Automotive Engineering - An Introduction to Electrical System.

Introduction to Automobile 
Electronics

~Jay Mehta

Electric Power: -

• In cars electric power can be made available in two ways:
• -       Storage battery.
• -       Generators
• Storage batteries need to be charged so that vehicle can move when it departs.
• Generators attached to the road wheels would just provide electricity when the car is moving.
• Usually, we use a combination of two to overcome individual disadvantages present in them.

Main Problems Faced in Electrical Supply in Automobile: -

• -       Ensure that battery is charged properly.
• -       It must not be overcharged or its life will be shortened.
• -       If it is undercharged it might not be able to start the engine.
• -       Overcharging during a running of the vehicle should be avoided.
• -       Reduce the weight of the battery and generator.
• -       High reliability, low maintenance cost.
• -       The power requirement of the electrical equipment must be kept low.

The Circuit: -

The metal work of the vehicle is used as a part for current to flow so as to complete the circuit.

British practice made the earthing positive but the change to negative earthing is still in progress.

Earth Return: -

Two necessary wires ‘go’ and ‘return’. In most of the cars, the connection is done via just one wire and other connections are completed through the steelwork or the metal body of the car.

Usually, the positive side of the battery is connected to earth. But nowadays, the negative side is being connected to earth for safety reasons. 

This single wiring has an advantage of cost and proper insulation can be assured. Proper insulation is a must in wires for waterproofing and electromagnetic losses – thus providing better connectivity and conductivity of current.

With earth being in the return circuit there may be a point where the connection with car meets corrosion which may cause a defective circuit.

Return to earth circuit convention is unlikely to change as the headlights and other devices which are connected by two insulated wires. Hence, we must accept the scheme of return to earth and think of problems that may arise.

Voltage and Current in Circuit: -

Current-Water Analogy.

Voltage is the pressure that allows current to flow.

Voltage exists on every device where electricity is conducted. Resistance is compared to the rusty and smooth pipes.

Ohms Law: -

V = I x R.

Current flowing through a conductor is directly proportional to the voltage across it, with constant of proportionality being the resistance which depends on temperature, pressure, cross-sectional area and other physical parameters.

The wattage of the headlight in a car is the measure of the amount of power consumed by it. 

Given by P = V x I. Where, P represents power consumed.

Ohms Law needs some modification when we deal with AC circuits of the car.

In general, electrical engineers use highest possible voltage which is practically possible as it provides low current and high power at the same time; and we know as current increases the gauge of wire needs to be decreased that a thick wire needs to be taken. Hence, we go for high voltage low current scheme to get the max power as it will help us take a wire with high gauge (thin wire) leading to cost reduction. This helps us save a large amount of cost that could have been wasted in wiring harness of the automobile.

Avoiding Wastage of Energy: -

The aim is to provide as little power dissipation possible hence, choosing a low resistive wire material for the wiring harness. When current passes through the conductor energy is consumed to heat up the conductor.

Choice of Voltage and System: -

• -       To use battery while the vehicle is stationary, choosing appropriate voltage range for the system becomes extremely narrow.
• -       Storage batteries work on direct current and are made up of individual cells; each giving a voltage of about 2.16 V.
• -       Each cell irrespective of its size can deliver only 2.16 V.
• -       We can think of making a single 2 V battery large enough to power the whole vehicle or merge various 2 V batteries by connecting them in series. The former is possible theoretically only, as other factors affect the practicality.

Lower the Voltage Thicker the Wire Needed: -

• -       Why don’t we use 240 volts supply?
• -       We don’t use 240 volts supply directly because though the size of cells will be reduced to a great extent but the number of cells increases to 120 cells leading to adding weight to the car by making it bulky. Moreover, chances of fire are increased as the smallest spark near the carbonator can lead to a massive fire in a petrol-laden atmosphere.
• -       Hence, we choose an optimal solution by taking batteries of voltage ranging from 6 to 24 V. This range fits into the maximum number of criteria and allows the proper functioning of all electrical networks in the automotive.

The Elements of The System: -

• -       Source of power: - Engine driven generator.
• -       Storage of power: - Storage battery (accumulator).
• -       Control of power: - battery charging, regulator equipment, fuses, wires, switches etc.
• -       Utilization of power: - ignition, engine starting, lighting, heat fans, windscreen wipers, other auxiliary devices.

~Jay Mehta

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Jay Mehta.

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