Impending emissions regulations for diesel engines, specifically exhaust particulate emissions have caused engine manufacturers to once again examine the potential of alternative fuels. Much interest has centered around compressed natural gas (CNG) due to its potential for low particulate and NOx emissions. Natural gas engine development projects have tended toward the use of current gasoline engine technology (stoichiometric mixtures, closed-loop fuel control, exhaust catalysts) or have applied the results of previous research in lean-burn gasoline engines (high-turbulence combustion chambers). These technologies may be inappropriate for foreseeable emissions targets in heavy-duty natural gas engines. This paper describes the development of a mechanically simple CNG conversion of a 3.7L engine for Hercules Engines, Inc. which has met 1994 heavy-duty emissions targets on the heavy-duty diesel FTP test cycle, and discusses the relationship between in-cylinder gas temperature and NOx emissions as well as the effects of air-fuel ratio, spark timing, and combustion chamber shape on exhaust emissions. A low-turbulence combustion chamber was designed for this engine which takes advantage of these relationships.