Automatic Timing Advance Unit

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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,