Definitions of efficiencies in I.C. engines The ratio of indicated thermal efficiency to the corresponding air-standard (ideal cycle) efficiency is termed as which engine performance metric?

Introduction / Context:
Separating real-engine losses from ideal-cycle limits helps diagnose where performance is being lost. Relative efficiency compares the indicated thermal efficiency of the real engine to the theoretical air-standard cycle efficiency under similar compression ratio and conditions.

Given Data / Assumptions:

• Indicated thermal efficiency accounts for in-cylinder conversion of fuel energy to indicated work.
• Air-standard efficiency is derived from the corresponding ideal cycle (e.g., Otto) at the same compression ratio.
• Comparison excludes mechanical losses (that is handled by mechanical efficiency).

Concept / Approach:
Relative efficiency = eta_indicated / eta_air-standard. It expresses how closely the real combustion and heat-transfer processes approach the ideal assumptions. It is sometimes called efficiency ratio. It does not include drivetrain or accessory losses.

Step-by-Step Solution:

Verification / Alternative check:
Textbook performance maps typically report mechanical efficiency separately as BP/IP, while relative efficiency benchmarks indicated performance against the ideal limit.

Why Other Options Are Wrong:

Mechanical efficiency compares BP to IP; overall efficiency compares BP to fuel energy; volumetric efficiency compares actual to theoretical air charge; isentropic efficiency applies to turbomachinery stages.

Common Pitfalls:
Assuming relative efficiency includes friction. It does not; friction is outside indicated processes.

Final Answer:

relative efficiency (efficiency ratio)