Thermal performance definitions: The indicated thermal efficiency of a steam engine is defined as the:

Introduction / Context:
Multiple efficiencies are used when evaluating steam engines: indicated thermal, brake thermal, mechanical, boiler, and overall efficiencies. Clear definitions are essential for correct performance analysis and comparison across machines.

Given Data / Assumptions:

• I.P. (indicated power) is power developed in the cylinder from the indicator diagram.
• Energy supplied in steam refers to the enthalpy flow available to the engine.
• Steady operation with measurable mass flow and state points.

Concept / Approach:
Indicated thermal efficiency η_it compares the useful power developed inside the cylinder (indicated power) to the rate of thermal energy supplied in the incoming steam. Mathematically, η_it = (heat equivalent of I.P.) / (steam energy rate). This differs from brake thermal efficiency (uses B.P.) and mechanical efficiency (B.P./I.P.).

Step-by-Step Solution:
Identify numerator: I.P. converted to kJ/s (1 kW = 1 kJ/s).Identify denominator: m_dot * (h_in − h_ref) for the engine boundary.Form the ratio: η_it = I.P. / steam heat rate.Select the option that states this definition explicitly.

Verification / Alternative check:
Cross-check with related definitions: η_mech = B.P./I.P.; η_bt = B.P./(steam heat rate); η_overall = B.P./(fuel heat rate).

Why Other Options Are Wrong:
Options (a) and (d) are not standard definitions; (b) is mechanical efficiency; (e) mixes unrelated plant-level metrics.

Common Pitfalls:
Using brake power instead of indicated power; mixing engine-boundary efficiency with plant-wide overall efficiency.

Final Answer:
ratio of heat equivalent to indicated power to the energy supplied in steam