Ignition system — spark plug voltage level in petrol (S.I.) engines The typical high voltage required at the spark plug during normal operation is of the order of:

Introduction / Context:
In spark-ignition engines, the ignition coil boosts battery voltage to a much higher level so that a spark can jump the plug gap under cylinder pressure. Understanding the order of magnitude helps diagnose ignition faults and select components.

Given Data / Assumptions:

• Battery provides around 12 volts nominal.
• Coil step-up plus switching (breaker or electronic) generates several thousand volts.
• Required voltage depends on plug gap, mixture, pressure, and temperature.

Concept / Approach:
Typical required voltage during running is often between 10 kV and 20 kV, with spikes higher during cold starts or lean conditions. Values like 1 kV or 2 kV are far too low to consistently ionize the gap under compression, whereas 11 kV lies squarely within the common operational range for many engines and is an appropriate 'order of' estimate.

Step-by-Step Solution:
Recognize that the spark must bridge roughly 0.6 to 1.0 mm gap under high cylinder pressure.Recall typical coil output and plug firing voltage: about 10 kV to 20 kV.Select a value within this band as the order-of-magnitude: 11 kV.

Verification / Alternative check:
Oscilloscope patterns of secondary ignition commonly show firing voltages around 10 kV with higher peaks during acceleration or cold conditions, validating the selection.

Why Other Options Are Wrong:
1 kV and 2 kV are insufficient under compression; 22 kV can occur but is on the higher side and not the typical order-of-magnitude for normal warmed-up running.

Common Pitfalls:
Confusing coil maximum capability with typical firing voltage; ignoring that required voltage increases with plug wear, lean mixtures, or boosted engines.

Final Answer:
11 kilovolts