GDI HCCI: Effects of Injection Timing and Air Swirl on Fuel Stratification, Combustion and Emissions Formation

HCCI-combustion with direct injection of gasoline using a standard GDI-injector is investigated in this work. The test engine is a 6-cylinder heavy-duty diesel engine with one cylinder operating in HCCI-mode. Exhaust gases from one of the diesel cylinders serve as simulated EGR. Electric heaters are used to raise the inlet temperature when no EGR is applied. The piston bowl is modified to match the hollow-cone spray better than the original re-entrant piston. Spray imaging outside the engine shows the characteristics of the fuel spray.

Injection timing sweeps show that a homogeneous charge is created when the injection is performed in the middle of the intake stroke for a moderate fuel/air-equivalence ratio of 0.29. This leads to low emissions of NO_x and Smoke.

Using a homogeneous mixture when the fuel/air-equivalence ratio is reduced to 0.20 leads to low combustion efficiency with associated high levels of CO and HC emissions. The combustion efficiency can for this fuelling rate be improved by retarded injection that creates fuel stratification at the time of autoignition. However, retarding the injection too much results in high NO_x production and propensity for knocking combustion. A trade-off between the combustion efficiency and the emissions of NO_x is recognized.

Increased air swirl decreases HC, CO and Smoke emissions, especially when the injection timing is earlier or later than the optimal. This is attributed to improved fuel/air mixing and demonstrates the role of air swirl in a directly injected HCCI combustion system.