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Various configurations of high-pressure-loop EGR system for a heavy-duty turbocharged diesel engine have been studied through CFD and performance simulation. Dual-EGR piping between the intake- and exhaust-manifolds with the ‘exhaust-pulse ejector’ at the junction of the exhaust manifold were found promising with respect to higher EGR ratio at high load, less pumping loss and higher volumetric efficiency. A reed valve in each EGR passage was effective in that respect. It was also found that combining with venturi tubes or intake-resonance-charging could enhance the above-mentioned effects. Some of these effects have been experimentally proven on a real engine.

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.

To find more effective lean mixture preparation methods for smokeless and low NOx combustion, a numerical study of the effects of in-cylinder flow field before injection on mixture formation in a premixed compression ignition engine was conducted. Premixed compression ignition combustion is a very attractive method to reduce both NOx and soot emissions, but it still has some problems, such as high HC and CO emissions. In case of early direct injection, it is important to avoid wall wetting by spray impingement, which can cause higher HC and CO emissions. Since it is not easy to examine the effects of initial flow and injection parameters on mixture formation over the wide range by practical engine tests, a computer program named “GTT (Generalized Tank and Tube)” code was used to simulate the in-cylinder phenomena before autoignition.

The effects of fuel injection timing on fuel-air mixture formation, combustion and emissions for a PREmixed lean DIesel Combustion (PREDIC) engine has been studied numerically by the KIVA II modeling package. The software was modified with an improved autoignition and combustion submodel, which describes the formation of combustible or ignitable fuel-air mixtures by turbulent mixing, and describes four chemical reactions, including low-temperature oxidation. The results indicate that the present computational model reproduces major features of two-stage autoignition and experimentally observed trends in NO and unburned fuel emissions. The relationships among in-cylinder distributions of fuel sprays, fuel-air equivalence ratio, temperature and mass fractions of NO and unburned fuel were demonstrated by graphically imaged results. A method of fuel-air mixture characterization has been introduced and used to analyze the numerical results.

Low NOx combustion is possible by PREDIC (PREmixed lean DIesel Combustion) in which fuel is injected at a very early stage of the compression stroke and the combustion starts at near the top dead center by self-ignition. To simplify the phenomenon of the PREDIC process, the test engine was operated with gaseous fuels added to intake air to realize combustion of a perfectly homogeneous mixture. The rich limit was observed around λ=2.0∼2.4. This limit was determined by considering the increase in NOx, and the steep pressure rise. During high load operations is not only the ignition timing but also the combustion rate should be controlled. By comparing the homogeneous charge and direct injection case, the mixture heterogeneity could be found to have an influence on the ignition timing and combustion rate, the engine speed and injection timing also had an influenced on these.

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.

A study of the mixing mechanism of fuel with surrounding air is necessary in order to clarify the combustion process. In this study, the flow field near non-evaporating diesel spray as well as spray surface were observed and analyzed using a Nd-YAG laser light sheet. A single shot fuel spray was injected into a high pressure vessel and photographed under double-pulse laser illumination. The images of dispersed particles in the vessel were processed and velocity vectors were obtained by the auto-correlation method. Measured results showed temporal variation in the air movement around the spray. Just after the start of injection, air near the nozzle was pushed outward by the spray tip, after which the flow direction reversed. The air velocity ahead of spray tip was very low compared to fuel spray tip velocity. At a stable injection condition, air near the nozzle tip was pulled by the spray movement and flowed uniformly, and the spray-air boundary was smooth.

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.