Fuel rating for compression-ignition (C.I.) engines In diesel (C.I.) engines, the standard scale used to express the ignition quality of the fuel is called:

Introduction / Context:
Engine fuels are graded on different scales depending on the combustion principle. Spark-ignition (petrol) engines rely on a controlled flame initiated by a spark, whereas compression-ignition (diesel) engines rely on self-ignition due to high temperature reached by compressing air. This question tests whether you can identify the correct rating scale for diesel fuel quality.

Given Data / Assumptions:

• C.I. engines depend on auto-ignition delay characteristics of the fuel.
• Rating scales differ between S.I. and C.I. engines.
• Common rating terms include octane number, cetane number, and performance number.

Concept / Approach:
For diesel fuel, ignition quality is measured by cetane number. A higher cetane number indicates shorter ignition delay and smoother pressure rise, aiding quieter operation, easier cold starting, and reduced white smoke. Octane number, by contrast, measures resistance to knock in petrol engines. Performance number is a related term used historically for high-octane aviation fuels, not for diesel.

Step-by-Step Solution:
Identify the engine type: compression-ignition (diesel).Recognize the key desired property: prompt auto-ignition (short ignition delay).Recall the rating that quantifies this in diesel fuels: cetane number.Select the option explicitly stating cetane number.

Verification / Alternative check:
Diesel fuel specifications from standards (e.g., EN 590, ASTM D975) list minimum cetane numbers to ensure satisfactory ignition delay and emissions performance, confirming the correct property and scale name.

Why Other Options Are Wrong:
Octane number applies to spark-ignition anti-knock quality, not diesel. Performance number relates to very high octane petrol ratings. 'None of these' is incorrect because a correct named scale exists.

Common Pitfalls:
Confusing knock in petrol with diesel knock; assuming both engines share the same rating term; overlooking that higher cetane is desirable, whereas higher octane means greater knock resistance in petrol.

Final Answer:
cetane number