Thermodynamics troubles

  • Thread starter morry
  • Start date
Hi guys,

Im trying to calculate the heat input and brake specific fuel consumption for an engine. However, Im having troubles with units lining up.

Heat Input= reaction enthalpy (J/kg) x fuel flow rate per injector (kg/s) x 4 (4 cyl engine) x 0.5 (4 stroke)

Doing it like that gives me J/s = power, which is what I want, but shouldnt the engine speed have something to do with this?

Next problem is the BSFC. Im after units of kg/MJ. It looks simple to just multiply the flow rate by time and multiply the reaction enthalpy by mass. Then divide one by the other. This would give me the correct units, however I dont really know what to do.

Can someone point me in the right direction?

As the flowrate of fuel is already mentioned, you need not worry about the engine speed(alternatively, it is mass of fuel per one power stroke times the rpm incase of a two stroke engine and mass of fuel times the rpm/2 incase of a 4 stroke engine).

The reciprocal of the reaction enthalpy (I take it as calorific value) gives you the specific consumption in terms of Indicative Power. When you apply efficiency, you will get specific consumption in terms of brake power.
Ok, so the fuel flowrate is good.

The BSFC still confuses me a bit. It says in my book that it is fuel used per joule of output. So units of kg/MJ. I know the fuel flow rate, but this is in kg/pulse. Not kg. This is what confuses me. To work out the other half of the equation is simply the reciprocal of the calorific value like you mentioned.

Thanks quark.
Anyone else got any ideas?

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