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TKART magazine Tech Talk | The spark plug
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03 March 2016
Small, but fundamental: this is the component that brings the engine to life and gets your kart going
The two-stroke internal combustion engines engines used in competitive kart racing are extremely sophisticated “machines” that guarantee elevated performance standards. Yet, it takes just a few key components to make them work. The spark plug is the part that literally turns the engine on, by igniting a spark that sets off fuel combustion in the cylinder head chamber. Since the high number of engine revolutions don’t leave fuel a lot of time to burn, the spark has to be intense enough to cause quick and proper ignition.
While spark plugs have evolved in terms of materials, over the decades not much has changed in terms of how they’re built and how they work.
The key factor is the full transfer of the electric charge from the plug’s central electrode to the engine, both of which are made of metal
(and so are excellent conductors). This is why much of the plug’s body is surrounded by a ceramic casing that works as an insulator. The casing’s insulating efficiency is further enhanced by its ribbed outline: forcing the current to cover a longer path reduces the risk of energy leaks. The threaded metal shell allows the spark plug to connect to the engine cylinder head and its reach (1.25 mm on plugs made for kart engines) is enough to disperse some of the heat generated by combustion.
The other key elements of a spark plug are the electrodes: the main central one featuring a highly positive charge and the terminal electrode with a null charge. The difference between the electrodes’ electric potential the measure of accumulated energy) is what generates the spark.
The first spark plug ever sold was the one designed by Robert Bosch in 1902.Early-generation plugs generated 15 sparks per second, today their efficiency is 5 times that. Peak temperatures have gone from 600 to 900°C and voltage from 10,000 to more than 30,000 volts.
Spark plugs are of two kinds: hot or cold. In hot plugs, the inside of the threaded metal case provides a smaller contact surface with the ceramic casing, causing lower heat dispersion and so a higher operative heat range.
Spark plugs are of two kinds: hot or cold. In cold plugs the greater heat dispersion is fruit of a greater ceramic/metal contact surface. The operative heat range of spark plugs is key in both 2 and 4 stroke engines.
The diagram offers a simplified illustration of a kart engine’s ignition circuit. The ignition system, installed on the crankshaft opposite to the sprocket, generates a low voltage current. The ignition coil converts this into high voltage tension that reaches the plug’s central electrode (positive element) and discharges as a spark onto the terminal electrode (null element). This is the element that rounds off the circuit through the threads that fit to the engine, acting as the ground point.
The spark plug is part of an electrical circuit that includes an ignition system and an ignition coil. Current is first converted from low to high and when it reaches the plug it discharges in the form of a spark. Getting an elevated voltage (about 20,000 volts) before the current reaches the plug is key to get it past the dielectric resistance that characterizes the gap between the two electrodes, which is created by the fuel/air blend and acts as an insulating element.
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