1996-02-01

Dripping and Slow Spray in a Disk-Type Gasoline Injector 960116

The disk-type gasoline injector uses a flat disk with a seal ring to control the valve opening and the flow rate. Though the disk-type injectors provide several advantages over the pintle-type injectors, some disk injector drips and produces undesired large droplets after several minutes of operation. High-speed photography shows that the injector dripping problem could be a result of slow droplets coming out of the injector at the end of the injection cycle. Namely, a second slow spray is generated.
Purpose of this work is to employ the Computational Fluid Dynamics (CFD) techniques to identify the causes of the slow droplets and to improve the injector design. The CFD analysis thus focuses only on the closing stage of the injection cycle. The computational domain in the valve decreases with the time; the transient grid sizes and locations are determined by a constant valve closing velocity.
CFD results show that the slow spray and therefore the injector dripping are caused by a sudden pressure oscillation when the disk starts to close the valve. The initial pressure surge in the valve accelerates the spray. Later the decreased pressure slows down the fuel at the exit and makes it unable to follow the previous spray. This slow fuel produces the slow droplets.
One way to ease off the pressure oscillation is to provide more free volume above the valve. Three valve designs with different ventilation hole sizes and numbers are used to study the effect of free volume. The peak pressure decreases substantially when the ventilation holes are changed from two to three.

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