The Cummins PT fuel system (fig. 5-28) is exclusive to Cummins diesel engines; PT stands for “Pressure – Time”. 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.

• Cummins Pt Fuel Pump Calibration Manual Transmission
• Cummins Pt Fuel Pump
• Cummins Pt Fuel Pump Manual
• Cummins Pt Fuel Pump Calibration Manuals
• Cummins Pt Fuel Pump Calibration Manual Diagram

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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 Cummins PT 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 Cummins PT Fuel 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.

Cummins Pt Fuel Pump Calibration Manual Transmission

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 of Cummins PT Fuel System, always consult the manufacturer’s service manual.

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In last article, we talk about CCEC Cummins diesel engines operating principles. Well, in this article, you will learn what are the major components of its PT Fuel System.

Fuel System

The PT fuel system is used exclusively on Cummins Diesels. The identifying letters, “PT” are an abbreviation for “pressure-time”.

The operation of the Cummins PT Fuel System is based on the principle that the volume of liquid flow is proportionate to the fluid pressure, the time allowed to flow and the passage size through which the liquid flows. To apply this simple principle to the Cummins PT Fuel System, it is necessary to provide:

1. A fuel pump.

2. A means of controlling pressure of the fuel being delivered by the fuel pump to the injectors so individual cylinders will receive the right amount of fuel for the power required of the engine.

Fig.5-1. Fuel flow schematic N/NT-855 Engine

1). PT (TYPE G) FUEL PUMP

2). SHUT-DOWN VALVE

3). FUEL CONNECTOR

4). INJECTOR

5). INJECTOR RETURN

6). FROM TANK

7). FUEL FILTER

3. Fuel passages of the proper size and type so fuel will be distributed to all injectors and cylinders with each pressure under all speed and load conditions.

4. Injectors to receive low-pressure from the fuel pump and deliver it into the individual combustion chambers at the right time, in equal quantities and proper condition to burn.

The PT fuel system consists of the fuel pump, supply lines, drain lines, fuel passages and injectors.

Fuel Pump

The fuel pump is coupled to the fuel pump drive which is driven from the engine gear train. Fuel pump main shaft in turn drives the gear pump, governor and tachometer shaft assemblies.

Fuel injection pumps must be supplied with fuel under pressure because they have insufficient suction ability. All diesel injection systems require a supply pump transfer fuel from the supply tank through the filters and lines to the injection pump.

Gear Pump And Pulsation Damper

The gear pump is driven by the pump main shaft and contains a single set of gears to pick up and deliver fuel throughout the fuel system. Inlet is at the rear of the gear pump. A pulsation damper mounted to the gear pump contains a steel diaphragm which absorbs pulsations and smooths fuel flows through the filter screen and to the governor assemblies.

Throttle

The throttle provides a means for the operator to manually control engine speed above idle as required by varying operating conditions of speed and load.

In the fuel pump, fuel flows through the governor to the throttle shaft. At idle speed, fuel flows through the idle port in the governor barrel, past the throttle shaft. To operate above idle speed, fuel flows through the main governor barrel port to the throttling hole in the shaft.

Cummins Pt Fuel Pump

PT(type G) VS Fuel Pump

The PT (type G) VS fuel pump, is made up of four main units; the gear pump, standard governor, throttle and a VS (Variable Speed) governor.

Governors

The “Standard” governor is actuated by a system of springs and weights, and has two functions:

1. The governor maintains sufficient fuel for idling with the throttle control in idle position.

2. It cuts off fuel to the injectors above maximum rated rpm.

During operation between idle and maximum speeds, fuel flows through the governor to the injectors. This fuel is controlled by the throttle and limited by the size of the idle spring plunger counter bore. When the engine reaches governed speed, the governor weights move to governor plunger, and fuel passages to the injectors are shut off. At the same time another passage opens and dumps the fuel back into the main pump body.

In this manner, engine speed is controlled and limited by the governor regardless of throttle position.

The VS governor, in the upper portion of the fuel pump housing, operates in series with the standard governor to permit operation at any desired (near constant) speed setting within the range of the standard governor. Speed can be varied with the VS speed control lever, located at top of pump. This pump gives surge free govern ring throughout the engine speed range with a speed droop smaller than the standard governor and is suited to the varying speed requirements of power take off, etc.

Note: When operating the PT(type G) VS fuel pump at any desired constant speed, the VS governor lever should be placed in operating position and the throttle locked in full open position to allow a full flow of fuel through the standard governor.

PT (tyep D) Injectors

The injector provides a means of introducing fuel into each combustion chamber. It combines the acts of metering, timing and injection. Principles of operation are the same for Inline and V-engines but injector size and internal design differs slightly.

Fuel supply and drain flow are accomplished through internal drillings in the cylinder heads. A radial groove around each injector mates with the drilled passages in the cylinder head and admits fuel through an adjustable orifice plug in the injector body. A fine mesh screen at each inlet groove provides final fuel filtration.

The fuel grooves around the injectors are separated by 0-rings which seal against the cylinder head injector bore. This forms a leak-proof passage between the injectors and the cylinder head injector bore surface.

Fuel flows from a connection atop the fuel pump shutdown valve through a supply line into the lower drilled passage in the cylinder head. A second drilling in the head is aligned with the upper injector radial groove to drain away excess fuel. A fuel drain allows return of the unused fuel to the fuel tank.

The injector contains a ball check valve. Ad the injector plunger moves downward to cover the feed opening, an impulse pressure wave seats the ball and at the same time traps a positive amount of fuel in the injector cup for injection. As the continuing downward plunger movement injects fuel into the combustion chamber, it also uncovers the drain opening and the ball rises from its seat. This allows free flow through the injector and out the drain for cooling purposes and purging gases from the cup.

Fuel Lines, Connections And Valves

Supply And Drain Lines - Fuel is supplied through lines to cylinder heads. A common drain line returns fuel not injected, to supply tank.

Connections - Fuel connectors are used between the Inline engine cylinder heads to bridge the gap between each supply and drain passage.

Shut-Down Valve - Either a manual or an electric shut-down valve is used on Cummins Fuel Pumps.

Cummins Pt Fuel Pump Manual

Fig. 5-2, Fuel pump manual override knob

Attention: With a manual valve, the control lever must be fully clockwise or open to permit fuel flow through the valve.

Cummins Pt Fuel Pump Calibration Manuals

With the electric valve, the manual control knob must be fully counterclockwise to permit the solenoid to open the valve when the “switch key” is turned on. For emergency operation in case of electrical failure, turn manual knob clockwise to permit fuel to flow through the valve.

Cummins Pt Fuel Pump Calibration Manual Diagram

No matter it is engine’s fuel system or other parts, it is very necessary and important to know about the related knowledge if you want to prolong the generator set service life. Also, familiar with the operation, it is convenient to do the maintenance in the future.