SPARK PLUG WORKING AND TYPES

 

SPARK PLUG WORKING, TYPES, CLEANING THE SPARK PLUG AND FAILURE.

WORKING:  The function of the spark plug is to produce an electric spark to ignite the compressed air fuel mixture inside the cylinder. It must produce the spark  at the correct movement at the end of the compression stroke. A proper gap is to be maintained between the two electrode of the spark plug so that the sparking may take place. When the spark plug is screwed in the cylinder head, the ground electrode is said to be connected with ground. The terminal of the centre electrode is directly connected with H.T leads of the ignition coil in case of single cylinder engine, or through the distributor in case of multi cylinder engine. The secondary circuit ot the electrical system to be completed through the gap between the electrodes. When the H.T. current passes through the circuit, it jumps the gap producing a , spark, which ignites the compressed air fuel mixture in the cylinder.

Spark plug with a resistor

SPARK PLUG GAP:  The gap between the centre electrode and the ground electrode is called the spark plug. This is adjusted to recommend specifications by bending the ground electrode. It varies from 0.4mm to 1.0mm. It is measured with a filler gauge. The electrical resistance of the spark plug depends upon the nature and compression of the fuel mixture and also upon the gap. Too large and too small gap reduce the efficiency of the entire ignition system, which in turns causes losses in engine power and operating efficiency. It is therefore of great importance to maintain the proper spark plug gap. No foreign materials, such as  carbon, should be deposited in the electrode, which will reduce the gap. Also, the electrode should not corrode which will increase the gap. The carbon deposited on the outside insulator between the plug terminal and the shell allows some of the high voltage current to bypass the gap. This weakness the intensity of the spark causing losses in combustion efficiency and a decrease in a engine power. If electrical leaks become excessive, the plug will not produce spark at all. Cracked insulator are usually caused by careless installation of the plug or by careless adjustment of the plug gap.

TYPES OF THE SPARK PLUG.

Spark plug are two types according to heat range :

·        Hot spark plug

·        Cold spark plug

Hot spark plug :  heat range is a means of  designating how hot plug will run in operation. It refer to the ability of the spark plug to heat transfer from the firing tip of the insulator to the cooling system the engine. The temperature that a spark plug will attain depends upon the distance through which the heat is transfer. If the path of heat travel is long, the will run hotter than if the path is short. Shot spark plug has longer path of heat travel and runs hotter than the cold spark plug which has shorter path of heat travel and runs coller.

Hot and cold spark plug

Cold spark plug : Cold spark plugs are used in heavy duty or continuous running high speed engines to avoid overheating. When a plug runs too cold, the carbon deposit on the insulator around the centre electrode. A plugs that runs hot will wear more rapidly, since the high temperature cause the electrode to burn away from quickly. If a plug runs too hot, the insulator may take on white or greyish cast and may appear blistered. Low speed medium duty engines running at colder operating conditions require a hot spark plug. Different engines working in various conditions need a plug having a specific heat range and manufactures producing plugs  in several heat ranges. In between the hot and cold spark plugs, medium hot medium cold spark plugs are also available to suit the specific condition of the engine. The projected core nose type of plug has the plug tip cooled by the incoming charge at high engine speed, enabling the plug to run hotter at low speeds so extending the heat range 

Cleaning the spark plug : Due to the combustion of fuel in the cylinder, carbon particles deposit on and around the electrode, which not only reduce the plug gap but also prevent the spark to occur. If the spark is still occurring, it is too weak that it cannot ignite the fuel. Hence the spark plug is to be cleaned. There is no specified period after which the spark plug is cleaned. It depends upon the carbon deposit which can take place due to any reason like, nature of fuel mixture strength lubricating oil, etc. The spark plug can be cleaned either by a sand paper or by sand blasting. For this purpose remove it from cylinder head, clean it correct the gap, check carefully if any sand particles is sticking on it and then fit it again in the cylinder head properly.  If the insulator or sealing is broken, the spark plug should be replaced. The plug should be given an engine performance test after it is replaced in the cylinder. It should be checked carefully that the centre of electrode end is perfectly flat and not round. If it is round it should be filled flat and the gap adjusted to specification.

SPARK PLUG

Spark plug failure : A spark plug will fail due to the following reasons.

1.      Plug fouled by engine oil entering combustion chamber.

2.      Plug fouled by too rich mixture.

3.      Plug badly covered with carbon from poor ignition.

4.      Plug gap incorrect.

5.      Burned electrodes or broken lower insulator caused by overheating.

6.      Red, brown or yellow oxide deposit on plug interior that short the insulator.

7.      Accumulation of dirt or moisture on the outside of the insulator that short the plug by grounding the high voltage.

8.      Cracked or broken insulator sealing.