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Performance of an Internal Combustion engine by heat balance sheet


This article gives an overview of the performance of an Internal Combustion engine by using a heat balance sheet which is nothing but an energy balance as per the first law of thermodynamics. The heat produced during combustion is balanced in different ways.

I C Engine means Internal Combustion Engine. The petrol engine or diesel engine of an automobile we come across in daily life is an example of an IC Engine. The purpose of the IC Engine is to produce the power or rotation of the shaft(flywheel) at an expense of fuel (petrol, diesel or Compressed Natural Gas (CNG)). Thus, we can say that the IC Engine is a prime mover. Each machine works based on its construction and working principle along with necessary requirements. Thus, it becomes necessary to assess the performance of a machine that is in operation. The assessment gives us an idea for further modification, update or rejection.

The IC Engine performance is generally taken from the heat balance sheet. Like the balance sheet, it will contain the credit (heat supplied) and debit (utilized) heat during the operation of an engine.

The main components of this heat balance sheet are:


  1. Heat equivalent to brake power or output power available at flywheel/crankshaft

  2. Heat rejected to cooling medium like water in the radiator

  3. Heat carried away by exhaust gases

  4. Unaccounted heat may include radiation losses, heat loss from various parts of IC engine and incomplete combustion.

The balance sheet contains 100 % heat credit as heat produced by the complete combustion of the fuel during a cycle. This heat liberated from the combustion will be distributed to four energy debits mentioned above. This is an example of a principle of energy conservation or the law of thermodynamics.

Exergy-Heat balance sheet of an IC engine

  • Credit: Qtotal in kJ = Mass of fuel (kg) x Calorific value of a fuel (kJ/kg)
  • Debit1: The first part of the heat is absorbed by the cooling medium like water. The heat carried away is equal to the sensible heat rise of water i.e. mass of water circulated x heat capacity of water x temperature rise of water

  • Debit 2: The second part of the total heat is carried away by the exhaust gases. One can calculate it by use of calorimeter which contains water in which one has to balance the heat rejected by exhaust gas = heat gain by water in the calorimeter. Both are sensible heats.
    Mass of water x specific heat of water x temperature rise of water = Mass of exhaust gas x specific heat x temperature loss of exhaust gas.

  • Debit 3: The last and most important part of the total heat is brake power or heat equivalent to brake work

  • Thus, Credit = Debit 1 + Debit 2 + Debit 3 + Unaccounted heat losses
    The heat of combustion = Heat carried away by cooling water + Heat carried away by exhaust gases + Heat equivalent to brake power + unaccounted heat losses

    Normally, 1/3 of the heat is utilized to obtain useful work and thus, the efficiency of an IC engine is approximately 33 %. The other 67 % may be contributed to heat loss in the cooling medium, Heat carried away by exhaust gases and unaccounted heat losses. Thus, we can say that useful work obtains from an IC engine is only 1/3rd of the total heat produced. Thus, the exergy of an IC engine is about approximately 33 %. The lost work or energy of an IC engine is approximately 67 %. The useful work can be increased if we reduce the heat losses other than the brake power output.


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