Definition of overall (brake thermal) efficiency: What do we call the ratio of useful work obtained at the crankshaft in a given time to the fuel energy supplied during the same time?

Introduction / Context:
Several efficiency metrics exist for internal combustion engines. Distinguishing overall (brake thermal) efficiency from mechanical and indicated efficiencies is essential for interpreting dynamometer tests and fuel economy.

Given Data / Assumptions:

• Useful output measured as brake power at the crankshaft.
• Fuel energy based on mass flow multiplied by lower calorific value.
• Steady operation over the measurement period.

Concept / Approach:

Overall (brake thermal) efficiency relates shaft output to chemical input: η_overall = B.P. / (ṁ_f * LCV). It captures all losses from combustion irreversibility, heat rejection, incomplete expansion, and mechanical friction indirectly because only brake output appears in the numerator.

Step-by-Step Solution:

Verification / Alternative check:

Using brake specific fuel consumption, bsfc = ṁ_f / B.P., we have η_overall = 1 / (bsfc * LCV) with consistent units, confirming the same definition.

Why Other Options Are Wrong:

Mechanical efficiency = B.P. / I.P., comparing shaft to indicated power. Indicated thermal efficiency = I.P. / (ṁ_f * LCV). Volumetric efficiency concerns air induction, not energy conversion. Combustion efficiency quantifies heat release from fuel but not shaft delivery.

Common Pitfalls:

Mixing up brake and indicated quantities; using higher heating value instead of LCV without accounting for water vapour condensation assumptions.

Final Answer:

overall efficiency