Examples of this type of application include the DDC/MTU Series 2000 and MTU/DDC Series 4000 engines. This is the case in large marine and stationary power plants where the sheer size of the engine prohibits the use of short injection lines. In some cases, the shortest possible line may still be too long for an in-line pump to operate effectively. In their attempt to minimize complications from line dynamics, designers strive to keep the total line length as short as possible. With highly pulsating systems and pressure waves traveling through narrow pipes, line dynamics can be difficult to manage and may cause erratic injection behavior at the nozzle. Engine and fuel system designers strive to have the pump location such that all of the injection lines are equal in length between the injection pump and the entry to the injectors. The pump is usually gear-driven by the crankshaft and is positioned in a central location relative to the engine assembly. In-line pumps, serving multi-cylinder engines, house as many pumping elements as there are cylinders in the engine. Most P-L-N systems can be classified into three categories, based on the type of the injection pump, as follows: A variety of P-L-N configurations have been developed, with different technical and/or economic justifications. The pump-line-nozzle injection system is so-called for producing high fuel pressure in a pumping element, transferring the fuel pressure pulse through a high pressure injection line, and then spraying this fuel into the cylinder via the nozzle of an injector. Due to its historical significance, knowledge of the P-L-N system is essential for understanding the principles and the ongoing evolution of the diesel injection system. While the P-L-N system has been displaced by common rail and unit injector type fuel injection systems in new engine designs for markets with the most stringent emission standards, this fuel system does remain popular in markets with less stringent emission standards.
The pump-line-nozzle (P-L-N) system, also called the pump-pipe-nozzle system, was for many decades the dominant type of diesel injection system in practically all diesel engine applications.