Homogeneous Charge Compression Ignition (HCCI) combustion can be made to occur in a four-stroke engine with smooth and even combustion under some circumstances. It offers the possibility of light load operation without throttling, thus giving fuel economy like a diesel, in the same engine allowing full load operation with homogeneous charge, thus giving a power density comparable to a gasoline engine.This paper gives results of an experimental program in which the ranges of permissible values of the operating parameters were defined for HCCI operation of a four-stroke engine. It was found that HCCI required high exhaust gas recirculation (EGR) rates (in the range of 13 to 33 percent) and high intake temperatures (greater than 370°C). Under the right conditions HCCI combustion produced fuel economy results comparable with a D.I. diesel engine (ISFC in the range 180 to 200 g/kWh).It is suggested that a passenger car engine could be designed that would use HCCI at idle and light load to obtain economy like diesel, along with smooth operation, while switching to conventional gasoline engine operation at full power for good specific power output.