PV diagrams for heat engines are legitimate ?

  • Thread starter miko1977
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  • #1
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PV diagrams for heat engines are "legitimate"?

Hello forum

Since my student days i was wondering about how is it possible to represent the cycle of the gas in a heat engine (e.g. otto cycle) in a PV or whatever diagram when it is obvious that the gas is not in equilibrium during the process. The piston moves so fast therefore it is impossible for the gas to be in thermodynamic equilibrium during the process and so it is wrong to draw continuous lines between the states because in that way we assume that it is always in equilibrium. I could accept it if the cycle was very slow but obviously it is superfast. It is unfortunate that not a single text book doesn't give a good explanation about this very important (in my opinion) subject so i am relying in this forum to enlighten me!!

Thanks
 

Answers and Replies

  • #2
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Actually the speed of the engine doesn't effect thermodynamic equilibrium unless the pistons are moving faster than the speed of sound. If they are moving faster than the speed of sound then yes your correct equilibrium will not be met becuase the air molecules ,so to speak, dont "see" the piston coming. But when it moves slower than the speed of sound the air molecules can "see" the piston coming and the system adjust in pressure.

If you asking if the entire system adjust to pressure at the same time when the piston moves up and down the answer is yes. Or rather its so close to happening at the same time that we neglect the difference.

While the idealized cycle are not representative of real life cycles 100% they are close and take a relatively similar shape. Your right the lines between states are not linear but their are not oscillations or anything.
 
  • #3
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Thanks man!! I am suprised that the speed of sound is related to this!!! But it makes sence.
 

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