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Fluids

Heat engine


Now that we have collected all ingredients for a machine which transfers heat into work, let us study one complete cycle of such heat engine. We have in mind a quantity of gas closed inside a cylinder under a movable piston. The piston is connected to a wheel as is shown in the figure below, which we have borrowed from the Heat Engines: the Carnot Cycle page of Michael Fowler from the Department of Physics of the University of Virginia.



When the gas expands it makes the wheel perform half a turn in one direction. Once it is moving, the wheel pushes the piston back in the second half of its movement. In between there are two parts of a complete cycle where the volume of the gas hardly changes. The pressure-volume diagram below approximates to a high degree of simplification such process as we will explain in the following.



Let us start the cycle at (V1,P1), which is in the lower left corner of the diagram, and subsequently follow the cycle as indicated by the arrows.
  • In the isochoric process (V1,P1)(V1,P2) heat is supplied to the gas, thereby increasing the pressure of the gas from P1 to P2 at constant volume V1. This part of the cycle compares to the explosion of gasoline vapor in one of the cylinders of your vehicle, right at the point where the volume of the gas is at its minimum.
  • Next, we arrive at (V1,P2) in the upper left corner of the diagram. The wheel is now positioned such that it is ready to receive the principal push from the expanding gas, while the gasoline explosion still continues to deliver heat. The gas expands from volume V1 to the larger volume V2 at the constant pressure P2. It is in that part of the process that work is delivered.
  • We arrive at (V2,P2) in the upper right corner of the diagram. The explosion has finished and heat is being transferred to the interior of the vehicle in order to create a pleasant ambience for the driver and her passenger, despite the freezing atmospheric conditions. In cooling down the gas, its pressure decreases from P2 to the lower pressure P1 at the constant volume V2.
  • We arrive at (V2,P1) in the lower right corner of the diagram. From here, the moving wheel gives through its inertia a push to the piston such that the volume of the gas is reduced from V2 to the lower volume V1 at the constant pressure P1. In this part of the cycle the wheel excerts work on the gas, whereas moreover more heat is channeled to the interior of the vehicle.
We arrive at (V1,P1) in the lower left corner of the diagram. From there, the cycle starts again.
In the adjacent animation the system performs one more cycle in order to fill the cylinder with fuel.
The above animation stems from Oracle Education Foundation, ThinkQuest, Projects by students for students. More on heat engines can be found at




The energy balance