What does a spark plug do?

How a spark plug makes its spark

A spark plug does not make electricity. The car's battery supplies only 12 volts, far too little to jump through air. The ignition coil, one per plug in most modern engines, turns that into a short pulse of 10,000 to 40,000 volts and sends it down the plug's centre electrode at the moment the engine computer orders.

The plug's job is to give that pulse a safe path and a gap to jump. A tube of ceramic insulates the centre electrode from the metal body, which is screwed into the cylinder head and so is connected to the engine's ground. At the tip, the centre electrode ends a short distance from the ground electrode, a small hook welded to the body: about 1 mm in most car engines, anywhere from 0.6 to 1.8 mm depending on the engine. The voltage builds until the gas in the gap breaks down and conducts, and a spark jumps across.

The spark is over in an instant, but it heats a tiny ball of mixture so far that it starts to burn by itself. From this flame kernel a flame front spreads through the whole combustion chamber. The plug only lights the fire; the burning fuel does the work.

Ignition timing: why the spark moves

The moment of the spark is the ignition timing, given in degrees of crank rotation before TDC. Real petrol engines fire somewhere between about 10° before TDC at idle and about 30° at high rpm. Load matters too: with the throttle wide open the cylinder is fuller, the mixture burns faster and the engine is closer to knocking, so it needs less advance than when cruising on light throttle.

Get it wrong and the engine notices. Fire too early and the pressure rises while the piston is still coming up; the unburnt rest of the mixture can then explode on its own, which you hear as a metallic pinging or knock and which can damage pistons. Fire too late and the peak comes when the piston is already far down: the engine loses power, uses more fuel and the exhaust runs hot. Older engines set the timing with a distributor; today the engine computer works it out for every spark from rpm, load and temperature, and knock sensors on the block let it pull the timing back the moment knocking starts.

Inside a spark plug

A spark plug is built in layers around the centre electrode. At the top is the terminal, where the coil or its boot clips on. Below it, the white ceramic insulator, usually aluminium oxide, holds the centre electrode and keeps tens of thousands of volts from leaking to the body; the ribs on its outside lengthen the path along the surface so the current can't creep over it. Many plugs have a resistor inside the insulator that damps radio interference from the spark.

The metal shell holds everything together. Its hexagon takes the spark plug wrench, its thread screws into the cylinder head, and a sealing ring or a tapered seat keeps the combustion gas in. At the bottom, the ground electrode bends over towards the end of the centre electrode, leaving the gap the spark jumps. The plug also has to carry heat away from its tip into the cylinder head, which is where heat range comes in.

Heat range: hot enough to clean itself, cool enough not to glow

The insulator tip sits in the fire, so its temperature matters. Below about 450 °C, soot and oil deposits don't burn off and build up on the tip until they form a conducting path, and the spark leaks away instead of jumping the gap: the plug fouls. Above about 850 °C, the tip itself gets hot enough to light the mixture before the spark does. That glow ignition, or pre-ignition, comes too early and can melt electrodes and pistons. A plug should run between these two limits.

How hot a plug runs is its heat range. A hot plug has a long insulator nose that takes a long path to shed heat, so it stays hot and cleans itself, which suits engines that run cool and slow. A cold plug has a short nose that passes heat quickly to the head, which suits engines that run hot under high load, such as turbocharged ones. Makers number heat ranges differently, so the right plug is the one the engine maker specifies.

Copper, platinum or iridium, and when to change them

Every spark wears away a little electrode metal, so the gap slowly grows and the coil needs more voltage to jump it. Plain plugs with a nickel-alloy electrode around a copper core are cheap and conduct heat well but wear fastest: they are usually changed every 30,000 to 50,000 km (about 20,000 to 30,000 miles). Plugs with a thin platinum or iridium tip on the centre electrode resist wear far better and last about 100,000 km (60,000 miles) and more; many modern engines come with them from the factory.

These figures are typical ranges, not rules. The interval that counts is the one in the car's service schedule, and it is worth fitting the plug type the maker specifies, because a different gap or heat range can make the engine run worse.

Signs of a worn spark plug

A worn or fouled plug sparks weakly or not every time. The cylinder it serves then misfires: the engine shakes or idles roughly, hesitates when you accelerate, starts badly, especially in the cold, and uses more fuel because unburnt mixture goes out of the exhaust. The check-engine light often comes on with a misfire code for that cylinder, and the unburnt fuel can overheat the catalytic converter.

A mechanic can read a lot from the plug's face: light grey-brown deposits mean it is running right, black soot points to a rich mixture or a plug that runs too cold, and white, blistered or melted tips to overheating.

FAQ

Frequently asked questions