In liquid fuels for spark-ignition engines, the “performance number” (an extension beyond the octane number scale) is primarily related to which property: wax content, spontaneous ignition temperature, knocking tendency, or sulphur content?

Introduction / Context:
For spark-ignition (SI) engines, resistance to knock is the key quality metric. Octane number quantifies this resistance up to 100 using isooctane as the high reference fuel. Highly knock-resistant fuels such as certain aviation gasolines exceed this scale; their quality is expressed as “performance number,” conceptually extending octane rating to values above 100 by comparing indicated performance relative to a reference blend.

Given Data / Assumptions:

• Applies to SI engine fuels (gasoline/avgas), not diesel.
• Performance number correlates with anti-knock behavior.
• Engine testing conditions mirror controlled reference procedures.

Concept / Approach:
Knock arises from end-gas autoignition; fuels that delay or suppress this reaction allow higher compression or advanced timing, improving efficiency. Performance number therefore extends the anti-knock measurement beyond the octane number scale, not to be confused with thermal stability, wax content, or sulphur level. Although spontaneous ignition temperature influences knock kinetics, the standardized metric relates directly to observable knocking behavior and engine performance rather than a single physicochemical property.

Step-by-Step Solution:

Verification / Alternative check:
Fuel standards and aviation references equate higher performance number with higher anti-knock performance and improved detonation margins in SI engines.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing performance number with cetane number (diesel ignition quality for CI engines).

Final Answer:
Knocking tendency