DIESEL FUEL SYSTEM

The primary job of the diesel fuel system is to inject a precise amount of atomized and pressurized fuel into each engine cylinder at the precise time. The major parts of the diesel system are the fuel tank, fuel transfer pump, fuel filters, injection pump, and injection nozzles (fig. 1-29).

Fuel Transfer Pump

The fuel transfer pump is normally used on modern high-speed diesel engines. It can be driven by either engine or battery voltage. The fuel transfer pump can be located on the outside of the fuel tank in the supply line, submerged within the fuel tank, or mounted on the backside of the injection pump. The fuel pump pushes or draws the fuel through the filters where the fuel is cleaned.

Injection Pump

Several types of injection pumps are used on diesel engines. Each has its own unique operating principles.

The primary function of the injection pump is to supply high-pressure fuel for injection.

Injection Nozzles

A wide variety of injector nozzles are in use today. All are designed to perform the same basic function which is to spray the fuel in atomized form into the combustion chamber of each cylinder.

Cold Weather Starting Aids

Diesel fuel evaporates much slower than gasoline and requires more heat to cause combustion in the cylinders of the engine. For this reason, preheater and starting aids, called glow plugs, are installed on equipment equipped with diesel engines.

PREHEATERS.- Preheaters are normally installed in the intake manifold; however, in a two-stroke cycle engine, they are placed in the air passages surrounding the cylinders. The preheater burns a small quantity of diesel fuel in the air before the air is drawn into the cylinders. This burning process is accomplished by the use of either a glow plug or an ignition coil that produces a spark to ignite a fine spray of diesel fuel. The resulting heat warms the remaining air before it is drawn into the cylinders.

Figure 1-30.-Glow plug.

GLOW PLUGS.- Glow plugs (fig. 1-30) and the injection nozzle are installed in the precombustion chamber of the cylinder head. The glow plug is turned on when you turn on the ignition switch. On some equipment a light on the dashboard signals that the glow plug is cycling which signals you to wait between 15 to 30 seconds before cranking the engine. The heat, created by electrical resistance in the glow plug, heats the fuel and air mixture. The heat generated by the glow plug and the heat generated by compression allow the fuel to ignite.

AIR INDUCTION SYSTEMS

The function of an air intake system is to supply the correct amount of air needed to increase the combustion and the efficiency of an engine. On a diesel engine, the air intake system cleans the intake air, silences the intake noise, furnishes air for supercharging, and supplies scavenged air in two-stroke engines.

The three major components of the air induction system are blowers, turbochargers, and superchargers. They may be of the centrifugal or rotary type, or they may be gear-driven directly from the engine, belt or chain-driven, or driven by the flow of exhaust gases from the engine.

BLOWERS

The scavenging process, used in the two-stroke cycle diesel engine, is simply a charge of air forced into the cylinder by the blower. As this charge of air is forced into the cylinder, all the burnt gases are swept out through the exhaust valve ports. This air also helps cool the internal engine parts, particularly the exhaust valves.

Figure 1-31.-Blower air intake system.

The blower shown in figure 1-31 provides the forced-air induction for the scavenging process. Two rotors are closely fitted in a housing that is bolted to the engine. The rotor lobes provide continuous and uniform displacement of air as the rotors revolve. Blower rotors either have two lobes or three lobes, depending on the type.

TURBOCHARGERS

The four-stroke cycle engine uses two methods of air induction: naturally aspirated and turbo charged. The naturally aspirated system depends on atmospheric pressure to keep a constant supply of air in the intake manifold. The turbocharger is designed to force air into the cylinder and aid in scavenging the exhaust gases. The turbocharger differs from the blower in that the turbocharger uses the energy of exhaust gases to drive a turbine wheel (fig. 1-32).

The hot exhaust gases from the engine go through the exhaust inlet, across the turbine wheel, and out the exhaust outlet. The force of the exhaust turns the turbine wheel and shaft. This action rotates the compressor wheel (impeller) that is attached to the opposite end of the turbine shaft. As the impeller rotates, it draws air into the housing. The air is then compressed and forced into the intake manifold.