How ignition works

A petrol engine squeezes fuel and air in each cylinder, then lights it with a spark. The ignition system makes that spark, in the right cylinder, at the right moment, thousands of times a minute.

It needs four parts:

• The battery supplies 12 volts, far too few to make a spark on their own.
• The ignition coil turns those 12 volts into many thousands. Most modern engines have one coil sitting right on top of each spark plug.
• The ECU switches each coil on and off, and so decides exactly when each spark comes.
• The spark plug screws into the top of the cylinder. Its two metal tips, the electrodes, sit about 1 mm (0.04 in) apart, and the spark jumps that gap.

Diesel engines don’t have spark plugs. They squeeze the air so hard that it gets hot enough to light the fuel the moment it’s injected.

The demo shows one cylinder at 3,000 rpm with your foot flat down. Step through with the numbered buttons, then move the spark earlier and later and watch the pressure graph.

1. Coil. A coil has two windings of copper wire round an iron core: a first winding of a few hundred turns and a second of many thousands. For about 3 thousandths of a second before the spark, the ECU lets current from the battery flow through the first winding. That builds up a magnetic field in the core.
2. Spark. The ECU cuts the current. The magnetic field collapses through the second winding, and because it has so many more turns, it makes many thousands of volts. The voltage rises until it is enough to jump the plug’s gap, about 18,000 volts here, and the spark crackles across.
3. Flame. The spark lights a tiny ball of mixture between the electrodes. A flame spreads out from it across the cylinder, taking about 3 thousandths of a second to burn the lot.
4. Push. The burning gas heats up, its pressure soars and it drives the piston down.

Because the burn takes time, the spark has to come before the piston reaches the top. The best moment is the one that makes the pressure peak about 15° of crank turn after the top, just as the piston starts down. Here, that means sparking 22° before the top.

• Too late, and the piston is already running away from the pressure. You get less push, and more of the heat goes out with the exhaust.
• Too early, and the pressure builds while the piston is still rising, so it fights it. Worse, the pressure and heat can make the last of the mixture explode on its own before the flame reaches it. That sharp bang is knock, sometimes called pinking, and it can crack pistons.

The best timing changes all the time. At light load the thin mixture burns slowly, so the ECU fires earlier; at high revs there’s less time for the burn, so it fires earlier too. At full load the mixture is dense and burns fast, so it fires later to keep clear of knock. The ECU lesson shows it doing this.

Try Worn plug. Its gap has worn to 2 mm (0.08 in), which needs more volts than the coil can give. There’s no spark, nothing burns and the piston gets no push: a misfire.

Older cars had one coil for the whole engine. A spinning distributor sent its spark to each cylinder in turn, through thick HT leads (high tension, meaning high voltage). Inside it, a pair of contacts called points switched the coil. They wore as they opened and closed, so they needed adjusting at every service, and the timing drifted between services.

From the 1970s, electronics took over the switching, and later the ECU took over the timing as well. Many engines then dropped the distributor for one coil per pair of cylinders. Most modern engines go further, with one small coil on top of each plug, called coil-on-plug. With no moving parts and no long leads to lose voltage along, the spark is stronger and timed far more precisely.

• Worn spark plugs. Every spark wears a little metal off the electrodes, so the gap slowly grows and needs more volts. Worn plugs misfire first under heavy load, when the dense mixture makes the spark hardest to jump. Plugs are changed at a service: long-life iridium and platinum plugs last many years; older copper plugs need changing far more often.
• A failing coil. Coils get hot, and in time their windings break down. One cylinder then misfires, so the engine shakes, feels down on power and may be slow to start.
• Misfires show up on the dashboard. The ECU watches how smoothly the crankshaft turns and notices when one cylinder stops pushing. It stores a fault code naming the cylinder, and a code reader on the OBD port tells you which one. A flashing engine management light means a bad misfire: unburnt fuel is reaching the catalytic converter and can overheat it, so ease off and get it checked soon.
• Knock. A knock sensor, like a tiny microphone bolted to the engine, listens for knock. When it hears it, the ECU fires the sparks later until it stops. That protects the engine, but you lose some power. Filling up with lower octane fuel than the maker recommends makes this happen more.
• Fouled plugs. Plugs that are coated in soot or oil can’t spark well. The colour of a plug’s tip tells a mechanic a lot about how the engine is running.

• Timing is a big part of a remap. Firing the spark earlier, up to the edge of knock, is one of the ways a remap adds power. Higher octane fuel resists knock, which is why some remaps need it.
• Boost needs a smaller gap. A turbocharged engine squeezes more air into each cylinder, and the denser mixture makes the spark harder to jump. Tuned turbo engines often run a smaller plug gap, and sometimes stronger coils, so the spark doesn’t get blown out at high boost.
• Colder plugs. Every plug has a heat range. A “colder” plug carries heat away from its tip faster, so a hard-working tuned engine can’t make the tip glow hot enough to light the mixture before the spark.
• “Performance” plugs and leads on a standard engine make little or no difference. A healthy standard spark already lights the mixture fully; what matters is fitting the right plug, at the right gap, and changing it on time.

Octane: what it actually means covers knock and octane in more detail.