To ignite the mixture of air-petrol inside the engine cylinder,a spark plug is used.A spark plug which is screwed into the cylinder head is usually designed to withstand a pressure about 35 kilogram per centimeter square and operate under 10000 to 30000 volts.
Construction Of Spark Plug:-
The core insulating assembly is housed in the steel shell assembly.The lower threaded portion of the shell which has 18 millimeter or 14 millimetre diameter thread on standard plugs has ground electrode welded to it.The centre electrode is screwed into the insulator at its upper position.The upper end of the centre electrode is connected to the terminal which communicates with the distributor.
When high voltage is applied,the spark jumps in the small gap which is provided between the lower end of the centre electrode and the ground electrode.This spark is responsible for starting ignition in the cylinder.
Platinum,nickel are generally used as electrode material due its capacity to withstand corrosives,high temperature and has good thermal conductivity.The insulator should have a good thermal conductivity,chemical inertness to gases of combustion,high resistance,be non-brittle and able to withstand high temperature and pressure.In recent practice is to use aluminium and silicon oxides also.
In order to prevent the escaping of gases from the cylinder to the atmosphere, suitable sealing gaskets are provided between the shell and the core insulator.Another steel gasket is also provided at the area where the steel shell is to contact cylinder head when the spark plug is screwed in its proper position.The leakage of the hot gases past a defective or worn out gasket is likely to raise the temperature of the plug.
Generally 0.3 to 0.6 millimeter gap is kept in the spark plug.Under special circumstances a 0.7 millimeter gap can also be kept.The efficiency of the entire ignition system is greatly reduced both by too large or too small gap.Therefore,the gap must be kept in the desired limits.
The deposit of foreign materials such as carbon between the electrodes or on the outside of the insulator between the plug terminal and the shell are source of nuisance and permit some of the high voltage to by-pass gap resulting in a reduced intensity of spark giving decreased engine efficiency.To avoid carbon deposits,we should have to operate spark plug above 400 degree centigrade.By bending the ground electrode,we can adjust the gap of plug.
There are two types of spark plugs used for automotive purpose,
01) Hot Plug:-
In this spark plug,the path traced by the heat received by the plug,to reach the Cooling cylinder head water is longer so that the plug can be maintained at a high temperature to burn carbon deposits.
02) Cold Plug:-
It has a shorter insulating nose which increase the rate of heat transfer due to short conducting path.This type of spark plugs are suitable for the engines,running a high speed and having high compression ratio.
You may also interested in following........
Different Ignition Systems Of Four Cylinder Engine.
Troubleshooting Of Diesel Engine.
Working Principle Of Simple Carburettor Used In Petrol Engine.