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Thread: can the pV=nRt be used to describe an engine?

  1. #1 can the pV=nRt be used to describe an engine? 
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    I'm currently doing a project, working towards a degree in engineering and i'm not too sure if equation can be used or not.

    I need to describe the effect of adding more air into an internal combustion engine and thought to use the equation pV=nRt.

    Taking the volume of the cylinder, the constant for air and the temperature as constant (as the engine is not being modified, just the air intake is increasing)

    re-aranging to give p=(nRt)/v

    can i then increase n (as the higher intake increases the number of moles) to prove that pressure will increase?

    or does this mean that volume will also change?
    or am i way of the mark and this equation is not suited to this situation?


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  3. #2  
    Forum Radioactive Isotope MagiMaster's Avatar
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    I can't say for sure overall, but I can say that, assuming only two variables can change, then changing one will change the other. So if the temp and the volume are constant, then adding air will raise pressure.


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  4. #3  
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    Idealy i would only change the number of moles as the input to the engine cylinders. But does changing this mean i also need to change the volume?

    Or is volume only the volume of the cylinder, and not the volume that the air is occupying
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  5. #4  
    Forum Isotope Bunbury's Avatar
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    No you don't need to change the volume because the volume of the cylinder is fixed. The air is occupying the whole cylinder but because it's mixed with fuel its partial pressure is less than the total pressure in the cylinder.

    I think your assumption of constant temperature is wrong. If the air is compressed as if by a supercharger it will be warmer than if it was aspirated normally.
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  6. #5  
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    If the initial pressure remains constant, increasing the amount of air will decrease the amount of fuel.
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  7. #6 Re: can the pV=nRt be used to describe an engine? 
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    Quote Originally Posted by moogan
    I'm currently doing a project, working towards a degree in engineering and i'm not too sure if equation can be used or not.

    I need to describe the effect of adding more air into an internal combustion engine and thought to use the equation pV=nRt.

    Taking the volume of the cylinder, the constant for air and the temperature as constant (as the engine is not being modified, just the air intake is increasing)

    re-aranging to give p=(nRt)/v

    can i then increase n (as the higher intake increases the number of moles) to prove that pressure will increase?

    or does this mean that volume will also change?
    or am i way of the mark and this equation is not suited to this situation?
    :P an engine cycle has four states:intake of air (energy required can be neglected),compression air (requires energy) and fuel injection, expansion combusted gasses
    (delivering of energy),expelling combusted gasses (energy can be neglected)
    the higher the intake pressure, the higher the temperature and pressure at end of compression cycle. the more air you put in the engine, the more fuel you can combust.
    if compression is done with a turbo, you recovering energy from your exhaust gas
    (efficiency increase +-10%)

    to work out the pressure and temp at end of compression use:
    T2=T1(V1/V2) ^k-1 (for air k=1,4)
    P2=P1(V1/V2) ^k
    to work out the power increase of the engine,you need to use some integrals...
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