Spark-ignition (petrol) engines vs. compression-ignition (diesel): Why is the compression ratio in petrol engines intentionally kept lower than in diesel engines? Choose whether you agree with the statement: a higher compression ratio in petrol engines would promote pre-ignition/knock, so it must be limited.

Introduction / Context:
This item tests core thermodynamics and combustion knowledge for internal combustion engines. Petrol (spark-ignition, SI) engines and diesel (compression-ignition, CI) engines operate with different ignition modes. Understanding why SI engines use lower compression ratios clarifies the link between compression, end-gas temperature, and abnormal combustion (pre-ignition/knock).

Given Data / Assumptions:

• Petrol engines ignite the mixture by a timed spark.
• Diesel engines ignite fuel by air heating due to compression alone.
• Raising compression ratio increases end-gas temperature and pressure.
• Knock in SI is auto-ignition of the unburned end-gas ahead of the flame front.

Concept / Approach:
Compression ratio influences peak temperature prior to ignition. In SI engines, too high a compression ratio drives the unburned end-gas toward its auto-ignition limit, causing detonation/knock or even pre-ignition from hot spots. Diesel engines demand high compression to achieve auto-ignition of injected fuel; the fuels also differ in knock resistance (octane for SI, cetane for CI).

Step-by-Step Solution:

Verification / Alternative check:
Practical SI engines use compression ratios roughly 8:1 to 12:1 (varies with fuel and technologies like EGR/direct injection), while diesels often range 15:1 to 20+:1, confirming the principle.

Why Other Options Are Wrong:

• Disagree: Contradicts well-established knock behavior in SI engines.
• Agree only at high altitude / only with alcohol fuels: Altitude and fuel type shift limits, but the fundamental reason (avoid auto-ignition) universally applies.

Common Pitfalls:
Confusing SI knock with diesel knock; in SI, higher octane resists knock, whereas diesels need high cetane for short ignition delay. Also, modern knock sensors allow operation near limits but do not change the underlying constraint.

Final Answer:
Agree