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Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some problems still remain, such as higher fuel consumption, a lack of ignition timing control, and a large increase in THC and CO, compared to conventional diesel combustion. Appropriate mixture formation is necessary to solve these problems. In this paper, the influence of mixture formation on PREDIC was investigated. It was found that the pintle type injection nozzle was shown to be suitable for PREDIC, because it produced a comparatively uniform mixture in the combustion chamber and avoided collision of the fuel spray with the cylinder liner. Modeling by the KIVA-II software package was carried out to improve our understanding of the mixture formation process.

In Japan there is currently a strong social demand for exhaust emissions reduction from heavy-duty diesel engines. Therefore, new Long-Term Regulation will come into effect in October 2005, setting the NOx standard at 2.0 g/kWh and the PM standard at 0.027 g/kWh. At the same time, customers always demand exceptional fuel economy from heavy-duty commercial vehicles. A urea-based Selective Catalytic Reduction (SCR) system was developed to satisfy both these demands, and will be introduced in the fall of 2004. The operating conditions of vehicles in Japan are different from those in the US and Europe. Basically, average vehicle speeds are significantly lower. To improve the low temperature SCR performance, an oxidation catalyst was located upstream of the SCR, and an additional oxidation catalyst was located downstream of the SCR for emergency NH3 slip. The muffler size with all three catalysts was similar to a conventional muffler.

Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some points of concern remained unsolved, such as a large increase in THC and CO, higher fuel consumption, and an operating region narrowly limited to partial loads, compared to conventional diesel operation. In this paper, the causes of PREDIC's problem areas were analyzed through engine performance tests and combustion observation with a single cylinder engine, through fuel spray observation with a high-pressure vessel, and through numerical modeling. Subsequently, measurable improvements were achieved on the basis of these analyses. As a result, the ignition and combustion processes were clarified in terms of PREDIC fuel-air mixture formation. Thus, THC and CO emissions could be decreased by adopting a pintle type injection nozzle, or a reduced top-land-crevice piston.