Marine Fuel Injection Pumps
There are two basic types of fuel injection pumps:
- Helix Control: Simple and less expensive. Cavitation erosion can alter control characteristics.
- Valve Control: More Expensive. Plunger less prone to wear. Valves can be overhauled or exchanged.
Valve Controlled Fuel Pump
A suction valve is lifted off its seat when the plunger is at bottom dead center. As the plunger moves upwards the suction valve will be lowered onto its seat by the lever. The spill valve remains closed. This is the point of fuel injection.
The plunger continues to rise until it reaches a point where the spill valve opens. Fuels spill back to the suction side of the pump signalling the end of injection. As the plunger moves down its stroke the pressure difference across the suction valve causes it to lift off its seat which admits fuel to the pump barrel. Further down the stroke the suction valve lifts mechanically via its push rod and lever.
The volume of the fuel delivered is controlled by the position of the spill valve eccentric shaft. This is connected to the fuel governor linkage.
Helix Controlled Fuel Pump
A fuel pump with helix control is shown in figure 2 above. The pump consists of a liner or a barrel in which the plunger is moved up and down by the fuel cam and return spring. The suction port is in the barrel. The plunger is free to rotate in the barrel and the rotation is achieved with a rack and pinion assembly.
The quantity of the fuel injected is controlled by two milled channels at the top of the plunger. Each channel has a helical control edge. Figure below shows how the amount of fuel injected is regulated.
Consider full fuel delivery first. In the figure 3 above, the plunger (1), is at the bottom of its stroke. The ports in the barrel are uncovered allowing fuel to enter the space above the plunger. As the plunger rises on the fuel cam (2), the upper edge of the machines helix covers the ports, thereby sealing off the space above the plunger. The pressure rapidly rises to injection pressure. Refer figure 4. This is the pressure at which the injector is set to open, just before the fuel is injected. Fuel pressure continues to rise as the plunger rises.
Once the lower edge of the helix uncovers the ports (3) in figure 3, the fuel is vented to the pump return line and the injection pressure falls. The injector will close once the pressure falls below the injector opening pressure.
The upper edge of the helix controls the commencement of fuel delivery. The lower edge controls the end of delivery.
Less fuel needs to be injected for part load operation. The plunger is free to rotate in the barrel via a rack and pinion assembly connected to the plunger foot. As the plunger rotates the position of the helix relative to the port in the barrel will change, as shown in figure 3, (4). In this case the plunger does not travel so far before the port, hence less fuel is injected.
If the plunger rotates so that the vertical milled groove is in line with one of the ports, the plunger will be at zero delivery as the pump is vented to the suction side during its entire stroke.
Considering the plot of fuel pressure against crank angle shown in figure 4, fuel pressure rapidly due to the shape of the fuel cam. When the injector opening pressure is reached, fuel injection commences. Thereby a slight drop in fuel pressure at the instant the valve opens, but the pressure will continue to rise to the maximum injection pressure.
"Operation and Maintenance of Machinery in Motorships" by N.E Chell
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