You have probably heard the statement, "The fuel injection system is the actual heart of the diesel engine." When you consider that indeed a diesel could not be developed until an adequate fuel injection system was designed and produced, this statement takes on a much broader and stronger meaning.
In this section you will learn about various methods of mechanical injections and metering control. There have been many important developments in pumps, nozzles, and unit injectors for diesel engines over the years, with the latest injection system today relying on electronic controls and sensors.
Diesel fuel injection systems must accomplish five particular functions: meter, inject, time, atomize, and create pressure.
Atomization is generally obtained when liquid fuel, under high pressure, passes through the small opening (or openings) in the injector or nozzle. As the fuel enters the combustion space, high velocity is developed because the pressure in the cylinder is lower than the fuel pressure. The created friction, resulting from the fuel passing through the air at high velocity, causes the fuel to break up into small particles.
If the atomization process reduces the size of the fuel particles too much, they will lack penetration. Too little penetration results in the small particles of fuel igniting before they have been properly distributed or dispersed in the combustion space. Since penetration and atomization tend to oppose each other, a compromise in the degree of each is necessary in the design of the fuel injection equipment, particularly if uniform distribution of fuel within the combustion chamber is to be obtained.
Diesel engines are equipped with one of several distinct types of fuel injection systems: individual pump system; multiple-plunger, inline pump system; unit injector system; pressure-time injection system; distributor pump system; and common rail injection system.
The individual pump system is a small pump contained in its own housing, and supplies fuel to one cylinder. The individual plunger and pump barrel are driven off of the engine’s cam shaft. This system is found on large-bore, slow-speed industrial or marine diesel engines and on small air-cooled diesels; they are not used on high-speed diesels.
Multiple-plunger, inline pump systems (Figure 4-9) use individual pumps that are contained in a single injection pump housing. The number of plungers is equal to the number of cylinders on the engine, and they are operated on a pump camshaft. This system is used on many mobile applications and is very popular with several engine manufacturers.
Figure 4-9 — Multiple plunger, inline pump system.
The fuel is drawn in from the fuel tank by a pump, sent through filters, and delivered to the injection pump at a pressure of 10 to 35 psi. All pumps in the housing are subject to this fuel. The fuel at each pump is timed, metered, pressurized, and delivered through a high-pressure fuel line to each injector nozzle in firing order sequence.
The unit injector systems utilize a system that allows timing, atomization, metering, and fuel pressure generation that takes place inside the injector body and services a particular cylinder. This system is compact and delivers a fuel pressure that is higher than any other system today.
Fuel is drawn from the tank by a transfer pump, filtered. and then delivered. The pressure is 50 – 70 psi before it enters the fuel inlet manifold located within the engine’s cylinder head. All of the injectors are fed through a fuel inlet or jumper line. The fuel is pressurized, metered, and timed for proper injection to the combustion chamber by the injector. This system uses a camshaft-operated rocker arm assembly or a pushrod- actuated assembly to operate the injector plunger.
The pressure-time injection system (PT system) got its name from two of the primary factors that affect the amount of fuel injected per combustion cycle. Pressure, or “P,” refers to the pressure of the fuel at the inlet of the injector. Time, or “T,” is the time available for the fuel to flow into the injector cup. The time is controlled by how fast the engine is rotating.
The PT system uses a camshaft-actuated plunger. This changes the rotary motion of the camshaft to a reciprocating motion of the injector. The movement opens and closes the injector metering orifice in the injector barrel. Fuel will flow only when the orifice is open; the metering time is inversely proportional to engine speed. The faster the engine is operating, the less time there is for fuel to enter. The orifice opening size is set according to careful calibration of the entire set of injection nozzles.
The distributor pump systems are used on small to medium-size diesel engines. These systems lack the capability to deliver high volume fuel flow to heavy-duty, large displacement, high-speed diesel engines like those used in trucks. These systems are sometimes called rotary pump systems. Their operating systems are similar to how an ignition distributor operates on a gasoline engine. The rotor is located inside the pump and distributes fuel at a high pressure to individual injectors at the proper firing order.
The common rail injection is the newest high-pressure direct injection fuel delivery system. An advanced design fuel pump supplies fuel to a common rail that acts as a pressure accumulator. The common rail delivers fuel to the individual injectors via short high-pressure fuel lines. The system’s electronic control unit precisely controls both the rail pressure and the timing and duration of the fuel injection. Injector nozzles are operated by rapid-fire solenoid valves or piezo-electric triggered actuators.
With the exception of common rail injection systems, all of the systems described previously were designed to operate without the use of electronic controls. To meet modern performance, fuel efficiency, and emission standards, unit injectors, multiple- plunger, inline pumps, and distributor pump injection systems have all been adapted for use with various levels of electronic controls. Of these systems, electronically controlled and actuated unit injectors have become the prominent choice in heavy-duty engine design.
The Caterpillar diesel engine uses the pump and nozzle injection system. Each pump measures the amount of fuel to be injected into a particular cylinder, produces the pressure for injection of the fuel, and times the exact point of injection. The injection pump plunger is lifted by cam action and returned by spring action. The turning of the plungers in the barrels varies the metering of fuel. These plungers are turned by governor action through a rack that meshes with the gear segments on the bottom of the pump plungers. Each pump is interchangeable with other injection pumps mounted on the pump housing.
The sleeve metering and scroll-type pumps that are used by Caterpillar operate on the same fundamentals, a jerk pump system (where one small pump contained in its own housing supplied fuel to one cylinder). Individual "jerk" pumps that are contained in a single injection pump housing with the same number of pumping plungers as that of the engine cylinders are commonly referred to as inline multiple-plunger pumps.
The sleeve metering fuel system was designed to have the following seven advantages:
The term “sleeve metering” comes from the method used to meter the amount of fuel sent to the cylinders. Rather than rotate the plungers to control the amount of fuel to be injected, like most pump and nozzle injection systems, the use of a sleeve system (Figure 4-10) is incorporated with the plunger. The sleeve blocks a spill port that is drilled into the plunger. The amount of plunger travel with its port blocked determines the amount of fuel to be injected. Basic operation is as follows:
Figure 4-10 — Sleeve metering barrel and plunger assembly.
Figure 4-11 — Injection pump operating cycle.
To increase the amount of fuel injected, raise the sleeve through the control shaft and fork so that the sleeve is effectively positioned higher up on the plunger. This means that the spill port will be closed for a longer period of time as the cam lobe is raising the plunger. Increasing the effective stroke of the plunger (the time that both ports are closed) will increase the amount of fuel delivered.
Electronic unit injection has proven to be the most adaptable fuel injection system available. Fuel enters the injector through two filters screens. Fuel not used for injection cools and lubricates the injector before exiting through the return port on its way back to the fuel tank.
The electronic unit injection system uses mechanical action to create the pressures needed for injection. The fuel enters the injector through an inlet to the electronically controlled poppet valve. The valve is held open by spring pressure; the fuel simply flows into the opening. When the piston is approximately 60 degrees BTDC on its compression stroke, the camshaft pivots the rocker arm through its roller follower. When the solenoid is energized, the armature is pulled upward, closing the poppet valve. This forces the injector follower down against its external return spring. This action raises the trapped fuel to a pressure sufficient to lift the injector needle valve off its seat. The strength of the needle valve spring determines when the valve will open. Opening pressures of 2,800-3,200 psi are common. When the needle valve unseats, fuel flows through the opening in the injector; this increases the fuel pressure to approximately 20,000 psi.
The distributor-type fuel system is found on small- to medium-sized diesel engines. Its operation is similar to an ignition distributor found on a gasoline engine. A rotating member within the pump, called a rotor, distributes fuel at high pressure to the individual injectors in engine firing order sequence.
There are several manufacturers of distributor-type fuel injection systems. The distributor-type fuel system that will be discussed is the DB2 Roosa Master diesel fuel- injection pump, manufactured by Stanadyne's Hartford Division.
The Roosa Master fuel-injection pump is described as an opposed plunger, inlet metering, distributor-type pump. Simplicity, the prime advantage of this design, contributes to greater ease of service, low maintenance cost, and greater dependability. Before describing the injection pump components and operation, let us familiarize ourselves with the model numbering system.
The main components of the DB2 fuel-injection pump are the drive shaft, distributor rotor, transfer pump, pumping plungers, internal cam ring, hydraulic head, end plate, governor, and housing assembly with an integral advance mechanism. The rotating members that revolve on a common axis include the drive shaft, distributor rotor, and transfer pump.
The drive shaft is the driving member that rotates inside a pilot tube pressed into the housing. The rear of the shaft engages the front of the distributor rotor and turns the rotor shaft. Two lip-type seals prevent the entrance of engine oil into the pump and retain fuel used for pump lubrication.
The distributor rotor is the drive end of the rotor, containing two pumping plungers located in the pumping cylinder. Slots in the rear of the rotor provide a place for two spring-loaded transfer pump blades. In the rotor, the shoe, which provides a large bearing surface for the roller, is carried in guide slots. The rotor shaft rotates with a very close fit in the hydraulic head. A passage through the center of the rotor shaft connects the pumping cylinder with one charging port and one discharging port. The hydraulic head in which the rotor turns has a number of charging and discharging ports, based on the number of engine cylinders. An eight-cylinder engine will have eight charging and eight discharging ports. The governor weight retainer is supported on the forward end of the rotor.
The transfer pump is a positive displacement, vane-style unit, consisting of a stationary liner with spring-loaded blades that ride in slots at the end of the rotor shaft. The delivery capacity of the transfer pump is capable of exceeding both pressure and volume requirements of the engine, with both varying in proportion to engine speed. A pressure regulator valve in the pump end plate controls fuel pressure. A large percentage of the fuel from the pump is bypassed through the regulating valve to the inlet side of the pump. The quantity and pressure of the fuel bypassed increase as pump speed increases.
The operation of the model DB2 injection is similar to that of an ignition distributor. However, instead of the ignition rotor distributing high-voltage sparks to each cylinder in firing order, the DB2 pump distributes pressurized diesel fuel as two passages align during the rotation of the pump rotor, also in firing order. The basic fuel flow is as follows:
The maximum amount of fuel that can be injected is limited by maximum outward travel of the plungers. The roller shoes, contacting an adjustable leaf spring, limit this maximum plunger travel. At the time the charging ports are in register, the rollers are between the cam lobes; therefore, their outward movement is unrestricted during the charging cycle except as limited by the leaf spring.
To prevent after-dribble and therefore un-burnt fuel at the exhaust, the end of injection must occur crisply and rapidly. To ensure that the nozzle valve does, in fact, return to its seat as rapidly as possible, the delivery valve, located in the drive passage of the rotor, acts to reduce injection line pressure. This occurs after fuel injection, and the pressure is reduced to a value lower than that of the injector nozzle closing pressure. The valve remains closed during charging and opens under high pressure, as the plungers are forced together. Two small grooves are located on either side of the charging port or the rotor near its flange end. These grooves carry fuel from the hydraulic head charging posts to the housing. This fuel flow lubricates the cam, the rollers, and the governor parts. The fuel flows through the entire pump housing, absorbs heat, and is allowed to return to the supply tank through a fuel return line connected to the pump housing cover, thereby providing for pump cooling.
In the DB2 fuel pump, automatic advance is accomplished in the pump by fuel pressure acting against a piston, which causes rotation of the cam ring, thereby aligning the fuel passages in the pump sooner. The rising fuel pressure from the transfer pump increases the flow to the power side of the advance piston. This flow from the transfer pump passes through a cut on the metering valve, through a passage in the hydraulic head, and then by the check valve in the drilled bottom head locking screw. The check valve provides a hydraulic lock, preventing the cam from retarding during injection. Fuel is directed by a passage in the advance housing and plug to the pressure side of the advance piston. The piston moves the cam counterclockwise (opposite to the direction of the pump rotation). The spring-loaded side of the piston balances the force of the power side of the piston and limits the maximum movement of the cam. Therefore, with increasing speed, the cam is advanced and, with decreasing speed, it is retarded.
We know that a small amount of fuel under pressure is vented into the governor linkage compartment. Flow into this area is controlled by a small vent wire that controls the volume of fuel returning to the fuel tank, thereby avoiding any undue fuel pressure loss. The vent passage is located behind the metering valve bore and leads to the governor compartment by a short vertical passage. The vent wire assembly is available in several sizes to control the amount of vented fuel being returned to the tank. The vent wire should NOT be tampered with, as it can be altered only by removing the governor cover. The correct wire size would be installed when the pump assembly is being flow- tested on a pump calibration stand.
The DB2 injection pump can be used on a variety of applications; therefore, it is available with several options as required. The options are as follows:
Turning in the torque screw moves the fuel metering valve toward its closed position. The torque screw controls the amount of fuel delivered at full-load governor speed.
If additional load is applied to the engine while it is running at full-load governed speed, there will be a reduction in engine rpm. A greater quantity of fuel is allowed to pass into the pumping chamber because of the increased time that the charging ports are open.
Fuel delivery will continue to increase until the rpm drops to the engine manufacturer’s predetermined point of maximum torque.
Do NOT attempt to adjust the torque curve on the engine at any time. This adjustment can only be done during a dynamometer test where fuel flow can be checked along with the measured engine torque curve on a fuel pump test stand.
The DB2 fuel injection pump uses a mechanical type governor (Figure 4-12). As you learned earlier, the function of the governor is to control the engine speed under various load settings. As with any mechanical governor, it operates on the principle of spring pressure opposed by weight force, with the spring attempting to force the linkage to an increased fuel position at all times. The centrifugal force of the rotating flyweights attempts to pull the linkage to a decreased fuel position.
Figure 4-12 — Fuel injection pump with governor assembly.
Rotation of the governor linkage varies the valve opening, thereby limiting and controlling the quantity of fuel that can be directed to the fuel plungers. The position of the throttle lever controlled by the operator's foot will vary the tension of the governor spring. This force, acting on the linkage, rotates the metering valve to an increased or decreased fuel position as required.
At any given throttle position the centrifugal force of the rotating flyweights will exert force back through the governor linkage which is equal to that of the spring, resulting in a state of balance. Outward movement of the weights acting through the governor thrust sleeve can turn the fuel-metering valve by means of the governor linkage arm and hook. The throttle and governor spring position will turn the metering valve in the opposite direction.
The governor is lubricated by fuel received from the fuel housing. Fuel pressure in the governor housing is maintained by a spring-loaded ball-check return fitting in the governor cover of the pump.
The injector nozzle used with the DB2 fuel-injection pump is opened outward by high fuel pressure and closed by spring tension. It has a unique feature in that it is screwed directly into the cylinder head. An outward opening valve creates a narrow spray that is evenly distributed into the precombustion chamber. Both engine compression and combustion pressure forces assist the nozzle spring in closing an outward opening valve. These factors allow the opening pressure settings of the nozzle to be lower than those of conventional injectors.
During injection, a degree of swirl is imparted to the fuel before it actually emerges around the head of the nozzle. This forms a closely controlled annular orifice with the nozzle valve seat, which produces a high velocity atomized fuel spray, forming a narrow cone suitable for efficient burning of the fuel in the precombustion chamber.
The nozzle has been designed as basically a throwaway item. After a period of service, the functional performance may not meet test specifications. Nozzle testing is comprised of the following checks:
Each test is done independently of the others (for example, when checking the opening pressure, do not check for leakage). If all the tests are satisfied, the nozzle can be reused. If any one of the tests is not satisfied, replace the nozzle. For testing procedures, consult the manufacturer’s service manual.
Over the years Cummins has produced a series of innovations, such as the first automotive diesel, in addition to being the first to use supercharging and then turbocharging. All cylinders are commonly served through a low-pressure fuel line. The camshaft control of the mechanical injector controls the timing of injection throughout the operating range. This design eliminates the timing-lag problems of high-pressure systems.
To meet Environmental Protection Agency (EPA) exhaust emissions standards, Cummins offers the Celect (electronically controlled injection) system. Since the Celect system did not start production until 1989, there are literally thousands of Cummins with pressure-time (PT) fuel systems.
The pressure-time (PT) fuel system (Figure 4-13) is exclusive to Cummins diesel engines; it uses injectors that meter and inject the fuel with this metering based on a pressure-time principle. A gear-driven positive displacement low-pressure fuel pump 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.
Figure 4-13 — Pressure-time fuel system.
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. Because 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 over-speeding 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 PT fuel system has three main advantages:
The fuel pump commonly used in the pressure-time system is the PTG-AFC pump (PT pump with a governor and an air-fuel control attachment) (Figure 4-14). 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.
Figure 4-14 — Pressure-time gear pump.
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:
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:
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. Fuel 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.
A PT injector (Figure 4-15) 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, as follows:
Figure 4-15 — Pressure- time injector operation.
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. Always 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:
Proper injector adjustment and maintenance will ensure a smooth running engine as long as the following factors are met:
For required adjustments and maintenance schedules, always consult the manufacturer’s service manual.
The mechanical electronic unit injector is a common unit injector with an electronic solenoid that is controlled by the ECM. Mechanical pressure is created by the camshaft moving a roller and a pushrod, and a follower pressing on top of the injector unit. The rate and amount of fuel injected into the cylinder is controlled by the opening and closing of the solenoid that is controlled by the ECM.
The hydraulic electronic unit injectors use high pressure engine oil to provide the force needed to complete injection. Many of the mechanical drive components found in standard mechanical or electronic unit injection systems, such as cam lobes, lifters, push rods, and rocker arms, are not used in this system.
A solenoid on each injector controls the amount of fuel delivered by the injector. A gear- driven axial pump raises the normal pressure to the levels required by the injectors. The ECM sends a signal to an injection pressure control valve to control pressure, and another signal to each injector solenoid to inject the fuel.
Pressure in the engine oil manifold is controlled by the ECM through the use of an injection pressure control valve. The injection pressure control valve, or dump valve, controls the injection pump outlet pressure by dumping excess oil back to the sump.
The ECM monitors pressure in the manifold through an injection pressure sensor. The ECM measures the pressure sensor signal to the desired injection pressure. Based on this measurement, the ECM changes the oil pressure in the high pressure manifold.
High pressure oil is routed from the pump to the high pressure manifold through a steel tube. From there it is routed to each injector through shorter jumper tubes.
4. (True or False) Atomization occurs when the fuel enters the combustion chamber because the pressure in the cylinder is lower than the fuel pressure.
5. What manufacturer produced the first automotive diesel?