Fuels and knocking resistance A petrol (gasoline) that detonates easily in a spark-ignition engine is referred to as what?

Introduction / Context:
Detonation (knock) is spontaneous, uncontrolled end-gas combustion in a spark-ignition engine, producing pressure waves that can damage components and reduce efficiency. The fuel’s resistance to knock is quantified by its octane rating.

Given Data / Assumptions:

• Spark-ignition (petrol) engine operating with fixed compression ratio.
• Octane number indicates knock resistance under standardized test conditions.
• Other descriptors (unleaded, premium) may correlate with additives and marketing but are not strict measures of knock resistance unless a number is specified.

Concept / Approach:
Lower octane fuels auto-ignite more readily under pressure and temperature, promoting detonation. Higher octane fuels can withstand higher compression and advanced ignition timing without knocking. Therefore, a petrol that “detonates easily” is low octane petrol.

Step-by-Step Solution:
Identify property controlling detonation: octane rating.Relate “detonates easily” to “low knock resistance.”Low knock resistance corresponds to a low octane rating.Thus, the correct description is “low octane petrol.”

Verification / Alternative check:
Engines designed for high octane fuel often knock when run on lower octane; ECU knock sensors may retard timing to protect the engine, demonstrating dependence on octane.

Why Other Options Are Wrong:
High octane petrol resists knock; it does not detonate easily.

Unleaded refers to absence of tetraethyl lead, not directly to knock behavior unless octane is specified.

Blended or “premium” fuels may have higher octane, but the term alone is not definitive.

Common Pitfalls:
Confusing “power” or “energy content” with octane; octane rating does not measure energy per litre but knock resistance. Assuming all premium fuels are the same across regions despite differing octane scales (RON, MON, AKI).

Final Answer:
low octane petrol