An injector in an engine is used to propel or inject amount of fuel into the piston section of the engine. Inside an in-line injection pump, there is a pumping element and a delivery valve for each cylinder of the engine. The element has a barrel and a plunger inside. The barrel usually has two ports called the inlet port and the spill port respectively. They connect the inside of the barrel with the gallery. And at the top of the barrel, there is a delivery valve 8N7005, a delivery valve holder, and a pipe used to carry fuel to each cylinder.

Normally, the combustion chamber is filled with high-pressure and heated air. When the plunger is below the ports, fuel from the gallery enters the barrel above the plunger which makes sure that the barrel is full of fuel. When the camshaft, the thing that moves the plunger in a reciprocating motion, rotates, the plunger is pushed past the ports. And then the surfaces produce a sealing effect, which traps the fuel above the plunger. By the time the plunger is moved further, it raises the pressure of the fuel. And this forces the fuel out past the delivery valve, along the fuel line to the injector.

When the plunger moves towards the top of a barrel, the injection will begin by the time the plunger has closed the spill ports and the pressure raises. As soon as the helix passes the spill port, the pressure above the plunger will drop although the plunger is still moving towards the top. It is evident that the amount of the fuel that is injected into the cylinder is depended on the position of the helix according to the spill port.

The plunger is machined to the fine tolerance such as the barrel in which it always reciprocates. Wear from abrasive particles in the fuel will make the pump take longer time to build up the pressure required.The plunger rotates to deliver fuel to different cylinders. In a four-cylinder system, for example, the plunger makes four quick, 90-degree rotations to inject fuel into each cylinder and then repeat the cycle as long as the engine is running. The mechanisms that control the plunger, such as the spring that provides its force, are always located nearby, but the actual system layout can change with each design.