Operating controls that reduce SI knocking Which single adjustment will reliably decrease knocking tendency in a spark-ignition engine (all else equal)?

Introduction / Context:
Engine knock control uses multiple levers. Some adjustments reduce end-gas temperature/pressure or shorten residence time, lowering the chance of auto-ignition. Understanding which operational change helps is critical for diagnosis and calibration.

Given Data / Assumptions:

• Conventional SI engine with fixed compression ratio.
• Same load and fuel unless otherwise stated.
• We compare single parameter changes.

Concept / Approach:

Retarding spark timing delays peak pressure closer to or just after top dead center, reducing end-gas temperature and pressure before spontaneous ignition can occur. Conversely, advancing timing, raising intake temperature, increasing compression ratio, or lowering fuel octane all increase knock propensity by elevating end-gas reactivity or pressure–temperature history.

Step-by-Step Solution:

Verification / Alternative check:

Knock sensors and closed-loop ignition control in modern ECUs routinely retard timing upon knock detection, proving its effectiveness as a first-line mitigation.

Why Other Options Are Wrong:

Advancing timing, higher compression, higher intake temperature, or lower octane each increases end-gas reactivity or load, thus promoting knock.

Common Pitfalls:

Over-retarding reduces efficiency and increases exhaust temperature; it is a mitigation, not a cure for fundamental design/fuel mismatches.

Final Answer:

Retarding the ignition timing