PV diagrams for heat engines are legitimate ?

In summary, the discussion revolves around the representation of gas cycle in a heat engine and the concept of thermodynamic equilibrium. The speed of the engine does not affect equilibrium unless it is faster than the speed of sound, which leads to air molecules not being able to adjust to the piston's movement. However, in most cases, the system adjusts to pressure at a similar time and the idealized cycle closely resembles real-life cycles. The lines between states may not be linear, but there are no oscillations.
  • #1
miko1977
13
0
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 textbook 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
 
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  • #2


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, don't "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


Thanks man! I am suprised that the speed of sound is related to this! But it makes sence.
 

Related to PV diagrams for heat engines are legitimate ?

1. What is a PV diagram for a heat engine?

A PV diagram for a heat engine is a graphical representation of the pressure-volume relationship during the operation of a heat engine. It shows the changes in pressure and volume as the engine goes through its different stages.

2. How is a PV diagram used to analyze heat engines?

A PV diagram is used to analyze heat engines by providing a visual representation of the thermodynamic processes happening within the engine. It can help determine the efficiency and performance of the engine, as well as identify any potential issues or improvements.

3. Are PV diagrams accurate representations of real-life heat engines?

Yes, PV diagrams are accurate representations of real-life heat engines as they are based on the laws of thermodynamics and are used to analyze and predict the behavior of actual engines. However, they may not account for all real-world factors and may need to be adjusted accordingly.

4. Can PV diagrams be used for all types of heat engines?

Yes, PV diagrams can be used for all types of heat engines, including both internal combustion and external combustion engines. They may look slightly different depending on the engine type, but the principles and analysis methods remain the same.

5. How can understanding PV diagrams help in the design of heat engines?

Understanding PV diagrams is crucial in the design of heat engines as it allows for the optimization of the engine's performance. By analyzing the diagram, engineers can identify areas for improvement and make adjustments to increase efficiency and overall performance.

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