ignition

Ignition is the controlled initiation of fuel combustion inside an engine cylinder. It is the system that produces a spark (in gasoline engines) or heat (in diesel engines) at precisely the right moment in the compression stroke, allowing the fuel-air mixture to burn and drive the piston down. Without ignition, there is no combustion; without timing, there is no usable power.

In spark-ignition engines, the ignition system consists of a battery, distributor or electronic control unit, ignition coil, spark plugs, and associated wiring. The coil steps up battery voltage to 10,000 to 40,000 volts; this high voltage jumps across the spark plug gap (typically 0.8 to 1.3 millimeters) to ignite the mixture. The timing of this spark must advance as engine speed increases and retard under heavy load, a function traditionally managed by mechanical centrifugal and vacuum mechanisms and now handled by electronic modules reading engine sensors.

Variants and failure modes

In compression-ignition (diesel) engines, there is no spark; instead, fuel injected into air heated by compression to 400 to 600 degrees Celsius self-ignites. The ignition delay, the period between injection and combustion, is critical. Too much delay and pressure rises too quickly, causing rough running and mechanical stress; too little and combustion is incomplete. Additives in diesel fuel raise the cetane number to improve ignition characteristics.

Common ignition failures include fouled spark plugs (carbon or oil buildup preventing spark), failed ignition coils, broken plug wires, and worn distributor contacts. In modern vehicles, the engine control unit monitors spark timing and adjusts it thousands of times per second based on inputs from knock sensors, oxygen sensors, and throttle position. A misfire, where ignition does not occur or occurs too late, raises engine temperature, damages the catalytic converter, and triggers a check-engine light.

The term "ignition" also refers colloquially to the electrical switch that energizes the engine's starting and running circuits. A vehicle's ignition key or button controls power to the fuel pump, engine computer, and starter motor before ignition of combustion actually occurs.