A diesel pump's full name is a diesel injection pump, and like the name suggests, is responsible for injecting fuel into the firing cylinders of diesel engines. It is important to remember that, unlike gasoline-powered engines, diesel engines do not use spark plugs to ignite the fuel in the firing cylinders. They rely entirely on the high compression pressure of the fuel in the cylinder to result in combustion. As a result, diesel injection pumps are extremely important and must be built tough to produce the compression rates of up to 15,000 psi necessary for the engine's functioning.
Distributor Injection Pumps
Distributor diesel injection pumps are also called rotary injection pumps. They are comprised of a hollow cylinder called a rotor as it sits on a cam which connects to the ports of the distributor head. The rotor sits between the firing cylinders in the engine block, and has a small hole in one side. Unlike a gas engine, this distributor is more similar to a carburettor in that it pushes blasts of fuel through to the rotor in time with the rise and fall of the engine's pistons. Sitting on a cam as it is, the rotor turns toward the firing cylinder when it's about to fire, aligns its hole with the port on the side of the cylinder, and sprays in the fuel just in time for the combustion, before turning again toward the next cylinder to repeat the process.
Inline Injection Pumps
Inline diesel injection pumps were among the first design to be created, and are generally considered very effective. They involve a layout similar to that of the actual engine. A cylinder of the pump sits atop each one of the engine's firing cylinders. They even have a set of pistons like the engine, but in the pump the pistons point downward while the engine's pistons point upward. A rotating cam sits atop of the length of the pump's cylinders and connects to the engine's serpentine belt. A fuel line connects into the sides of each of the pump's cylinders. When the engine starts, the cam atop the pump turns, forcing one piston down after another in the exact same sequence as the pistons in the engine's firing cylinders. Each pump piston forces fuel into the engine in a highly-concentrated spray. This combined with the engine's air intake design creates the necessary compression in the firing cylinders for combustion. When the pump pistons draw back, simple suction pulls more fuel from the lines into the pump's cylinders.