3508, 3512 and 3516 Engines Caterpillar


Fuel System

Usage:

3508 5PW



Illustration 1g00322667

Fuel Flow Schematic (Typical Example)

(1) Fuel manifolds. (2) Fuel filter. (3) Fuel priming pump. (4) Fuel injectors. (5) Pressure regulating valve. (6) Fuel return to tank. (7) Primary fuel filter. (8) Fuel transfer pump. (9) Fuel line to filter. (10) Fuel supply line from the primary fuel filter. (11) Fuel line to priming pump.

Fuel transfer pump (8) is located on the right side of the engine. The lower shaft of the engine oil pump drives the gear type transfer pump. Fuel from the supply tank is pulled through primary fuel filter (7) by fuel transfer pump (8) and sent to fuel filter (2) .

Fuel transfer pump (8) has a check valve and a bypass valve. The check valve is located in the pump head assembly. The pump head assembly is located behind the connection for line (9). The check valve prevents fuel flow back through the transfer pump when fuel priming pump (3) is used. The bypass valve is located behind a cap (plug) in the drive end of the pump. The bypass valve limits the maximum pressure of the fuel. The bypass valve will open the outlet side of the pump to the pump inlet if the fuel pressure exceeds 860 kPa (125 psi). This process helps prevent damage to the fuel system components from too much pressure.




Illustration 2g00316127

Right Front Of Engine (Typical Example)

(1) Fuel manifolds. (2) Fuel filter. (8) Fuel transfer pump. (9) Fuel line to filter. (10) Fuel supply line from the primary fuel filter. (11) Fuel line to the priming pump.

The transfer pump pushes fuel through the fuel filter (2) on the front of the engine to fuel manifolds (1). The fuel flows through the manifold to inlet fuel line (13) connected to the right side of each cylinder head. Filter screens are located in the ports of the unit injector. A drilled passage (14) in cylinder head (15) takes fuel to a circular chamber around the injector. The chamber is made by the O-rings on the outside diameter of fuel injector (4) and by the injector bore in the cylinder head.




Illustration 3g00322726

Cylinder Heads

(4) Injector. (12) Outlet fuel line. (13) Inlet fuel line.

Only part of the fuel in the chamber is used for injection. Approximately three to five times the amount of fuel that is needed for normal combustion flows through the chamber. This fuel then flows to a drilled passage in the left side of the cylinder head. This passage is connected by outlet fuel line (12) to the bottom section of the fuel manifold. This constant flow of fuel around the injectors helps to cool the injectors.




Illustration 4g00322727

Fuel Flow Through Injector

(4) Fuel injector. (12) Outlet fuel line. (13) Inlet fuel line. (14) Drilled passage. (15) Cylinder head. (16) Cylinder.

The fuel flows back through the fuel manifold. The fuel flows to pressure regulating valve (5) on the front of the right fuel manifold. The fuel flows through this valve. Fuel then flows back to the tank.

Check each engine installation for excess fuel flow. The excess fuel flow is based on the amount of fuel that is consumed. The minimum flow is three times the amount of fuel that is consumed. Excess fuel is then returned to the fuel tank. This will make sure that any air in the system will be removed before the fuel is sent back to the injectors.

Pressure regulating valve (5) has an arrangement of a spring and a plunger. This arrangement is between the bottom section of the fuel manifolds and the line that returns fuel to the tank. The valve keeps the pressure of the fuel between 415 to 450 kPa (60 to 65 psi).

Fuel Transfer Pump




Illustration 5g00307484

(1) Check valve. (2) Bypass valve.

The fuel transfer pump is located on the right side of the engine. The lower shaft of the engine oil pump drives the gear type transfer pump. Fuel from the supply tank is pulled through the primary fuel filter by the fuel transfer pump and sent to the fuel filter housing.

The fuel transfer pump has a check valve (1) and a bypass valve (2). The check valve is located in the pump head assembly. The check valve prevents fuel flow back through the transfer pump when the fuel priming pump is used. The bypass valve is located behind a cap (plug) in the drive end of the pump. The bypass valve limits the maximum pressure of the fuel. The bypass valve will open the outlet side of the pump to the pump inlet if the fuel pressure exceeds 860 kPa (125 psi). This process helps prevent damage to the fuel system components from too much pressure.

Fuel Injection Control Linkage




Illustration 6g00322845

Fuel Injector Control Linkage

(1) Fuel injector. (2) Control shaft. (3) Bellcrank. (4) Control rod. (5) Rack. (6) Lever. (7) Governor shaft. (8) Control shaft. (9) Cross shaft.

A fuel injector (1) is located in each cylinder head. The position of rack (5) controls the amount of fuel that is injected into the cylinder. Pull the rack out of the injector for more fuel. Push the rack in the injector for less fuel. The rack position is changed by bellcrank (3). The bellcrank is moved by control rod (4). The control rods have an adjustment screw on the top. The adjustment screw is used to synchronize the injectors. The control rods are spring loaded. If the rack of one injector sticks, it will still be possible for the governor to control the other racks so the engine can be shutdown. Each control rod on the right side of the engine is connected by a lever (6) to control shaft (8). When the governor shaft (7) is viewed from the front of the engine and the governor shaft rotates in a clockwise direction, the action of the governor linkage moves control shaft (8) in a counterclockwise direction. This counterclockwise direction is direction of the fuel "ON".

Right control shaft (8) and left control shaft (2) are connected by cross shaft (9). The linkage between the injectors on the left side of the engine and control shaft (2) is similar to the linkage on the right side.

The weight of the control linkage will move the fuel injector racks to the "SHUTOFF" fuel position and the engine will stop, if the linkage becomes disconnected from the governor.

Fuel Injector




Illustration 7g00322846

Fuel Injector Operation

(1) Screw. (2) Rocker arm. (3) Clamp. (4) Control rod. (5) Rack. (6) Drilled passage. (7) Upper port. (8) Lower port. (9) Plunger. (10) Barrel. (11) Fuel passage. (12) Needle valve. (13) Lifter assembly. (14) Camshaft. (15) Piston.

The fuel injector is held in position by clamp (3). Fuel is injected when rocker arm (2) pushes the top of the fuel injector downward. The movement of the rocker arm is controlled by the camshaft through lifter assembly (13) and control rod (4). The amount of fuel that is injected is controlled by rack (5). Movement of the rack causes the rotation of a gear that is fastened to plunger (9). Rotation of the plunger changes the effective stroke of the plunger.

Injection timing is a product of two factors. The two factors include the angular location of camshaft (14) and the location of plunger (9). The angular location of the camshaft is controlled by the camshaft drive gears at the rear of the engine. The location of the plunger can be adjusted with screw (1) .

Fuel Injection Cycle

When the plunger is at the top of a stroke, fuel flows from the chamber for the fuel supply, around the injector, and through the lower ports and through the upper ports of barrel (10). As plunger (9) is moved downward by rocker arm (2), fuel is pushed back into the supply chamber through lower port (8). Fuel can also go through a drilled passage (6) in the center of the plunger, around the relief groove, and through upper port (7) of the barrel. As the lower port (8) is closed by the bottom of plunger (9), fuel can still flow through upper port (7) until the port is closed by the upper edge of the relief groove on plunger (9). At this point, injection starts and the effective stroke of the plunger begins.

During the effective stroke, fuel is injected into the cylinder until the downward movement of plunger (9) allows the scroll (helix) to open the lower port. This downward movement of plunger (9) also releases the fuel pressure. The amount of fuel that is injected during the effective stroke is determined by the position of the scroll in relation to the lower port.

Fuel goes through the center passage (6) of the plunger and fuel goes through the lower port (8) during the remainder of the downstroke. As the lower port is opened, the sudden release of very high pressure causes the fuel to hit the deflector for spills with a high force. The deflector for spills gives protection to the injector housing from erosion because of the force of the released fuel. On the return stroke, the injector barrel is filled with fuel again from the fuel supply chamber.

The plunger can be turned by rack (5) while the plunger is moved upward and the plunger is moved downward by rocker arm (2). The upper part of the plunger has a flat side that fits through the gear. The gear is engaged with the rack. The plunger slides upward and the plunger slides downward in the gear. As the rack moves, the gear and the plunger rotate together. This rotation of plunger (9) controls the fuel output of the injector. The rotation of the plunger changes the relation of the scroll to the lower port in the barrel. The rotation of the plunger also increases or the rotation decreases the length of the effective stroke for injection. The scroll can set the amount of fuel per injection stroke. Therefore, the fuel rate to the engine can be controlled in relation to different engine loads.

No injection takes place during the downstroke of the plunger when rack (5) is moved all the way against the injector body. This position is the fuel "shutoff". Fuel injection begins when the rack is moved outward by a small distance. As the rack continues to move outward, the amount of fuel that is injected into the cylinder is increased until the maximum fuel position is reached.

During the fuel injection stroke, fuel passes from the barrel chamber through a valve assembly. The valve assembly has a spring-loaded needle valve (12) and fuel flows through fuel passages (11) around the needle valve to the valve chamber. The fuel pressure lifts the needle valve off the seat. The fuel can now flow through the orifices in the tip into the combustion chamber.

If needle valve (12) is held open between injection cycles by small debris, gases from combustion could enter the injector. The gases will cause damage. A flat check valve is used above the needle valve in order to keep these high pressure gases from combustion out of the injector. The injector operates with the flat check valve until the foreign particle has been washed away through the orifices by the fuel and normal operation again takes place.

The tip of the injector extends a short distance below the cylinder head into the combustion chamber. The tip has several small orifices that are evenly spaced around the outside diameter in order to spray fuel into the combustion chamber. The top surface of the piston (15) is designed with a shaped crater that causes the air to swirl. The mixture improves as the fuel is sprayed into the air that is swirling for more complete combustion.

2301 Electronic Governor System

The 2301 Electric Governor Control System consists of the components that follow:

  • 2301 Electric Governor Control (EGC)

  • Actuator

  • Magnetic Pickup




Illustration 8g00322848

2301 Electric Governor Control (EGC)

The 2301 Electric Governor System gives precision engine speed control. The 2301 Control measures the engine speed constantly. The 2301 Control makes the necessary corrections to the engine fuel setting through an actuator. The actuator is connected to the fuel system.




Illustration 9g00316125

Typical Magnetic Pickup Location

(1) Magnetic pickup.

The engine speed is detected by a magnetic pickup. This pickup makes an AC voltage that is sent to the 2301 Control. The 2301 Control now sends a DC voltage signal to the actuator.




Illustration 10g00322852

Front Of Engine (Typical Example)

(2) EG6P Actuator.

The actuator changes the electrical input from the 2301 Control to a mechanical output. The mechanical output is connected to the fuel system by a linkage. For example, if the engine speed is more than the speed setting, the 2301 Control will decrease the output. The actuator will now move the linkage in order to decrease the fuel to the engine.

Magnetic Pickup




Illustration 11g00285114

Schematic of Magnetic Pickup

The magnetic pickup is a permanent magnet generator with a single pole. The magnetic pickup is made of wire coils around a permanent magnet pole piece. As the teeth of the flywheel ring gear go through the magnetic lines of force around the pickup, an AC voltage is made. The ratio between the frequency at this voltage and the speed of the engine is directly proportional.

This engine speed frequency signal (AC) is sent to the 2301 Control Box. A conversion is made to DC voltage. The DC signal is sent in order to control the actuator. This voltage is inversely proportional to engine speed. Therefore, the voltage output to the actuator decreases if the engine speed increases. When engine speed decreases, the voltage output to the actuator increases.

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