Mechanical efficiency is the ratio of:

Question

BlackTom AI · Accepted Answer

Mechanical efficiency is the ratio of brake horsepower to indicated horsepower.

Relevant knowledge points and concepts:
- Indicated horsepower (IHP) is the gross power developed inside the engine cylinders, calculated from pressure-time (indicator) diagrams; it represents the theoretical power produced by combustion before mechanical losses.
- Brake horsepower (BHP) is the usable power delivered at the engine’s crankshaft (or output shaft) measured by a dynamometer; it is IHP minus mechanical losses (friction, pumping, accessory drives).
- Frictional horsepower (FHP) quantifies those internal mechanical losses. The basic relationship is: IHP = BHP + FHP.
- Mechanical efficiency (ηm) expresses what fraction of the developed (indicated) power becomes usable output: ηm = BHP / IHP.

Step-by-step reasoning:
1. Start from the power balance: IHP = BHP + FHP.
2. Rearranged, mechanical efficiency ηm = BHP / IHP.
3. Alternatively ηm = 1 − (FHP / IHP), since FHP = IHP − BHP.

Numeric example with intermediate steps:
- Suppose IHP = 400 hp and internal friction losses FHP = 40 hp.
- Then BHP = IHP − FHP = 400 hp − 40 hp = 360 hp.
- Mechanical efficiency ηm = BHP / IHP = 360 / 400 = 0.90 = 90%.

Why this answer is correct:
- Mechanical efficiency measures how effectively the engine’s internally generated power (IHP) is converted to usable output (BHP). Thus the correct ratio is BHP to IHP.

Why the other options are incorrect:
- "Fuel to air" describes fuel–air ratio or mixture, not mechanical efficiency.
- "Torque to RPM" is related to power calculation (Power = Torque × RPM × constant) but not a measure of mechanical efficiency.
- "Power to weight" is power-to-weight ratio (specific power), used for performance, not internal mechanical efficiency.

Conclusion: Mechanical efficiency equals brake horsepower divided by indicated horsepower, which matches the provided correct answer: Brake horsepower to indicated horsepower.

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