Compression-ignition engines: What is the effect of higher combustion-chamber wall temperature on knocking tendency?

Introduction / Context:
Diesel (CI) knock is linked to ignition delay: longer delay allows more fuel to accumulate before rapid combustion, producing an abrupt pressure rise and characteristic knock. Chamber temperature influences chemical kinetics of auto-ignition and thus affects knock propensity.

Given Data / Assumptions:

• Ignition in a CI engine occurs when injected fuel auto-ignites in hot compressed air.
• Ignition delay shortens as ambient (wall/charge) temperature increases.
• All else equal (fuel cetane, injection timing, air–fuel ratio), delay dominates knock intensity.

Concept / Approach:
Higher wall and charge temperatures promote faster pre-flame reactions, reducing ignition delay. With shorter delay, less fuel accumulates before combustion starts, softening the pressure rise rate and reducing knock severity. Conversely, cold starts lengthen delay and aggravate knock, which is why glow plugs are used in small diesels.

Step-by-Step Solution:

Verification / Alternative check:
Practical observations: winter operation yields harsher diesel combustion sounds; warming the chamber (glow plugs, block heaters) improves smoothness and reduces knock, supporting the theory.

Why Other Options Are Wrong:

• Increase: Opposite of the established ignition-delay relationship.
• Not effect: Temperature is a primary variable in auto-ignition kinetics.
• Increase only at full load: Load modifies temperature and injection, but the base trend (higher temperature reduces knock) holds broadly.

Common Pitfalls:
Confusing SI knock (end-gas) with CI knock; mechanisms differ though both are forms of abnormal/undesirable combustion.

Final Answer:
reduce