Question

The ratio of brake power to indicated power of an internal combustion engine is called:

• A. brake thermal efficiency
• B. volumetric efficiency
• C. relative efficiency
• D. mechanical efficiency

Hint:

Think of it like a paycheck. Indicated Power is your "Gross Pay" (what you earned), and Brake Power is your "Net Pay" (what you actually get to take home). The Mechanical Efficiency tells you how much of your total earnings weren't lost to "taxes" (friction)!

Answer 1

The Correct Option is D

1. Understanding Power Types:

• Indicated Power (IP): The total power developed inside the engine cylinder by the combustion of fuel. This is calculated from indicator diagrams.

• Brake Power (BP): The actual useful power available at the engine's crankshaft. This is measured using a dynamometer (brake).

• Friction Power (FP): The power lost due to friction between moving parts (pistons, bearings) and driving auxiliaries.
The relationship is: $BP = IP - FP$.

2. Defining Mechanical Efficiency:

Mechanical efficiency ($\eta_{mech}$) is the ratio that describes how well the engine converts the power developed in the cylinder into useful output power at the shaft. $$\eta_{mech} = \frac{\text{Brake Power (BP)}}{\text{Indicated Power (IP)}}$$

3. Why it matters: A high mechanical efficiency (typically $70\%$ to $90\%$ in modern engines) indicates that the engine has low internal friction. As the engine load increases, mechanical efficiency generally increases because the fixed friction losses become a smaller percentage of the total power.