Superiority of Newly Developed Direct-Injection Engine over Conventional Engine for Personal Watercraft

A new engine of which the fuel is injected directly into the has been developed cylinder as an environmental-friendly engine for personal watercraft(PWC). This new engine surpasses the conventional carbureted engine in performance, yet achieves 75% reduction of THC+NOx emission and 30% improvement of fuel consumption in the ICOMIA mode. This paper describes the system characteristics of this newly developed DI engine, with reference to its combustion and exhaust gas emission properties. This newly developed engine employs a fuel injection system that injects the fuel directly into the cylinder without a high-pressure fuel pump. To analyze the reliability of its unique fuel injection system, the cyclic injection rate is measured the Bosch method. The result is that the fuel injection volume per cycle fluctuates very little even under such extreme conditions as a condition of highly repetitive high injection volume equivalent to an engine running at 7000 rpm with wide-open throttle (WOT). To clarify the difference between stratified charge combustion, which is a definitive property of this DI engine, and homogeneous charge combustion in a carbureted engine, these engines are compared by measuring in-cylinder pressure and analyzing combustion in the ICOMIA mode. The result reveals that, compared with combustions in a carbureted engine, stratified charge combustion maintains higher combustion stability by reducing fluctuations of the indicated mean effective pressure per cycle, even when operated under partial loads, including idling. A comparison of exhaust gas emissions by the DI engine and the conventional carbureted engine reveals that the DI engine is clearly superior as the benefit of its stable fuel injection and stratified charge combustion.