The Importance of Injection System Characteristics on Hydrocarbon Emissions from a Direct-Injection Stratified-Charge Engine

The effects of injection variability, low velocity fuel injection, and injector orifice size on unburned hydrocarbon emissions were studied in a direct-injection stratified-charge (DISC) engine. The engine incorporated a combustion process similar to the Texaco Controlled Combustion System (TCCS) and was operated with gasoline. The variability in the amount of fuel injected per cycle was found to have a negligible effect on HC emissions. Changing the amount of fuel injected at low velocity at the end of injection impacted the HC emissions by up to 50%. A positive pressure differential between the injection line and the combustion chamber when the injector needle closed resulted in more fuel injected at low velocity and increased HC emissions. High speed single frame photography was used to observe the end of injection. Injectors with smaller orifices had substantially lower HC emissions than the baseline injector. Smaller injector orifices favored lower emissions because of both lower residence times of unburned fuel in the combustion chamber and a lower portions of the fuel injected at low velocity at the end of injection. These studies were done without considering the constraint of satisfactory operability over the full load and speed range of the engine. They suggest that a solution to the high HC emissions from the DISC engine at low loads can be found with improvements to the injection system to span high and low power operation without degradation in the injection process.