In an Atkinson or Miller cycle engine, the expansion ratio is designed to be greater than the compression ratio primarily to:
Options:
(A) Extract additional mechanical work from expanding combustion gases down to lower pressure, thereby improving thermal efficiency
(B) Eliminate the need for engine oil lubrication
(C) Increase the maximum cylinder firing pressure above 250 bar
(D) Convert the engine into an external combustion steam engine
Answer Key:
A
Step-by-Step Solution & Explanation
By delaying intake valve closing (Miller/Atkinson cycle), the effective compression ratio is lowered while the expansion ratio remains large. This allows more work to be extracted from the high-temperature expansion gases, reducing exhaust blowdown energy losses and raising cycle thermal efficiency.