The first documented use of a spark plug in an internal combustion engine was attributed to the Belgian Engineer Jean Joseph Étienne Lenoir in 1859. Lenoir is known for developing the first internal combustion engine, which burned a mixture of coal gas and air. The air-fuel mixture it aspirated was ignited by a “jumping spark” ignition system, which he patented in 1860.
Lenoir’s ignition system created sparks by using high voltage electricity to jump an air gap. This was accomplished by sending mechanically generated low voltage pulses through a type of electrical transformer known as a Ruhmkorff coil. The coil would transform the low voltage pulses into lower current, high voltage pulses, suitable for spark generation.
Reliably igniting over 20 million combustion cycles while surviving exposure to the extreme temperatures and pressures of ignited fuel would prove to be a formidable challenge.
All spark plugs are fundamentally composed of two electrodes separated by an insulator. These electrodes converge at a «spark gap», where spark generation occurs. As the initial current flows from the ignition coil to the spark plug’s electrodes, the flow of electricity is initially blocked by the insulating properties of the air-fuel mixture within the gap. As the voltage pulse ramps up, the potential created between the electrodes begin to restructure the gases within the spark gap. As the voltage increases further, the insulating limit or the dielectric strength of the spark-gap gases begin to break down, causing it to ionize.
The first spark plugs had a very minimal set of operational requirements. Their main design concerns were the plug’s fit and position and its ability to maintain an operating temperature range that would allow the plug end to self-clean by burning off deposits.
The thermal properties of a spark plug are designated by a relative heat range.
The emergence of leaded gasoline in the 1930s would also cause aggressive deposit buildup on the mineral insulator ends.
To keep up with this, construction was shifted towards a single piece design composed of a ceramic called sintered alumina.
Sintered alumina plugs operated at much higher temperatures, which helped counteractact the fouling issues caused by leaded fuels via deposit burn-off. It’s electrical insulation properties also allowed much higher voltages to be used, tolerating up to 60,000 volts. This would be further improved by the addition of ribs which increased the surface area of the insulator. Modern spark plug still used sintered alumina and can tolerate voltages well past 100,000 volts.
The next big change in spark plug design would occur in the form of copper core plugs during the 1970s as a direct result of policy changes. In 1974, the US government began to impose fuel mandates and stricter emissions regulations which prompted the removal of lead from gasoline, the introduction of catalytic converts, and the move to smaller more efficient engine designs.
By the 1990s computer controlled ignition systems were becoming common and the need for more energetic spark generation with newer higher compression and forced induced engines was becoming apparent. This was accomplished by moving ignition coils into assemblies that sat directly above the spark plug. Known as coil-on-plug ignition, the one coil per cylinder configuration coupled with the shorter direct path of current flow allows for extremely high voltages to be used, often well past 100,000 volts.
On modern fuel injected cars, higher compression ratios as well as tighter control of combustion timing is used to extract as much energy as possible, increasing power and efficiency. As an engine’s rotating speed increases, triggering an ignition event slightly before the point of maximum compression within a cylinder, or advancing timing is done to give the combustion process more time to occur.
Under certain conditions uncontrolled combustion can be triggered as smalls pockets of air-fuel mixture explode outside the envelope of the normal spark triggered combustion front. This is known as detonation and it can occur when timing is advanced too aggressively or the air-fuel trim is mismatched for the conditions within the cylinder. Though this can be mitigated by using a higher octane fuel, which lowers gasoline’s volatility, the engine control unit is generally tasked with finding a balance point between preventing detonation and achieving ideal combustion timing.
Engine knock sensors were developed for this task and they functioned as highly tuned microphones, listening for the tones of sound produced on an engine block as it experiences detonation. The ability to accurately manage detonation also kept combustion chamber designs relatively conservative. During the late 1990’s several manufacturers were researching better methods to detect detonation. The advent of ionic detonation detection used spark plugs to sense chamber ionization.