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Fuel Injection Strategies for Reducing NOx Emissions from
Direct-Injected Gasoline Engines, 03-9139

Principal Investigator
Rudolf H. Stanglmaier (Terry Alger)

Inclusive Dates: 04/01/97 - 07/01/99

Background - Direct-injected, spark-ignition (DISI) engines offer the potential for improved performance and fuel economy over their port-injected counterparts. However, DISI engines emit higher levels of oxides of nitrogen (NOx) and hydrocarbons (HC) than their port-injected counterparts. The goal of this project is to investigate an alternative technique for obtaining the fuel-efficiency benefits of lean gasoline combustion, but with low engine-out NOx emissions.

Approach - The relatively high levels of engine-out NOx emissions from DISI engines are directly associated with the combustion method employed during low-load operation (flame propagation through a stratified fuel/air mixture). It is believed that most NOx emissions are generated in the rich region of the combustion chamber where the flame is initiated. The stategy pursued in this project was to alter the low-load combustion characteristics of a direct-injected gasoline engine, such that a nearly homogeneous, lean-overall mixture was combusted at a uniformly low temperature. This combustion method is commonly referred to as homogeneous-charge compression ignition, or HCCI.

Accomplishments - A single-cylinder research engine was successfully operated in HCCI mode, using commercial gasoline as a fuel, and it was demonstrated that it is possible to operate a gasoline engine on HCCI combustion mode at light loads. Engine efficiency comparable to a DISI engine was obtained by operating the engine unthrottled and controlling the load through the fuel injection quantity. While in HCCI combustion mode, the fuel injection timing was used to control the phasing of the combustion event, relative to the engine cycle, within a reasonable range. In addition, it was discovered that the fuel-injection timing could be used to trade off some of the NOx benefit for low HC emissions. A range of operation was identified in which this engine could operate at much lower engine-out NOx and HC emissions than a contemporary DISI engine. In general, these encouraging results have generated significant interest in this new engine concept.

Emissions index of NOx versus HC, comparing the DI-HCCI engine with state-of-the-art port-injected and direct-injected gasoline engines.

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