By clicking the diagram the names will be clearly visible
The function of the fuel injection system is to provide the right amount of fuel at the right moment and in a suitable condition for the combustion process. There must therefore be some form of measured fuel supply, a means of timing the delivery and the atomisation of the fuel.
The injection of the fuel is achieved by the location of cams on a camshaft. This camshaft rotates at engine speed for a two-stroke engine and at half engine speed for a four-stroke. There are two basic systems in use, each of which employs a combination of mechanical and
hydraulic operations.
The most common system is the jerk pump; the other is the common rail. This camshaft rotates at engine speed for a two-stroke engine
and at half engine speed for a four-stroke. There are two basic systems
in use, each of which employs a combination of mechanical and
hydraulic operations.
The most common system is the jerk pump.
Jerk pump system:
In the jerk pump system of fuel injection, a separate injector pump exists
for each cylinder. The injector pump is usually operated once every
cycle by a cam on the camshaft. The barrel and plunger of the injector
pump are dimensioned to suit the engine fuel requirements. Ports in the
barrel and slots in the plunger or adjustable spill valves serve to regulate
the fuel delivery (a more detailed explanation follows). Each injector
pump supplies the injector or injectors for one cylinder.
The needle valve in the injector will lift at a pre-set pressure which ensures that the fuel will atomise once it enters the cylinder.There are two particular types of fuel pump in use, the valvecontrolled discharge type and the helix or helical edge pump.
Valve controlled pumps are used on slow-speed two-stroke engines and the helix type for all medium- and high-speed four-stroke engines.
Helix-type injector pump:
The injector pump is operated by a cam which drives the plunger up and down. The timing of the injection can be altered by raising or lowering the pump plunger in relation to the cam. The pump has a constant stroke and the amount of fuel delivered is regulated by rotating the pump plunger which has a specially arranged helical groove cut into it.
The fuel is supplied to the pump through ports or openings at B. As the plunger moves down, fuel enters the cylinder. As the plunger moves up, the ports at B are closed and the fuel is pressurised and delivered to the injector nozzle at very high pressure.When the edge of the helix at C uncovers the spill port D pressure is lost and fuel delivery to the injector stops.
A non-return valve on the delivery side of the pump closes to stop fuel oil returning from the injector. Fuel will again be drawn in on the plunger downstroke and the process will be repeated.
The plunger may be rotated in the cylinder by a rack and pinion arrangement on a sleeve which is keyed to the plunger.
This will move the edge C up or down to reduce or increase the amount of fuel pumped into the cylinder.The rack is connected to the throttle control or governor of the engine. This type of pump, with minor variations, is used on many four-stroke diesel engines.
Valve-controlled pump:
In the variable injection timing (VIT) pump used in MAN B&W engines the governor output shaft is the controlling parameter. Two linkages are actuated by the regulating shaft of the governor. The upper control linkage changes the injection timing by raising or lowering the plunger in relation to the cam.
The lower linkage rotates the pump plunger and thus the helix in order to vary the pump output In the Sulzer variable injection timing system the governor output is connected to a suction valve and a spill valve. The closing of the pump suction valve determines the beginning of the injection. Operation of the spill valve will control the end of injection by releasing fuel pressure. No helix is therefore present on the pump plunger.
Read also: Information About Common Rail System.
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