Fuel knock tolerance — Highest Useful Compression Ratio (HUCR) HUCR is defined as the highest compression ratio at which the __________.

Introduction / Context:
Knock resistance of spark-ignition fuels is characterized by octane rating and also by Highest Useful Compression Ratio (HUCR), a concept used in standardized knock testing.

Given Data / Assumptions:

• Standardized CFR (Cooperative Fuel Research) test engine conditions.
• Focus on spark-ignition knock, not diesel auto-ignition quality.

Concept / Approach:
HUCR is the maximum compression ratio at which a fuel can operate in the test engine without audible knock under specified operating settings. It is a practical measure of a fuel’s anti-knock quality.

Step-by-Step Solution:

Verification / Alternative check:
Correlation exists between higher octane numbers and higher HUCR values, consistent with anti-knock performance.

Why Other Options Are Wrong:

• Maximum output or peak efficiency (B/C): not the definition of HUCR.
• Simple continuous running (A): too vague.
• Perfect combustion and zero emissions (E): unrealistic and unrelated to HUCR.

Common Pitfalls:
Confusing HUCR with mechanical compression ratio limits of a particular production engine; HUCR is a test-fuel property under standardized conditions.

Final Answer: