Spark Plug | Working of a Spark Plug

Spark plug The spark plug is fitted to the cylinder head. Its function is to conduct the high potential into the combustion chamber and produce a spark.

The cross section of a typical spark plug can be seen in the picture The spark plug has two electrodes, a steel shell assembly and an insulator unit.


The steel shell assembly houses the core insulator unit. The lower portion of the shell is threaded. The ground electrode is welded to the lower portion of the threaded section. The upper portion of the shell is hexagonal is shape, like a bolt head. This hexagonal portion provides a surface for the spark plug spanner. With the help of the spanner, the plug can be screwed into and out of the threaded hole in the cylinder head.

The core insulator is designed to withstand high voltages, pressures as high as 45 kscm and temperatures approximating 2000°C.

The center electrode is housed within the insulator. This is so made without causing distortion, shrinkage or cracking. The electrode is cemented or screwed into the insulator. Sometime, the electrode is secured within the insulator by a fused vitreous seal. This seal ensures perfect sealing under expansion and contraction. The upper end of the center electrode is connected to the terminal of the spark plug wire. The lower end of the center electrode projects beyond the insulator. This end forms a small gap with the ground electrode.

Sealing gaskets are incorporated between the insulator and the shell of the spark plug. These gaskets prevent the escape of gases under various temperature and pressure conditions. A copper gasket is incorporated between the plug shell and the cylinder head. This gasket provides a seal between the two units and helps in the conduction of heat.

The size of the electrode must be fairly substantial to resist burning. The size of the electrode also determines heat transfer. The electrode temperature must be sufficiently high to burn off rapidly any sooty or oily deposit. A compromise is made in deciding the size of the electrodes. As such, the electrode size will vary for different types of engines.

The shape of the electrode should be such as to allow any oil to drain away from the spark plug. This is important in the case of two stroke engines which use petroil (petrol mixed with lubricating oil) lubrication.


The spark plug electrodes are made of platinum tungsten or iridium alloys or more usually of nickel chromium barium alloys.

In the past, porcelain or mica were generally used for the spark plug insulator. Presently, oxides of aluminium and silicon are used. The insulator is glazed with a silica coating, expect near the tip. This coating resists adherence of carbon on the surfaces exposed to the combustion gases.

Chassis | Components of a Chassis | Automobile System

Components of the chassis

The various components of the chassis and their location in a vehicle can be seen in this picture


Engine produces power required to move the vehicle at the desired speed, overcoming the external resistances. This can be a SI engine or CI engine or gas engine.

Radiator is a device used to re-cool the hot engine cooling water for recirculation purpose. The radiator is connected by rubber hoses to the engine, to allow the cooling system water to circulate between them. Other parts of the cooling system are fan, fan belt drive, and water circulating pump.

Clutch helps to isolate the engine from the transmission system as and when required. It also permits the vehicle to be started from rest, smoothly and with jerks.

Gearbox provides the torque of the required amount at the driving road wheels. Whenever the resistance to be overcome by the vehicle alters, the gear ratio of the power train is to be changed. The gear box serves precisely this purpose. The gearbox also helps to reverse the vehicle.

Universal joints cum propeller shaft transmits the torque from the gearbox shaft to the final drive. The gearbox is usually attached to the vehicle frame. The final drive, differential and rear axle are connected to the frame by means of springs. As the tyres of the vehicle hit the bumps in the road, the rear axle moves up and down. The universal joints help the propeller shaft to assume different inclination. The propeller shaft has a sliding arrangement within itself. This helps the shaft to have different lengths when the shaft assumes different inclinations.

Final drive transmits the torque from the longitudinal universal joints cum propeller shaft to the transverse driving half axle shafts and the wheels. The final drive also multiplies the driving torque.

Differential allows the driving wheels on the two sides of the vehicle to rotate at the same speed when moving over a straight road and at different speeds whenever the vehicle makes a turn.

Half-axle drive shafts transmit the driving torque from the final drive and differential unit to the driving road wheels.

Frame-supports the engine, vehicle body, wheels and other components.

Spring and wheels transmit the vehicle load from the frame to the road. They also damp jerks and shocks when the vehicle wheel hits bumps in the road.

Special dampers dampen the resultant oscilations of the springs when the vehicle wheel hits bumps in the road.

Electrical system in the chassis consists of a battery, starting motor, generator/alternator, and controls, and in the case of a spark ignition power plant the engine ignition system and lighting arrangement.

Controls in a vehicle include steering system, brake system, engine control and power train control.

Steering system is used to change the direction of motion of the vehicle by turning the front wheels. The front wheels are linked to the steering wheel which is operated by the driver, by a system of levers and rods.

Brake system ensures safe driving of the vehicle. With the brake system, the vehicle can be stopped quickly, or slowed down while going down a slope. The brakes are mounted on all the wheels of a vehicle. The brakes are connected with the brake pedal or lever by means of mechanical, hydraulic or air operated devices.

Engine control refers to the mechanism which permits a variation in the revolutions of the engine crankshaft in accordance with demands. In a vehicle this refers to the accelerator pedal and the connected system.


Power train control boils to engaging and disengaging the clutch and the gearbox. These controls, thereby, make the speed and torque variation possible.

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