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A reverse uniflow-type two-stroke gasoline direct injection engine, which has potentials of high power weight ratio, high thermal efficiency and low exhaust gas emissions, has been developed and tested. In this study, one of the features of this engine: very low cycle-to-cycle combustion variation at idling condition, is focused to clarify the reasons. To achieve this, a transparent cylinder model engine was designed and built to visualize the in-cylinder mixture formation process, and the free spray characteristics of a swirl-type injector were examined using a large chamber with changing the injection pressure, environmental gas pressure, and the gas temperature. As a result, the reasons of stable idling operation were deduced.

The authors have been engaged in developing a new-generation two-stroke gasoline engine which could be employed ultimately for automobiles. By investigating the defects of the Schnurle-type two-stroke gasoline engine, a reverse uniflow-type direct injection engine has been developed and built. The newly introduced system employs stratified charge combustion in light to medium load conditions by using the technology already developed for the four-stroke direct injection gasoline engines while it can supply the maximum power output by using a super-charger and attaining homogeneous combustion. Engine performance is being tested experimentally. In order to analyze the performance test results, numerical analysis of in-cylinder phenomena, such as gas-exchange, gas motion, fuel spray formation, and mixture formation is carried out in this paper.

Because the two-stroke gasoline engine has a feature of high power density, it might become a choice for automobiles' power train if the high HC exhaust emissions and high fuel consumption rate could be improved. As the GDI technology is quite effective for two-stroke engines, a Schnurle-type small engine was modified to a GDI engine, and its performance was tested. Also, numerical analysis of the mixture-formation process was carried out. Results indicated it was possible to reduce both the HC emissions and fuel consumption drastically with the same maximum power as a carbureted engine at WOT condition. However, misfiring in light load condition was left unresolved. Numerical analysis clarified the process of how the mixture formation got affected by the injector location, injection timing, and gas motion.

High specific power output two-stroke engine for snowmobile use was converted to a Direct Fuel Injection (DFI) engine, in order to achieve lower HC emission by avoiding fuel short circuiting and to obtain higher specific power output by increasing induction air and optimizing A/F. High pressure single fluid fuel system was chosen because of extremely high fuel delivery rate. The fuel injector and its location were investigated and optimized for better mixture formation and lower HC emission. 140kW/L level specific power output, maximum engine speed of 9000 rpm, and 1/2 the level of HC emissions were obtained.

An attempt to reduce the HC emission of a two-stroke engine was carried out. A simple homogeneous charge combustion created with a Direct Fuel Injection (DFI) system was applied to a Personal Water Craft (PWC) engine. 1/4 HC emission of the base carbureted engine was obtained in International Council of Marine Industry Association (ICOMIA) driving mode due to the exclusion of fuel short-circuiting. Then stratified charge combustion was introduced. A numerical simulation of air and spray motion was performed and mixture formation was optimized. The low load misfiring was completely overcome and finally, less than 1/8 HC emission was achieved.