Reducing knock in spark ignition (SI) engines Which measures help reduce knocking tendency in SI engines under otherwise similar conditions?

Introduction / Context:
Knock in SI engines arises from auto-ignition of the end-gas ahead of the flame front. It depends on pressure, temperature, and time available for pre-flame reactions. Control strategies modify one or more of these factors to reduce end-gas auto-ignition propensity.

Given Data / Assumptions:

• Conventional SI engine with throttled or modern load control.
• Same fuel and compression ratio for the comparison.
• Focus on operating settings (timing and speed) rather than design changes.

Concept / Approach:

Retarding spark timing shifts the pressure rise closer to or slightly after top dead center, lowering end-gas temperature and pressure before auto-ignition can occur. Increasing engine speed shortens the total time available for end-gas reactions, also reducing knock likelihood for the same load and timing. Hence both measures mitigate knock.

Step-by-Step Solution:

Verification / Alternative check:

Dynamometer tests show reduced knock counts when spark is retarded and when operating at higher rpm with constant load, until other limits (e.g., cycle-by-cycle variability) appear.

Why Other Options Are Wrong:

Advancing timing at low speed can aggravate knock due to higher end-gas pressures and temperatures.

Common Pitfalls:

Over-retarding damages efficiency and increases exhaust temperatures; mitigation must balance performance and durability.

Final Answer:

both (a) and (b)