A mechanical diesel injection pump is designed to pressurize, meter, and distribute the diesel fuel that's injected into the engine to create the optimum power output.
A mechanical injection system needs to be capable of pressurizing the fuel sufficiently to open the injectors (nozzles) and deliver the fuel to the cylinders in an atomized form. Mechanical injector opening pressures are usually between 3,000 and 5,000 psi.
The amount of fuel metered by the injection pump is the precise control of fuel quantity. Diesel engines operate with an excess of air in the cylinders so there is always more air present than is necessary to completely combust the fuel.
Distribution of the fuel is the last job of the mechanical injection pump. The fuel system needs to be phased (sequenced) to deliver the fuel to each cylinder at the correct time and in the correct firing order. Proper fuel system phasing is required to balance the engine output (all cylinders producing the same amount of power). Thus, if the injection event timing is set at 3 degrees before top dead center (BTDC), all cylinders must be fueled at the same respective crank angle.
A complete cycle of a diesel engine is 720 degrees of crankshaft rotation, and the fueling pulse has only a maximum of 40 crank angle degrees to be accomplished. For example, a four-stroke diesel engine running at 2,000 rpm produces approximately 17 power strokes in each of its cylinders every second, so the pumping, metering, and injection cycle are all measured in milliseconds, which is 1/1,000 second.