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Sleeve Metering Fuel System

Sleeve Metering Fuel System

The sleeve metering fuel system (fig. 5-9) was designed to have the following seven advantages

  1. To have fewer moving parts and fewer total parts.
  2. Simple design with compactness.
  3. It can use a simple mechanical governor. No hydraulic assist required.
  4. The injection pump housing is filled with fuel oil, rather than crankcase oil for lubrication of all internal parts.
  5. The plunger, barrel, and sleeve design used in all Caterpillar sleeve metering units follows a common style.
  6. The transfer pump, governor, and injection pump are mounted in one unit.
  7. Uses a centrifugal timing advance for better fuel economy and easier starts.

The term sleeve metering comes from the method used to meter the amount of fuel sent to the cylinders—a sleeve system (fig. 5-10). Rather than rotate the plungers to control the amount of fuel to be injected, like most pump and nozzle injection systems, the use of a sleeve is incorporated with the plunger. The sleeve blocks a spill port that is drilled into the plunger. The amount of plunger travel with its port blocked determines the amount of fuel to be injected. Basic operation is as follows:

  1. Fuel is drawn from the fuel tank by the transfer pump through the fuel/water separator and the primary and secondary filters.
  2. Fuel from the transfer pump fills the injection pump housing at approximately 30 to 35 psi with the engine operating under full load. Any pressure in excess of this will be directed back to the inlet side of the transfer pump by the bypass valve. A constant-bleed valve is also used to allow a continuous return of fuel back to the tank at a rate of approximately 9 gallons per hour, so the temperature of the fuel stays cool for lubrication purposes and assist in maintaining housing pressure.
  3. Since the injection pump is constantly filled with diesel from the transfer pump under pressure, any time the fill port is uncovered, the internal drilling of the plunger will be primed by the incoming fuel caused by the downward moving plunger relative to pump camshaft rotation (fig. 5-11).
  4. At the correct moment, the rotation of the pump cam lobe begins to force the plunger upward until the fill port is closed, as it passes into the barrel. At the same time the sleeve closes the spill port. The pump, line, and fuel valves are subjected to a buildup in fuel pressure and injection will begin (fig. 5-11).
  5. Injection of the fuel will continue as long as both the fill port and spill ports are completely cover by the barrel and sleeve (fig. 5-11).
  6. Injection ends the moment that the spill port starts to edge above the sleeve, releasing the pressure in the plunger and letting fuel escape from the pump back into the housing. Also, at the end of the stroke, the check valve closes to prevent the fuel from flowing back from the injector fuel line (fig. 5-11).

To increase the amount of fuel injected, raise the sleeve through the control shaft and fork so that the sleeve is effectively positioned higher up on the plunger. This means that the spill port will be closed for a longer period of time, as the cam lobe is raising the plunger. Increasing the effective stroke of the plunger (time that both ports are closed) will increase the amount of fuel delivered.

NOTE

For procedures on removing, replacing, and servicing the injection pumps in a sleeve metering fuel system, refer to the manufacturer’s service manual.

 

GOVERNOR ACTION.—The governor on a Caterpillar sleeve metering fuel system is a mechanical governor and acts throughout the entire speed range of the engine. The majority of the sleeve metering fuel system uses three springs-a low-idle (inner) spring, a high-idle (outer) spring, and a dashpot spring. When the operator requires more power from the engine, he/she steps on the throttle. This causes the governor control lever to apply pressure that compresses the governor spring and to transfer this motion to the thrust collar. Since governor action from the spring and weight motion is of the back and forth variety, an additional linkage between the injection pumps and the governor transforms this sliding horizontal governor movement from the thrust collar into a rolling motion at the sleeve control shaft. A simple connecting lever commonly known as a bell crank lever accomplishes this action.

The bell crank lever contacts the thrust collar on one end and the governor sleeve control shaft on the other end. The bell crank pivots on a fixed vertical bell crankshaft to gain mechanical advantage through the lever principle. At the sleeve shaft end, it rides in a ball-and-socket joint that holds it in place and minimizes linkage movement. Therefore, any horizontal movement at the governor weight shaft and spring will cause an equally precise movement at the ball-and-socket joint, leading to reposition of the sleeves. If, in this case, the operator has increased the throttle position, the sleeves would be lifted, thereby covering the spill port for a longer overall effective plunger stroke.

As with any mechanical governor, an increase in either the throttle position or load will cause a speed change to the engine. Spring pressure is always trying to increase the fuel delivered to the engine, while centrifugal force of the rotating weights is always trying to decrease the amount of fuel going to the engine. Somewhere within the throttle range, however, a state of balance between these two opposing forces will exist as long as the engine speed is capable of overcoming the load placed on it to keep the spring and weight force in a state of balance.

When the engine is stopped, the action of the governor spring force places the thrust collar and the sleeve control shaft to the full-fuel position; therefore, easier starting is accomplished Once the operator cranks and starts the engine, centrifugal force will cause the flyweights to move outward, which now opposes the spring force, and the thrust collar and spring seat will come together, as they are pushed to a decreased fuel position. When the force of the weights equals the preset force of the spring established by the idle adjusting screw, these forces will be in a state of balance, and the engine will run at a steady idle speed with the throttle at a normal idle position.

Governor action will operate from idle throughout the speed range of the engine. A load stop pin controls the maximum speed of the engine. Rotation of the throttle lever causes the load stop lever to lift the load stop pin until it comes in contact with the stop bar or screw, thereby limiting any more fuel to the engine.

The purpose of the dashpot governor spring is to prevent any surging or irregular speed regulation of the engine by the fact that the piston either pulls fuel into or pushes fuel out of its cylinder through an orifice. The dashpot governor spring force varies with the piston movement, and as the engine load is increased or decreased, fuel is drawn into the piston cylinder through the orifice. This action gives the effect of a high governor spring rate that minimizes speed variations through oscillation during load changes of the engine. At any time the ignition switch is turned off or the governor speed control lever is moved to the OFF.position, the sleeve levers move the sleeves down, cutting off fuel to the cylinders.

NOTE

Any and all adjustments to the governor and governor controls should be made according to the manufacturer’s manual and specifications.

AUTOMATIC TIMING ADVANCE UNIT.— All current Caterpillar engines use some form of automatic timing for the fuel injection pump. On sleeve metering injection systems, this advance is mounted on the front end of the camshaft of the engine. The gear of the automatic advance unit meshes with and drives the fuel injection pump camshaft. The principal parts of the advance unit are the slides, the springs, and the weights.

Operation of the automatic advance-timing unit is as follows:

  • The slides are located and driven by two dowels, attached to the engine camshaft gear. The slides, in turn, fit into notches within the weights, thereby transferring their drive from the engine camshaft gear to the weights.
  • With the engine running, centrifugal force exerted by the rotating weight assemblies cause them to act against the force of the springs.
  • Since the weights are designed with notches in them, as they move outward under centrifugal force, they cause the slides to effect a change in the angle between the timing advance gear and the two drive dowels of the engine camshaft.
  • This relative movement of the timing advance unit gear will, therefore, automatically advance or retard the timing of the fuel injection pump in relation to the engine speed and load.

However, built into the advance unit is a maximum timing variation of 5 degrees with the timing change starting at approximately low idle rpm and continuing on up to the rated speed of the engine; therefore, you cannot adjust the automatic timing advance unit. The timing unit is lubricated by engine oil under pressure from drilled holes at the engine camshaft front bearing.

SCROLL METERING FUEL SYSTEM.— The scroll metering fuel system is similar to the sleeve metering fuel system in that it uses a plunger and barrel to create high pressure for injection. This system was designed to create higher injection pressure on direct-injection engines, offering an approximate 10 percent fuel economy improvement over precombustion-type engines, along with the ability to meet long-term EPA exhaust emissions regulations and better overall engine performance, as well as the ability to provide greater part commonality between different series engines.

In a scroll system two helix cut ports are used—the bypass closed port and the spill port Fuel is supplied from the transfer pump to an internal fuel manifold in the injection pump housing at approximately 35 psi. When the pump plunger is at the bottom of its stroke, fuel at transfer pump pressure flows around the pump barrel and to both the bypass closed port and spill port, which are both open at this time to allow fuel to flow into the barrel area above the plunger. The pump plunger is moved up and down by the action of a roller lifter, riding on the injection pump camshaft, which rotates at one-half of engine speed. As the injection pump camshaft rotates and the plungers rises, some fuel will be pushed back out of the bypass closed port until the top of the plunger eventually closes both the bypass closed port and the spill port. Further plunger movement will cause an increase in the trapped fuel pressure, and at approximately 100 psi, a check valve will open and fuel will flow into the injection line to the injection nozzle.

The fuel pressure of 100 psi is not enough to open the injection nozzle, which has an opening pressure of between 1,200 and 2,350 psi for a 3300 series engine and between 2,400 and 3,100 psi on 3406 engines. However, as the plunger continues to move up in its barrel, this fuel pressure is reached very quickly.

A high-pressure bleed-back passage and groove machined around the barrel are in alignment during the effective stroke to bleed off any fuel that leaks between the plunger and the barrel for lubrication purposes.

When the upward moving plunger uncovers the spill port, injection ceases, and although the plunger can still travel up some more, this is simply to allow most of the warm fuel (due to being pressurized) to spill back into the manifold. As the plunger moves downward in the barrel, it will once again uncover the bypass closed port and cool fuel will fill the area above the plunger for the next injection. When the spill port is opened, pressure inside the barrel is released and the check valve is seated by its spring.

Within the check valve assembly is a reverse flow check valve that opens when fuel pressure in the injection line remains above 1,000 psi and closes as soon as the fuel pressure drops to 1,000 psi. This will keep the fuel lines filled with fuel at 1,000 psi and ready for the next injection. This provides for a consistent and smooth engine power curve.

TRANSFER PUMP.—With the introduction of the scroll metering fuel system, the gear-type fuel transfer pump that had been used for years by Caterpillar was superseded by the use of a piston-type transfer pump. Current scroll metering fuel systems use a single-piston, double-acting pump with three one-way check valves.

The transfer pump is bolted to the low side of the injection pump housing. It is capable of delivering up to 51 gallons of fuel per hour at 25 psi. There is no need for a relief valve in this transfer pump due to the fact that maximum pressure is controlled automatically by the force of the piston return spring.

The transfer pump is activated by an eccentric (a device that converts rotary motion into reciprocating motion) on the injection pump camshaft, causing the pushrod to move in and out, as the engine is running. This action causes the piston to move down against the force of the piston return spring inside the transfer pump housing. The downward movement of the piston will cause the inlet check valve and the outlet check valve to close, while allowing the pumping check valve to open to allow fuel below the piston to flow into the area immediately above the downward piston.

As the injection pump camshaft eccentric rotates around to its low point, the transfer pump spring pushes the piston up inside its bore, causing the pumping check valve to close, and both the out and inlet valves are forced open. Fuel above the piston will be forced through the outlet check valve and the pump outlet port at approximately 35 psi. As this occurs, fuel will also flow through the pump inlet port and the inlet check valve to fill the area below the piston and the pump will repeat the cycle.

GOVERNOR.—The governor assembly used with the scroll metering fuel system is a hydra-mechanical servo-type unit. The reason for using a servo-valve is to provide a "boost" to the governor.

Without the servo-valve, both the governor spring and flyweights would have to be very large and heavy. With the use of the servo assist, little force is required to move both the accelerator and the governor control lever.

Basically, the governor assembly consists of three separate components:

  1. The mechanical components of the governor, such as the weights, springs, and linkage.
  2. The governor servo that provides hydraulic assistance through the use of pressurized engine oil to provide rapid throttle response and to reduce overall size requirement of the flyweights and springs.
  3. The dashpot assembly that is designed to provide stability to the governor during rapid load/throttle changes.

FUEL INJECTOR NOZZLE.—The fuel injector nozzle, used with the scroll metering fuel system, is a multiple-hole design, inward-opening, non-leakoff type. There are minor changes between the earlier nozzles and current models. Older nozzles are identified by the use of a color-coded black or blue washer, while the newer ones use a copper washer.

The nozzle is a multiple-hole design since it is used in direct injection engines only. The number and size of the holes will vary between different series of engines. For example, the 3306 engine nozzle uses a nine-hole tip, while the nozzle in the 3406 uses a six-hole tip. These different nozzles cannot be intermixed in the same engine or switched from one series engine to another.

The nozzle is designed for injection pressures of 15,000 psi and short injection duration to prevent a loss in fuel economy due to stringent EPA emission requirements. The nozzle incorporates a carbon dam on the lower end of the pencil part of the body and a seal washer on the upper end. The carbon dam prevents carbon blow-by into the nozzle bore in the cylinder head, while the upper seal prevents compression leakage from the cylinder. Injector nozzle operation is as follows:

  • The nozzle receives high-pressure fuel from the fuel pump through the inlet passage and filter screen and into the fuel passage.
  • When fuel pressure is high enough, the injector valve is lifted against the force of the return spring and fuel is injected through the multiple holes in the spray tip. This causes an increase in fuel pressure and the fuel to be finely atomized spray for penetration of the compressed air in the combustion chamber.
  • When fuel pressure drops below injection pressure, the return spring closes the fuel valve.

NOTE

For information on the removal and repair of the fuel injector nozzle, consult the manufacturer’s service manual.

Figure 5-9.—Sleeve metering fuel pump assembly.

Figure 5-10.—Sleeve metering barrel and plunger assembly.

Figure 5-11.—Injection pump operating cycle.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
All rights reserved