multiple injections.