Ignition source in petrol (spark-ignition) engines In a conventional petrol engine, the premixed air–fuel charge in the cylinder is ignited by which device?

Introduction / Context:
Petrol (spark-ignition) engines rely on externally initiated ignition of a premixed air–fuel charge. The reliability and timing of the ignition source are crucial for power, efficiency, and emissions control.

Given Data / Assumptions:

• Engine type: spark-ignition gasoline engine.
• Mixture is air plus atomised fuel prepared by a carburettor or fuel injection.
• Goal: identify the component that actually initiates combustion.

Concept / Approach:
Spark plugs provide a high-voltage electrical discharge across electrodes, igniting the compressed mixture at a precise crank angle. Other listed components either meter fuel (pump/injector) or are relevant to diesel engines (glow plug) and do not create the timed ignition spark in SI engines.

Step-by-Step Solution:
Determine which device delivers a controlled spark → the spark plug.Eliminate components that deliver fuel but do not ignite it (pump, injector).Eliminate diesel-specific aids (glow plug) and implausible items (heated valve).

Verification / Alternative check:
Ignition systems comprise ignition coil, control electronics or distributor, leads, and spark plugs to ensure reliable kernel formation.

Why Other Options Are Wrong:

• Fuel pump/injector: regulate fuel delivery, not ignition.
• Glow plug: assists cold starting in diesel, not used to trigger SI combustion.
• Heated valve: not an ignition device.

Common Pitfalls:
Assuming fuel injectors always imply compression ignition; many SI engines are fuel-injected but still require a spark plug.

Final Answer:
Spark plug