The fuel system is one of the most complicated of all engine systems; therefore, special care must be exercised when making adjustments and repairs. Even though manufacturers have designed many different fuel systems, the basic principle involved is the same in all of them. If you understand the basic principle for one system, you will have no difficulty in becoming familiar with other systems. The procedures for the maintenance and repair of the various systems are also similar.

Lets review briefly not only the function of a fuel system but also the various types of fuel systems. As you know the function of a fuel injection system is to deliver fuel to the engine cylinders under specific conditions: at a high pressure, at the proper time, in the proper quan-tities, and properly atomized. This function may be carried out by either one of two types of systems: the air injection type or the solid injec-tion type. Since there are few air injections systems now in use, we will consider only the solid (mechanical) injection type systems.

Solid injection systems may be classified as jerk pump systems and common rail systems. Variations are to be found in each of these systems. The following examples show some of the basic differences between the various solid injection systems.

Systems of the JERK PUMP type may be identified as either individual pump systems or unit injection systems. Some jerk pump systems use a separate pump and fuel injector for each cylinder, while the unit injection systems combine the pump and injector into a single unit.

The Bosch system is an example of an in-dividual pump system. The pump is a cam-actuated, constant stroke, lapped plunger and barrel pump. The pump times, meters, distributes, and provides the necessary pressure to inject the fuel into the cylinder through a separate nozzle. The General Motors unit injector is an exam-ple of a unit injection system. It embodies a cam-actuated, constant stroke, lapped plunger and bushing, a high pressure pump, and an injection nozzle, all in one unit.

In the Cummins injection system, a cam-actuated injector and nozzle assembly is mounted in each cylinder. This system employs a common metering device that distributes a measured quan-tity of fuel to each of the injectors. The Cummings injection system embodies characteristics of the unit injector and is sometimes classified as such, although it is also called a distributor system.

The Fairbanks-Morse injection system is another example of a jerk pump system. The injection system known as the COMMON RAIL system includes two types: the basic common rail system and the modified common rail system.

The fuel injection systems used on Atlas engines and some older models of Cooper-Bessemer engines are of the basic type. In this system one untimed, high-pressure pump supplies fuel at injection pressure to a main header (com-mon rail). The fuel flows from the header to the injector valves and nozzles at each cylinder. The injector valves are cam-operated and timed. Metering of the fuel is controlled by the length of time the nozzle remains open and by the pressure maintained by the high-pressure pump in the common rail.

The modified common rail system (constant pressure), found on newer models of Cooper-Bessemer engines, uses a high-pressure pump to maintain fuel at the injection pressure in an accumulator bottle. The fuel is metered by individual valves mounted on the side of the engine; it then flows to the pressure-operated nozzles in the cylinder head, to be atomized and distributed in the cylinder.

Since complete details for the maintenance and repair of each of the various fuel systems in serv-ice are beyond the scope of this book, specific information on a particular fuel injection system must come from the appropriate manufacturers technical manual.