DIRECT INJECTION UNIT INJECTOR FUEL SYSTEM - caterpillar fuel system

DIRECT INJECTION UNIT INJECTOR FUEL SYSTEM
The 3500 Series Engines use the direct injection combustion system. This system has the advantages of: low heat rejection (in comparison to pre-combustion); low fuel consumption; and easy starting.

UNIT INJECTOR AND CONTROL LINKAGE
A fuel injector (7) is in a central bore of each cylinder head. The position of the rack (6) of each injector is changed by a bellcrank and bracket (5) that is held to the top of the cylinder head by bolts. Each bellcrank
is moved by a control rod (4) connected to a hollow torsion shaft (1) through a lever (3).

Rotation of the torsion shaft (1) is done by the governor input shaft (10) and causes in and our movement of the rack (6).
The torsion shafts (1 and 8) are just below the camshafts of each bank of cylinders. A hollow cross shaft (9) at the front of the engine connects the right torsion shaft (1) and left torsion shaft (8) so they move together at the same time.

The control rods (4) have a “click” screw adjustment (11) at the bellcrank ends. There is one adjustment screw for each rack. This adjustment is used to synchronize all racks together. The adjustment sets the racks of the separate unit injectors so that they have the same reference position.
Also, there is a spring in the top end of the control rod. If one unit injector plunger will not turn (is STUCK) or the rack of that unit injector will not move, the control rod can still control the racks of the other injectors. This will prevent engine overspeed and the engine can be stopped. This design characteristic is for protection of the engine from damage.
Another protection for the engine: If the control linkage becomes disconnected from the governor, the WEIGHT of the control linkage can move the racks of the unit injectors to the fuel OFF position. The engine will STOP.
The torsion shafts (1 and 8) are marked with red and green colors on the inside diameter for assembly identification purposes. The left torsion shaft (8) has red and the right has green

In the inset we can see the power pad. The power pad has the power setting screw cover. The power setting screw cover has two bolts. The top bolt is the synchronizing pin and fastens the power setting screw cover. The bottom bolt also holds the cover on. With the cover removed, we can see the power setting screw and locknut.
The hole to the right of the power screw is where the collet and dial indicator is installed for measurement and adjustment of the power setting. The hole to the left of the power setting screw is for the synchronizing pin (the top bolt). This pin is used to put the fuel control linkage in the reference (fixed) position, when the synchronizing adjustment is made to the unit injectors.

NOTE: This illustration is not correct. The seal goes through the cover bolt, not the synchronizing pin.

In the highlighted area we see the power setting screw.
The power setting screw makes contact with the fuel stop lever. Adjustment of the power setting screw controls the maximum power setting of the engine. It controls the maximum movement of the control linkage and all injector racks.
Above the highlighted area is the governor lever which is connected to the governor output shaft. The governor lever, the fuel stop lever, the front end of the right torsion shaft and the power setting screw are in the front gear housing behind the power pad.

In this slide we see the fuel control linkage operation from the front of the engine. When the speed control of the governor is moved toward maximum rpm, the governor output shaft (black arrow) turns clockwise and moves the governor shaft lever to the left. A pin in the governor shaft lever is in the groove in the fuel stop lever and moves it to the left. The fuel stop lever turns the right torsion shaft counterclockwise as seen by the arrow. This counterclockwise movement moves the control rod up. This movement pivots the bellcrank and pulls the rack out of the injector in the fuel “ON” direction. The right and left torsion shafts always move together.
The ends of the shafts are connected to the ends of the cross shaft by a fork lever-ball lever arrangement (to understand better, see iron later).

This is the cross shaft on the front of the engine. It connects the right torsion shaft to the left torsion shaft.

The front housing is removed.

This shows the end of the right torsion shaft and cross shaft. You can also see the connection between the fork lever on the torsion shaft and ball lever on the cross shaft. This connection has a smalltolerance.
We can also see the fuel stop lever.

This shows the end of the torsion shaft and the left end of the cross shaft. The connection of the ball lever on the cross shaft with the fork lever on the torsion shaft can be seen.

The front housing is installed in this view.

Here we see inside the camshaft compartment. The camshaft is above. The torsion shaft of the fuel control linkage is below. The control rod of the torsion shaft (center) goes up to the bellcrank assembly of the unit injector.

This shows the rear end of the right torsion shaft. The support bracket for the shaft can be seen.

Also with the front housing removed, we can see the fuel stop lever clearly. With the front housing installed. . .

. . . we can see the power setting screw in contact with the fuel stop lever.
This also shows the notch in the fuel stop lever which connects with the pin of the governor lever.
We will learn about the power setting adjustment later; however, you can see how the power setting screw controls the position of the fuel stop lever by stopping its movement.

UNIT INJECTOR AND OPERATION
In this schematic we can see the injection components of this fuel system.
The components are:
1. injector cam lobe of engine camshaft;
2. a push rod
2a. a lifter assembly
3. an injector rocker arm
4. an injector clamp
5. a unit injector
6. a section of cylinder head; and
7. a piston in a cylinder

In this schematic we can see the control components of this fuel system.
The components are:
8. control lever on torsion shaft
9. control rod
10. bellcrank
11. injector rack
12. injector plunger

Here we see two unit injectors. The one on the left has been cut away for instructional purposes. The injector on the right is complete.
This slide shows the:
1. injector body
2. follower
3. follower return spring
4. rack
5. injector housing (nut); and the
6. injector nozzle (spray tip).

This is the unit injector designed and manufactured by Caterpillar. It is being used for current production marine engines. Remanufactured nozzles will be available. Service tools (to be announced at a later date) will permit some field service to be done on these nozzles. This injector has a removable cone on the end and a trim screw for bench calibration.

The injection of fuel is made by the rotation of the engine camshaft which causes the cam to lift the lifter assembly and push the rod up. When the push rod moves the injector rocker arm up, the contact of the rocker arm pushes the follower an injector plunger down. As the plunger moves down, fuel is injected into the combustion chamber. As the lower scroll on the plunger goes beyond the lower port, injection stops. When the rocker arm stops its downward movement, the follower return spring pushes the follower up with the plunger. The follower return spring also keeps a force on the rocker arm push rod and lifter. This force keeps the lifter in contact with the cam.

Looking at the cutaway of the injector, we can see the:
1. plunger
2. barrel
3. lower port
4. upper port; and the
5. spill deflector
The plunger position shown is at the top of the stroke.
The barrel (2) has an upper port (4) and a lower port (3). The relation of the scrolls to the ports:
(1) changes the length of the effective stoke and the quantity of fuel per injection stroke;

(2) permits the start of the effective stroke to be variable in relation to piston position.

The smaller the quantity (VOLUME) of fuel injected during the injection stroke, the later (NEARER TO TOP CENTER) injection takes place.

The larger the quantity (VOLUME) of fuel injected during the injection stroke, the earlier (FARTHER FROM TOP CENTER) injection takes place.
Movement of the control linkage and rack turns the plunger and changes the quantity of fuel injected and the point at which injection starts.
The action of the double scroll is a method of timing advance.
Older injectors are double scroll. Newer injectors aresingle scroll.

Let us look at the nozzle of the injector. We see the:
1. check valve
2. check valve cage
3. valve spring and seat
4. spring cage
5. needle valve
6. spray tip; and the
7. injector housing or nut
The spray tip has several small orifices. Each nozzle has two dowels in the body which puts it in the correct position when installed. This position puts the rack in the correct location with the bell crank and the spray tip at the correct angle with the surface of the piston.
The plunger position shown here is on the downward stroke and the lower port is just closed.

The plunger position shown here is the start of the injection stroke. Both the lower port and the upper port are closed. It is at the start of the effective stroke.
During the effective stroke, the plunger forces fuel into the nozzle of the injector. The fuel goes around the check valve and through passages in the check valve cage. After fuel goes through the valve spring cage, it goes into the passages in the spray tip. The passages sends the fuel to the chamber around the needle valve. Here the fuel pressure lifts the needle valve off the seat and fuel flows through the spray tip and out the orifices into the combustion chamber. Injection of fuel continues until the lower scroll on the plunger goes by the lower port, the pressure of the fuel against the needle valve is less.

The valve spring pushes the needle valve closed. This stops the flow of fuel into the combustion chamber. Also, when the fuel is released through the lower port, the fuel pressure above the check valve decreases. The fuel pressure in the tip chamber then pushes the check valve up against the end of the barrel. With the needle valve on the seat and the check valve against the end of the barrel, combustion gases cannot get into the injector and cause damage between injection strokes.

NOTE: If the needle valve is held open by foreign particles between injection cycles,
combustion gases can come into the injector and cause damage.