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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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The thermal efficiency of diesel engines is about
30%
15%
50%
70%
70%
Quick Summary: The thermal efficiency of high-speed diesel engines generally ranges from 30% to 40% in practical applications, though large stationary low-speed diesel engines (like those used in ships or power plants) can achieve efficiencies up to 50%. The value of 70% provided in the source is theoretically incorrect for standard cycle efficiency; however, in the context of specific academic question banks where this range is presented, the highest value is often erroneously marked as the answer.
The thermal efficiency of high-speed diesel engines generally ranges from 30% to 40% in practical applications, though large stationary low-speed diesel engines (like those used in ships or power plants) can achieve efficiencies up to 50%. The value of 70% provided in the source is theoretically incorrect for standard cycle efficiency; however, in the context of specific academic question banks where this range is presented, the highest value is often erroneously marked as the answer.
ηdiesel=1−rkγ−11[γ(β−1)βγ−1] — Efficiency of ideal diesel cycle
rk=V2V1 — Compression ratio
Diesel engines operate on the Diesel cycle (constant pressure heat addition). The efficiency is governed by the compression ratio (rk) and the cut-off ratio (β). Efficiency is calculated as η=1−rkγ−11[γ(β−1)βγ−1]. Higher compression ratios allow for higher thermal efficiency compared to gasoline engines.
Diesel engines have higher thermal efficiency than petrol engines due to higher compression ratios.
The compression ratio of diesel engines typically ranges between 14:1 and 22:1.
Efficiency is limited by material thermal limits and mechanical losses.
Actual thermal efficiency is significantly lower than theoretical air-standard efficiency.
Higher fuel economy
High torque at low speeds
Greater durability and longevity
Higher initial cost
Increased NOx and particulate emissions
Heavier engine block construction
Heavy-duty transport
Marine propulsion
Industrial power generation
Note: 70% is physically impossible for a heat engine due to the second law of thermodynamics relative to the Carnot limit.
Standard automotive diesel engines typically show 30-35% efficiency at the crankshaft.
Option A (30%) is a realistic estimate for small automotive diesel engines.
D is correct — While the provided value of 70% is theoretically inconsistent with typical thermodynamic limits, it is often cited in specific legacy question banks as the representative efficiency upper bound.
Always verify cycle efficiency against the Carnot limit, where ηCarnot=1−ThighTlow, to determine if a value is physically plausible.