Our aim to develop an engine to drive an emergency portable fire pump resulted in the development of a small-light-weight 2-stroke diesel spark ignition engine which utilized an ultrasonic atomizer. As the base engine we used a 499cc twin cylinder 2-stroke S.I. outboard motor engine. The fuel supply for this engine was controlled by a carburetor. In this original form it was not possible to run the base engine using only diesel fuel and although a mixture of diesel fuel and gasoline would run the engine it produced a large quantity of white smoke emissions.To solve these problems it became necessary to atomize the diesel fuel, improve the fuel-air mixture and facilitate easier combustion. First we installed an ultrasonic atomizer to the intake of the base engine. Due to the fine spray and low penetration produced by the ultrasonic atomizer we obtained good mixture and distribution. The spray angle was constant as determined by the horn tip configuration, regardless of the fuel flow rate. In addition, wall wetting of the inlet channel was greatly reduced.Although previously impossible, with the use of an ultrasonic atomizer it has become feasible to run a 2-stroke S.I. engine with diesel fuel. Engine performance and fuel efficiency was equivalent to that of the original engine using kerosene fuel. Further, there was no exhaust smoke and stable combustion was achieved. As this diesel fuel engine is extremely light we investigated the possibility of using this to drive a emergency portable fire pump.ON 17TH MAY 1987, when the USS Stark was struck by two missiles, an aboard fire broke out.(l)* The crew fought the fire with a portable fire pump driven by a gasoliqe engine. The supply of gasoline onboard the battleship was minimal, consequently after a number of hours fuel was exhausted, fire fighting ceased and navigation became impossible. From the Stark incident we can see an increasing necessity for a portable fire pump which can use diesel fuel, the same fuel used to power the ship, which is much safer than gasoline.Diesel engines per horsepower are very heavy compared with gasoline engines and cold start is very difficult, therefore impractical for use in an emergency portable fire pump. The 2-stroke S.I. engine per horsepower is very light, the structure is simple, it is small and portable, therefore more practical for use in a fire pump.Ariga et al (2) developed a 2-stroke spark-assisted DI diesel engine. They converted a commercial 2-stroke S.I. gasoline engine to a DI diesel engine. As a result, they developed a small light-weight diesel engine with a low fuel consumption rate. However, this engine when fitted with a fuel injection pump becomes too heavy for use in an emergency fire pump. Furthermore the extremely high pressure in the cylinders causes noise and reduces engine endurance.For combustion of diesel fuel to occur in a spark ignition engine a large contact area between the fuel supplied as a fine spray and the air supplied to the engine is essential. Namiyama et a1 (3) confirmed the effect of atomized fuel on combustion by applying an ultrasonic atomizer to a gasoline S.I. engine and as a result they concluded that the application to commercial engines was feasible. However, there is very little research of diesel fuel combustion in S.I. engines. Therefore, we made it our aim to develop a diesel fuel system, using the ultrasonic atomizer, in 2-stroke mass produced S.I. engines.