Post on 16-Apr-2022
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HEAT ENGINE ------- Interactive Physics simulation -------
Page 01
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How does a heat engine work?
The heat engine shown here converts heat from firewood into useful work, in this case pumping water out of the ground. This is an example of an external combustion engine - the flame is outside the engine, unlike in a typical gasoline-powered car. Converting heat energy to steady motion often requires a working fluid. In this case, the working fluid is hot air trapped in the engine. A displacer moves the air around from the hot side to the cold side of the engine, and back. The air in the engine is heated when the displacer is highest and the air is near the flame. The hot air pushes on a piston when the displacer is on its way back down. The hot air cools when the displacer is lowest and the air is farthest from the fire. The piston is used to produce a vacuum - water rises out of the ground to fill this vacuum. The efficiency of the engine is the ratio of the work done to the heat input. No engine can be 100% efficient - there is always waste heat.
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Stroke - This slider adjusts how much the total volume of the working fluid can change due to the piston moving up and down. For a small stroke, the piston won't move up & down very far, and the volume won't change much. Since the work done by the engine is the product of force and distance, a bigger stroke will lead to more work.
Heat Input - This slider adjusts how much heat is entering the system at the furnace end. When the displacer inside the engine is in its "up" position, the air at the bottom is heating; when the displacer inside the engine is in its "down" position, the air is at the top of the engine and is cooling. More heat input will allow the temperature of the air on the hot end to be higher.
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Pressure vs. Volume - This graph shows the engine cycle for this heat engine. In the course of one full cycle of the engine, the pressure and volume vary in a loop as shown on this diagram. The top and bottom parts of the loop represent isotherms, during which the temperature of the working fluid in the engine is constant (and either high (top) or low (bottom)). The left and right parts of the loop represent isochores, during which the volume of the working fluid is constant and it is either being heated or cooled. This cycle is an idealized cycle - realistic cycles are less rectangular in shape. The area inside this cycle is equal to the net output work of the engine.
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Adjust the sliders to maximize the energy input of the system. (Adjust the heat input slider so that the Q in, found in the upper left graph, is at its maximum amount.)
Adjust the sliders to maximize the work done by the system. (Adjust the heat input slider and the stroke slider so that the W out, found in the upper left graph, is at its maximum amount.)
Adjust the sliders so that the efficiency of the heat engine is 23%. (Adjust the heat input and stroke sliders so that the efficiency measured in the upper left graph reads 23%.)
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Challenge ME!
Increasing the heat input will increase the work done - but what happens to the efficiency?
During which of the four phases of the thermodynamic cycle does the engine do work?
How is the area of the bounded region on the PV diagram related to the engine's performance?
Need Help?
Check out the Heat Engine Walkthrough video at: https://youtu.be/PpFLjyOTSgk
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How is a car engine different?
In a car engine, air and fuel are mixed within the cylinder. Upon combustion, the dramatic increase in pressure inside pushes the piston. The basic thermodynamic principles are similar.
What happens if we run an engine in reverse?
Turning the flywheel allows you put in mechanical energy and leverage it to extract heat from the cool end of the system. This is the basic concept upon which refrigerators and air conditioners operate.
Can I build one of these on my own?
Yes! Try searching for tin-can Stirling engine models. There are many different plans out there. It is important to be safe - wear eyewear, have fire safety and first aid equipment handy, and keep your work area properly ventilated. Building one of these models is challenging but immensely satisfying.
Can we make a 100% efficient engine?
No, the laws of thermodynamics forbid it. The young physicist Sadi Carnot determined the maximum possible efficiency, now known as the Carnot efficiency, which depends solely on the relative temperature of the high and low end.
Physics Concepts | Click on the link below to learn more.
Heat Engine - http://www.ck12.org/physics/Heat-Engine/
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