1674 DIESEL TRUCK ENGINE Fuel System Caterpillar


Fuel System
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1.1. Fuel Injection Pump Operation
2.2. Fuel Pump
3.1. Governor Operation

The fuel system is a pressure type with a separate injection pump and injection valve for each cylinder. Fuel is injected into a precombustion chamber, not directly into the cylinder.

A transfer pump supplies fuel filtered through primary (optional) and final filters to a manifold in the injection pump housing.

The transfer pump supplies more fuel than is required for injection, so a bypass valve is built into the pressure side of the pump. The valve limits the maximum pressure within the supply system.

An air vent valve located on the fuel filter base permits the removal of air in the injection pump supply system after servicing the filter or whenever air is allowed to enter the system. Air is removed by opening the valve and pressurizing the fuel system. The priming pump. The vent valve must be open and the pump operated until a stream of fuel, without air bubbles, flows from the vent line.

Fuel Injection Pump Operation

Fuel enters the fuel injection pump housing from the fuel filter through the fuel manifold and enters the fuel injection pump through the inlet port. The injection pump plungers and lifters are lifted by the cam lobes on the fuel system's own camshaft and always make a full stroke. The lifters are held against the cam lobes by springs. Each pump measures the amount of fuel to be injected into its respective cylinder and delivers it to the fuel injection nozzle.

The amount of fuel pumped per stroke is varied by turning the plunger in the barrel. The plunger is turned by the governor action through the gear segmented sliding rack which turns the gear segment on the bottom of the pump plunger. The position of the scroll on the plunger determines the amount of fuel injected into the cylinders.

Fuel Pump

Figures A, B and C illustrate the functioning 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. The fuel is trapped and will be forced through a check valve, fuel line, and injection valve.

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

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

In Fig. D the plunger has been 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 has been rotated into the idling position. The narrow part of the plunger formed by the helix will cover the port for only a short part of the stroke. This permits only a small amount of fuel to be injected per stroke.

In Fig. F the plunger has been rotated into the full load position. The wide part of the plunger formed by the helix covers the port for a longer part of the stroke. This permits a larger amount of fuel to be injected per stroke.

Fuel, under high pressure from the injection pumps, is transferred through the injection lines to the injection valves. As high pressure fuel enters the nozzle assembly, the check valve within the nozzle opens and permits the fuel to spray into the precombustion chamber where it mixes with the compressed air.

The precombustion chamber serves several purposes: It provides an effective chamber for the mixing of fuel and air before it enters the cylinder for final combustion, thus permitting the use of a wider range of fuels. It permits the use of easy replaceable fuel nozzles and lends itself to the use of glow plugs to assist in cold weather starting.

These engines can be started in low temperatures with the use of glow plugs, which provide heat for initial ignition of the fuel in the precombustion chamber.

The nozzle assembly requires no adjustment and can be removed and a new one installed quickly and economically.

Governor Operation

The full range hydro-mechanical governor operates over the entire speed range. The governor compensates automatically for minor grades and eliminates the need for continuous accelerator correction to maintain a constant speed. This governor requires low accelerator pedal pressure and less driver effort. This hydro-mechanical governor allows the use of smaller flyweights for quick engine response.

When the engine is operating, the balance between the centrifugal force of revolving weights and the force of the accelerator pedal on the compressed spring, controls the movement of a valve and indirectly, the fuel rack. The valve directs pressure oil to either side of a rack positioning piston. Depending on the position of the valve, the rack is moved to increase or decrease the fuel to the engine to compensate for load variation.

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

When the engine load increases, the revolving weights slow down. The weights move toward each other and allow the governor spring to move the valve forward.

As the valve moves, an oil passage around the piston opens to pressure oil. The oil flows through this passage and fills the chamber behind the piston. The pressure forces the piston and rack forward, increasing the amount of fuel to the engine. Engine RPM increases until the revolving weights rotate fast enough to balance the force of the governor spring.

When the engine load decreases, the revolving weights speed-up and the toes on the weights move the valve left, allowing the oil behind the piston to flow through a drain passage opened at the rear of the piston. At the same time, the pressure oil between the sleeve and the piston forces the piston and rack to the left. This decreases the fuel to the engine and the engine slows down. When the force of the revolving weights balances the governor spring force, the RPM of the engine will be the same as before.

When the engine is started, a speed limiter plunger restricts the movement of the accelerator until the crankcase lubricating oil reaches operating pressure. The plunger in the speed limiter retracts when the operating oil pressure is reached, then the accelerator can be depressed.

The governor valve is shown in the position when the force of the weights and the force of the spring are balanced.

Oil from the engine lubricating system lubricates the governor and drains into the fuel injection pump housing.

A fuel-air ratio control is mounted on the end of the governor housing and is a device which restricts the movement of the fuel rack during acceleration. It coordinates the movement of the fuel rack with the amount of air available in the inlet manifold allowing the engine to burn the fuel injected into the engine under all operating conditions and thus will keep exhaust smoke to a minimum.

When the operator depresses the accelerator to increase engine speed, the fuel rack collar contacts a bolt. The bolt restricts the movement of the rack until the turbocharger boost of air pressure in the cover forces the diaphragm against spring pressure and moves the bolt head away from the collar. This will allow the fuel rack to move toward more fuel position as the intake manifold pressure increases with the increase in engine speed. The adjustment of the spring is such that a balance is maintained between increase of fuel, increase of load, increase of speed and increase of air for an efficient fuel-air combustion ratio and smoke-free operation.