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The field of diesel combustion research is producing numerous reports on studies of premixed combustion, which promises simultaneous reduction of both NOx and soot, in order to meet increasingly stringent regulations on harmful emissions from automobiles. However, although premixed combustion can simultaneously reduce both NOx and soot, certain issues have been pointed out, including the fact that it emits greater quantities of unburned HC and CO gases and the fact that it limits the operating range. Furthermore, this combustion method sets the ignition delay longer with the aim of promoting the mixing of fuel and air. This raises issues with the product due to the combustion instability and sensitivity to the uneven fuel properties that are found on the market, the capability of the engine response under transient conditions, the deterioration in combustion noise, and so on.

Multiple injections that are necessary to reduce emissions, noise, and vibration of diesel engine have been accomplished by common-rail fuel injection system. Especially a fuel amount of each injection has to be tightly controlled, but actually there is a difference between an actual amount and a target one. Injection period will be determined according to a map in that the relation between injected amount and injection period is stated. But in the case of multiple injections, pressure wave caused by previous injections remain in a common-rail system at the time an injection second or later starts. Therefore, actual amounts injected will be different from target ones. In order to compensate the difference, the method that the influence of pressure wave on fuel amount injected is also investigated and injection period will be corrected is realized in an actual engine control system. Meanwhile, pressure wave propagation in common-rail has been studied.

For reduction of NOx and soot emission with conventional diesel diffusion combustion, the authors focused on enhancement of the rate of injection (hereafter referred to as RoI) to improve air availability, thus enhancing the fuel distribution and atomization. In order to increase opening ramp of the RoI (hereafter referred to as fast injection rate), a hydraulic circuit was improved and nozzle geometries were optimized to make the greatest use of the advantages of the hydraulic circuit. Two different common rail injectors were prepared for this research. One is a mass production-type injector with piezo actuator that achieved the EURO-V exhaust gas emission standards, and the other is a prototype injector equipped with the new hydraulic circuit. The nozzle needle of the prototype injector is directly actuated by high-pressure fuel from common rail to improve the RoI.