Spark Ignition and Combustion Timing

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.