Before discussing spark plugs, we need to know what spark-ignition engines are. IC engines can be divided into two depending on the type of ignition: Spark-ignition and compression ignition. Spark ignition engines use an electric spark to ignite the air-fuel mixture while in the latter, the air-fuel mixture can ignite itself. Engines that run on petrol are Spark ignited while those that run on diesel are compression ignited.
The reason behind it is that petrol is a highly volatile fuel and the temperature required for it to self-ignite is higher. Hence an electric spark is introduced to ignite it. But in the case of diesel, it can self-ignite at a very low temperature. When it is compressed, along with the pressure, the temperature also increases and ignites itself. There are a lot more differences between spark-ignited and compression-ignited engines but let’s just save it for another day.
Spark plugs are basically a device that introduces the electric current to the combustion chamber of spark-ignition engines to ignite the air-fuel mixture. It delivers the electric spark from the ignition system to the combustion chamber and delivers it without any pressure leakage from the chamber.
The spark plug has a central electrode and an outer shell separated by a porcelain insulator. The central electrode is connected to the output of the ignition system by means of highly insulated wire and resistors. The metal shell has threads and is screwed onto the cylinder head electrically grounding itself and the central electrode will be protruding inside the combustion chamber. The outer shell will have a hook-like structure that reaches the top of the central electrode. There will be a tiny gap between the central electrode and the hook-like structure of the outer shell and that is where the spark is produced. This gap distance is crucial for the proper performance of the vehicle.
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As the current flows from the ignition coil, a potential difference develops between the central electrode and the outer electrode or side electrode. The gases between the electrodes are initially insulators but as the voltage rises, the gases get ionized and become a conductor. Similar to what happens during lightning and thunder, this allows the flow of current between the electrodes. As the current in the gap rises, the temperature also increases causing the gas to expand suddenly resulting in a small explosion
and finally combustion of the air-fuel mixture. Spark plugs require a voltage of a minimum of 12000 to fire properly and this is done by the ignition system using the power from the battery.
As mentioned before, the gap between the electrodes is very crucial for the proper working of the engine. Too small of a gap can result in a weak spark which might not ignite the air-fuel mixture properly and if the gap is too large, it can result in no spark or misfire which can ignite the mixture properly but can affect the performance and efficiency of the engine adversely.
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The top of the spark plug has a terminal to connect to the wires from the ignition system. Most of the passenger cars will have the terminal made in such a way that the wires can snap onto it. The insulator is a ceramic material that provides mechanical support in addition to electrical insulation. The spark plug seals the combustion chamber and hence proper seals are also necessary to prevent leakage from the cylinder. Usually, the seals are made of compressed glass/metal powder. The outer shell is provided with threads to screw it into the cylinder head. They remove heat from the insulator and pass it on to the cylinder head and also act as a ground for the sparks passing from the central electrode to the side electrode. The thread length or reach is also very important. Spark plugs need to be installed with the exact reach as required. The central electrodes are, nowadays, made of noble metals such as iridium, tungsten, palladium etc. These electrodes are used because of their high melting points and durability.
Spark plugs are cleaned using sandblasting or wire brushes but wire brushes might deposit small metal deposits on it affecting combustion. There are gauges to check the spark plug gap while replacing or cleaning.