4.1 Diesel Fuel Systems

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 4-1.

Figure 4-1 — 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

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 Oil Grades

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 as follows:

Cetane Number

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 Content

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 and Pour Point

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 and Stability

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.

Combustion Chamber Design

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 Combustion Chamber

Direct injection is the most common combustion chamber (Figure 4-2, 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.

Figure 4-2 — Combustion chambers.

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 Combustion Chamber

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.

Pre-combustion Chamber

Precombustion chamber design involves a separate combustion chamber located in either the cylinder head or wall. As Figure 4-2, 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 precombustion chamber. When fuel is injected into the precombustion chamber, it partially burns, building up pressure. This pressure forces the mixture back into the combustion chamber, and complete combustion occurs.

Combustion Chamber

Swirl chamber systems (Figure 4-2, 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.

Engine Governors


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.

Term Definition
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 Is a repeated and sometimes rhythmic variation of speed due to over control by the governor. Also called speed drift.
Sensitivity  Is an expression of how quickly the governor responds to a change in speed.
Response time Is 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.

Types of Governors

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:

Mechanical Governors. In most governors installed on diesel engines, 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 4-3), 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 4-3 — 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 as follows:

The disadvantages are as follows:

Hydraulic Governors. 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 4-4), 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 4-4 — 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).

Electronic Governors. 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.

Diesel Fuel System Components

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 4-5. 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 4-5— 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.

Tank and Cap

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 4-6) 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.

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.

Figure 4-6 — Fuel tank construction.

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.

Supply Pump

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.

Fuel Filters

Diesel fuel filters (Figure 4-7) 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 4-7 — 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.

Water Separators

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 4-8) 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. Their basic operation is as follows:

Figure 4-8 — Water separators.

Injection Pump

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.

Return Line

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?

A. 4D
B. 3D
C. 2D
D. 1D

2. Cloud point is the temperature at which _____ in the fuel begins to settle out, with the result that the fuel filter becomes clogged.

A. cetane
B. octane
C. wax
D. water

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?

A. 4
B. 3
C. 2
D. 1