TUBE 3090802 - Caterpillar

Fuel Ratio Control
Sealed Unit
Fuel Ratio Control (Sealed Unit)
(1) Governor (2) Fuel ratio controlThe fuel ratio control (FRC) restricts the amount of fuel to the combustion chambers until sufficient boost has been achieved to ensure clean combustion on turbocharged engines. Unlike the earlier FRC designs, this control is a sealed unit. Individual parts of the unit are not serviceable.Earlier FRC designs required a static lever setting adjustment inside the governor housing. The sealed fuel ratio control is used with a lever assembly that is activated by oil pressure. No static lever setting is required with the sealed FRC.
Fuel Ratio Control and Governor
(3) Port (4) Air chamber (5) Diaphragm (6) Retainer assembly (7) Rod (8) Lever assembly (9) Springs (10) Shaft (11) Lever assembly (12) Servo valve (13) Tangs (14) SpringAt engine startup, the governor causes servo valve (12) and the fuel rack to move in the FUEL ON direction (to the left). [Refer to subject Governor in the Service Manual for operation of servo valve (12) and the fuel rack.] As servo valve (12) moves left, if forces lever assembly (11) to rotate clockwise against the light tension of spring (14).After engine startup, servo valve (12) travels right to an idle rack position. As the servo valve moves, spring (14) causes lever assembly (11) to rotate counterclockwise until a crossdrilled hole in the sleeve of lever assembly (11) aligns with piston cavity ports in lever assembly (8). Engine lube oil, feeding from the governor housing into the center passage of shaft (10), now travels through a crossdrilled hole in the shaft to enter a piston cavity in lever assembly (8). The pressure of the oil against the piston causes lever assembly (8) to slide sideways and compress spring (14). Tangs (13) of lever assemblies (8) & (11) are now overlapped, restricting clockwise rotation of lever assembly (11). With lever assembly (11) restricted, the FUEL ON travel of servo valve (12) is also restricted.During engine operation, any increase in intake manifold boost pressure is sent to chamber (4) through port (3). When boost is sufficient, diaphragm (5) forces retainer assembly (6) and rod (7) to the right. Lever assemblies (8 & 11) now rotate clockwise, allowing servo valve (12) to move left for increased fuel.When boost pressure drops, springs (9) return retainer (6) and rod (7) to normal position. FUEL ON movement of the servo valve is again restricted until sufficient boost is achieved.When the engine is shut off, oil pressure is released from the piston cavity of lever assembly (8). Spring (14) forces lever assembly (8) away from lever assembly (11). Lever assembly (11) is no longer restricted. The fuel ratio control is ready for the next engine startup. A variation of this design is used for engine applications where unrestricted rack travel is NOT desirable for startup. In those applications, spring (14) is removed and lever assembly (8) is modified so that lever tangs (13) are always overlapped. Servo valve and rack travel at startup are limited