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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. 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. 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.
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 . A variety of P-L-N configurations have been developed, with different technical and/or economic justifications. Most P-L-N systems can be classified into three categories, based on the type of the injection pump, as follows:
In-line pumps, serving multi-cylinder engines, house as many pumping elements as there are cylinders in the engine. The pump is usually gear-driven by the crankshaft and is positioned in a central location relative to the engine assembly. 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. 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. In their attempt to minimize complications from line dynamics, designers strive to keep the total line length as short as possible. In some cases, the shortest possible line may still be too long for an in-line pump to operate effectively. 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. Examples of this type of application include the DDC/MTU Series 2000 and MTU/DDC Series 4000 engines. In older versions of these engines, unit pump systems were used to maintain short injection lines between the pump and injector. Each unit pump is installed on the engine in close proximity to the cylinder it serves and is driven by the camshaft of the engine. Since the unit pump system uses a separate pump for each cylinder, this configuration falls in fact somewhere in between the P-L-N and the unit injector systems; we will discuss the unit pump system in the Unit Injector/Pump paper.
High pressure pumping elements consisting of plunger and barrel combinations are made of high strength tool steel and extremely close tolerances are kept between the sliding/rotating parts. This high precision machining is required throughout the mechanical components of the injection system to maintain accurate metering and injection timing within 1° crankangle. The cost of such fuel systems is rather high and is difficult to justify especially in small passenger car size engines. A solution to this problem is the distributor pump, where one central pumping element is used to produce the high injection pressure. This high pressure fuel is then introduced into a commutator head or distributor assembly that diverts it to the proper injector and cylinder according to the engine firing order. Reducing the number of pumping elements for a multi-cylinder diesel engine application to only one reduces the cost of the expensive high precision machined parts of the pumping element and makes its cost more appropriate for the small car market.
For several decades mechanical control was featured in P-L-N injection systems. Sophisticated mechanical devices have been developed—such as governors and timing, boost and torque control devices—to control engine speed and a number of other parameters. Since the late 1970s, the P-L-N system was modernized through an evolutionary process where the initial steps were simply to use electrical components to replicate functions that were previously performed by mechanical components. The introduction of electronics to the diesel engine industry was slow, largely due to the negative cost implications as well as doubts about the reliability of electronics in the rugged applications of the diesel engine. Uncertainty about whether electronics would really be required to meet emission regulations while helping maintain good engine performance further delayed progress toward adopting electronics in heavy-duty diesel fuel systems. Emission regulations, however, continued on an increasingly stringent path, forcing more demands on the fuel injection system. Further, early demonstrations of what electronics could do helped focus attention on these developments and direct more capital and resources into research efforts. A somewhat detailed description of the “electronification” of the in-line and the distributor/rotary pump systems with a special treatment of some of its basic, as well as new functions is given in the last section of this paper.