We have discussed at length, the means of delivering a high voltage surge to the spark plugs in order to produce a spark which can ignite the compressed fuel/air mixture. We’ve even talked about the spark, but not specifically about the spark plug.
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A spark plug’s purpose, as we know, is to ignite the fuel/air mixture. Hence, it will be operating under specific conditions and these have been considered in its design.
About A spark plug
porcelain insulator – The insulator is held in a heavy metal case which is screwed into the cylinder head and makes electrical contact to ground.
On the metal case is the ground electrode. It is from the center electrode to the ground electrode that the spark will jump.
You can see from the picture that the plug is of heavy and durable construction. It must be to withstand the high pressure and heat developed in the combustion chamber.
Obviously, the heat from combustion will cause the plug to get hot,but it can be so constructed that it will retain more or less of the heat which it picks up.
Plugs are referred to as being hot or cold, depending upon their relative retained running temperature.
The heat range of a spark plug is probably the most important factor in its design.
Hot operating plugs are constructed with long ceramic insulators, while the cooler plugs have relatively short insulators.
The length of the insulator determines the plug heat range because it must pass its heat to the heavy metal case, where it, in turn, passes the heat to the nearby coolant chamber.
Therefore, the shorter the conduction path, the cooler the plug operation.
Manufacturers recommend which plug to use for the make, model and year of the automobile and always base their recommendation on average driving conditions.
Therefore, it is always important to inspect the spark plugs when they are removed from the engine. An engine driven under severe operating conditions requires cooler running spark plugs than do engines running at continuously low speeds.
A plug that is too cool for the operating conditions of the engine will soot up with oil or carbon and will eventually short out.
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A plug that is too hot will blister and ignite the mixture prematurely by its white hot insulator.
A properly operating plug will retain just enough heat to burn off the small amount of oil which is always present in the combustion gasses, the insulator will be relatively clean and have a grayish tan color.
A cold plug is one which readily passes heat to the surrounding material in the engine head, as compared to a hot plug which retains more heat. It runs at a higher temperature than a similar sized colder plug.
The distance between the center electrode and the ground electrode is referred to as the spark plug gap. And obviously this gap determines the length of the spark. As you will see, the gap is also a compromise when average driving conditions are encountered.
At idle speeds because the mixture is not well distributed and contains a high fuel/air ratio, a long spark is needed for efficient ignition.
A long gap provides a smooth idle but reduces top speed. The reason top speed is reduced using a wide plug gap is that higher voltage is required to Jump the wider gap and at high speeds, coil output drops off.
Hence, a wide gap produces a good idle but poor top speed and a small gap causes a rough idle but increases top engine speed. Therefore, automotive manufacturers specify a gap which is most suitable for all driving ranges. An example of a common gap size is .035 inch for a 1964 Ford Galaxie.
For more info about spark plugs read this wiki Spark plugs
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.