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To reduce exhaust emissions and fuel consumption, the effect of high pressure fuel injection was investigated with in-cylinder fuel spray observation and single cylinder engines. Spray impingement on the cavity wall promotes mixing with air and reduction in the nozzle area extends this wall impingement as a result of increasing both fuel injection pressure and injection period. There exists an optimum range for the injection period. Increased injection pressure by modifying injection rate of fuel pump and nozzle area, improves smoke and fuel consumption at low and medium speeds in particular. To extend these effects of high pressure injection, more optimized combustion system and minimized injection equipment drive torque must be required. To resolve the problem of high pressure injection such as higher combustion noise and increase in NOx emissions, the combination with pilot injection must be one of the most effective ways.