Power of Combustion Engine

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Equation / Last modified by KurtHeckman on 2016/05/04 14:59
`"Power" = `
MichaelBartmess.Power of Combustion Engine

The Combustion Engine Power equation computes the power of a combustion engine based on the rate of energy input (H) and the indicated fuel efficiency (IEF).


This equation can be used to determine the power for any engine:  for automobiles, engines that are used for drilling oil and gas, or any combustion engine.


The following are inputs to this equation:

  • H  - the rate of energy input, the amount of equivalent heat energy in BTUs that is fed to the engine
  • IFE - the indicated fuel efficiency


The work done by combusting gases in the engine's cylinders is sometimes called "indicated work" and thus we derive the name for the thermal efficiency ratio as the Indicated Thermal Efficiency (IFE). IFE measures the efficiency ratio, the ratio of the work done by the fuel combusting during a period of time to the input potential energy, the equivalent of the potential heat energy of the fuel supplied during the same period of time.  This efficiency ratio is analogous to the completeness of fuel combustion.

The mechanical energy equivalent of heat can be expressed as units of foot-pounds, British Thermal Units (BTUs), and horsepower-hours:

  • 1 BTU = 778  ft-lb
  • 1 hp-hr = 2545 BTUs

So, the equation can be shown with all units represented:

Engine Power = `("IFE" * H "BTU"/"hr") / "conversion factor"`

Where the conversion factor is 2509.62675902397 (BTU/hr) /  hp


If all the potential energy in the fuel could be converted directly to work, meaning there were no losses in the system, the IFE would be 100%. If the amount of fuel delivered to the cylinders were known, the total energy content of the injected fuel can be determined and and from this the thermal efficiencies can be calculated.    If the amount of fuel delivered to the engine is measured over time, the rate at which the corresponding amount of energy is delivered to the engine can be converted  to potential power.