The diesel fuel injection system is a major component of a properly operating engine. An engine out of adjustment can cause excessive exhaust smoke, poor fuel economy, heavy carbon buildup within the combustion chambers, and short engine life.
In this manual you will learn the major components of the diesel fuel system and how they operate so you may better maintain or have the knowledge to repair a diesel engine.
When you have completed this manual, you will be able to:
Like the gasoline engine, the diesel engine is an internal combustion engine using either a two- or four-stroke cycle. Burning or combustion of fuel within the engine cylinders is the source of the power. The main difference in a diesel engine is that the diesel fuel is mixed with compressed air in the cylinder as shown in Figure 1.
Figure 1 — Diesel and gasoline engines intake strokes.
Compression ratios in the diesel engine range between 6:1 for a stationary engine and 24:1 for passenger vehicles. This high ratio causes increased compression pressures of 400 to 600 psi and cylinder temperatures reaching 800°F to 1200°F. At the proper time, the diesel fuel is injected into the cylinder by a fuel-injection system, which usually consists of a pump, fuel line, and injector or nozzle. When the fuel oil enters the cylinder, it will ignite because of the high temperatures. The diesel engine is known as a compression-ignition engine, while the gasoline engine is a spark-ignition engine.
The speed of a diesel engine is controlled by the amount of fuel injected into the cylinders. In a gasoline engine, the speed of the engine is controlled by the amount of air admitted into the carburetor or gasoline fuel injection systems.
Mechanically, the diesel engine is similar to the gasoline engine. The intake, compression, power, and exhaust strokes occur in the same order. The arrangement of the pistons, connecting rods, crankshaft, and engine valves is about the same. The diesel engine is also classified as in-line or v-type.
In comparison to the gasoline engine, the diesel engine produces more power per pound of fuel, is more reliable, has lower fuel consumption per horsepower per hour, and presents less of a fire hazard.
These advantages are partially offset by higher initial cost, heavier construction needed for its high compression pressures, and the difficulty in starting which results from these pressures.
Diesel fuel is heavier than gasoline because it is obtained from the residue of the crude oil after the more volatile fuels have been removed. As with gasoline, the efficiency of diesel fuel varies with the type of engine in which it is used. By distillation, cracking, and blending of several oils, a suitable diesel fuel can be obtained for all engine operating conditions. Using a poor or improper grade of fuel can cause hard starting, incomplete combustion, a smoky exhaust, and engine knocks.
The high injection pressures needed in the diesel fuel system result from close tolerances in the pumps and injectors. These tolerances make it necessary for the diesel fuel to have sufficient lubrication qualities to prevent rapid wear or damage. It must also be clean, mix rapidly with the air, and burn smoothly to produce an even thrust on the piston during combustion.
Diesel fuel is graded and designated by the American Society for Testing and Materials (ASTM), while its specific gravity and high and low heat values are listed by the American Petroleum Institute (API). Each individual oil refiner and supplier attempts to produce diesel fuels that comply as closely as possible with ASTM and API specifications. Because of different crude oil supplies, the diesel fuel may be on either the high or low end of the prescribed heat scale in BTU per pound or per gallon. Because of the deterioration of diesel fuel, only two grades of fuel are considered acceptable for use in high-speed heavy-duty vehicles. These are the No. 1D or No. 2D fuel oil classification. Grade No. 1D comprises the class of volatile fuel oils from kerosene to the intermediate distillates. Fuels within this classification are applicable for use in high-speed engines in service involving frequent and relatively wide variations in loads and speeds. In cold weather conditions, No. 1D fuel allows the engine to start easily. In summary, for heavy-duty high-speed diesel vehicles operating in continued cold-weather conditions, No. 1D fuel provides better operation than the heavier No. 2D.
Grade No. 2D includes the class of distillate oils of lower volatility. They are applicable for use in high-speed engines in service involving relatively high loads and speeds. This fuel is used more by truck fleets due to its greater heat value per gallon, particularly in warm to moderate climates. Even though No. 1D fuel has better properties for cold weather operations, many still use No. 2D in the winter, using fuel heater/water separators to provide suitable starting, as well as fuel additive conditioners, which are added directly into the fuel tank.
Selecting the correct diesel fuel is a must if the engine is to perform to its rated specifications.
Generally, seven factors must be considered in the selection of a fuel oil:
Other considerations in the selection of a fuel oil are:
Cetane number is a measure of the fuel oil’s volatility; the higher the rating, the easier the engine will start and the smoother the combustion process will be within the ratings specified by the engine manufacturer. Current 1D and 2D diesel fuels have a cetane rating between 40 and 50.
Cetane rating differs from the octane rating used in gasoline in that the higher the number of gasoline on the octane scale, the greater the fuel resistance to self ignition, which is a desirable property in gasoline engines with a high compression ratio. Using a low octane fuel will cause premature ignition in high compression engines. However, the higher the cetane rating, the easier the fuel will ignite once injected into the diesel combustion chamber. If the cetane number is too low, you will have difficulty in starting. This can be accompanied by engine knock and puffs of white smoke during warm-up in cold weather.
High altitudes and low temperatures require the use of diesel fuel with an increased cetane number. Low temperature starting is enhanced by high cetane fuel oil in the proportion of 1.5°F lower starting temperature for each cetane number increase.
Fuel volatility requirements depend on the same factors as cetane number. The more volatile fuels are best for engines where rapidly changing loads and speeds are encountered. Low volatile fuels tend to give better fuel economy where their characteristics are needed for complete combustion, and will produce less smoke, odor, deposits, crankcase dilution, and engine wear.
The volatility of a fuel is established by a distillation test where a given volume of fuel is placed into a container that is heated gradually. The readiness with which a liquid changes to a vapor is known as the volatility of the liquid. The 90 percent distillation temperature measures volatility of diesel fuel. This is the temperature at which 90 percent of a sample of the fuel has been distilled off. The lower the distillation temperature, the higher the volatility of the fuel. In small diesel engines higher fuel volatility is needed than in larger engines in order to obtain low fuel consumption, low exhaust temperature, and minimum exhaust smoke.
The viscosity is a measure of the resistance to flow of the fuel, and it will decrease as the fuel oil temperature increases. What this means is that a fluid with a high viscosity is heavier than a fluid with low viscosity. A high viscosity fuel may cause extreme pressures in the injection systems and will cause reduced atomization and vaporization of the fuel spray.
The viscosity of diesel fuel must be low enough for it to flow freely at its lowest operational temperature, yet high enough to provide lubrication to the moving parts of the finely machined injectors. The fuel must also be sufficiently viscous so that leakage at the pump plungers and dribbling at the injectors will not occur. Viscosity also will determine the size of the fuel droplets, which in turn govern the atomization and penetration qualities of the fuel injector spray.
Recommended fuel oil viscosity for high-speed diesel engines is generally in the region of 39 SSU (Seconds Saybolt Universal), which is derived from using a Saybolt Viscosimeter to measure the time it takes for a quantity of fuel to flow through a restricted hole in a tube. A viscosity rating of 39 SSU provides good penetration into the combustion chamber, atomization of fuel, and suitable lubrication.
Sulfur has a definite effect on the wear of the internal components of the engine, such as the piston ring, pistons, valves, and cylinder liners. In addition, a high sulfur content fuel requires that the engine oil and filter be changed more often because the corrosive effects of hydrogen sulfide in the fuel and the sulfur dioxide or sulfur trioxide that is formed during the combustion process combine with water vapor to form acids. High additive lubricating oils are desired when high sulfur fuels are used. Refer to the engine manufacturer’s specifications for the correct lube oil when using high sulfur fuel.
Sulfur content can be established only by chemical analysis of the fuel. Fuel sulfur content above 0.4% is considered as medium or high, and anything below 0.4% is low. No. 2D contains between 0.2 and 0.5% sulfur, whereas No. 1D contains less than 0.1%.
Sulfur content has a direct bearing on the life expectancy of the engine and its components. Active sulfur in diesel fuel will attack and corrode injection system components and contribute to combustion chamber and injection system deposits.
Cloud point is the temperature at which wax crystals in the fuel (paraffin base) begin to settle out with the result that the fuel filter becomes clogged. This condition exists when cold temperatures are encountered and is the reason that a thermostatically controlled fuel heater is required on vehicles operating in cold weather environments. Failure to use a fuel heater will prevent fuel from flowing through the filter and the engine will not run. Cloud point generally occurs 9-14°F above the pour point.
Pour point of a fuel determines the lowest temperature at which the fuel can be pumped through the fuel system. The pour point is 5°F above the level at which oil becomes a solid or refuses to flow.
Cleanliness is an important characteristic of diesel fuel. Fuel should not contain more than a trace of foreign substances; otherwise, fuel pump and injector difficulties will develop, leading to poor performance or seizure. Because it is heavier and more viscous, diesel fuel will hold dirt particles in suspension for a longer period than gasoline. Moisture in the fuel can also damage or cause seizure of injector parts when corrosion occurs.
Fuel stability is its capacity to resist chemical change caused by oxidation and heat. Good oxidation stability means that the fuel can be stored for extended periods of time without the formation of gum or sludge. Good thermal stability prevents the formation of carbon in hot parts such as fuel injectors or turbine nozzles. Carbon deposits disrupt the spray patterns and cause inefficient combustion.
The fuel injected into the combustion chamber must be mixed thoroughly with the compressed air and distributed as evenly as possible throughout the chamber if the engine is to function at maximum efficiency and exhibit maximum drivability. A well designed engine uses a combustion chamber designed for the intended usage of the engine. The injectors used should complement the combustion chamber. The combustion chambers described in the following sections are the most common, and cover virtually all of the designs that are currently in use.
Direct injection is the most common combustion chamber (Figure 2, View A) and is found in nearly all engines. The fuel is injected directly into an open combustion chamber formed by the piston and cylinder head. The main advantage of this type of injection is that it is simple and has high fuel efficiency.
In the direct combustion chamber, the fuel must atomize heat, vaporize, and mix with the combustion air in a very short period of time. The shape of the piston helps with this during the intake stroke. Direct injection systems operate at very high pressures of up to 30,000 psi.
Indirect injection chambers were previously used mostly in passenger cars and light truck applications because of lower exhaust emissions and quietness. In today’s technology with electronic timing, direct injection systems are superior. Therefore, you will not see many indirect injections system on new engines; they are still on many older engines, however.
Figure 2 — Combustion chambers.
Pre-combustion chamber design involves a separate combustion chamber located in either the cylinder head or wall. As Figure 2, View B shows, this chamber takes up from 20% - 40% of the combustion chambers TDC volume and is connected to the chamber by one or more passages. As the compression stroke occurs, the air is forced up into the pre-combustion chamber. When fuel is injected into the pre-combustion chamber, it partially burns, building up pressure. This pressure forces the mixture back into the combustion chamber, and complete combustion occurs.
Swirl chamber systems (Figure 2, View C) use the auxiliary combustion chamber that is ball-shaped and opens at an angle to the main combustion chamber. The swirl chamber contains 50% - 70% of the TDC cylinder volume and is connected at a right angle to the main combustion chamber. A strong vortex (mass of swirling air) is created during the compression stroke. The injector nozzle is positioned so the injected fuel penetrates the vortex and strikes the hot wall, and combustion begins. As combustion begins, the flow travels into the main combustion chamber for complete combustion.
A governor is a device that senses engine speed and load, and changes fuel delivery accordingly. All diesel engines use some sort of governor, whether it is mechanical, servo-mechanical, hydraulic, pneumatic or electronic. A governor is needed to regulate the amount of fuel delivered at idle to prevent it from stalling. It is also required so it can cut off the fuel supply when the engine reaches its maximum rated speed. Without a governor, a diesel engine could reach maximum RPM and destroy itself quickly. The governor is often included in the design of the fuel injection system. The main reason that a diesel requires a governor is that a diesel engine operates with excess air under all loads and speeds.
Even though it is not part of the fuel system, a governor is directly related to this system since it functions to regulate speed by the control of fuel or of the air-fuel mixture, depending on the type of engine. In diesel engines governors are connected in the linkage between the throttle and the fuel injectors. The governor acts through the fuel injection equipment to regulate the amount of fuel delivered to the cylinders. As a result, the governor holds engine speed reasonably constant during fluctuations in load.
To understand why different types of governors are needed for different kinds of job, you will need to know the meaning of several terms used in describing the action of the governor in regulating engine speed (Table 5-1).
Table 5-1 — Terms used to explain governor operation.
|Maximum no-load speed||The highest engine rpm obtainable when the throttle linkage is moved to its maximum position with no load applied to the engine.|
|Maximum full-load speed||Indicates the engine rpm at which a particular engine will produce its maximum designed horsepower setting as stated by the manufacturer.|
|Idle or low-idle speed||Indicates the normal speed at which the engine will rotate with the throttle linkage in the released or closed position.|
|Work capacity||Describes the amount of available work energy that can be produced to the output shaft of the governor.|
|Stability||Refers to the ability of the governor to maintain speed with either constant or varying loads without hunting.|
|Speed droop||Expresses the difference in the change in the governor rotating speed which causes the output shaft of the governor to move from its full-open throttle position to its full-closed position or vice versa.|
|Hunting||A repeated and sometimes rhythmic variation of speed due to over control by the governor. Also called speed drift.|
|Sensitivity||An expression of how quickly the governor responds to a change in speed.|
|Response time||Normally the time taken in seconds for the fuel linkage to be moved from a no-load to a full-load position.|
|Isochronous||Indicates the zero-droop capability. In others words, the full-load and no-load speeds are the same|
|Overrun||Expresses the action of the governor when the engine exceeds its maximum governed speed.|
The type of governor used on a diesel engine is dependent upon the application required. The six basic types of governors are mechanical, pneumatic, servo, hydraulic, electric, and electronic. While electronically-controlled fuel governing systems are being used on nearly all late-model engines, there are millions of the other governor types still in service. The durability and rebuild capability of the diesel engines has ensured that mechanical and other types of governors have many more years of service to come.
The governors used on heavy-duty truck applications and construction equipment fall into one of two classifications:
Other classifications of governors used on diesel engines are as follows:
In most governors installed on diesel engines used by the Navy, the centrifugal force of rotating weights (flyballs) and the tensions of a helical coil spring (or springs) are used in governor operation. On this basis, most of the governors used on diesel engines are generally called mechanical centrifugal flyweight governors
In mechanical centrifugal flyweight governors (Figure 3), two forces oppose each other. One of these forces is tension spring (or springs) which may be varied either by an adjusting device or by movement of the manual throttle. The engine produces the other force. Weights attached to the governor drive shaft are rotated, and a centrifugal force is created when the engine drives the shaft. The centrifugal force varies with the speed of the engine.
Figure 3 — Mechanical governor.
Transmitted to the fuel system through a connecting linkage, the tension of the spring (or springs) tends to increase the amount of fuel delivered to the cylinders. On the other hand, the centrifugal force of the rotating weights, through connecting linkage, tends to reduce the quantity of fuel injected. When the two opposing forces are equal, or balanced, the speed of the engine remains constant.
To show how the governor works when the load increases and decreases, let us assume you are driving a truck in hilly terrain. When the truck approaches a hill at a steady engine speed, the vehicle is moving from a set state of balance in the governor assembly (weights and springs are equal) with a fixed throttle setting to an unstable condition. As the vehicle starts to move up the hill at a fixed speed, the increased load demands result in a reduction in engine speed. This upsets the state of balance that had existed in the governor. The reduced rotational speed at the engine results in a reduction in speed, and, therefore, the centrifugal force of the governor weights. When the state of balance is upset, the high-speed governor spring is allowed to expand, giving up some of its stored energy, which moves the connecting fuel linkage to an increased delivery position. This additional fuel delivered to the combustion chambers results in an increase in horsepower, but not necessarily an increase in engine speed.
When the truck moves into a downhill situation, you are forced to back off the throttle to reduce the speed of the vehicle; otherwise, you have to apply the brakes or engine/transmission retarder. You can also downshift the transmission to obtain additional braking power. However, when you do not reduce the throttle position or brake the vehicle mass in some way, an increase in road speed results. This is due to the reduction in engine load because of the additional reduction in vehicle resistance achieved through the mass weight of the vehicle and its load pushing the truck downhill. This action causes the governor weights to increase in speed, and they attempt to compress the high-speed spring, thereby reducing the fuel delivery to the engine. Engine over-speed can result if the road wheels of the vehicle are allowed to rotate fast enough that they, in effect, become the driving member.
The governor assembly would continue to reduce fuel supply to the engine due to increased speed of the engine. If over-speed does occur, the valves can end up floating (valve springs are unable to pull and keep the valves closed) and striking the piston crown. Therefore, it is necessary in a downhill run for you to ensure that the engine speed does not exceed maximum governed rpm by application of the vehicle, engine, or transmission forces.
Favorable as well as unfavorable characteristics are found in mechanical governors. The advantages are
The disadvantages are:
Although hydraulic governors have more moving parts and are generally more expensive than mechanical governors, they are used in many applications because they are more sensitive, have greater power to move the fuel control mechanism of the engine, and can be timed for identical speed for all loads.
In hydraulic governors (Figure 4), the power which moves the engine throttle does NOT come from the speed-measuring device, but instead comes from a hydraulic power piston, or servomotor. This is a piston that is acted upon by fluid pressure, generally oil under the pressure of a pump. With appropriate piston size and oil pressure, the power of the governor at its output shaft (work capacity) can be made sufficient to operate the fuel-changing mechanism of the largest engines.
Figure 4 — Hydraulic governor.
The speed-measuring device, through its speeder rod, is attached to a small cylindrical valve, called a pilot valve. The pilot valve slides up and down in a bushing which contains ports that control the oil flow to and from the servomotor. The force needed to slide the pilot valve is very little; a small ball head is able to control a large amount of power at the servomotor.
The basic principle of a hydraulic governor is very simple. When the governor is operating at control speed or state of balance, the pilot valve closes the port and there is no oil flow.
When the governor speed falls due to an increase in engine load, the flyweights move inward and the pilot valve moves down. This opens the port to the power piston and connects the oil supply of oil under pressure. This oil pressure acts on the power piston, forcing it upward to increase the fuel.
When the governor speed rises due to a decrease of engine load, the flyweights move out and the pilot valve moves up. This opens the port from the power piston to the drain into the sump. The spring above the power piston forces the power piston down, thus decreasing the speed.
Unfortunately, the simple hydraulic governor has a serious defect which prevents its practical use. It is inherently unstable, that is, it keeps moving continually, making unnecessary corrective actions. In other words it hunts. The cause of this hunting is the unavoidable time lag between the moment the governor acts and the moment the engine responds. The engine cannot come back to the speed called for by the governor.
Most hydraulic governors use a speed droop to obtain stability. Speed droop gives stability because the engine throttle can take only one position for any speed. Therefore, when a load change causes a speed change, the resulting governor action ceases at a particular point that gives the amount of fuel needed for a new load. In this way speed droop prevents unnecessary governor movement and overcorrection (hunting).
The recent introduction of an electronically controlled diesel fuel injection system on several heavy-duty high-speed truck engines has allowed the speed of the diesel engine to be controlled electronically rather than mechanically. The same type of balance condition in a mechanical governor occurs in an electronic governor. The major difference is that in the electronic governor, electric currents (amperes) and voltages (pressure) are used together instead of mechanical weight and spring forces. This is possible through the use of a magnetic pickup sensor (MPS), which is, in effect, a permanent magnet single-pole device. This magnetic pickup concept is being used on all existing electronic systems, and its operation can be considered common to all of them. MPSs are a vital communications link between the engine crankshaft speed and the onboard computer (ECM). The MPS is installed next to a drive shaft gear made of a material that reacts to a magnetic field. As each gear tooth passes the MPS, the gear interrupts the MPS’s magnetic field. This in turn produces an AC current signal, which corresponds to the rpm of the engine. This signal is sent to the ECM to establish the amount of fuel that should be injected into the combustion chambers of the engine. Electronic speed governing systems are set up to provide six basic governing modes:
Each of the control modes above is described in more detail below.
The major advantage of the electronic governor over the mechanical governor lies in its ability to modify speed reference easily by various means to control such things as acceleration and deceleration, as well as load.
Before discussing the various types of fuel injection systems, let us spend some time looking at the basic components that are necessary to hold, supply, and filter the fuel before it passes to the actual injection system as shown in Figure 5. The basic function of the fuel system is to provide a reservoir of diesel fuel, to provide sufficient circulation of clean filtered fuel for lubrication, cooling, and combustion purposes, and to allow warm fuel from the engine to re-circulate back to the tank(s). The specific layout and arrangement of the diesel fuel system will vary slightly between makes and models.
Figure 5— Diesel fuel injection system.
The basic fuel system consists of the fuel tank(s) and a fuel transfer pump (supply) that can be a separate engine-driven pump or can be mounted on or inside the injection pump. In addition, the system uses two fuel filters—a primary and secondary filter—to remove impurities from the fuel. In some systems you will have a fuel filter/water separator that contains an internal filter and water trap.
Fuel tanks used today can be constructed from aluminum or alloy steel. Baffles are welded into the tanks during construction. The baffle plates are designed with holes in them to prevent the fuel from sloshing while the vehicle is moving. The fuel inlet and return lines should be separated by a baffle in the tank and be at least twelve inches apart to prevent warm return fuel from being sucked right back up by the fuel inlet line. Both the inlet and return lines should be kept at least 1 inch above the bottom of the tank so sediment or water is not drawn into the inlet.
A well designed tank (Figure 6) will contain a drain plug in the base to allow for fuel tank drainage. This allows the fuel to be drained from the tank before removal for any service. Many tanks are equipped with a small low-mounted catchment basin so that any water in the tank can be quickly drained through a drain cock which is surrounded by a protective cage to prevent damage.
Figure 6 — Fuel tank construction.
The diesel fuel tank is mounted directly on the chassis because of its weight (when filled) and to prevent movement of the tank when the equipment is operated over rough terrain. Its location depends on the type of equipment and the use of the equipment. On equipment used for ground clearing and earthwork, the tank is mounted where it has less chance of being damaged by foreign objects or striking the ground.
The fuel tank filler cap is constructed with both a pressure relief valve and a vent valve. The vent valve is designed to seal when fuel enters it due to overfilling, vehicle operating angle, or a sudden jolt that would cause fuel slosh within the tank. Although some fuel will tend to seep from the vent cap, this leakage should not exceed 1 ounce per minute.
Fuel injection pumps must be supplied with fuel under pressure because they have insufficient suction ability. All diesel injection systems require a supply pump to transfer fuel from the supply tank through the filters and lines to the injection pump. Supply pumps can be either external or internal to the injection pump. There are several types of supply pumps used on diesel engines.
The remaining task to be accomplished by the fuel system is to provide the proper quantity of fuel to the cylinders of the engine. This is done differently by each manufacturer and is referred to as fuel injection.
Diesel fuel filters (Figure 7) must be capable of trapping extremely small contaminants. The porosity of the filter material will determine the size of the impurities it can remove. Typical fuel injector nozzles are measured in microns. Therefore, it is necessary to filter very small impurities out of the fuel before it gets to the injector and plugs it. Diesel fuel filter elements fall into two categories of construction, depth filters and surface filters.
Figure 7 — Fuel filters.
Depth filters are made of woven cotton. The most popular material used for these filters is cotton thread that is blended with a springy supporting material. Depth element filters can be used either in a shell base bolt-on assembly or as a spin-on application. These filters are typically used as a primary filter and are located between the fuel tank and the transfer pump.
Surface filters are made of pleated paper that is made from cellulose fiber. The fiber is treated with a phenolic resin that acts as a binder. The physical properties of the paper-- thickness, porosity, tinsel strength, basic weight, and micron rating--can be very closely controlled during the manufacturing process.
The purpose of a fuel filter is mainly to remove foreign particles as well as water. However, too much water in a fuel filter will render it incapable of protecting the system. So to ensure this does not happen, most diesel engine fuel systems are now equipped with fuel filter/water separators (Figure 8) for the main purpose of trapping and holding water that may be mixed in with the fuel. Generally, when a fuel filter/water separator is used on a diesel engine, it also serves as the primary filter. There are a number of manufacturers who produce fuel filter/water separators with their concept of operation being common and only design variations being the major difference.
Figure 8 — Water seperators.
Their basic operation is as follows:
A fuel injection pump is the pump that takes the fuel from the fuel manifold and pushes it under high pressure through the fuel lines to the fuel injectors. The fuel injection pump, or metering pump, boosts low and medium fuel pressures to the high pressures needed for injection.
The fuel return line returns fuel to the tank and deposits it into the open space above the fuel. This allows the air bubbles to be vented. It should also be inserted to the tank at least 12 inches away from the fuel pickup point so that the returned fuel will not be picked up before the air is vented.
There are three basic types of electric fuel gauges: the balancing coil, the thermostatic, and the electronic (digital) gauge system. Most gauge systems include a sending unit in the fuel tank and a fuel gauge on the instrument panel.
The balancing coil fuel gauge system has a sliding contact in the tank that moves back and forth as the position of the float changes. The resistance in the unit changes as the contact moves. When the tank is full, current flows through both coils, but the stronger field is around the full coil and the needle is pointed to the full mark. As the tank is emptied, the float moves down, the resistance decreases, and the flow of electricity moves easier through the tank unit and ground. Therefore, the magnetic pull of the full coil weakens, and the magnetic field around the empty coil increases. This pulls the needle to the empty mark.
The thermostatic fuel gauge system contains a pair of thermostat blades. Each blade has a heating coil connected in series through the ignition switch to the battery. As the tank blade heats up, the dash blade heats up as well, the movement corresponding with the tank blade. The dash blade movement goes through a linkage to the indicator, which moves to the appropriate position on the gauge dial.
The digital fuel gauge system consists of a fuel sensor which reads the amount of fuel in the tank and sends a signal to the gauge through a computer by an electrical pulse indicating how much fuel is in the tank.
|Test Your Knowledge
1. What grade of diesel fuel is used in warm and moderate climates?
2. Cloud point is the temperature at which ______ in the fuel begins to settle out, with the result that the fuel filter becomes clogged.
3. The fuel tank filler cap is constructed with both a pressure relief valve and a vent valve. The rate of leakage should not exceed how many ounces per minute?
- To Table of Contents -
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.
Inline Pump System Multiple-plunger, inline pump systems (Figure 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 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 pushrodactuated 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, multipleplunger, 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 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.
Figure 10 — Sleeve metering barrel and plunger assembly.
Basic operation is as follows:
Figure 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 fuelinjection pump, manufactured by Stanadyne's Hartford Division.
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:
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 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 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 pre-combustion 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. ***27
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 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 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 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 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:
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 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:
Figure 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.
|Test Your Knowledge
4. 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?
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Supercharging and turbocharging are methods of increasing engine volumetric efficiency by forcing the air into the combustion chamber, rather than merely allowing the pistons to draw it naturally. Supercharging and turbocharging, in some cases, will push volumetric efficiencies over 100 percent.
A supercharger is an air pump that increases engine power by pushing a denser air charge into the combustion chamber. With more air and fuel, combustion produces more heat energy and pressure to push the piston down in the cylinder.
The term supercharger generally refers to a blower driven by a belt, chain, or gears. Superchargers are used on large diesel and racing engines.
The supercharger raises the air pressure in the engine intake manifold. When the intake valves open, more air-fuel mixture can flow into the cylinders. An intercooler is used between the supercharger outlet and the engine to cool the air and to increase power (cool charge of air carries more oxygen needed for combustion).
A supercharger will instantly produce increased pressure at low engine speed because it is mechanically linked to the engine crankshaft. This low-speed power and instant throttle response are desirable for passing and for entering interstate highways.
The centrifugal supercharger has an impeller equipped with curved vanes (Figure 16). As the engine drives the impeller, it draws air into its center and throws it off at its rim. The air then is pushed along the inside of the circular housing. The diameter of the housing gradually increases to the outlet where the air is pushed out.
Figure 16 — Centrifugal supercharger.
A turbocharger is an exhaust-driven supercharger (fan or blower) that forces air into the engine under pressure (Figure 17). Turbochargers are frequently used on small gasoline and diesel engines to increase power output. By harnessing engine exhaust energy, a turbocharger can also improve engine efficiency (fuel economy and emissions levels).
Figure 17 — Turbocharger.
A turbocharger is located on one side of the engine. An exhaust pipe connects the exhaust manifold to the turbine housing. The exhaust system header pipe connects to the outlet of the turbine housing.
Theoretically, the turbocharger should be located as close to the engine manifold as possible. Then a maximum amount of exhaust heat will enter the turbine housing. When the hot gases move past the spinning turbine wheel, they are still expanding and help rotate the turbine.
The turbocharger consists of three major components: a radial inward flow turbine wheel and shaft, a centrifugal compressor wheel, and a center housing that supports the rotating assembly, bearings, seals, turbine housing, and compressor housing. The center housing also has connections for oil inlet and oil outlet fittings.
The turbine wheel is located in the turbine housing and is mounted on one end of the turbine shaft. Exhaust gases enter the turbine housing and spin the turbine wheel.
The compressor wheel is located on the turbine shaft on the opposite end of the turbine wheel. As the gases spin the turbine wheel, the turbine shaft spins the compressor wheel.
The turbine housing is made of a heat-resistant alloy casting that encloses the turbine wheel and provides a flanged exhaust gas inlet and an axially-located turbocharger exhaust gas outlet.
The basic operation of a turbocharger is as follows:
The turbocharger offers a distinct advantage for a diesel engine operating at higher altitudes. The turbocharger automatically compensates for the loss of air density. An increase in altitude also increases the pressure drop across the turbine. Inlet turbine pressure remains the same, but outlet pressure decreases as the altitude increases. Turbine speed also increases as the pressure differential increases.
Turbocharger lubrication is required to protect the turbo shaft and bearings from damage. A turbocharger can operate at speeds up to 100,000 rpm. For this reason, the engine lubrication system forces oil into the turbo shaft bearings. Oil passages are provided in the turbo housing and bearings, and an oil supply line runs from the engine to the turbocharger. With the engine running, oil enters the turbocharger under pressure. A drain passage and drain line allow oil to return to the engine oil pan after passing through the turbo bearings.
Sealing rings (piston-type rings) are placed around the turbo shaft at each end of the turbo housing, preventing oil leakage into the compressor and turbine housings.
While there are many types of turbocharger controls, they fall into two groups: those that limit turbocharger speed and those that limit compressor outlet pressure, or boost. Controls that limit turbocharger speed keep the turbocharger from destroying itself. Those that limit boost keep the turbocharger from damaging the engine. Since the modern turbocharger can produce more pressure than the engine can use, most controls are designed to limit the amount of boost. One of the most common methods of limiting the boost is with a waste gate valve.
A waste gate limits the maximum amount of boost pressure developed by the turbocharger. It is a butterfly or poppet-type valve that allows exhaust to bypass the turbine wheel.
Without a waste gate, the turbocharger could produce too much pressure in the combustion chambers. This could lead to detonation (spontaneous combustion) and engine damage.
A diaphragm assembly operates the waste gate. Intake manifold pressure acts on the diaphragm to control waste gate valve action. The valve controls the opening and closing of a passage around the turbine wheel. Under partial load, the system routes all of the exhaust gases through the turbine housing. The waste gate is closed by the diaphragm spring. This assures that there is adequate boost to increase power.
Under a full load, boost may become high enough to overcome spring pressure. Manifold pressure compresses the spring and opens the waste gate. This permits some of the exhaust gases to flow through the waste gate passage and into the exhaust system. Less exhaust is left to spin the turbine. Boost pressure is limited to a preset value.
The use of a turbocharger increases the temperature of the intake air. This increase in temperature is because the turbocharger compresses the air. To help counteract this increase in temperature, an intercooler or aftercooler is installed. There are two types of aftercoolers being used today: coolant aftercoolers and air-to-air aftercoolers.
In coolant aftercoolers, engine coolant flows through the aftercooler core tubes. As the hot compressed air from the turbocharger passes around the tubes, it is dropped to the temperature of the coolant.
In air-to-air aftercoolers, the air is a heat exchanger that cools the air entering the engine. It is a radiator-like device mounted at the pressure outlet of the turbocharger.
Outside air flows over and cools the fins and tubes of the intercooler. As the air flows through the intercooler, heat is removed. By cooling the air entering the engine, engine power is increased because the air is denser (contains more oxygen by volume). Cooling also reduces the tendency for engine detonation.
Turbo lag refers to a short delay before the turbocharger develops sufficient boost (pressure above atmospheric pressure).
As the accelerator pedal is pressed down for rapid acceleration, the engine may lack power for a few seconds. This is caused by the impeller and turbine wheels not spinning fast enough. It takes time for the exhaust gases to bring the turbocharger up to operating speed. To minimize turbo lag, the turbine and impeller wheels are made very light so they can accelerate up to rpm quickly.
|Test Your Knowledge
6. Which part does NOT drive a supercharger?
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Diesel fuel evaporates much slower than gasoline and requires more heat to cause combustion in the cylinder of the engine. For this reason, preheating devices and starting aids are used on diesel engines. These devices and starting aids either heat the air before it is drawn into the cylinder or allow combustion at a lower temperature than during normal engine operation.
In cold weather, coolant heaters are used to keep the fuel flowing freely. There are three common types of heaters used on mobile diesel engines: immersion block heaters, circulating tank heaters, and fuel-fired heaters.
The immersion block heater is installed directly into the engine block in a location predetermined by the manufacturer. The warmed coolant circulates around the cylinders in the block by convection. The heater is powered by either 120 or 240 volts.
Circulating tank heaters are installed in a way that creates circulation. The coolant leaves through the bottom of the engine block and travels to the heater. The heated coolant rises and is transferred back to the top of the engine block. These types of heaters should be of a higher wattage than immersion block heaters because some of the heat is lost through the hoses and tank. Circulating heaters are available in 120 or 240 volts.
Fuel-fired heaters are used to heat the engine and cab unit via the coolant by burning diesel fuel. These units burn less fuel than the engine would if left idling. The unit operates like a kerosene-fired space heater. The flame warms the coolant that is flowed through the space.
Most oil heaters are electric-powered immersion heaters that are installed in the oil sump through the drain plug or the dipstick opening. A thermostat can also be installed as part of the heating unit. This type of heater is designed to keep the oil pan warm; the heat radiates upward to warm the entire engine.
The purpose of a glow plug is to heat up the air that is drawn into the pre-combustion chamber to assist starting, especially in cold weather. Operating temperatures of 1500ºF can be reached in a matter of seconds. Glow plugs are common on pre-combustion chamber engines, but not on direct injection diesels because they use shaped piston crowns that produce a very effective turbulence to the air in the cylinder. Direct injection engines also have less immediate heat loss to the surrounding cylinder area than in a pre-combustion engine and generally have a higher injection spray-in pressure.
Figure 18 — Glow plugs.
A glow plug is used for each cylinder located just below the injection nozzle and threaded into the cylinder head (Figure 18). The inner tip of the glow plug extends into the pre-combustion chamber. The glow plugs may be turned on using the ignition switch with the length of time being controlled from an electronic module. During colder weather, the system may have to be cycled more than once to start the engine.
Glow plugs are not complicated and are easy to test. Disconnect the wire going to the glow plug and use a multimeter to read the ohms resistance of the glow plug. Specifications for different glow plugs vary according to the manufacturer. Be sure and check the manufacturer’s service manual for the correct ohms resistance value.
Starting fluids must never be used if the engine is equipped with either glow plugs or an electric intake heater.
Ether is a highly volatile fluid that is injected into the intake manifold as you crank the engine. It is found in an aerosol or capsule-type container. Since ether has a low ignition point, the heat generated in the combustion chamber is able to ignite it. Heat from this ignition then ignites the diesel fuel, and normal combustion takes place. Once the diesel engine starts, no more fluid is required. Use of spray cans of ether is discouraged. The only method of safely using ether is with a closed dispensing system.
Automatic ether systems are wired into the cranking circuit and dispense starting fluid when the cranking circuit is energized. Flow of the ether is controlled by a valve orifice or rapid cycling of the valve, resulting in continuous atomizer flow. An engine temperature switch prevents operation when the engine is warm.
The nozzle directs the flow of starting fluid into the airstream and determines the rate at which it is injected. The direction of the spray should be against the flow of incoming air to maximize mixing with the air.
Cold starting aids such as ether should be used only in extreme emergencies. Too much ether may detonate in the cylinder too far before top dead center (BDTC) on the compression stroke. This could cause serious damage, such as broken rings, ring lands, pistons, or even cracked cylinder heads. If you must use ether, the engine has to be turning over before you spray it into the intake manifold.
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If all diesel engines had nearly identical fuel system trouble, diagnosis and maintenance procedures could follow a general pattern. But, with the exception of similar fuel tanks and basic piping system, diesel fuel systems differ considerably. Consequently, each engine manufacturer recommends different specific maintenance procedures. However, the tune-up and maintenance procedures described are representative of the job you will do. For all jobs, refer to the manufacturer’s service manual for the fuel system you are servicing, even if you fully understand all procedures.
Many diesel engine operating troubles result directly or indirectly from dirt in the fuel system. That is why proper fuel storage and handling are so important. One of the most important aspects of diesel fuel is cleanliness. The fuel should not contain more than a trace of foreign substance; otherwise, fuel pump and injector troubles will occur. Diesel fuel, because it is more viscous than gasoline, will hold dirt in suspension for longer periods. Therefore, every precaution should be taken to keep the fuel clean.
If the engine starts missing, running irregularly, rapping, or puffing black smoke from the exhaust manifold, look for trouble at the spray nozzle valves. In this event, it is almost a sure bet that dirt is responsible for improper fuel injection into the cylinder. A valve held open or scratched by particles of dirt so that it cannot seat properly will allow fuel to pass into the exhaust without being completely burned, causing black smoke. Too much fuel may cause a cylinder to miss entirely. If dirt prevents the proper amount of fuel from entering the cylinders by restricting spray nozzle holes, the engine may skip or stop entirely. In most cases, injector or valve troubles are easily identified.
Improper injection pump operation, however, is not easily recognized. It is more likely caused by excessive wear than by an accumulation of dirt or carbon, such as the spray nozzle is subjected to in the cylinder combustion chambers. If considerable abrasive dirt gets by the filters to increase (by wear) the small clearance between the injector pump plunger and barrel, fuel will leak by the plunger instead of being forced into the injector nozzle in the cylinder. This gradual decrease in fuel delivery at the spray nozzle may remain unnoticed for some time or until the operator complains of sluggish engine performance.
Although worn injector pumps will result in loss of engine power and hard starting, worn piston rings, cylinder liners, and valves (intake and exhaust) can be responsible for the same conditions. However, with worn cylinder parts or valves, poor compression, a smoky exhaust, and excessive blow-by will accompany the hard starting and loss of power from the crankcase breather.
It requires only a little water in a fuel system to cause an engine to miss, and if present in large enough quantities, the engine will stop entirely. Many fuel filters are designed to clog completely when exposed to water, thereby stopping all fuel flow. Water that enters a tank with the fuel or that is formed by condensation in a partially empty tank or line usually settles to the lowest part of the fuel system. This water should be drained off daily.
Air trapped in diesel fuel systems is one of the main reasons for a hard starting engine. Air can enter the fuel system at loose joints in the piping or through a spray nozzle that does not close properly. Letting the vehicle run out of fuel will also cause air to enter the system. Like water, air can interfere with the unbroken flow of fuel from the tank to the cylinder. A great deal of air in a system will prevent fuel pumps from picking up fuel and pushing it through the piping system. Air can be removed by bleeding the system as set forth in the procedures described in the manufacturer’s maintenance manual.
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When troubleshooting a diesel engine, keep in mind that problems associated with one make and type of engine (two-stroke versus four-stroke) may not occur exactly in the same way as in another. Specifically, particular features of one four-stroke-cycle engine may not appear on another due to the type of fuel system used and optional features on that engine. Follow the basic troubleshooting steps listed below before rolling up your sleeves and trying to pinpoint a problem area.
One of the easiest methods to use when troubleshooting an engine for a performance complaint is to visually monitor the color of the smoke coming from the exhaust stack. There are four basic colors that may exit from the exhaust system at any time during engine operation—white, black, gray, or blue. The color of the smoke tips you off to just what and where the problem might lie.
White smoke is generally most noticeable at engine start-up, particularly during cold conditions. As the combustion and cylinder temperatures increase during the first few minutes of engine operation, the white smoke should start to disappear which indicates the engine is sound. However, if the white smoke takes longer than 3 to 5 minutes to fade away, a problem exists. The problems white smoke may indicate are as follows:
Black or gray smoke generally is caused by the same conditions—the difference between the colors being one of opacity or denseness of smoke. Black or gray smoke should be checked with the engine at operating temperature of 160°F. Abnormal amounts of exhaust smoke emission is an indication that the engine is not operating correctly, resulting in a lack of power, as well as decreased fuel economy. Excessive black or gray exhaust smoke is caused by the following:
Blue smoke is attributed to oil entering the combustion chamber and being burned or blown through the cylinder and burned in the exhaust manifold or turbocharger. Remember: always check the simplest things first, such as too much oil in the crankcase or a plugged crankcase ventilation breather. The more serious problems that can cause blue smoke are as follows:
Listed below are several quick and acceptable checks that can be performed on a running engine to determine if one or more injectors are at fault on any type of engine.
On four-stroke-cycle engines with a high-pressure in-line pump or distributor system, such as Caterpillar and Roosa Master, you can loosen off one injector fuel line, one at a time, about one-half turn as you hold a rag around it while noting if there is any change in the operating sound of the engine. If the injector is firing properly, there should be a positive change to the sound and rpm of the engine when you loosen the line, since it prevents the delivery of fuel to the cylinder.
On an engine with the PT fuel system, a cylinder misfire can be checked by running the engine to a minimum of 160°F. Remove the rocker covers and install a rocker lever actuator over an injector rocker lever. Hold the injector plunger down while the engine is running at low idle. This will stop the fuel flow to that injector. If the engine speed decreases, the injector is good. If the engine rpm does not decrease, replace the injector.
On the two-stroke-cycle non-electronic Detroit diesel engines, remove the rocker cover; then, using a large screwdriver push and hold down the injector follower while the engine is idling. This action is like shorting out a spark plug on a gasoline engine, since it prevents fuel from being injected into the combustion chamber. If there is no change to the sound and speed of the engine, the injector is not firing. There should be a definite change to indicate that the injector was in fact firing.
The dead cylinder Test is another name for the quick injector misfire check. It is performed in the same manner. If you experience a problem while performing this test, you have a “dead cylinder”. ***41
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In this manual you have learned about the diesel fuel system and its components, different methods of injection, superchargers, turbochargers, and cold starting devices, and have been briefly introduced to some troubleshooting techniques. Because there are newer and better innovations every day, you should also refer to the manufacturer’s guide for specific systems. Your knowledge of the diesel fuel system will enable you to evaluate certain engine problems with confidence that the fuel system can be diagnosed.
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1. What factor makes it possible to ignite the air-fuel mixture of a diesel engine without the use of a spark plug as required in a gasoline engine?
2. What action controls the speed of a diesel engine?
3. Which characteristic is one advantage of the diesel engine over the gasoline engine?
4. What agency is responsible for grading diesel fuel?
5. What grade of diesel fuel is used in truck fleets because of its greater heat value?
6. Which factor must be considered when selecting a fuel oil?
7. The measure of the volatility of a diesel fuel is known as the _______ number.
8. If the cetane number of a diesel fuel is too low, which condition can result?
9. Current diesel fuels have a cetane rating that ranges between _______.
10. Low volatile fuels tend to provide better fuel economy and produce _______.
11. Which property has a direct bearing on the life expectancy of the engine and its components?
12. Which combustion chamber design is the most common?
13. When precombustion chambers are used on a diesel engine, which factor causes the greatest amount of fuel atomization?
14. What component is designed to prevent an engine from over-speeding and allow the engine to meet changing load conditions?
15. At what location is the governor connected on a diesel engine?
16. What type of governor prevents an engine from exceeding a specified maximum speed?
17. What type of governor maintains any specified engine speed between idle and maximum speed?
18. What type of governor provides a regular or stable engine speed, regardless of load conditions?
19. What part of a spring-loaded mechanical governor does the manual throttle directly adjust?
20. The tension of the spring in the mechanical flyweight governor has a tendency to _______.
21. For engine speed to stabilize, what condition must exist within the governor?
22. Which characteristic is NOT an advantage of a mechanical governor?
23. The hydraulic governor is inherently unstable. To maintain stability, hydraulic governors employ _______.
24. In an electronic governor, at what location is the magnetic pickup sensor installed?
25. Sediment or water is prevented from entering the fuel system because the inlet fuel line is how far from the bottom of the tank?
26. Why is it necessary to have a supply pump to transfer fuel from the tank to the injection pump of a diesel engine?
27. What are the five functions of a diesel fuel injection system?
28. The rate at which fuel is injected also determines the rate of_____.
29. What type of injection system is used on Caterpillar diesel engines?
30. What action varies the metering of fuel in a Caterpillar injection system?
31. With the engine operating at full load, the transfer pump fills the injection pump housing with fuel at approximately _______ psi.
32. At approximately what rate, in gallons per hour, does the constant bleed valve return fuel back to the fuel tank?
33. What type of governor is used on the sleeve metering fuel system?
34. The sleeve metering fuel system uses what for lubrication?
35. What type of seal is used to prevent engine oil from entering the DB2 fuel pump?
36. What positive displacement type of transfer pump is used in the DB2 fuel pump?
37. What action limits the maximum amount of fuel that can be injected by the DB2 fuel pump?
38. In the PTG-AFC fuel pump, what determines the AFC plunger position?
39. After replacing the injectors in a Cummins PT fuel system, you should readjust them after the engine has been warmed up to within what temperature range?
40. When overhauling a set of PT fuel injectors, you must keep them together because they are ______ sets.
41. What drives a turbocharger?
42. What type of cold weather starting devices uses convection as a means to transfer heat?
43. Blue smoke coming from the exhaust indicates the existence of which condition?
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