How does a Fuel Gauge Work?

A Fuel gauge is used to indicate to the driver the level of the fuel present in the tank. It is mounted on the instrument board in front of the driver. In this article, I will discuss the details of the fuel gauge working principle. 

How does a fuel gauge work?

How does a fuel gauge work?
How does a fuel gauge work

Typically it is a gauge that measures the amount of fuel remaining in the fuel tank before it runs empty. It will usually have graduated markings on it to give a high-level indication of precisely how much fuel is left.

Newer cars might have an electronic gauge as part of the car diagnostics display – it won’t have a “traditional” gauge as such, but rather a readout that indicates the distance left until the tank runs empty. Some cars might have both a digital readout and a “traditional” gauge.

Working Principle- 

The gauge is also a simple device. The current from the sender passes through a resistor that either wraps around or is located near a bimetallic strip. The bimetallic strip is hooked up to the needle of the gauge through a linkage.

The bimetallic strip is a piece of metal made by laminating two different types of metal together. The metals that make up the strip expand and contract when they are heated or cooled. Each type of metal has its own particular rate of expansion. The two metals that make up the strip are chosen so that the rates of expansion and contraction are different.

When the strip is heated, one metal expands less than the other, so the strip curves, with the metal that expands more on the outside. This bending action is what moves the needle.

As resistance increases, less current passes through the heating coil, so the bimetallic strip cools. As the strip cools, it straightens out, pulling the gauge from full to empty.

Some newer cars, instead of sending the current directly to the gauge, use a microprocessor that reads the output of the resistor and communicates with the dashboard. These systems actually help improve the accuracy of the gauge.

Types of fuel Gauge- 

Fuel gauges are an important part of any vehicle and can help ensure that you always have enough fuel to get to where you need to go. There are many different types of fuel gauges, each with its own unique features and benefits. Let’s take a look at the different types of fuel gauges available.

  1. Ac electrical fuel gauge.
  2. Bimetal-type electrical fuel gauge.
  3. Thermal type electrical fuel gauge.
  4. Thermostatic type electrical fuel gauge. 

Ac electrical fuel gauge- 

Ac electric fuel  gauge.
Ac electric fuel gauge.

This is an Ac electric fuel gauge with balanced coils. It consists of two units-tank unit and a dash unit. The two units are connected by a single wire. The tank unit comprises a rheostat(resistance coil and movable contact) and a float. The rheostat contact is actually by float to assume a position on the resistance corresponding to the part of the float in the fuel tank. This movable contact is earthed. The dash unit or the instrument board unit consists of two coils spaced 90° apart with an armature and pointer assembly provided at the intersection of the coil axes. The right-hand coil is earthed. 

Working principle- 

When the ignition switch is turned on, current from the battery flows through the two coils. This produces a magnetic field that acts on the armature to which the pointer is attached.

When the tank is full and the float is up, the resistance in the tank unit is high. The current flowing through the E (empty) coil also flows through the F (full) coil. Thus, the armature is pulled to the right so that the pointer indicates the F (full) side of the dial.

When the tank begins to empty, the float goes down and the resistance of the tank unit drops. Thus, more of the current flowing through the E coil passes through the tank unit. Since less current is flowing through the F coil, its magnetic field is weaker and the armature is pulled to the left so that the pointer indicates towards the E (empty side of the dial).

The fuel gauge operates only when the ignition switch is turned on. The current consumption of the gauge is so low (0.15 amp) as to cause no appreciable drain on the electric system.

Vibrations in voltage supplied to the gauge do not affect their accuracy as their operation depends upon the proportion of electric current flowing through the coils of the dash unit, rather than on the actual strength of their magnetic fields.

The construction of the fuel gauge depends upon the tank. For small tanks, the movable contact is actuated directly by the float arm. The float may be either single or double. For large tanks, the movable contact may be mounted on a vertical axis and driven from the float arm by bevel gears. Many fuel gauges are designed to provide approximately five or fewer liters of reserve fuel when the pointer indicates the fuel tank is empty.

Bimetal type electric fuel gauge- 

Bimetal type electric fuel gauge.
Bimetal-type electric fuel gauge

This is a type of bimetal type electric fuel gauge. It also consists of two units — the sender unit and the dash unit or receiver unit. The sender unit is located in the fuel tank. It consists of a heating coil wound around a bimetal strip and an external float mechanism that acts through a cam and flexible diaphragm so that the position of the float varies the height of earthed contact which in turn varies the tension in the bimetal strip. The receiver unit is located on the instrument panel in front of the driver. The two units are connected by a wire and receive current from the battery of the generator.

Working principle- 

When the fuel tank is empty, the float is down and the two contacts in the sender unit are just touching. In this position, when the ignition switch is turned on, the current flows through the circuit, and heat is generated in the heating coil causing the bimetal to bend. As soon as the bimetal bends, it opens the contact, and the circuit breaks. Then the heating coil and bimetal cool; and the spring returns to its former position where contact is again made. Since the heating coils of the sender and receiver units are connected in series, similar slight bending of the bimetal in the receiver unit takes place, which pulls the pointer to the zero position of the receiver unit.

When the fuel tank is full, the float is up, and the eccentric shaft raises the earthed contact against the insulated bimetal contact, bending the bimetal in the sender unit. As the bimetal is under tension. more current is required to heat it so as to bend it sufficiently to break contact. Due to the increased current, a similarly increased bending of the bimetal in the receiver unit occurs which pulls the pointer to the “full” position. The cycle of opening and closing the contacts is continuously repeated.

Thermal type electric fuel gauge- 

Thermal type electric fuel gauge.
Thermal-type electric fuel gauge

This is a thermal-type electric fuel gauge. It consists of two units—a dash unit and a tank unit. The dash unit is exactly like the receiver unit of the bimetal-type electric fuel gauge. The tank unit consists of a rheostat as a sliding contact on a resistance wire, which is moved by the float. The two units are connected in series to a constant voltage regulator, which is energized when the ignition switch is turned on.

Working principle- 

When the fuel tank is full and the float is up, the circuit has the least resistance in the tank unit resistance wire and more current flows through the circuit. This causes heating of the bimetal arm so that it bends and moves the needle towards the “full” position.

When the tank begins to empty, the float drops. It increases the resistance which reduces the current to a value that causes the bimetal arm to force the needle toward the “empty” position.

Thermostatic type electric fuel gauge- 

Thermostatic type electric fuel gauge.
Thermostatic-type electric fuel gauge

This is a thermostatic-type electric fuel gauge. . It is an early type gauge still used on some late-model cars. It uses two wires to connect the dash unit and tank unit, instead of one wire as in other gauges. The dash unit consists of a pair of thermostat blades, each with a heating coil. The coils are connected in series through the ignition switch to the battery. When the ignition switch is turned on, the current passing through the coils heats them. The heat causes the blades to bend so as to move an indicator over a scale. The tank unit consists of a float that actuates a cam. The cam, on turning, imposes bending on the tank thermostat blade.

Working principle- 

when the tank is full and the float is up the cam puts a considerable bend in the blade. The current flowing through the heater coils, when the ignition switch is turned on, heats the tank blade. It bends farther so that the contact separates. Then the blade cools and the points re-open. This action continues as long as the ignition switch is on. Meanwhile, the blade in the dash unit is heated and bends the same amount. Movement of this blade is carried through linkage to the pointer which moves to indicate a “full” position. 

When the tank is empty and the float is down, the cam bends the tank thermostat blade only a little. Thus. only a small amount of heating is enough to turn the blade farther and open the contacts. The dash unit blade bends only a little and the pointer indicates the “empty” position.


The fuel gauge is very much responsible for the perfect quantity of fuel present in the fuel tank. So, I think, this article will clear all your doubts about Fuel Gauge. 

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