These terms are commonly used in internal combustion engines (IC engines) and other machinery to describe different power measurements.
1. Indicated Power (IP)
Definition: The theoretical power developed inside the engine cylinder due to the combustion of fuel. It is calculated from the indicator diagram (pressure-volume curve).
Formula:
For a multi-cylinder engine:
Where:
= Mean Indicated Pressure (Pa or N/m²)
= Stroke Length (m)
= Cross-sectional area of piston (m²)
= Engine speed (RPM)
= Number of cylinders
Key Points:
Measured using an indicator device (e.g., pressure sensor).
Represents total power before mechanical losses (friction, heat, etc.).
2. Brake Power (BP)
Definition: The actual power available at the engine crankshaft (output shaft) for useful work. Measured using a dynamometer (hence also called shaft power).
Formula:
Where:
= Engine speed (RPM)
= Torque (Nm)
Alternative (Dynamometer Method):
= Brake load (N)
= Brake drum radius (m)
= Constant (depends on dynamometer type)
Key Points:
Lower than IP due to friction & other losses.
Used to calculate mechanical efficiency.
3. Relationship Between IP, BP & Friction Power (FP)
Where:
FP (Friction Power) = Power lost due to friction, pumping, etc.
4. Mechanical Efficiency (η)
Shows how well the engine converts indicated power into usable power.
Typical values: 75–90% for diesel engines, lower for petrol engines.
Summary Table
| Parameter | Definition | Measurement Method |
|---|---|---|
| Indicated Power (IP) | Power generated inside cylinder | Indicator diagram |
| Brake Power (BP) | Power at crankshaft (usable) | Dynamometer |
| Friction Power (FP) | Power lost due to friction |
Example Calculation
If an engine has:
Then:
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