Which Engine Achieves Carnot (Maximum Reversible) Efficiency? Among engine types, only a fully reversible engine operating between two reservoirs can, in principle, achieve Carnot’s maximum efficiency limit.

Introduction / Context:
The Carnot theorem states that no heat engine operating between two thermal reservoirs can be more efficient than a reversible engine working between the same reservoirs—and all reversible engines between the same temperatures have the same efficiency. This question asks which engine type fits that idealization.

Given Data / Assumptions:

• Two thermal reservoirs at fixed temperatures T_H and T_L.
• Engine cycle can be idealized as reversible or irreversible.
• No restrictions on working fluid; reversibility is the key criterion.

Concept / Approach:

Real petrol (Otto) and diesel (Diesel) engines are inherently irreversible due to friction, finite-rate combustion, pressure losses, and heat transfer across finite differences. Therefore they cannot achieve the Carnot limit. Only a hypothetical reversible engine—one that operates quasi-statically with no entropy generation—matches Carnot efficiency.

Step-by-Step Solution:

Verification / Alternative check:

Exergy analysis of actual engines shows positive entropy generation and associated exergy destruction, proving eta < eta_C. Laboratory approximations can approach but never equal the reversible limit.

Why Other Options Are Wrong:

Petrol and diesel engines are specific real cycles (Otto/Diesel) with irreversibilities; irreversible engines by definition cannot reach Carnot’s maximum; “any real engine” contradicts the second law.

Common Pitfalls:

Assuming higher compression ratio or better fuel alone allows surpassing fundamental reversible limits; confusing high component efficiencies with cycle reversibility.

Final Answer:

reversible engine