A staged combustion engine has been evaluated in which pairs of cylinders are coupled in series. The first cylinder of the pair inducts and burns a homogeneous, fuel-rich mixture which produces exhaust products containing substantial amounts of combustibles (CO, H2, and HC) and only small quantities of NOX. These products are then cooled, mixed with additional air, and inducted into another cylinder for a second stage of combustion. Additional work is extracted in this second stage, where substantial cleanup of CO and HC occurs while maintaining a low level of NOX.Experiments with a two-cylinder research engine showed that low NOX emission could be obtained without sacrificing engine efficiency. However, approximately 40 percent more displacement is required to produce the same power as conventional SI engines. The sources of HC, CO, and NOX emissions were investigated, as were the effects of major engine variables on these exhaust emissions and fuel consumption.The staged combustion concept was implemented using a 7.46 litre V-8 engine, for which the necessary control systems were devised and built. This engine was evaluated in an experimental vehicle on a chassis dynamometer with a 2041 kg (4500 lb) inertia weight. Present FTP emission levels are 0.87 g/mi HC, 8.7 g/mi CO, and 0.34 g/mi NOX at low mileage using a catalytic converter. FTP fuel economy is 4.9 km/ℓ (11.5 mpg), but acceleration performance is poorer than comparable production cars. Adequate evaluations of production-related factors such as driveability, durability, and fuel requirements have not been made. Although the low NOX emission demonstrated by this engine concept is noteworthy, the engine is presently precluded from serious contention by the stringent limitations which current statutes place on allowable HC and CO emissions during the 1975 FTP.