The traditional style of injection pump is the in-line pump. They have been used for many decades and are still commonly found on agricultural and stationary diesels. They are also found in many older model diesel road vehicles.

In-line pump systems have a separate pump plunger for each nozzle cylinder of the engine. The pump is run at half engine crankshaft speed and has a central shaft cam lobes attached. Each plunger is operated by its cam once every two crankshaft revolutions, coinciding of course with the power stroke of its engine cylinder. Each plunger also has a spiral groove or helix machined on its side and cut through to the top of the plunger. When operating, the plunger is pushed up by its cam lobe. At a certain point the spiral groove will line up with a spill port on the side of the plunger cylinder and the rest of the fuel is spilled.

Because the groove is a spiral shape, the point in the plunger stroke when it uncovers the spill port will vary as the plunger is rotated a few degrees either way. This adjusts the spill point and hence, the volume of its fuel charges to the engine cylinder.

Since all of the engine cylinders receive the same sized fuel charge, all of the plungers are rotated together. This is achieved by each plunger having gear teeth machined to its circumference, which are engaged by a common gear rack which runs through the pump body. As the rack moves back and forth, all four plungers are rotated together. The extremes of travel of the pump rack control the minimum and maximum fuel charge quantities the pump is capable of delivering.

In-line pumps have the advantage of a straightforward method of operation but suffer a couple of important disadvantages. They consist of a large number of moving parts which have to be operating in close match to give efficient operation. When it comes to high performance applications, it becomes increasingly difficult to keep the numerous DllA138S1191 plungers and control collars exactly matched in their delivery, especially if they begin to wear unevenly. As a result, many higher performance applications these days use a type of single plunger distributor pump.