Compression ratio and knock tendency (petrol engines): Using a fixed-octane fuel, increasing the compression ratio in a spark-ignition petrol engine will ________ the tendency to knock.

Introduction / Context:
Engine knock (auto-ignition) in spark-ignition engines depends on mixture temperature, pressure, and fuel octane. Compression ratio strongly influences end-gas conditions; higher ratios increase the likelihood that the unburned end gas reaches auto-ignition before the flame front arrives.

Given Data / Assumptions:

• Fuel octane rating is fixed (no change in fuel anti-knock quality).
• Other variables held constant (intake temperature, spark timing, etc.).
• Naturally aspirated operation for simplicity.

Concept / Approach:
Higher compression ratio raises the temperature and pressure of the air–fuel mixture at the end of compression. Since auto-ignition propensity increases with temperature and pressure for a given octane, the probability of knock rises. Therefore, the statement that increasing compression ratio will not affect knocking tendency is incorrect; it does affect it adversely (increased knock tendency) unless compensated by higher octane fuel, retarded spark, cooler charge, or knock-mitigation strategies.

Step-by-Step Solution:

Verification / Alternative check:
Engine calibration practice ties allowable spark advance and compression ratio to octane. Higher compression requires higher octane or conservative spark timing to avoid knock, directly showing the dependency.

Why Other Options Are Wrong:

• Agree: Opposite of well-established thermodynamic behavior.
• Conditional options: The dependency is general; operating condition changes merely shift the threshold.

Common Pitfalls:
Assuming modern knock sensors eliminate the problem; they mitigate damage by retarding spark, but efficiency may suffer. The physical tendency still increases with compression ratio for a fixed fuel.

Final Answer:
Disagree