Diesel ignition quality: The cetane number requirement for high-speed diesel (HSD) fuels used in automotive engines must be at least how much?

Introduction:
Cetane number measures ignition quality in compression-ignition (diesel) engines. High-speed diesel used in automotive applications requires a minimum cetane number to ensure quick ignition delay, smooth running, and low noise.

Given Data / Assumptions:

• Context is typical exam/industry baseline for HSD.
• We distinguish between minimum specification values and premium/modern regional norms.

Concept / Approach:
Historically and in many standard question banks, the minimum acceptable cetane for HSD is set at or above 45. Some modern specifications in various jurisdictions target higher values (e.g., around 51) to reduce emissions and enhance cold starting, but the canonical minimum often cited for HSD is 45.

Step-by-Step Solution:

Verification / Alternative check:
Older BIS/API style texts and refinery training materials frequently reference a 45 minimum for HSD, with improved grades exceeding this.

Why Other Options Are Wrong:

• 30: Poor ignition quality, unsuitable for HSD.
• 75 or 95: Far above commercial diesel ranges.
• 51: Regionally common newer target, not the classic minimum asked here.

Common Pitfalls:
Mixing up contemporary regional specs with the baseline figures used in standard MCQs.

Final Answer:
45