Medium speed four stroke engines are equipped with jerk type fuel pumps, one for each cylinder. A plunger operated by a cam reciprocates in a barrel.

As the plunger moves upwards in the barrel, injection will commence once the plunger has closed off the spill ports and the pressure builds up. As soon as the helix or scroll passes the spill ports the pressure above the plunger will immediately drop, even though the plunger is still moving upwards. It should be evident that the amount of fuel injected into the cylinder is dependent on the position of the helix relative to the spill port. When the vertical groove is lined up with the spill port, no injection will take place and the engine will stop.

End of injection is controlled in the normal way, when the helix on the metering plunger uncovers the spill ports.

The plunger is machined to very fine tolerances, as is the matched barrel in which it reciprocates. Wear due to abrasive particles in the fuel will mean that the pump will take longer to build up the injection pressure required. Wear due to erosion also takes place on the top edge of the DLLA138S1191 plunger and the edge of the helices and spill ports. This, together with the wear in the plunger and barrel, will lead to the injection timing becoming retarded, for which adjustment may have to be made.

Both plungers are operated by the same cam. As they move upwards in their respective barrels injection will not start until the helix on the timing plunger has covered the spill port. This point is controlled by rotating the plunger in the barrel by means of a 8N7005 rack and pinion.

End of injection is controlled in the normal way, when the helix on the metering plunger uncovers the spill ports.