Fundamental Study for Optimizing Fuel Injection Rate Shaping of Diesel Engines
Akihiko AZETSU* and Yoshifumi WAKISAKA*
The effects of fuel injection rate shaping on spray and combustion were examined using an electronically controlled fuel injection system. The injection rate shaping used in this study were triangular with different rates of injection rate increase, and quasi-steady injection with periodical fluctuation of injection rate. In addition to the experiments, CFD calculations for non-evaporating spray were performed to examine the effect of the rate of injection rate increase on fuel distribution.
From the experiments and calculations on sprays with triangular injection rate shaping, it was pointed out that the rate of injection rate increase affected not only temporal spray evolution but also spatial fuel distribution in sprays, which greatly affected temporal flame evolution. From the experiments on spray with periodical fluctuation of injection rate, it was confirmed that the spray dispersion could be controlled by the amplitude of fluctuation. Also the spray with fluctuation was found to have shortened dense liquid core.
Key Words: Diesel Engine, Diesel Spray, Fuel Injection Rate Shaping, Fuel Distribution
Fuel injection rate shaping, the temporal change of the injection rate during the injection period, determines the temporal and spatial distributions of fuel within the engine cylinder, and has been considered to have a large influence on combustion in diesel engines. Therefore optimal injection rate shaping is one of the key techniques of improving spray combustion and reducing harmful exhaust emissions such as NOx and particulate matter.
There were many researches which examined the effect of injection rate shaping on diesel combustion -. However a more systematic study was thought to be necessary for exploring the effect fundamentally, since injection conditions, e.g., injection period and maximum injection rate, were changed with the variation of injection rate shaping in many of these researches. From this point of view, we have developed an electronically controlled fuel injection system which can control injection pattern, injection period, peak injection rate and amount of fuel injected independently . By using this injection system, we have examined the effect of the injection rate shaping on spray evolution  and spray combustion - experimentally.
In this paper, we focus mainly on the effects of injection rate shaping on fuel distribution in sprays. One of the injection rate shaping examined in this study was triangular, a conventional injection rate shaping in diesel engines, with different rates of injection rate increase. The other was a quasi-steady injection with periodical fluctuation of injection rate, investigating the applicability for spray control. In addition to the experiments, CFD calculations for non-evaporating spray were performed to examine the effects of the rate of injection rate increase on fuel distribution.
2. EXPERIMENTAL APPARATUS AND PROCEDURE
The injection system used in this study is an accumulator type . Therefore the fuel supply pressure is constant during the injection period. In order to control the injection, a piezoelectric actuator is attached on an extended pressure pin of a conventional diesel injection nozzle. A schematic diagram of the injector is shown in Fig.1. Owing to the fast response of the piezoelectric actuator, the movement of the nozzle needle, i.e., the cross-sectional area of the flow passage around the nozzle sheet, can be controlled during the injection, enabling the fuel injection rate to be set arbitrarily.