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.

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.

Ac electric fuel gauge.

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

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

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.

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. 

Conclusion-