Overall (Brake Thermal) Efficiency – Definition for Heat Engines For an internal combustion engine, overall efficiency is defined as the ratio of useful work obtained at the crankshaft over a given time to the total fuel energy supplied in the same time. Is this definition correct?

Introduction / Context:
Overall efficiency, often called brake thermal efficiency, is a cornerstone metric for heat engines. It compares the useful shaft output delivered at the crankshaft to the chemical energy supplied by the fuel during the same duration. This provides a practical measure of how effectively an engine converts fuel energy into mechanical work that can drive a vehicle or generator.

Given Data / Assumptions:

• Useful output is brake power measured at the crankshaft via a dynamometer.
• Energy input is fuel mass flow multiplied by lower calorific value over the same time.
• Steady-state operation during the measurement interval.

Concept / Approach:

By definition, overall efficiency evaluates the entire energy conversion chain inside the engine, including combustion, thermodynamic cycle, and mechanical transmission to the shaft. It therefore uses brake power in the numerator, not indicated power. Using the supplied fuel energy in the denominator ensures the metric accounts for all losses including incomplete combustion, heat rejection, and frictional losses between piston and crankshaft.

Step-by-Step Solution:

Verification / Alternative check:

Brake specific fuel consumption provides the inverse relationship: bsfc = fuel flow / brake power. Using bsfc, eta_overall can be cross-checked as eta_overall = 1 / (bsfc * LCV) with consistent units, confirming the same concept.

Why Other Options Are Wrong:

Limiting the definition to diesel or full load is unnecessary; it applies to any heat engine and at any load. Supercharging does not alter the definition, only the values.

Common Pitfalls:

Confusing overall efficiency with indicated thermal efficiency (which uses indicated power) or with mechanical efficiency (which is B.P. / I.P.). Another pitfall is mixing time bases; numerator and denominator must correspond to the same interval.

Final Answer:

True