NUT-WELD PILOT 3794141 - Caterpillar

Figure 3-6a. Schematic Diagram, Fuel Limiter and LinkageWhen the fuel limit lever contacts the fuel-limit nut on the shutdown bushing, it begins lifting the shutdown rod to recenter the governor pilot-valve plunger. The upward movements of the fuel-limit and feedback levers continue until the left end of the feedback lever raises far enough to recenter the amplifier-pilot-valve plunger and stop the flow of oil to the amplifier piston. At this point, the fuel-limit lever recenters the governor pilot-valve plunger, stopping the upward movement of the governor power piston. This limits the amount of fuel to provide a proper fuel/air ratio for efficient burning. Although the governor flyweights are in an underspeed condition at this time, the power piston remains stationary until manifold air pressure rises.As engine speed and load increase, manifold air pressure rises after a short time lag. The increase in manifold air pressure produces a proportionate increase in the sensing-bellows force. The bellows force, now greater than the restoring-spring force, causes the bleed-valve diaphragm to move further off its seat. This allows a greater flow of oil to sump than is admitted through the orifice pack. Governor oil pressure acting on the upper side of the sensor piston forces the piston (and cam) downward and, in the process, further compresses the restoring spring. The piston continues its downward movement until the net increase in restoring-spring force equals the net increase in bellows force. This restores the bellows and bleed-valve diaphragm to their original positions. At this point, the outflow of oil is again equal to the inflow, and the piston stops moving.As the sensor piston and cam move downward in response to a rise in manifold air pressure, the bellcrank rotates in a cw direction. This allows the floating-lever pivot point, the left end of the lever, and in turn the hydraulic-amplifier pilot-valve plunger to rise.The loading spring under the pilot-valve plunger maintains a positive contact between the plunger, levers, bellcrank, and cam. When the pilot-valve plunger rises above center, the oil under the amplifier piston bleeds to sump through a drilled passage in the center of the plunger. The passage in the plunger restricts the rate of oil flow to sump and decreases the rate of movement of the amplifier piston to minimize hunting. As the amplifier piston moves downward, the left end of the fuel-limit lever also moves downward. This lowers the shutdown rod which in turn lowers the governor pilot-valve plunger and increases engine fuel.The above events occur in continuous and rapid sequence. Normal governor operation is overridden during an acceleration transient and engine fuel is scheduled as a function of manifold air pressure, regardless of governor speed setting. To prevent interference with normal governing action during steady-state operation, the sensor piston and cam continue their downward movement until sufficiently below the effective limiting point.Conversely, a drop in manifold air pressure rotates the bellcrank ccw. This lowers the fuel-limit lever, depressing the pilot-valve plunger, and releases pressured oil to the underside of the