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In recent years, small diesel engines for industrial use are required to achieve still higher power output per engine size (high power density) and to have the performance to comply with stringent exhaust emissions standards as well. To meet these social demands and market requirements, there are various engineering tasks yet to accomplish. In this paper, two important technical issues have been focused, those are, combustion system improvement to enable lower exhaust emissions and higher power output than conventional models and new cooling system as a solution for increased heat load issue accompanying the progress of high power density. Firstly, for combustion system improvement, the swirl adjustment technique has been developed that will optimize the swirl ratio for each engine application with different load and speed condition.

With a drastic advance of computing software and hardware, recently, simulation techniques play snore important role in development and design of diesel engines. Especially, Finite Element Method (FEM) is indispensable for design of diesel engine components, as a tool of optimizing each part and selecting the Materials. Meanwhile, it is one of the greatest theme for all engine designers to raise compactability and high-performance of small engines. At the same time, we have to respond to social needs, that is safety, reliability, durability, and comfortability, for reduction of noise and vibration. Moreover, for design of engine components, we always try to reduce a period of development and make more economical components because we have to consider those cost. To meet this severe requirement, it is needed not to apply the simulation techniques just as they are, but to establish the higher and more useful ones.

In-Direct Injection (IDI) system are mainly used in off-road diesel engines with output of less than 19 kW. These engines generally employ a mechanical injection system. Since it is difficult for these engines to flexibly control the injection timing and injection quantity, there are restrictions on improving fuel efficiency and emission performance. Therefore, we have developed an electronically controlled fuel injection system that is optimal for small diesel engines. We adopted injectors used in relatively inexpensive direct-injection gasoline engines for automobiles, instead of injectors for common rail systems, which are often used in diesel engines. The adopted injector is a multi-hole nozzle, and its spray behavior is different from that of the pintle nozzle used in swirl-chamber diesel engines.