Preventing knock in spark-ignition engines To reduce knocking tendency in the end-gas region far from the spark plug, which properties should that charge region ideally have?

Introduction / Context:
Knock in spark-ignition (SI) engines results from auto-ignition of the unburned end-gas ahead of the flame front. Reducing end-gas reactivity is central to knock control, protecting the engine and improving drivability and efficiency.

Given Data / Assumptions:

• Premixed SI combustion initiated at the spark plug.
• End-gas conditions determine auto-ignition risk.
• Typical fuels with finite octane rating.

Concept / Approach:

Auto-ignition depends on temperature, pressure, and time for pre-flame chemistry. Lower density implies less mass and lower pressure rise; lower temperature slows chemistry; longer ignition delay (higher octane behavior) gives more time for the flame to consume the end-gas before it auto-ignites. Hence all three listed attributes reduce knock likelihood.

Step-by-Step Solution:

Verification / Alternative check:

Empirical controls that create these conditions include charge cooling (intercoolers, water injection), retarded spark (reduces pre-TDC compression heating), and using higher-octane fuel (increases ignition delay).

Why Other Options Are Wrong:

Choosing only one factor ignores the multi-parameter nature of auto-ignition. Residual gases can both cool and dilute but also reduce oxygen; taken alone, it is not the complete answer.

Common Pitfalls:

Assuming lean mixtures always reduce knock; at high load, excessively lean operation can raise end-gas temperature due to slower burning and may not help.

Final Answer:

All of these