Automobiles: Definition, Classification, and Components

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Automobile engineering is one of the best streams of mechanical engineering. It deals with the various types of automobile engineering, the mechanisms of transmission systems, and their applications. Automobiles are different types of vehicles used for the transportation of passengers, goods, etc.

Basically, all types of vehicles work on the principle of internal combustion processes, or sometimes the engines are called internal combustion engines. Different types of fuel are burned inside the cylinder at a higher temperature to get high transmission motion in the vehicles. Most automobile systems are internal combustion engines only. Therefore, every mechanical engineer and automobile engineer should have knowledge of automobile engineering, its mechanisms, and its various applications.


  • After studying this unit, you should be able to
  • define automobile engineering,
  • classify the vehicles,
  • list the various components of automobile, and
  • describes the function of components of automobile


Automobile engineering is the branch of engineering that deals with everything about automobiles and practises to propel them. An automobile is a vehicle driven by an internal combustion engine and used for the transportation of passengers, goods, and people on the ground. An automobile can also be defined as a vehicle that can be moved by itself. Examples: Car, jeep, bus, truck, scooter, etc.

Classification of Automobile

Automobiles or vehicles can be classified into different bases as given below:

On the Basis of Load

  • Heavy transport vehicles (HTV) and heavy motor vehicle (HMV), e.g. trucks, buses, car, jeep etc.
  • Light transport vehicle (LTV), e.g. pickup, station wagon, etc.
  • Light motor vehicle (LMV), e.g. cars, jeeps, etc.


  • Two-wheeler vehicle types, for example Scooter, motorcycle, scooty, etc.
  • The three wheel drive vehicle, for example: Auto rickshaw, three wheeler scooter for handicaps and tempo, etc.
  • Four wheeler vehicle, for the example: Car, jeep, trucks, buses, tempo etc.
  • Six wheeler vehicle, for the example: Big trucks with the two gear axles for each having four wheels drive.

Fuel Used

  • Petrol vehicle, e.g. motorcycle, scooter, cars, etc.
  • Diesel vehicle, e.g. trucks, buses, etc.
  • Electric vehicle (EV) which is the use battery to drive.
  • Steam vehicle, e.g. an engine which uses steam engine. These engines are now obsolete.
  • Gas vehicle, e.g. LPG and CNG vehicles, where is the LPG liquefied petroleum gas and CNG is the compressed natural gas.


On the basis of the body vehicles are classified as below :

  • Sedan with two doors
  • Sedan with four doors
  • Station wagon
  • Convertible, e.g. jeep, etc.
  •  Van
  • Special purpose vehicle, e.g. ambulance, milk van, etc.


  • Conventional vehicles with manual transmission, e.g. car with 5 gears.
  • Semi-automatic
  • Automatic: In an automatic transmission system, gears are not required to be changed manually. It automatically changes as per the speed of the automobile vehicles.

 Position of Engine

Engine in Front

Most of the vehicles have the engine in the front of vehicles. Example: most of the cars, buses, trucks, jeep,tempo in India.

The engine in the Rear Side

 Very few vehicles have the engine located on the rear side. Example: Nano car


The automobile system can be considered to consist of five basic components:

  • Is the Engine or Power Plant: It is the source of the power plant.
  • The Frame and Chassis: It is the supports engine, wheels, body, braking system, steering, etc.
  • The transmission transmits the power from the engine into the car’s wheels. It consists of the clutch, transmission, shaft, axles, and differential.
  • The body.
  • Accessories are including light, air conditioner/hearer, stereo, wiper, etc.


A brake is a mechanical device that inhibits motion, slowing or stopping a moving object and preventing its motion. The rest of this article is dedicated to the various types of vehicle brakes system.

Most commonly, brakes use the friction between the two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other methods of energy conversion may be employed. For example, regenerative braking converts so much of the energy to electrical energy, which may be stored for later use. Other methods convert kinetic energy into potential energy in stored forms, such as pressurised air or pressurised oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. The other braking system even transfers kinetic energy into different forms, for example, by transferring the energy to a rotating flywheel.

Brakes are generally applied to the rotating axles or wheels but may also take other forms such

as the surface of the moving liquid fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or aeroplanes with both wheel brakes and drag flaps increased into the air during landing.

Since kinetic energy increases quadratically with velocity (K = 1/2 mv2), an object moving at 10 m/s has 100 times as much energy as one of the same mass moving at 1 m/s, and consequently, the theoretical braking distance when braking at the friction limit is 100 times as long. In practice, fast vehicles usually have a significant air drag system, and the energy lost to air drag rises quickly with high speed. Almost all vehicles have a brake of some sort. Even baggage carts and shopping carts may have them for use as moving ramps. Most fixed-wing aircraft are fitted with wheel brakes on the undercarriage. Some aircraft also feature an air brake system designed to reduce their speed during flight.

When the brake pedal of the modern vehicle braking system with hydraulic brakes is pushed, ultimately a piston pushes the brake pad against the brake disc which slows the wheel down. On the brake drum system, it is similar to the cylinder pushing the brake shoes against the drum which also slows the wheel down. Brakes may be broadly defined as using friction, pumping, or electromagnetic.

One brake system may use several principles: for example, a pump may pass fluid through an orifice to create friction. Frictional brakes are most commonly used and can be broadly divided into “shoes” or “pads” brakes, which use an explicit wear surface, and hydrodynamic brakes, such as parachutes, which use friction in the working fluid and do not explicitly wear.

Typically the terms “friction brake system” it is used to mean pads/shoe brakes and excludes hydrodynamic brakes, even though hydrodynamic brakes use friction. Friction (pad/shoe) brakes are often rotating devices with a stationary pad and a rotating wear surface.

Common configurations include shoes that contract to rub on the outside of a rotating drum, such as a band brake; a rotating drum with shoes that expand to the rub inside of the drum, commonly called as a “drum brake system”, although other drum configurations are possible; and pads that pinch a rotating disc, commonly called a “disc brake”


The disc brake is a device for slowing or stopping the rotation of the wheel while it is in motion. A brake disc is generally use made of cast iron, but may in some cases be made of composites such as reinforced carbon-carbon or ceramic-matrix composites.

This is connected to the wheel and the axle. To stop the brake wheel, friction material in the form of brake system pads mounted on a device called a brake calliper is forced mechanically, hydraulically, pneumatically, or electromagnetically against both sides of the disc. The friction causes the disc and attached wheel to slow or brake. Brakes: both the disc and drum convert the motion to heat, but if the brakes get too hot, they will become less effective because they cannot dissipate enough heat. This condition of failure is known as the brake fade.


The most common arrangement of hydraulic brakes for passenger vehicles, motorcycles, scooters, and mopeds, jeeps, consists of the following.

  • Brake pedal or lever
  • A push-rod (also called an actuating rod)
  • The master cylinder assembly contains a piston assembly
  • Reinforced hydraulic lines
Braking system

Brake calliper assemblies usually consist of one or two hollow aluminum and chrome-plated steel pistons called the calliper pistons, a set of thermally conductive brake pads, and a rotor, also called the brake disc or drum brake, attached to an axle. The brake system is usually filled with glycol-ether-based brake fluid; other fluids may also be used.

At one time, passenger vehicles commonly employed drum brake systems on all four wheels. Later, disc brakes were used for the front and drum brakes for the rear. However, disc brakes have shown better heat dissipation and increased resistance to ‘fading’ and are therefore generally safer than drum brakes. Many 2-wheeler vehicle designs, however, continue to employ a drum brake for the rear wheel.

In the hydraulic braking system, when the brake pedal is pressed, a push-rod exerts force on the piston(s) in the master cylinder, causing fluid from the brake fluid reservoir to flow into a reassure chamber through a compensating port. This result is an increase in the pressure of the entire hydraulic braking system, forcing fluid through the hydraulic lines toward one or more calipers where it acts upon one or 2 caliper pistons sealed by one and more seated O-rings which prevent leakage of the fluid.

The brake calliper pistons are applied to force the brake pads against the spinning rotor, and the friction between the pads and the rotor causes the braking torque to be generated, slowing a vehicle. Heat generated by this friction is either dissipated through vents and channels in the rotor or is conducted through the pads, which are made of specialised heat-tolerant materials such as Kevlar or sintered glass.

Subsequent release of the brake pedal or lever allows the spring in my master cylinder assembly to return the master piston back into position. This action first relieves the hydraulic pressure on the brake calliper, then applies suction to the brake piston in the calliper assembly, moving it back into its housing and allowing the brake pads to release the rotor.


An air brake system, or more formally, a compressed air brake system, is a type of friction brake for vehicles in which compressed air pressing on a piston is used to apply the pressure to the brake paddle needed to stop the vehicle. Air brakes are used in large, heavy-loaded vehicles, particularly those having multiple trailers that must be linked into the brake system, such as trucks, buses, trailers, and semi-trailers, in addition to their use in railroad trains.


Air braking systems are typically used on heavy trucks and buses. The system consists of the service brakes, parking brakes, control pedal, and an air storage tank. For the parking brake, there is a disc brake or drum brake arrangement that is designed to be held in the ‘applied’ position by spring pressure. Air braking pressure must be produced to release these “spring break” parking brakes. For the service brakes, which are used while driving for slow or stop to be applied, the brake pedal is pushed, routing the air under a pressure of approx. 100–120 PSI and 690–830 Pascal (Kpa) to the brake chamber, causing the brake to be engaged.

Most types of trucks use air brakes with drum brake systems, though there is an increasing trend towards the use of disc brakes in this application. The compressor draws filtered air from the atmosphere and forces it into the high-pressure reservoirs at around 120 psi (830 kPa). Most heavy vehicles have a gauge within the driver’s view indicating the availability of the air pressure for safe vehicle operation, often including warning tones or lights. Setting the parking or emergency brake releases the pressurized air in the lines between the compressed air storage tank and the brakes, thus allowing the spring-actuated parking. Brake to engage. The sudden loss of air pressure would result in full spring-breaking pressure immediately.

The air-compressed brake system is divided into a supply system and a control system. The supply system compresses, stores, and supplies high-pressure system air to the control system as well as to additional air-operated auxiliary truck systems such as gearbox shift control, clutch pedal air assistance servo, etc.

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