The Timing Valve: Operation
We all are aware of the fuel pump which we have witnessed in present days. These, which are often called "jerk type" pumps, can pump the fuel to the injectors, the start and termination of fuel injection as well as meter the correct amount of fuel to the fuel injectors, with respect to the engine load. But with the "common rail" system of fuel injection, with emphasis on the "Doxford" engines, the fuel pump, which is usually a reciprocating pump driven by the main engine, pumps fuel oil at a very high pressure into the common rail, but does not decide the start and stop of fuel injection. This is done by the "timing valves", which are located in front of each unit. The timing valve controls the start/termination of fuel injection into the particular unit.
Referring to the figure beside, the timing valve and its cam are seen with the "common rail" high pressure line, on the discharge side of the reciprocating fuel pump. The reciprocating fuel pump is usually driven by the main engine itself and supplies fuel oil at a very high pressure to the common rail, which has a non-return valve and spill arrangement.
There is a separate accumulator bottle provided on the high pressure common rail line in order to avoid the pressure fluctuations or shock due to the change in load on the main engine. Then the high pressure fuel oil awaits for the opening of the timing valve (arrangement includes cam, lever, and valve block). When the cam peak lifts the lever up, the lever in turn opens the valve on the valve block, thus allowing the high pressure oil to reach the fuel injectors mounted on the cylinders of each unit. As the engine crankshaft rotates, thus making the cam to rotate causing the peak to move, the lever returns back from its lifted position, thus allowing the valve on the valve block to close,stopping further fuel oil flow to the fuel injectors.
The duration of the opening of the valve in the valve block determines the fuel injection period. The duration of the opening of the valve in the valve block can be altered by adjusting the fulcrum point on the valve lifting lever "L." At no delivery, the fulcrum remains at its lowest position. During this stage, the cam peak just oscillates the lever, but the valve is not opened in the valve block, thus no fuel is being injected.
The fuel pumps are capable of metering the correct amount of fuel to each unit. Based on the engine load, the governor controls the fuel rack of the fuel pump as seen in the adjacent figure. This regulated quantity of fuel is pumped by the fuel pump towards the timing valve. These fuel pumps have a helical slot in the plunger, the same as in conventional pumps. This ensures that the correct quantity of fuel is always injected with respective variation in engine load.