# Spark Ignition and Combustion Timing

> How a spark plug lights the compressed charge, why the spark is timed before top dead centre, how the flame spreads to build pressure, and what knock is.

- Level: Intermediate · about 12 minutes · Fuel & Ignition
- Lesson page: https://autoverse.maxrecursion.dev/topics/spark-ignition
- Interactive 3D lab: https://autoverse.maxrecursion.dev/labs/engine (Watch intake, compression, power and exhaust with crank, piston, cam and valves on one synchronised timeline.)

## What you will learn

- Describe the parts of a spark plug and how the spark is produced
- Explain why ignition is timed before TDC and why the advance changes with engine speed and load
- Describe how the flame spreads and when cylinder pressure peaks
- Explain what knock is, why it is harmful and how octane rating and timing relate to it
- Calculate how much time a given ignition advance represents at a given engine speed

## The idea, in plain language

In a petrol engine the fuel and air are squeezed together, but squeezing alone does not make them burn. Something has to light them, and that is the job of the spark plug.

The spark plug screws into the cylinder head with its tip inside the combustion chamber. An ignition coil turns the car's 12-volt supply into a pulse of many thousands of volts, which jumps across a tiny gap at the plug's tip as a spark.

The spark starts a small flame that then spreads through the whole mixture in a few thousandths of a second. That is fast, but the piston is moving fast too, so the spark is made a little before the piston reaches the top. That way the burning gas reaches its highest pressure just as the piston starts back down, and pushes it hardest.

If the mixture lights itself in the wrong place or too suddenly, it makes a metallic rattling sound called knock (or pinking). Modern engines listen for knock with a sensor and delay the spark slightly if they hear it.

## The engineering explanation

The ignition system stores energy in the primary winding of a coil; when the primary current is switched off the collapsing field induces a high secondary voltage that rises until the plug gap breaks down. The required voltage grows with gap size and cylinder pressure, typically several to a few tens of kilovolts. Once the gap ionises, a short arc deposits energy into a small kernel of mixture between the electrodes.

The kernel grows slowly at first, then a turbulent flame front sweeps across the chamber. A central plug in a compact pent-roof chamber keeps the flame path short. Because the burn takes a roughly constant time rather than a constant crank angle, the spark must be advanced further before TDC as rpm rises; light loads (lean, diluted charge, slower burning) also need more advance. The target is peak cylinder pressure around 15–20° after TDC: earlier and the piston fights the pressure rise on its way up; later and the gas expands with less useful work. The lab fires at 15° BTDC with an illustrative pressure peak near 20° ATDC.

Knock is auto-ignition of the unburned end gas ahead of the flame. Compressed and heated by the advancing flame, pockets of end gas ignite spontaneously, producing sharp pressure oscillations that ring the chamber and can overheat pistons and rings. Higher octane fuel resists auto-ignition. Knock is more likely with high compression, high load, hot intake air and excessive advance, so knock sensors let the engine run close to the limit and retard timing cylinder by cylinder when needed. Pre-ignition — the charge lit by a hot spot before the spark — is a different and more damaging event.

The amber glow in the lab is a conceptual visualisation of burning and expanding gas; the pressure trace is an illustrative model, not measured data.

## Step by step

1. **A compressed, ready charge.** The piston squeezes the fuel and air into the small chamber at the top. It is hot and tightly packed, but it will not burn until something lights it. *In engineering terms:* Compression raises charge temperature and density, which speeds up flame propagation. A petrol mixture is designed not to auto-ignite at this temperature — that is the opposite of a diesel.
2. **The spark — 15° before TDC.** The coil sends a high-voltage pulse to the plug and a spark jumps the gap, a little before the piston reaches the top. *In engineering terms:* Secondary voltage climbs until the gap breaks down. At 3000 rpm, 15° of advance is t = 15 / (6 × 3000) ≈ 0.83 ms ahead of TDC.
3. **The flame spreads.** A small flame grows from the spark and spreads across the chamber while the piston passes over the top. *In engineering terms:* An initial slow kernel-development phase is followed by rapid turbulent burning. Most of the mass burns within roughly 40–60 crank degrees at typical speeds; the lab's burn curve is a smooth illustrative S-curve.
4. **Peak pressure just after TDC.** The burning gas reaches its highest pressure just after the top, exactly when it can push the piston down hardest. *In engineering terms:* Peak pressure about 15–20° ATDC (around 40 bar in the illustrative curve) balances compression work against expansion work and gives the best torque for the fuel burned.
5. **Knock and timing limits.** If the spark is too early or the fuel is too weak, the last part of the mixture can explode by itself and make a rattling 'knock'. The engine computer backs the spark off when it hears it. *In engineering terms:* End-gas auto-ignition produces pressure oscillations at several kilohertz. Knock sensors on the block detect them and the controller retards timing in steps, then creeps it back.

## Key concepts

- **Ignition timing (advance)**: How many crank degrees before TDC the spark fires. More advance is needed at higher rpm because the burn takes roughly constant time.
- **Time per crank degree**: At N rpm the crank turns 6N degrees per second, so an advance of Δθ degrees is this many seconds. (`t = Δθ / (6·N)`)
- **Flame front**: The thin, moving boundary between burned and unburned gas. Turbulence from the intake port wrinkles it and makes it spread faster.
- **Knock**: Spontaneous ignition of unburned end gas ahead of the flame front, producing sharp pressure waves and an audible metallic rattle.
- **Octane rating**: A fuel's resistance to knock. Higher octane allows more compression or more advance before knock begins; it does not contain more energy.

## Common misconceptions

- **Myth:** Higher-octane fuel gives more power in any car. **Reality:** Octane only resists knock. An engine designed and tuned for regular fuel gains nothing unless it was knocking or retarding its timing.
- **Myth:** The spark should fire exactly at TDC. **Reality:** It fires before TDC so the slower early burn is complete in time for peak pressure shortly after TDC.
- **Myth:** Knock and pre-ignition are the same thing. **Reality:** Knock happens after the spark, in the end gas. Pre-ignition lights the charge before the spark from a hot spot and is usually more destructive.
- **Myth:** A bigger spark always means better combustion. **Reality:** Once the mixture reliably ignites, extra spark energy helps little; mixture quality, turbulence and chamber shape matter more.

## In the real world

- At 6000 rpm the crank turns 36,000° per second, so a 30° advance is under 1 ms — modern engine computers calculate and fire it every cycle.
- A worn plug with eroded electrodes needs more voltage to spark; misfires under hard acceleration are a classic symptom.
- Engines that recommend premium fuel may still run on lower octane because knock sensors retard timing — at some cost in power and economy.
- Older engines set static timing with a timing light aimed at marks on the crank pulley; modern engines control it electronically.

## Common questions

### Why does the spark plug fire before top dead centre?

The flame takes time to spread through the cylinder. Firing a little before the piston reaches the top means the burning gas reaches peak pressure just after top dead centre, when it can push the piston down most effectively. Engines advance the spark further at higher speeds.

### What is engine knock?

Knock is when part of the fuel-air mixture ignites by itself, ahead of the flame front, causing a sharp pressure spike that sounds like pinging or knocking and can damage pistons. Suitable fuel octane, a cooler charge and retarding the spark prevent it.

### What does an ignition coil do?

It steps the car's 12-volt supply up to the many thousands of volts needed to jump the spark plug gap. Current builds a magnetic field in the coil, and when it is switched off the collapsing field induces a very high voltage in the secondary winding.

## The story behind it

### Étienne Lenoir (1860)

Sold the first widely used gas engine, without compressing the charge. Lenoir's engine burned a mixture of coal gas and air, lit by sparks from a Ruhmkorff coil, and he patented it in 1860. Because the mixture was not compressed first it ran quietly but wastefully: real fuel use was far above the advertised figure, and it could overheat and seize without cooling water. Still, 143 had been sold in Paris alone by 1865, and in 1863 his gas-fuelled three-wheeled 'Hippomobile' covered the 18 km from Paris to Joinville-le-Pont in about three hours.

Sources: [Wikipedia: Étienne Lenoir](https://en.wikipedia.org/wiki/%C3%89tienne_Lenoir)

### Gottlob Honold (1901)

Worked out Bosch's brief for a hot, long-lasting spark from fixed electrodes. Honold (1876–1923) apprenticed in Robert Bosch's workshop in 1891, studied engineering, and came back to Bosch in 1901 to work on ignition. His task was an apparatus that would produce a hot spark of relatively long duration from non-moving electrodes; by December, working with Arnold Zähringer, he had a test model. He also conceived putting parabolic mirrors behind headlamps, and his father was a friend of Robert Bosch's father.

*Worth knowing:* This entry records only what the cited biography states: the 1901 brief and test model. It does not date a patent or name him as the spark plug's sole inventor.

Sources: [Wikipedia: Gottlob Honold](https://en.wikipedia.org/wiki/Gottlob_Honold)

## Related lessons

- [The Four-Stroke Cycle](https://autoverse.maxrecursion.dev/topics/four-stroke-cycle)
- [Fuel Injection](https://autoverse.maxrecursion.dev/topics/fuel-injection)
- [Diesel Engines](https://autoverse.maxrecursion.dev/topics/diesel-engines)
- [Valves, Camshafts and Valve Timing](https://autoverse.maxrecursion.dev/topics/valves-and-camshaft)
- [Battery and Starter Motor](https://autoverse.maxrecursion.dev/topics/battery-and-starter)
- [Torque and Power](https://autoverse.maxrecursion.dev/topics/torque-and-power)

## Further reading

- [How the ignition system works](https://www.howacarworks.com/basics/how-the-ignition-system-works) — How a Car Works
- [Cleaning and fitting spark plugs](https://www.howacarworks.com/ignition-system/cleaning-and-fitting-spark-plugs) — How a Car Works

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Original AutoVerse lesson (AutoVerse original content). Simulation values in AutoVerse labs are illustrative. Source page: https://autoverse.maxrecursion.dev/topics/spark-ignition