Abstract
The mixture preparation and combustion characteristics of spark ignition (SI) and spark-assisted controlled auto-ignition (SA-CAI) have been studied experimentally in two, direct injection, poppet valve, gasoline engines operating in both two stroke and four-stroke configurations. A thermodynamic firing engine was used to evaluate strategies to extend the SA-CAI part-load range in four-stroke operation. This enabled the minimum engine speed to be reduced to 1000 rpm and minimum load, to 1.5 bar net indicated mean effective pressure (NIMEP). Low peak lift, short period camshafts and negative valve overlap (NVO) timings were used to vary the quantity of trapped residual gases. Finally, the engine intake air temperature and pressure and the exhaust back pressure were varied. In parallel, an optical engine was modified to allow viewing access to the combustion chamber and upper region of the cylinder liner. Laser-based optical diagnostic measurements and in-cylinder pressure sampling were compared at an operating point of 1000 rpm and 3.0 bar NIMEP, for both SI and SA-CAI conditions. The fuel injection duration and ignition angles were varied. In the SA-CAI case, good combustion stability was achieved with predicted EGR rates in excess of 45%. The coefficient of variation (COV) in NIMEP was lower in the SA-CAI case for the same load conditions. The ignition delay angle was comparable between the two cases but the duration of the main burn was significantly greater in the SA-CAI case. These results were confirmed by lower bulk air velocities, weaker fuel concentration at ignition and slower flame propagation in the SA-CAI case. Although the cycle-to-cycle mixture homogeneity was greater in the SA-CAI case, peninsular-like flame propagation was observed in conjunction with individual ignition spots. Re-compression auto-ignition occurred in the period following early exhaust valve closure. The engine torque in the SA-CAI combustion case showed the greatest sensitivity to ignition timing advance at the highest levels of charge dilution. The dependency of SA-CAI operation upon ignition timing shows the potential for control of heat release phasing during the transition mode of a multi-mode combustion system.