Theoretically, the combustion of homogeneous, lean air-fuel mixtures offers potentially low emissions with low fuel consumption. An experimental investigation of a lean-combustion engine system, equipped with a lean thermal reactor, was conducted with a 1130 kg (2500 lb) vehicle. The effects of spark timing on vehicle emissions, fuel consumption, and lean reactor performance were determined.Retarded spark timing decreased HC and CO emissions, did not affect NOx emissions, and increased fuel consumption. Over-advanced spark timing decreased CO emissions, increased HC and NOx emissions, and increased fuel consumption.The vehicle emission levels were very sensitive to spark timing changes and carburetor calibration repeatability. Thus, implementation of such a vehicle would require careful control of these engine variables.