Detonation Tendency and Self-Ignition Temperature – True or False? Evaluate the statement: “A fuel will detonate less (has lower knock tendency) if it has a higher self-ignition temperature.”

Introduction / Context:
In spark-ignition engines, detonation (knock) occurs when the unburned end-gas auto-ignites before the flame front arrives. Fuel properties affecting auto-ignition strongly influence knock resistance, which is quantified by octane number. One such property is the self-ignition (auto-ignition) temperature—the temperature at which a mixture spontaneously ignites at a given pressure without an external spark.

Given Data / Assumptions:

• Comparing fuels under the same engine conditions (pressure, mixture strength).
• Self-ignition temperature is considered alongside other properties (octane sensitivity, evaporation characteristics).
• Focus on SI knock rather than CI ignition quality.

Concept / Approach:

A higher self-ignition temperature means the unburned end-gas must reach a higher temperature before auto-ignition can occur. For the same compression ratio and operating point, this delays or prevents end-gas auto-ignition, reducing knock tendency. Thus, fuels with higher auto-ignition temperature, all else equal, are more knock-resistant (higher octane behavior). Note this differs from diesel fuels, where easier auto-ignition (lower ignition delay) is desirable, associated with higher cetane number—not higher self-ignition temperature.

Step-by-Step Solution:

Verification / Alternative check:

Octane-boosting components generally raise resistance to auto-ignition. Correlations between octane quality and end-gas reactivity support the role of increased auto-ignition thresholds in reducing knock.

Why Other Options Are Wrong:

“False” contradicts the thermochemical basis. Conditions like mixture strength and EGR modulate knock but do not reverse the fundamental relation. The claim limited “only in diesel engines” is incorrect because diesel ignition quality follows a different metric (cetane) and aims for easier auto-ignition.

Common Pitfalls:

Mixing up SI octane with CI cetane: high octane fuels resist auto-ignition (good for SI), while high cetane fuels auto-ignite readily (good for CI). The properties and desired directions are opposite.

Final Answer:

True