Terminology check: relative efficiency of an I.C. engine State whether the following definition is correct: “The relative efficiency of an internal combustion engine is the ratio of its indicated thermal efficiency to the corresponding air-standard efficiency.”

Introduction / Context:
Naming conventions matter when comparing real engines to ideal cycles. Relative efficiency (also called efficiency ratio) normalizes indicated performance to the theoretical air-standard limit at the same basic cycle conditions.

Given Data / Assumptions:

• We compare indicated thermal efficiency to an ideal Otto/Diesel cycle efficiency.
• Air-standard model uses simplified assumptions (ideal gas, no pumping or heat losses).
• Applies to both SI and CI engines.

Concept / Approach:
Relative efficiency = eta_indicated / eta_air-standard. It evaluates how closely in-cylinder thermodynamic processes emulate the idealized cycle, independent of mechanical losses. It is a dimensionless ratio less than 1 for real engines.

Step-by-Step Solution:

Verification / Alternative check:
Engine texts list mechanical efficiency (BP/IP) and overall efficiency (BP/fuel energy) separately from relative efficiency, confirming the definition’s correctness.

Why Other Options Are Wrong:

It is not limited to one engine type; and it does not depend only on compression ratio, since combustion phasing, heat losses, and burn duration also matter.

Common Pitfalls:
Mixing up relative efficiency with mechanical or overall efficiency; they capture different loss mechanisms.

Final Answer:

Correct