HOW DOES SPARK PLUG WORK?

Almost everyone has heard about the word “spark plug” or even had had to change it at one time or the other. Spark plug dominates our everyday lives, from our cars to our lawn mower; there is none that doesn't make use of spark plug.
Basically all mechanism that uses internal combustion engine (spark ignition) uses spark plug. Let me digress a little to explain something here, there are two types of internal combustion engines: spark ignition engines (SI) and compression Ignition engines (CI).

Spark ignition engines are the engines that make use of spark plug to ignite the air- fuel mixture for combustion examples are petrol cars, bikes, petrol generating sets etc. While the compression ignition engines are the engines that make use of the heat generated from the compressed charge for combustion.
So going back to our spark plug and how it works, let’s know what a spark plug is first.

SPARK PLUG
picture of spark plug

Spark plug is a device that makes a high voltage to jump across a gap. When you look at the electrode side of the plug, you will discover that there is a space left in between(gap). This is the junction where the spark will jump thereby igniting anything around it.

The spark from the spark plug is introduced during the second phase (known as the compression stroke) in four stroke engines. Please note that the spark plug does not produce the spark itself, what produces the spark is the ignition coil.

parts of a spark plug

WORKING OF THE SPARK PLUG
During the compression stroke in a four stroke engine, the ignition coil which is part of the ignition system sends a very high voltage (between 40,000 volts to 100,000 volts) to the connector of the spark plug through the plug wire which terminate as a cap over the connector.

This is then transferred through the central electrode which point out just a little above the ceramic insulator, such a high voltage needs somewhere to discharge which of course will be the nearest metallic conductor near it.
Looking at the body of the spark plug in the picture above, it can be seen that the metallic body before the threaded section is clearly marked (+), thereby making it the positive terminal and the electrode the negative (-) terminal since no electric circuit is complete with just one terminal. Both terminals are insulated from each other by the ceramic.

So going back to the high voltage passed through the electrode, it will be attracted to the positive terminal which is just a few inches above it, thereby having to jump across the gap and in the process producing a spark which will ignite the air-fuel mixture introduced into the cylinder during the induction stroke. The washer acts as a seal to the combustion chamber.

HOW TO CHECK IF A SPARK PLUG IS STILL FIRING
Once a spark plug can’t fire again, it will lose it ability to ignite the air-fuel mixture thereby making the engine to under-perform or not to perform at all. So how can one check if the plug is bad?
This is very easy; just follow the simple steps below:

• Get a plug spanner and removed the plug, but remember that you have to disconnect the plug wire first before you can use your plug spanner. So just use your hand to pull out the plug wire and use your plug spanner to unscrew the plug.

• Then hold the plug and connect the plug wire back to the removed plug.

picture of how to check if a spark plug is bad or good

• Place the plug on the body of the engine and start the engine using the kick starter.
The placement is shown in the diagram above.

• If a blue spark can be seen, the plug is still in perfect order but if a yellow spark or no spark is seen then the plug needed to be changed.

Don’t worry, machines rarely develop fault if the maintenance procedures laid down by the manufacturer are rigidly followed, just do this and you won’t have to sweat in the middle of the night or if you like to try things out by yourself, anytime your mechanic is around make sure you grab your overall and work with him, you will learn one or two things from him before he is done.