Pressure-Time Fuel System

The pressure-time (PT) fuel system (fig. 5-28) is exclusive to Cummins diesel engines; it uses injectors that meter and injects the fuel with this metering based.on a pressure-time principle. A gear-driven positive displacement low-pressure fuel pump naturally supplies fuel pressure. The time for metering is determined by the interval that the metering orifice in the injector remains open. This interval is established and controlled by the engine speed, which determines the rate of camshaft rotation and consequently the injector plunger movement.

Since Cummins engines are all four-cycle, the camshaft is driven from the crankshaft gear at one-half of engine speed. The fuel pump turns at engine speed. ecause of this relationship, additional governing of fuel flow is necessary in the fuel pump.

A flyball type mechanical governor controls fuel pressure and engine torque throughout the entire operating range. It also controls the idling speed of the engine and prevents engine overspeeding in the high-speed range. The throttle shaft is simply a shaft with a hole; therefore, the alignment of this hole with the fuel passages determines pressure at the injectors.

A single low-pressure fuel line from the fuel pump serves all injectors; therefore, the pressure and the amount of metered fuel to each cylinder are equal.

The fuel-metering process in the IT fuel system has three main advantages:

1. The injector accomplishes all metering and injection functions.
2. The injector injects a finely atomized fuel spray into the combustion chamber at spray-in-pressures exceeding 20,000 psi.
3. A low-pressure common-rail system is used, with the pressure being developed in a gear-type pump. This eliminates the need for high-pressure fuel lines running from the fuel pump to each injector.

FUEL PUMP.—The fuel pump (fig. 5-29) commonly used in the pressure-time system is the PTG-AFC pump (PT pump with a governor and an air-fuel control attachment). The "P" in the name refers to the actual fuel pressure that is produced by the gear pump and maintained at the inlet to the injectors. The "T" refers to the fact that the actual "time" available for the fuel to flow into the injector assembly (cup) is determined by the engine speed as a function of the engine camshaft and injection train components.

The air-fuel control (AFC) is an acceleration exhaust smoke control device built internally into the pump body. The AFC unit is designed to restrict fuel flow in direct proportion to the air intake manifold pressure of the engine during acceleration, under load, and during lug-down conditions.

Within the pump assembly a fuel pump bypass button of varying sizes can be installed to control the maximum fuel delivery pressure of the gear-type pump before it opens and bypasses fuel back to the inlet side of the pump. In this way the horsepower setting of the engine can be altered fairly easily. The major functions of the PTG-AFC fuel pump assembly are as follows:

1. To pull and transfer fuel from the tank and filter
2. To develop sufficient fuel pressure to the fuel rail (common fuel passage) to all of the injectors
3. To provide engine idle speed control (governing)
4. To limit the maximum no-load and full-load speed of the engine (governing)
5. To allow the operator to control the throttle position and therefore the power output of the engine
6. To control exhaust smoke emissions to EPA specifications under all operating conditions
7. To allow shutdown of the engine when desired A major feature of the PT pump system is that there is no need to time the pump to the engine. The pump is designed simply to generate and supply a given flow rate at a specified pressure setting to the rail to all injectors. The injectors themselves are timed to ensure that the start of injection will occur at the right time for each cylinder.

The basic flow of fuel into and through the PT pump assembly will vary slightly depending on the actual model. A simplified fuel flow is as follows:

• As the operator cranks the engine, fuel is drawn from the fuel tank by the gear pump through the fuel supply line to the primary filter. This filter is normally a filter/water separator.
• The filter fuel then flows through a small filter screen that is located within the PT pump assembly, and then flows down into the internal governor sleeve.
• The position of the governor plunger determines the fuel flow through various governor plunger ports.
• The position of the mechanically operated throttle determines the amount of fuel that can flow through the throttle shaft.
• Fuel from the throttle shaft is then directed to the AFC needle valve.
• The position of the AFC control plunger within the AFC barrel determines how much throttle fuel can flow into and through the AFC unit and on to the engine fuel rail, which feeds the fuel rail.

The AFC plunger position is determined by the amount of turbocharger boost pressure in the intake manifold, which is piped through the air passage from the intake manifold to the AFC unit. At engine start-up, the boost pressure is very low; therefore, flow is limited. uel under pressure flows through the electric solenoid valve, which is energized by power from the ignition switch. This fuel then flows through the fuel rail pressure line and into the injectors.

A percentage of the fuel from both the PT pump and the injectors is routed back to the fuel tank in order to carry away some of the heat that was picked up cooling and lubricating the internal components of the pump and the injectors.

INJECTORS.—A PT injector is provided at each engine cylinder to spray the fuel into the combustion chambers. PT injectors are of the unit type and are operated mechanically by a plunger return spring and a rocker arm mechanism operating off the camshaft.

There are four phases of injector operation, which are as follows:

• Metering (fig. 5-30)—The plunger is just beginning to move downward and the engine is on the beginning of the compression stroke. The fuel is trapped in the cup, the check ball stops the fuel flowing backwards, and fuel begins to be pressurized. The excess fuel flows around the lower annular ring, up the barrel, and is trapped there.
• Pre-injection (fig. 5-30)—The plunger is almost all the way down, the engine is almost at the end of the compression stroke, and the fuel is being pressurized by the plunger.
• Injection (fig. 5-30)—The plunger is almost all the way down, the fuel injected out the eight orifices, and the engine is on the end of the compression stroke.
• Purging (fig. 5-30)—The plunger is all the way down, injection is complete, and the fuel is flowing into the injector, around the lower annular groove, up a drilled passageway in the barrel, around the upper annular groove, and out through the fuel drain. The cylinder is on the power stroke. During the exhaust stroke, the plunger moves up and waits to begin the cycle all over.

Injector adjustments are extremely important on PT injectors because they perform the dual functions of metering and injecting. Check the manufacturer’s manual for proper settings of injectors. On an engine where new or rebuilt injectors have been installed, initial adjustments can be made with the engine cold. lways readjust the injectors, using a torque wrench calibrated in inch-pounds after the engine has been warmed up. Engine oil temperature should read between 140°F and 160°F.

Anytime an injector is serviced, you must be certain that the correct orifices, plungers, and cups are used, as these can affect injection operation. You can also affect injection operation by any of the following actions:

• Improper timing.
• Mixing plungers and barrels during teardown (keep them together, since they are matched sets).
• Incorrect injector adjustments after installation or during tune-up adjustment.
• Installing an exchange set of injectors without taking time to check and correct other possible problems relating to injection operation. This is often overlooked.

Proper injector adjustment and maintenance will ensure a smooth running engine as long as the following factors are met:

1. Adequate fuel delivery pressure from the fuel pump to the fuel manifold.
2. Selection of the proper sizes of balance and metering orifices.
3. The length of time that the metering orifice is uncovered by the upward moving injector plunger.

NOTE

For required adjustments and maintenance schedules, always consult the manufacturer’s service manual.

Figure 5-28.—Pressure-time fuel system.

Figure 5-29.—Pressure-time (PT) gear pump.

Figure 5-30.—Pressure-time injector operation.