A Volvo 9.6L diesel engine was converted to run on 100 percent compressed natural gas in order to demonstrate “significantly reduced exhaust emissions.” A descendent of the natural gas 9.6L engine developed on this project is being used in bus applications in Göteborg, Sweden. At the time this paper was written, Volvo had manufactured two out of 20 gas buses for the city of Goteborg.A lean-burn, stratified-charge combustion system was originally chosen for this project which included a precombustion chamber (prechamber) located in the cylinder head. The prechamber was used to replace the original diesel fuel injector and was fitted with its own fuel supply and spark plug. A near stoichiometric air/fuel mixture was produced in the prechamber and ignited by the spark plug. As combustion progressed in the prechamber, a violent jet was produced that ignited a lean mixture in the main combustion chamber.After several months of prechamber design optimization, various shortcomings of the prechamber system were discovered. As a result, an open combustion chamber system was fitted to the engine for comparison purposes. This exercise produced results that led to the selection of the open combustion chamber system over the prechamber system.This paper focusses on the combustion development process for the lean-burn gas engine, and on a comparison of prechamber and open chamber results from the same engine. A brief discussion of the results of two different lean-burn oxidizing catalysts is also provided.