The increasing use of natural gas as a vehicle fuel has generated considerable research activity to characterize the performance and emissions of engines utilizing this fuel. However, virtually all of the results reported have been for pushrod OHV spark ignition engines or SI conversions of heavy-duty diesel engines. Because of the pressure to improve fuel economy imposed by CAFE requirements, passenger cars are increasingly tending toward high specific output, small displacement engines. These engines employ such features as four valves per cylinder and centrally located spark plugs which give them a different dependence on operating variables than traditional pushrod OHV engines.In this study, experiments were carried out with a two-liter four-cylinder Nissan SR20DE engine representative of modern design practice. The engine was operated on gasoline and natural gas at six different loads and three different speeds. Some tests were also done with isooctane. A feedback carburetion system was used with natural gas fueling to maintain stoichiometric operation. At each test point, exhaust emissions were measured. At selected points, the effect of fuel-air ratio, spark timing and EGR was investigated.The results show that natural gas fueling provides some improvement in brake specific energy consumption compared to gasoline fueling. Oxides of nitrogen, total hydrocarbons and carbon monoxide emissions were reduced with natural gas fueling. Differences in carbon monoxide emissions are believed to result mainly from differences in the control characteristics for the two fueling systems.