Boat Diesel – High pressure injection pump

Fuel is considered to be incompressible. When it is pumped into a closed system the pressure rises rapidly until the break setting of the injector is reached, which for non-turbo engines is normally approximately 140 times atmospheric pressure. The fuel then bursts through the injector and out into the combustion chamber in a fine atomised spray.

The pump timing has to be set so that injection starts just before the piston reaches the top of the compression stroke.

Diesel fuel is relatively thin, and the fit between the hardened steel pump piston and its housing has to be extremely close to prevent fuel leaking out and consequent loss of pressure.

The pistons and valves inside the pump which generate the pressure are therefore manufactured using hardened steel to very fine tolerances, with running clearances measured in thousandths of a millimetre (microns).

Sometimes individual pumps driven by a common camshaft supply individual injectors, or a single piston pump may supply several injectors by means of a distributor valve. In other cases several pumping elements are grouped together in a common housing.

In all cases the stroke of the piston, and therefore the amount of fuel delivered at pressure to the injector, is controlled mechanically. This controls the engine speed.

Most pumps incorporate a method of governing the engine by controlling the maximum fuel delivered. This provides constant speed regardless of load and also controls the maximum speed.

Slow speed setting (tickover) and fuel cutoff valves are also incorporated in this complex and highly engineered part of the fuel system.

Servicing and tuning the pump should only be carried out by qualified personnel. The best the owner can do is keep the pipe fittings leak-free. He can also ensure that the bolts holding the rotary pump to the engine are kept tight. If not the timing can be changed as the pump moves within the clearance of the bolt holes. Finally he can lubricate the linkages.

Any pump that is driven by a toothed belt should be inspected and the belt changed and tensioned according to the manufacturer’s instructions. Make sure that it does not slip otherwise incorrect timing will result in the engine failing to start, or running erratically.

Injector pumps rely on the diesel fuel passing through to lubricate them. Any water allowed into the pump will cause wear and possibly seizure of the parts.

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