A hybrid combustion engine in which a premixed gasoline homogeneous charge was ignited by a small amount of injected diesel fuel under a high compression ratio was made by modifying a 4-stroke cycle diesel engine and was tested on a bus.It was found that the premixed of gasoline was effective not only for decreasing the fuel consumption but also for reducing the smoke density during an acceleration road test. The effect of introducing a small amount of N2 gas for supressing the diesel knock under heavy load operation was examined.These days diesel engines are widely used because of their excellent fuel economy. Particulates emitted from a diesel engines, consist of dry soot observed as smoke and the Soluble Organic Fraction (SOF) containing Polynuclear Aromatic Compounds (PAC). It has been found that the PAC is mainly formed when the hydrocarbon fuel undergoes diffusion flame combustion such occurs in diesel engines and that the PAC is not formed in the case premixed flame combustion such as occurs in gasoline engines. In addition, it has been clarified that the SOF shows a strong mutagenic activity depending on the engine type and operating conditions (1). Based on such background, A. J. Dale and A. K. Oppenheim are proposing for engines a “homogeneous combustion process” rather than the diffusion flame (diesel) combustion process which provides exhaust products which might be hazardous to health, and they continue research studies to ignite a premixed lean homogeneous charge by a plasma jet igniter (2) (3) (4) (5).In a spark ignition engine which is a representative engine for using premixed flame combustion, because of the fixed point (spark plug) ignition the practical compression ratio is always limited due to the occurrence of knock. Therefore, a high compression ratio such as with a diesel engine cannot be used. In addition, since a throttle valve is indispensable, the fuel consumption becomes worse because of the increased pumping losses at partial loads and idle. Therefore, it is considered that when a premixed gasoline homogeneous charge is ignited using a small amount of injected diesel fuel, there is multi-point ignition produced by the compression ignition, and the occurrence of knock can be controlled by adjusting the amount and the injection timing of the diesel fuel even if a high compression ratio is used. Furthermore, this can also reduce the smoke density.Two kinds of 2-stroke cycle diesel engines and 4-stroke cycle diesel engines, one with an open combustion chamber (direct injection) and the other with a precombustion chamber, were modified to hybrid combustion engines and the effect of varying the mixing ratio of gasoline (as premixed homogeneous charge) to diesel fuel (as source of ignition), compression ratio, injection timing of diesel fuel on engine performance previously had been investigated. Excellent results were obtained by dynamometer tests (6) (7) (8).In this paper, a bus diesel engine of the bus was modified for hybrid combustion and the effect on the transient performance was investigated by the acceleration road test. As a result, the fuel consumption, the smoke density and the drivability of transient operation were improved similer to the result in the steady operation obtained in the dynamometer tests.* Numbers in parentheses designate reference at end of paper.