Emission and Deposit Formation in Direct Injection Stratified Charge SI Engines
Doctoral thesis, 2001
A promising approach to increase the efficiency of the spark ignition (SI) engine is to inject the fuel directly into the cylinder to form a stratified charge. There is a potential to reduce the fuel consumption at low speeds and loads by 20 - 25% compared with conventional SI engines with stoichiometric homogeneous charge. The main drawbacks to contemporary direct injection stratified charge (DISC) engines are high hydrocarbon (HC) and soot emissions.
An experimental investigation of the sources of HC emissions, from a production Mitsubishi GDI® direct injection stratified charge SI engine which uses wall-guided mixture preparation, is presented in this thesis. The engine-out HC emissions were 50% higher for stratified operation than for homogeneous operation for the engine tested under the low load and speed conditions investigated. A comparison of cycle-resolved HC and NOx emissions was made for homogeneous and stratified operation. Distinct differences observed in HC concentration traces indicate clearly that the sources of hydrocarbon emissions are not the same as in conventional SI engines. The measurements indicate that over mixing (quenching), at the boundaries of the air/fuel mixture cloud, and under mixing, both in the spray center and on the surface of the piston bowl, are the dominating mechanisms for HC emission formation in this engine. The engine tested is more sensitive to fuel composition than conventional SI engines. While there is only a marginal potential to reduce the HC emissions from DISC engines by altering the distillation curve of the fuel, there is much greater potential to decrease the emissions by improving the fuel and combustion systems.
Injector fouling was found to have a detrimental effect on HC emissions and engine stability when the engine was subjected to a 60 h deposit accumulation cycle. The frequency of misfires and/or partial burns became unacceptable after such short time intervals as 6 - 30 h for some of the gasolines tested. Although the deposit levels depend on fuel composition, it is concluded from this study that the formation of combustion chamber deposits did reduce HC emissions, due primarily to reduced heat transfer. It is clear from these tests that preventing injector deposit formation is essential for direct injection SI engines.