What is indicated power?

Short Answer:

Indicated power is the total power developed inside the engine cylinder due to the combustion of fuel. It represents the power generated by the expansion of gases on the piston before any mechanical losses occur. Indicated power is calculated from the pressure inside the cylinder and is an ideal measure of the engine’s capability.

It is usually determined using an engine indicator diagram that records pressure versus volume (P–V diagram). Indicated power helps in analyzing the efficiency and performance of an internal combustion engine before considering frictional or mechanical losses.

Detailed Explanation :

Indicated Power

Indicated power (IP) is the theoretical or total power developed by the working gases in an internal combustion (IC) engine cylinder. It represents the energy converted from chemical energy of the fuel into mechanical energy during combustion. However, it does not account for mechanical friction or other losses inside the engine. In simple terms, it is the power developed by the gases acting on the piston, and it can be measured using an indicator diagram that shows the relationship between pressure and volume in the cylinder.

Indicated power is always higher than the brake power because it includes the power lost due to friction and other mechanical resistances. The difference between indicated power and brake power is known as friction power. Thus, indicated power is a crucial parameter to determine the total engine efficiency and mechanical losses.

Formula for Indicated Power

Indicated power can be calculated using the following formula:

where:

  •  = Indicated mean effective pressure (kN/m²)
  •  = Stroke length (m)
  •  = Area of the piston (m²)
  •  = Number of power strokes per minute
  •  = Number of cylinders

This formula gives the indicated power in kilowatts (kW). It helps engineers find how much power is generated within the cylinders before accounting for mechanical losses.

Concept of Indicated Mean Effective Pressure (IMEP)

The indicated mean effective pressure is an average pressure that acts on the piston during the power stroke. It is derived from the indicator diagram and helps in simplifying the calculation of indicated power. The IMEP provides an idea of how effectively the pressure in the cylinder contributes to power generation. A higher IMEP means more efficient combustion and more power output.

The IMEP can be determined from the indicator diagram using:

This value, when substituted in the power formula, gives the indicated power for the engine.

Measurement of Indicated Power

The indicated power is measured using an engine indicator that records the pressure inside the engine cylinder as a function of piston displacement. The device produces an indicator diagram (P–V diagram). The area of this diagram represents the work done per cycle by the gases. By multiplying the work done per cycle by the number of working strokes per minute, the indicated power is obtained.

There are two common types of indicators used:

  1. Mechanical Indicators:
    These use springs, levers, and recording pens to draw the pressure-volume diagram on paper. They are mainly used in older engines.
  2. Electronic Indicators:
    These modern instruments use sensors and computers to record the pressure data accurately and calculate indicated power digitally.

Importance of Indicated Power

  1. Engine Performance Analysis:
    Indicated power helps in determining how effectively the fuel is being converted into useful work within the cylinder.
  2. Efficiency Calculation:
    It is used to calculate indicated thermal efficiency, which shows the percentage of energy from fuel converted into work inside the cylinder.
  3. Engine Design and Testing:
    During engine testing, indicated power helps engineers understand the combustion performance and optimize engine parameters.
  4. Loss Assessment:
    By comparing indicated power with brake power, mechanical losses (friction power) can be determined, allowing for engine improvement.

Relationship between Indicated Power, Brake Power, and Friction Power

The relationship is given by:

where,

  • IP = Indicated Power
  • BP = Brake Power (actual power available at the crankshaft)
  • FP = Friction Power (power lost due to friction, pumping, and mechanical resistance)

This equation helps in understanding how much of the total developed power is lost due to mechanical factors.

Factors Affecting Indicated Power

  1. Cylinder Pressure: Higher combustion pressure increases indicated power.
  2. Engine Speed: More cycles per minute produce more power.
  3. Fuel Quality: Fuels with higher calorific value provide more heat energy and hence more indicated power.
  4. Combustion Efficiency: Complete and uniform combustion leads to better power generation.
  5. Compression Ratio: A higher compression ratio increases the temperature and pressure of the gases, improving indicated power.
Conclusion

Indicated power is the total power developed inside the engine cylinder by the combustion of fuel before considering any frictional or mechanical losses. It is a key factor in evaluating the performance and efficiency of an engine. By comparing indicated power with brake power, engineers can identify losses and improve engine design for better performance. Thus, indicated power acts as a foundation for understanding the overall working efficiency of internal combustion engines.