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 order to realize combustion of a perfectly homogeneous mixture, the test engine was operated with gaseous fuels added to the intake air. The results show that low NOx combustion was also typical of operation near the rich limit, even when the mixture was provided as a uniformly aspirated gaseous fuel.
The effect of the addition of an oxygenated component to the diesel fuel was investigated to improve fuel consumption. Using an oxygenated fuel may offer the possibility of ignition timing control. Using ethanol, the ignition timing was delayed and fuel consumption was improved.