A gear pump is located at the rear of the fuel pump assembly. It is driven by the fuel pump main shaft at engine speed. Fuel from the tank enters the inlet or suction side of the gear pump and is carried around the outside of the two meshing gears to the outlet or pressure side of the gear pump.
A pulsation damper is connected to the pressure side of the gear pump to smooth the pressure pulsations created in the fuel by the rotating gear teeth. From the gear pump, fuel flows to the wire mesh magnetic filter.
From the filter, fuel flows to the inlet or supply passage of the automotive governor. The governor is a speed sensitive device that automatically controls or limits the engine speed. It does this by varying the fuel delivered to the engine under changing operating conditions.
Governor Assembly Functions and Components
The governor assembly performs three functions: pressure regulation, idle speed governing, and maximum speed governing. The makeup of the governor assembly includes components such as the weight assist spring, shims, governor plunger, weight carrier, flyweight, control spring, idle passage, supply passage, bypass passage, idle spring, and adjusting screws.
The flyweights are driven through gears by the fuel pump main shaft. The governor plunger is held between the flyweight feet and rotates with the flyweights. Any rotation creates a tendency for the flyweights to move away from their axis of rotation, which is the center line of the weight carrier shaft. This outward push is known as centrifugal force.
Governor Plunger Position and Torque Control
The rotating flyweights pivot on the flyweight pins and exert an axial force on the governor plunger. The position of the governor plunger is determined by the balance between the flyweight force on one end and the spring force applied to the opposite end of the plunger.
Torque depends on the quantity of fuel metered and injected per cycle. In the PT fuel system, torque is dependent on rail pressure. The control of rail pressure starts by regulating the fuel pressure supplied to the governor assembly, called supply pressure.
Supply Pressure Regulation
Supply pressure is controlled by a bypass type pressure regulator designed to unseat when a set supply pressure is reached. Excess fuel must be delivered to the governor assembly to allow some fuel to bypass, enabling pressure control.
When the fuel pressure on the button exceeds the force holding the button and plunger together, the button unseats and fuel is bypassed to the suction side of the gear pump. The pressure at which this happens is determined by the spring force divided by the recessed area of the button.
Changing the recessed area of the button changes the supply pressure curve height. Increasing speed changes plunger position, which alters the force on the button and supply pressure.
Engine Speed Governing
The automotive governor controls idle, govern, and maximum speed by positioning the governor plunger cutoff shoulder over the appropriate fuel passage on Cummins KTA19. At speeds near idle, flyweight force opposes the idle spring; near torque peak speed, it opposes the governor spring.
Between torque peak and rated speed, flyweight force balances the combined governor and torque springs. Once torque spring engages, it reduces flyweight force, affecting plunger position, and thus fuel delivery.
Throttle and Fuel Flow Control
The throttle shaft located between the governor and the fuel pump discharge allows the operator to reduce rail pressure and power. The throttle shaft has stop screws limiting travel and a fuel adjusting screw for maximum flow calibration.
When closed, a small amount of fuel (throttle leakage) flows through the throttle shaft to keep lines filled and lubricate injectors. Idle fuel flow comes from the governor through the idle passage around the throttle shaft.
Air Fuel Control Assembly
Fuel from the throttle shaft flows to the AFC section, which adjusts fuel pressure during acceleration on turbocharged engines to control black smoke. The AFC assembly includes components like the cover piston, diaphragm, plunger, spring retainer, and barrel spring.
Air pressure from the intake manifold acts on the diaphragm and plunger, opening fuel passages as pressure increases, removing fuel restrictions.
Dual Spring AFC Assembly
New PT fuel pumps use dual spring AFC assemblies that offer easier calibration, tamper resistance, and more repeatable settings. They come in standard and shaped port types, with the shaped port improving torque control and cold start response.
Shutdown Valve Function
The shutdown valve, controlled by a solenoid, prevents fuel flow when de-energized by seating a disc with a spring washer. Energizing the solenoid unseats the disc, allowing fuel flow to injectors.
Injector Operation
The injector plunger, actuated by the camshaft lobe, moves up and down to open and close the metering orifice, controlling fuel injection timing and volume. Fuel passes through a filter screen and adjustable orifice, with a check ball preventing reverse flow.
Metering occurs while the cam follower roller is on the inner base circle. Injection ends when the plunger bottoms in the injector cup and the cam roller passes over the cam nose.
Fuel returning from the injector flows through drain passages back to the tank.