How does the thermal efficiency of a crankcase-scavenged two-stroke petrol engine compare to a four-stroke engine with an equal compression ratio_____________________?
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Answer: B. lower
Explanation
Crankcase-scavenged two-stroke petrol engines use the crankcase to compress the incoming air-fuel charge before transfer. This method results in poor scavenging efficiency, allowing fresh mixture to escape directly into the exhaust port. Because of incomplete scavenging and short-circuiting fuel losses, their overall thermal efficiency is lower than four-stroke engines. Two-stroke engines complete a power cycle every single revolution, whereas four-stroke engines require two complete crankshaft revolutions. Crankcase scavenging mixes lubricating oil with fuel, leading to higher emissions and reduced combustion efficiency. Four-stroke engines feature dedicated intake, compression, power, and exhaust strokes with positive valve timing for better efficiency.
Option A of higher is incorrect because scavenging losses and incomplete expansion strokes reduce two-stroke thermal efficiency. Option C of same ignores the well-documented volumetric and thermal efficiency losses inherent in crankcase scavenging. Options D and E are incorrect because engine size does not overturn the fundamental thermodynamic limitations of crankcase scavenging.
- Subject
- Engineering
- Category
- Mechanical Engineering
- Topic
- —
- Difficulty
- medium
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