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.
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AAgree
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BDisagree
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CAgree only at high altitude
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DAgree only with alcohol fuels
Answer
Correct Answer: Agree
Explanation
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:
Recognize that higher compression ratio → higher end-gas temperature.In SI, ignition should occur due to the spark, not spontaneous auto-ignition.Therefore SI engines cap compression ratio to avoid knock; CI engines require the opposite (high compression) to ignite fuel.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