Heat Engine Cycles

Heat Engine Cycles

28 February 2016

Peterson T. G. Hauphaquer III

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Procedure

This week's lab examined the cyclic nature of a heat engine and the associated changes in internal energy (_U), heat (Q), and work (W) of an ideal gas during various phases of an engine's cycle. The heat engine consisted of an aluminum cylinder with a fixed volume of air, a connecting plastic tube, and a secondary air tank with a frictionless piston that was allowed to move freely to alter the overall volume that the gas occupied. The aluminum cylinder was submerged in cold or hot water during portions of the engine's cycle to provide isothermal conditions. A 200 gram mass was also intermittently placed on top of the piston to alter the pressure of the gas. The changes in the gas's temperature and pressure caused the piston in the cylinder to move up or down in order to increase or decrease the total volume that the gas occupied. To measure various states and processes of the experiment, we used a pressure sensor, two temperature sensors, and a rotary motion sensor that recorded the height of the piston (which could be used to calculate the changes in the total volume).

In the beginning of the experiment, we qualitatively examined the changes that occurred in pressure and volume of the gas when the portion of the gas in the aluminum cylinder was submerged in either a cold bath (ice-water) or a hot bath (near boiling water) and when the 200 g mass was placed on top of the piston. This allowed the characterization of the thermodynamic process that was occurring during each portion of the cycle. Once we had become familiar with the system and how it responded to each change in conditions, we attached the rotary sensor and began a quantitative analysis of the heat engine cycle using Logger Pro. The goal of our experiment was to produce a graph of the cycle showing a P-V diagram, where the gas's pressure and volume would return almost exactly to their starting points at the end of each cycle.

At the beginning of the cycle, we positioned the piston two centimeters from the bottom of the secondary cylinder. After recording the position of the piston, we placed the can into the cold water and allowed the gas to reach thermal equilibrium with the ice bath. Once the piston came to a stop, a 200 g mass was immediately added to the top of the piston that applied a force directly onto the piston, thus depressing the piston and altering the pressure of the contained gas. Next, we moved the aluminum cylinder from the cold water into a container holding hot water. Once again, the piston was allowed to come to rest. Finally, we removed the 200 g mass from the system and as the final step, the aluminum can was removed from the hot water and