Multiple Injection Strategies for Improved Combustion Stability under Stratified Part Load Conditions in a Spray Guided Gasoline Direct Injection (SGDI) Engine 2011-01-1228

Compared to conventional homogeneous direct injection or port-fuel injected engines, the second generation, spray guided, direct injection engine (SGDI) has the potential for significantly improved fuel economy during part load stratified charge operation. Multiple fuel injection strategies can be utilised to increase the unthrottled operating range, leading to further improvements in fuel economy. However, careful optimisation of these strategies is essential to ensure that benefits are maintained whilst further minimising emissions within combustion stability limits and consumer driveability demands.

The effects of multiple injection strategies upon fuel consumption, emissions and combustion stability were investigated in a single cylinder Ricardo Hydra engine with a spray guided combustion system. An outwardly opening piezoelectric actuated injector was employed. The fuel injection strategy utilised up to five injections per engine cycle. The injection sequence and injection duration was varied from a minimum of 0. 1ms per injection upwards. Other experimental variables included the split of injected mass between the injection events, fuel pressure, EGR quantity, and valve timing and lift to control air motion and internal residual gas fraction. Rapid testing methods, analysis and final optimisation of the engine was realised using DoE techniques.

For a given engine part load condition it was shown that multiple fuel injection events resulted in improved engine combustion stability when compared to a single injection event. Furthermore, combustion stability was also less sensitive to spark timing. The angle of 50% of mass fraction burnt in the multiple injection case was closer to the thermodynamic optimum than could be achieved in the single injection case. Low engine-out NOx emissions, an important attribute of a stratified combustion system due to the cost and regeneration fuel penalty associated with a LNT, was improved through multiple injections and external EGR.