Spark-ignition (petrol) engines — role of the ignition coil: In a conventional spark-ignition ignition system, the ignition coil must supply ________ voltage to the spark plug to jump the gap and initiate combustion in the cylinder.

Introduction / Context:
The ignition system of a spark-ignition (SI) petrol engine creates a controlled electrical discharge across the spark plug gap to ignite the air–fuel mixture. Central to this is the ignition coil, a transformer that elevates battery voltage to much higher levels so the spark can bridge the plug gap under high in-cylinder pressure.

Given Data / Assumptions:

• Battery supply is roughly 12 V (vehicle electrical system).
• The spark plug gap typically ranges from about 0.6 mm to 1.1 mm.
• In-cylinder gas pressure near firing is several bar, increasing the dielectric strength of the mixture, which requires higher breakdown voltage.

Concept / Approach:
Electrical breakdown across a gap depends on gas pressure, gap size, and mixture composition. A 12 V source is insufficient; rather, tens of kilovolts are needed. The ignition coil, acting as a step-up transformer, stores magnetic energy and then collapses the field to induce a high-voltage pulse on the secondary winding, producing the discharge at the plug.

Step-by-Step Solution:

Verification / Alternative check:
Service manuals and oscilloscope traces show secondary voltages commonly peaking between about 15 kV and 30 kV, varying with gap, pressure, and system design. This confirms that a high voltage is essential for consistent ignition under engine conditions.

Why Other Options Are Wrong:

• Low or medium (battery-level) only: 12 V cannot ionize the gap in a compressed gas mixture.
• Alternating low voltage: The spark requires a high-voltage pulse, not simply low-voltage AC.

Common Pitfalls:
Assuming plug firing voltage is constant; it rises with cylinder pressure (wide-open throttle, boosted engines) and with larger gaps. Weak coils or excessive gap increase misfire risk.

Final Answer:
high