What is heat balance sheet?

Short Answer:

A heat balance sheet is a detailed record that shows how the total heat energy supplied by the fuel in an internal combustion (IC) engine is distributed or used. It compares the input heat energy to the various forms of output, such as useful work and different types of losses.

In simple terms, the heat balance sheet helps to understand how efficiently the engine converts the heat from fuel into useful work and how much is wasted through exhaust gases, cooling water, or radiation. It is an important tool for evaluating the overall thermal performance of an engine.

Detailed Explanation :

Heat Balance Sheet

A heat balance sheet is a systematic representation of the distribution of heat energy in an internal combustion engine. It helps in analyzing how much of the heat produced from fuel combustion is converted into useful work and how much is lost in different forms. Since not all the heat supplied by fuel is converted into power, the heat balance sheet provides a clear picture of engine efficiency and energy losses.

It is called a “balance sheet” because it balances the total heat input (supplied by fuel) with the total heat output (useful work + losses). The aim of preparing this sheet is to determine how efficiently the heat energy is utilized within the engine.

The heat balance sheet is generally expressed in two forms:

  1. Heat Supplied (Input side):
    The total amount of heat energy obtained from burning the fuel.
  1. Heat Utilized and Lost (Output side):
    The energy output from the engine is divided into:

    • Useful heat equivalent to brake power (useful work).
    • Heat lost to cooling water.
    • Heat carried away by exhaust gases.
    • Unaccounted losses (radiation and convection losses).

Purpose of Heat Balance Sheet

  1. To find the efficiency of the engine:
    By comparing useful work output with total heat supplied.
  2. To locate heat losses:
    Helps identify where most energy is being wasted.
  3. To improve design and operation:
    Provides data to improve engine cooling, insulation, or combustion efficiency.
  4. To compare performance:
    Enables comparison of different engines under similar operating conditions.

Heat Distribution in an IC Engine

When fuel is burned in an internal combustion engine, the energy from the fuel is divided into several parts:

  1. Useful Heat (Brake Power):
    This is the portion of heat energy converted into mechanical work available at the crankshaft. It is the most desired output and represents the actual useful power of the engine.
  2. Heat Lost to Cooling Water:
    A significant portion of heat is absorbed by the cooling water to prevent the engine from overheating. Although necessary for engine operation, it represents energy that is not converted into useful work.
  3. Heat Lost in Exhaust Gases:
    The exhaust gases leaving the engine carry away a large amount of heat energy. This is one of the major sources of energy loss.
  4. Unaccounted or Miscellaneous Losses:
    These include heat lost by radiation, convection, friction, and incomplete combustion. Such losses are difficult to measure accurately and are often estimated.

Typically, in a normal petrol or diesel engine, the approximate heat distribution is as follows:

  • Useful work (Brake Power): 25–30%
  • Heat lost to cooling water: 25–30%
  • Heat lost in exhaust gases: 35–40%
  • Unaccounted losses: 5–10%

Procedure for Preparing Heat Balance Sheet

  1. Measure Input Parameters:
    • Amount of fuel consumed per second or per minute.
    • Calorific value (CV) of the fuel.
  2. Determine Heat Input:
  1. Calculate Useful Heat (Brake Power):
  1. Calculate Heat Lost to Cooling Water:

Where,

    •  = mass of cooling water per minute
    •  = specific heat of water
    •  and  = outlet and inlet temperatures of water
  2. Calculate Heat Lost in Exhaust Gases:

Where,

    •  = mass of exhaust gases
    •  = specific heat of exhaust gas
    •  = temperature of exhaust gas
    •  = atmospheric temperature
  2. Find Unaccounted Losses:
    These are obtained by subtracting all measured quantities from the total heat supplied.

Interpretation of Heat Balance Sheet

A heat balance sheet gives clear insight into how effectively the engine uses the supplied fuel energy. For example:

  • If the cooling losses are too high, it may indicate excessive heat removal, reducing efficiency.
  • High exhaust losses may suggest poor combustion or exhaust system design.
  • High unaccounted losses point to poor insulation or radiation losses.

Hence, the heat balance sheet not only provides numerical data but also serves as a diagnostic tool for improving overall thermal performance.

Practical Example (Conceptual)

Suppose an engine consumes 0.2 kg of fuel per minute with a calorific value of 42,000 kJ/kg. The total heat supplied per minute is:

If measurements show:

  • Useful work = 2400 kJ/min
  • Cooling water loss = 2500 kJ/min
  • Exhaust gas loss = 2800 kJ/min
    Then,

This gives the complete heat balance for the engine.

Conclusion

A heat balance sheet is an essential tool for analyzing the thermal performance of an internal combustion engine. It clearly shows how the heat energy from the fuel is divided into useful work and various losses. By preparing this sheet, engineers can identify inefficiencies and improve engine design, combustion process, and cooling systems. Thus, it plays a crucial role in achieving better fuel economy, reduced energy waste, and improved overall engine efficiency.