Typical ignition voltage requirement in a spark-ignition system: what voltage is needed across the plug gap to reliably produce a spark?

Introduction / Context:
Ignition systems in petrol engines transform low-voltage battery power into high voltage capable of breaking down the air-fuel mixture across the spark plug gap. Knowing the order of magnitude for breakdown voltage helps diagnose ignition faults and design insulation.

Given Data / Assumptions:

• Spark gaps typically 0.6–1.0 mm for conventional systems.
• Mixture pressure near end of compression increases breakdown voltage.
• Classic coil/ distributor systems; modern systems can supply even higher voltage.

Concept / Approach:
Paschen-type behavior indicates required voltage increases with pressure and gap. Traditional automotive systems are designed to deliver roughly 10–12 kV minimum under cylinder pressure. Modern coil-on-plug or CDI systems may reach 20–40 kV for reliability, but the textbook baseline for minimum reliable spark is in the 10–12 kV bracket.

Step-by-Step Solution:

Verification / Alternative check:
Service manuals list secondary ignition peak capability often above 20 kV to ensure margin, aligning with the minimum threshold near 10–12 kV for dependable ignition in standard conditions.

Why Other Options Are Wrong:

• 2–4 kV or 4–6 kV: Insufficient under compressed mixture conditions.
• 6–10 kV: May fire at low load but marginal at high pressure; not a robust design value.

Common Pitfalls:
Confusing no-pressure bench spark with in-cylinder pressure conditions; the latter demands much higher voltage.

Final Answer:
10 000 to 12 000 volts