Standard engine for knocking tests\nKnocking characteristics (octane/cetane-type tests) of petrofuels are measured in a ______ engine.

Introduction / Context:
Standardized test engines provide reproducible conditions to rate fuels for knock tendency (gasoline) or ignition quality (diesel). The CFR engine is the long-standing benchmark for octane and cetane-related procedures.

Given Data / Assumptions:

• ASTM standard methods for octane number rely on variable compression CFR spark-ignition engines.
• Cetane number tests likewise use CFR-type diesel test engines with controlled parameters.

Concept / Approach:
The CFR engine provides controlled compression ratio, speed, intake temperature, and standardized reference fuels. Octane number measures knock resistance in SI engines; cetane number measures ignition delay in CI engines. Both use CFR hardware adapted for their mode, not idealized thermodynamic cycles such as Carnot or Stirling.

Step-by-Step Solution:

Verification / Alternative check:
ASTM D2699/D2700 (RON/MON) and D613 (cetane) reference CFR engines explicitly.

Why Other Options Are Wrong:

• Carnot/Stirling are theoretical cycles, not test engines.
• “Diesel (unstandardized)” lacks the rigor of the CFR standard apparatus.

Common Pitfalls:
Assuming any engine can rate fuels; standardized conditions are essential for comparable numbers.

Final Answer:
CFR (Co-operative Fuel Research)