Engines today must satisfy stringent emission requirements but must at the same time have low fuel consumption. One method of approaching both of these goals in spark-ignited natural gas engines is with lean combustion. The use of as much as 80 percent excess air significantly reduces the peak combustion temperature and, as compared to a stoichiometric engine, reduces the NOx emissions by up to 90 percent and the fuel consumption by up to 15 percent. One limitation on lean combustion, however, is the high energy needed for ignition. In larger engines, a small prechamber containing an easily ignitable near-stoichiometric mixture has proved to be both successful and popular as one method of producing the necessary high ignition energy. Although this form of stratified charge combustion has been known for many years, its development has largely been the result of “cut and try” procedures. Lack of access for suitable instrumentation, combined with the difficulty of isolating the individual variables which affect performance, has limited the fundamental understanding of the mechanism of prechamber combustion. This paper summarizes results from a research program where a constant-volume combustion rig is used to simulate engine operation. Emphasis is placed on high-speed photography of the prechamber combustion. A second program on a single-cylinder prechamber spark-ignited gas engine and a third on a multiple-cylinder engine will be reported in subsequent papers.

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