This paper presents a follow on study from earlier work investigating the influence of fuel parameters on the deposit formation and emissions from a direct injection stratified charge spark ignition engine. It was shown that injector fouling was the main reason for the increase in unburned hydrocarbon emissions and spray visualizations supported these results. The hypothesis is that the deposit buildup in the injector caused the increased hydrocarbon emissions due to an increased wall film formation.
To further verify the findings, Phase Doppler Anemometry measurements at simulated engine conditions, were performed. Measurements recorded on the injector axis 20 mm downstream from the injector orifice, showed that the initial pre-jet velocity was 30% higher and the drop mean diameter was 5% larger in the case of a used injector compared to a new injector.
Based on these investigations, spray files were set-up in the 3-D CFD-code AVL FIRE™. A moving calculation mesh, capturing the main geometrical features from the Mitsubishi GDI® combustion system, was created. With this calculation mesh, the influence of important parameters on the mixture formation was studied for the two injectors.
Simulations showed that the mass of fuel captured in the wall film with a new injector was equivalent to 10% of the total injected fuel mass. With a fouled injector this rose to around 30%. This supports the hypothesis that the injector fouling makes a significant contribution to the increased amounts of hydrocarbon emissions found in engine tests.