1100 SERIES DIESEL TRUCK ENGINE Fuel System Caterpillar

Fuel System
1.1. Fuel Injection Pump Operation
2.2. Injector Operation

The fuel system consists of the transfer pump, fuel filter, shutoff solenoid, injection pumps, timing advance unit, governor, injection lines, nozzles and excess fuel return lines.

The diaphragm-type fuel transfer pump mounts on the fuel injection pump housing and is driven by a lobe on the injection pump camshaft. The pump draws fuel from the vehicle supply tank and delivers it to a spin-on throw-away type filter. The filter is a combination primary-secondary element.

Filtered fuel flows through a fuel shutoff solenoid, mounted on the fuel injection pump housing, into a fuel manifold. The solenoid operates electrically and stops fuel flow when the vehicle ignition system is off.

Fuel in the manifold flows through the barrel assembly inlet port into the area above the injection pump plunger. During injection, the camshaft forces the plunger upward in the barrel. The end of the plunger closes the inlet port and forces the fuel out through high pressure injection lines to the nozzles.

The injection nozzles are located under the valve cover and are held in place by clamps. The nozzle tip projects from the head into the cylinder bore. Atomized fuel is sprayed in a cone-shaped pattern through four orifices into the cylinder.

During injection, a small amount of fuel leaks past the valve guide in the nozzle body to lubricate its moving parts. Any excess leakage flows from the nozzle to a fuel return manifold under the valve cover of each cylinder head. External lines connect the manifolds and return the fuel to the tank.

Fuel Injection Pump Operation

Fuel enters the fuel injection pump housing from the fuel filter through the fuel manifold and goes to the fuel injection pump through inlet port. The injection pump plungers and lifters are lifted by the cam lobes on the injection pump camshaft and always make a full stroke. Springs hold the lifters against the cam lobes. Each pump meters the amount of fuel injected into its respective cylinder and delivers it to the fuel injector.

The amount of fuel pumped per stroke is varied by the turning of the plunger in its barrel. Each plunger is turned by the action of the governor through a gear segmented sliding rack turning the gear segment on the bottom of the pump plunger. The position of the scroll on the plunger determines the amount of fuel to be injected into the cylinder.

Figures A, B and C illustrate the function of an injection pump as the plunger makes a stroke.

In Fig. A the plunger is down and the inlet port is uncovered. Fuel flows into the space above the plunger, through the slot and into the recess around the plunger.

In Fig. B, the plunger has started up and the port is covered. Fuel is trapped and is forced through a check valve, fuel line, and injector as the plunger moves upward.

In Fig. C the plunger has risen until the port is uncovered by the recess in the plunger. The fuel now can escape back through the port into the fuel manifold and the injection ceases.

Note that the recess in the pump plunger forms a helix around the upper end of the plunger. Figures D, E and F illustrates that rotating the pump plunger affects the quantity of fuel injected.

In Fig. D the plunger is rotated into the shut-off position. The slot connecting the top of the plunger with the recess is in line with the port, therefore, no fuel can be trapped and injected.

In Fig. E the plunger is in the idling position. The narrow part of the plunger formed by the helix covers the port for a short part of the stroke. This allows a small amount of fuel to be injected for each stroke.

In Fig. F the plunger is rotated into the full load position. The wide part of the plunger formed by the helix covers the port for a longer portion of the stroke. This allows a larger amount of fuel to be injected for each stroke.

Fuel, under high pressure from the injection pumps, goes through injection lines to the injectors. As high pressure fuel enters the injector a valve in each injector opens and the fuel sprays into the cylinder to mix with compressed air.

Injector Operation

The nozzle body incorporates the inlet fitting, tip and valve guide. The valve is spring-loaded. A nylon compression seal under the inlet fitting "banjo" prevents engine compression leakage. The carbon dam at the lower end of the body retards carbon accumulation in the cylinder head bore.

Fuel, under pressure, flows through the inlet, around the valve, filling the nozzle body below the valve guide. When the fuel pressure, acting against the valve, overcomes spring force, the valve lifts off its seat. Fuel under high pressure sprays through four 0.012 in. (0,30 mm) orifices into the cylinder. When delivery to the nozzle ceases and pressure drops to the predetermined closing pressure, the spring returns the valve to its seat.

During injection, the excess fuel drains through a controlled clearance at the valve guide. Lubricating all moving parts. The excess fuel flows through a bleed boot at the top of the nozzle body and returns to the fuel tank.

Hydraulic Governor

The hydraulic governor maintains engine speed at the RPM selected by position of the accelerator pedal.

When the engine is operating, the balance between the centrifugal force of revolving weights and the force of the spring, control the movements of the valve. The valve directs pressure oil to either side of the rack-positioning piston. Depending upon the position of the valve, the rack moves to increase or decrease the amount of fuel to the engine to compensate for load variation.

Pressurized lubricating oil, directed through passages in the fuel injection pump housing, enters a passage to the governor cylinder. The oil encircles a sleeve within the cylinder. The oil is then directed through a passage in the piston to the valve.

As the engine load increases, the revolving weights slow down. The weights move toward each other causing the governor spring to move the valve forward. As the valve moves, a small passage in the piston begins to open to pressure oil. The pressure oil flows through this passage and to the chamber behind the piston. This pressure forces the piston and rack forward, increasing the amount of fuel going to the engine. Engine RPM increases until the revolving weights are rotating fast enough to balance the forces of the governor spring and weights.

As the engine load decreases, the revolving weights speed-up and the toes on the weights force the valve rearward, and the oil behind the piston flows through the drain passage into the valve. At the same time, the pressure oil between the sleeve and the piston forces the piston and rack rearward. Engine RPM will decrease until the force of the revolving weights balance the force of the governor spring.

At engine shut-down, a low idle spring returns the valve guide and valve to the full load position. This moves the rack to full travel position, and assures full fuel flow through the fuel injection pump at engine start-up.

A finger spring in the mid-range spring assembly allows for governor regulation between low idle and full load operation.