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THE SPARK PLUG

Carlo Forni

03 March 2016

Small, but fundamental: this is the component that brings the engine to life and gets your kart going

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

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.

HISTORICAL TIDBITS

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.

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

HOT SPARK PLUG COLD SPARK PLUG IGNITION CIRCUIT

HOT SPARK PLUG

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.

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COLD SPARK PLUG

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.

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IGNITION CIRCUIT

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.

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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.

WHAT IS IT?

HOW DOES IT WORK?

IN PRACTICE

When the difference in voltage between the two electrodes is high enough, the electric current gets past the resistance gap, discharges onto the end of the plug and generates a spark. Of course, the longer the gap between the two electrodes, the tougher the “leap”. Viceversa, if they are too close, the discharge could occur before the central electrode is sufficiently charged, generating a weak and premature spark and thus leading to improper fuel combustion.
When the spark flies, the position of the piston should be just before the top dead center (BTDC). The need to advance the spark’s timing is due to the fact that the fuel-air mixture in the combustion chamber does not completely burn the instant the spark fires. Advanced timing allows the piston to be already set in descending position when most of the fuel-air mix has burned, ready to set the engine in

motion. On kart engines, timing can be adjusted through the ignition starter motor.
Going back to how the plug works, the correct development of combustion depends on the molecular concentration and turbulence of the fuel-air mix and on the temperature inside the combustion chamber. Turbulence is especially important: the more the engine rotates, the more turbulent the blend gets and the less it takes to burn up. In other words, the two processes compensate each other.
If you know what you’re doing you can tinker around until you get a broad power curve – in other words, set advanced timing to optimize horsepower at a given rpm range. However, if you’re not a specialist, it’s best to forget experiments: you could compromise engine performance or cause severe damage (including seizure due to detonation).