What the ignition system does
In a petrol (gasoline) engine, the air and fuel mixture in each cylinder is squeezed by the piston and then lit by a spark. The ignition system’s job is to create that spark at the right moment, many times per second, in every cylinder. If the spark is weak, late, early or missing, the engine runs poorly.
The spark has to jump a tiny gap, and doing that through compressed gas requires a very high voltage, often tens of thousands of volts. A car battery supplies only about 12 volts, so the system includes a way to step it up.
The main parts
Ignition coil. A transformer that boosts the battery’s low voltage to the very high voltage needed for a spark. Older cars used one coil and a distributor to route the voltage to each cylinder in turn. Most modern cars use a coil for each cylinder, or one for each pair of cylinders, often mounted directly on top of the spark plug, a design commonly called coil-on-plug.
Spark plug. A small component screwed into the cylinder head. It has a central electrode and a side electrode with a small gap between them. When high voltage arrives, a spark jumps the gap and ignites the mixture. A ceramic insulator keeps the voltage from leaking away, and a metal shell seals the plug against the combustion pressure.
Engine control unit and sensors. The engine computer decides exactly when to fire each coil. It uses data from sensors that track crankshaft and camshaft position, engine load, air intake and temperature. Firing slightly earlier or later, known as ignition timing, affects power, efficiency and emissions. Modern systems adjust it constantly.
Wiring and connectors. On cars with coil packs, spark plug leads carry the voltage from the coil to the plug, and these can crack or deteriorate with age. Loose or corroded connectors can also cause intermittent problems.
Why spark plugs wear out
Every spark removes a minuscule amount of metal from the electrodes. Over tens of thousands of miles, the gap grows wider, and the electrodes become rounded. A larger gap requires more voltage to jump, which places more stress on the coil and makes weak sparks more likely. Plugs also collect deposits from fuel and oil, and heat cycles stress the materials.
Plug materials affect how long they last. Copper plugs are common and typically wear faster. Platinum and iridium plugs use harder metals on the electrode tips and are often designed to last much longer. The correct type for your engine is specified in the owner’s manual, so use what the manufacturer calls for. Replacement intervals vary considerably from one engine to another, so check the service schedule in your manual.
Signs of ignition trouble
When combustion fails in a cylinder, it is called a misfire. Depending on the severity, you may notice:
- Rough or shaky idle.
- Hesitation or stumbling when you accelerate.
- Loss of power, especially under load, such as climbing a hill.
- Poorer fuel economy than usual.
- Difficulty starting, or an engine not starting at all in bad cases.
- A flashing or steady check engine light. A flashing light usually means a severe misfire that can damage the catalytic converter, so reduce speed and have it checked promptly rather than continuing a long drive.
These signs can have many causes besides plugs, including fuel delivery problems, vacuum leaks, sensor faults or mechanical issues. A code reader can retrieve stored fault codes, which often identify which cylinder is misfiring, but the code is a starting point, not a final diagnosis.
Can you change them yourself?
On some engines, changing spark plugs is a manageable job with the right tools: a spark plug socket, a torque wrench and sometimes an extension. On others, plugs are buried under intake parts and are far harder to reach. A few important cautions:
- Work only on a cool engine, because aluminum cylinder heads can be damaged by removing plugs when hot.
- Keep dirt out of the cylinder when the plug is removed.
- Use the correct gap and tightening torque from the manual. Over-tightening can damage threads in the cylinder head, which is an expensive repair.
- Do not mix up wires or coil connectors.
If you are unsure, a qualified mechanic can do it quickly. It is a routine job that belongs in regular servicing.
What about diesel and electric?
Diesel engines do not use spark plugs. They ignite fuel by compressing air to a high temperature, then injecting fuel into it. Many diesels do have glow plugs, which are heaters that help them start when cold. Electric cars have no combustion at all, so there are no spark plugs, which is one reason EV maintenance looks different.
The takeaway
The ignition system is a chain: battery, coil, plug, and the computer that times everything. A worn plug is a cheap part, but ignoring it can lead to wasted fuel, rough running and damage to other components. Follow the interval in your manual, and take misfires seriously.