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.
Here, therefore, the authors have sought to reduce harmful emissions from conventional diesel diffusion combustion without relying on mixing enhancement that uses ignition delay. This was accomplished with a prototype injector that achieves a faster injection rate of approximately 80% or more than a mass production model piezo-driven common rail injector that complies with EURO 5 standards by enhancing the injection rate from the opening of the nozzle needle up to full lift (hereafter referred to as fast injection rate) while keeping the nozzle hole diameter small. The prototype injector achieved this by making the fuel-air mixture leaner without increasing the fuel pressure, or in other words without an adverse effect on fuel consumption due to increased work by the fuel pump, and thus yielded a reduction in harmful emissions. This was confirmed by evaluation of the spray characteristics and by experiments using a single-cylinder engine, the results of which were reported.
The further investigation here reproduced the combustion of a metal engine in an optical engine and used a high-speed video camera to capture time series flame images. These were used to conduct detailed analysis of flame behavior in the combustion chamber in time series. The results confirmed that enhancement of the penetrating capability of the spray due to the faster injection rate increased the spread of flame in the central portion of the combustion chamber and enhanced air utilization. The two-color method was also used to analyze the spatial distribution of the flame temperature and the KL factor, and its behaviors over time. This showed that the faster injection rate resulted in curbing combustion in the rear portion of the fuel spray, which is the portion where air is introduced to the spray, so that maximum temperature of the flame is reduced. It also showed that soot was oxidized sooner.
