Abstract
With direct injection diesel engines, the fuel spray characteristics determine the fuel combustion. The fuel spray characteristics are closely related to the injection hole shape of fuel injection nozzle. Research has been reported concerning the effect of nozzle diameter, nozzle internal flow on the fuel spray characteristics and the spray structure, but there has been little discussion of the spray characteristics in common rail injection systems.
In this research, the injection hole diameter, discharge coefficient, and other fundamentals parameters that determine the injection nozzle hole shape in a direct injection diesel engine using a common rail injection system were analyzed, including observation within the combustion chamber, to see how they related to the characteristics for the spray injected from the injection nozzle, how the engine performance changed. For analyzing under more realistic condition within the combustion chamber, a small fiber-scope, in which the fiber optics image and light guide were made into one unit, was applied to a production engine.
Furthermore on the basis of the obtained results, we examined the injection nozzle hole shape, which balances engines high output and reduced exhaust emissions.
As a result, we clarified as follows,
(1) For improving low-speed torque, enlarging the spray angle is effective, and for improving high-speed output, strengthening penetration is effective and for improving exhaust performance, promoting of fuel atomization is effective.
(2) The combustion within the cylinder was observed and the indicator diagram was analyzed. Enlarging the spray angle for low-speed 100% loads promotes the dispersion of the spray in the direction of the swirl current. Strengthening the penetration for the high-speed 100% load secures spray flying in the injection direction even for strong swirl and the enhanced mixing of air and fuel within the combustion chamber can suppress smoke. For medium-speed 25% loads, improved atomization of fuel promotes vaporization of fuel and the vaporization decreases particulate emissions caused by the improved fuel dispersion and the reduction of fuel adhesion of the combustion chamber wall.
(3) The combination of reducing the injection hole diameter and improving the flow coefficient is effective for balancing strengthening the penetration and atomization improvement.
(4) As a concrete plan of the improvement in a flow coefficient, round inlet injection hole and taper injection hole were evaluated. Both of them can improve the flow coefficient by reducing negative pressure domain at inlet part. Both combinations can realize a high flow coefficient further.