Engine performance terminology The ratio of brake power (measured at the shaft) to indicated power (calculated from the indicator diagram) is called:

Introduction / Context:
Performance of reciprocating engines is expressed using several efficiencies. Distinguishing between indicated and brake quantities is fundamental: indicated power is the power developed in the cylinder, while brake power is the usable output at the crankshaft after mechanical losses.

Given Data / Assumptions:

• Indicated power comes from pressure–volume (indicator) diagrams.
• Brake power is obtained from dynamometer measurements.
• Mechanical losses include friction, pumping losses, and auxiliaries.

Concept / Approach:
Mechanical efficiency quantifies how effectively the engine converts the gross cylinder power into net shaft power: η_mech = BP / IP. Overall or brake thermal efficiency compares brake power with fuel energy input; indicated thermal efficiency compares indicated power with fuel energy input. Volumetric efficiency concerns air-charge filling, not power conversion.

Step-by-Step Solution:
Define IP from area of indicator card and engine geometry.Measure BP using a dynamometer: BP = 2 * π * N * T.Compute η_mech = BP / IP.Recognize that η_mech < 1 due to friction and accessory loads.

Verification / Alternative check:
Test results typically show η_mech between 0.75 and 0.9 for healthy engines, aligning BP lower than IP.

Why Other Options Are Wrong:
Overall and brake thermal efficiencies relate to fuel energy; indicated thermal uses indicated power; volumetric efficiency is unrelated to power transmission losses.

Common Pitfalls:
Confusing brake and indicated values or mixing thermal and mechanical efficiency definitions.

Final Answer:
mechanical efficiency