Fuel consumption of the internal combustion engine was mainly understood as an economic point of view until several years ago, but it is becoming the issue of survival of human being in connection with the global warming phenomena, and the regulation on the fuel consumption of each government is becoming serious in these days either.
Fuel consumption, noise and vibration of direct injection diesel engine have been improved significantly during last decades by adoption of the common rail fuel injection system which is able to control fuel injection pressure, injection timing, pilot injection and multi stage injection precisely. As a result, market share of the vehicle powered with diesel engine is increasing rapidly.
Nevertheless, social request for the high fuel efficiency, high power output, compactness in size, light weight and high speed to the diesel engine is continuously increasing. In order to meet these requirements, air-fuel mixture should be made fast and efficiency, utilization of the air in the cylinder is to be maximized and ignition delay and flame duration should also be shortened.
In this paper, several methods have been devised and performed for the improvement of air-fuel mixing process and the characteristics of combustion, and the effects of improvement have been investigated through the qualitative and quantitative analysis, that is, analysis of the flame images taken by a high speed camera from the visible engine and measurement of ignition delay, flame duration and cylinder pressure.

The results are as the followings.

(1) Partial premixed compression ignition was performed by two-stage injection at visible engine and investigated through the flame images of combustion process and two-color method. Violent and luminous yellow flame over 2200K was occurred at the beginning stage of combustion, and flame contains particulate matters in case of traditional single stage ignition. However, flame area of high temperature was reduced and temperature distribution was even in the partial premixed compression ignition because several nuclei of ignition were occurred and small flames were developed.
The output of this study is that the method for reducing NOx and PM which are in trade-off relation was devised and investigated.
(2) In this paper, the combustion process of common rail diesel engine was studied by employing two kinds of pistons, one has several cavities on the piston crown to intensify the squish during the compression stroke in order to improve the atomization of fuel, we call this "multi-cavity piston" in this paper, the other is a toroidal single cavity piston which is generally used in high speed diesel engines.
In case of multi-cavity piston, ignition nuclei were occurred in early stage due to that fuel was injected to the several cavities and air flow was intensified, and diffusion of combustion to the whole combustion chamber also becomes very fast because each cavity expands independently after ignition.
In the quantitative aspect, peak combustion pressure in cylinder is increasing, and time to reach peak pressure, ignition delay & flame duration become shortened in multi-cavity piston. Therefore, it seems multi-cavity piston works better in improving power output & fuel efficiency and increasing engine speed.
(3) In this study, multi-cavity piston was applied to investigate the improvement of the delay in air-fuel mixture formation of bio-diesel fuel which has characteristic of high density especially in winter. In the case of multi-cavity piston, ignition nuclei were occurred in each cavity during early stage and flame diffusion became fast as in the case of diesel fuel. As a result, combustion process of BDF5 in the multi-cavity piston was improved as the case of diesel fuel in conventional single cavity piston.
Even though the effect of combustion improvement becomes lower as mixing rate of bio-diesel is increasing, it is a meaningful result to have found out a method for preventing power down and fuel consumption becoming worse.