Fuel octane effects in the partially premixed combustion regime in compression ignition engines

Previous work has showed that it may be advantageous to use fuels of lower cetane numbers compared to today's diesel fuels in compression ignition engines. The benefits come from the longer ignition delays that these fuels have. There is more time available for the fuel and air to mix before combustion starts which is favourable for achieving low emissions of NO_x and smoke though premixing usually leads to higher emissions of CO and unburned hydrocarbons. In the present work, operation of a single-cylinder light-duty compression ignition engine on four different fuels of different octane numbers, in the gasoline boiling range, is compared to running on a diesel fuel. The gasoline fuels have research octane numbers (RON) of 91, 84, 78, and 72. These are compared at a low load/low speed condition (4 bar IMEP / 1200 rpm) in SOI sweeps as well as at a higher load and speeds (10 bar IMEP / 2000 and 3000 rpm) in EGR sweeps. There is a NO_x advantage for the 91 RON and 84 RON fuels at the lower load. At the higher load, NO_x levels can be reduced by increasing EGR for all gasolines while maintaining much lower smoke levels compared to the diesel. In the conditions studied, the optimum RON range might be between 75 and 85.