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Internal combustion (I.C) engine formulas for calculations


How internal combustion engine work effectively to convert chemical energy to mechanical energy is depends on various parameters, those are

  • Power & mechanical efficiency
  • Mean effective pressure & torque
  • Specific output
  • Volumetric efficiency
  • Fuel air ratio
  • Specific fuel consumption
  • Thermal efficiency and heat balance
  • Exhaust smoke and other emissions
  • Specific weight

Power & mechanical efficiency

  • Indicated power (I.P)

The total power produced in the combustion chamber due to burning of fuel is called indicated power (I.P)


  • Brake power (B.P)

The power produced at output shaft of the engine is called brake power (B.P)


Mean effective pressure & torque

  • Pressure produced in the power stroke, the amount of pressure is acting on the piston is called mean effective pressure
  • Based on indicated power (I.P) pressure is called indicated mean effective pressure
  • Based on brake power (B.P) pressure is called brake mean effective pressure (Pm)
  • Depends on engine size torque and mean effective pressure will varies
  • The difference between indicated mean effective pressure and brake mean effective pressure is called friction mean effective pressure

Specific output

  • It is defined as the brake output per unit of piston displacement
  • Specific output = B.P/(AxL) = constant x Pm x r.p.m

Volumetric efficiency

  • The ratio of actual volume of the charge drawn in during the suction stroke to the swept volume of the piston is called volumetric efficiency

Fuel air ratio

  • The ratio of the mass of fuel to the mass of air in the fuel air mixture is called fuel air ratio

Specific fuel consumption (s.f.c)

  • The mass of fuel consumed per kW developed per hour, and is a criterion of economic power production
  • f.c = mass of fuel/B.P (kg/kwh)

Thermal efficiency and heat balance

  • The ratio of indicated work done to energy supplied by the fuel is called thermal efficiency


Heat balance sheet

  • Engine performance is represented by heat balance sheet
  • For heat balance sheet operate the I.C engine at constant load
  • Fuel quantity, time, calorific value, inlet and outlet temperature of cooling water and weight of exhaust gases are recorded

Exhaust smoke and other emissions

  • When air fuel mixture not burn completely then smoke will produce
  • Due to this incomplete charge burning will effects the output of the engine

Specific weight

  • The weight of the engine in kg for each B.P developed is called specific weight

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