Engine Cycles • The Sterling Cycle differs from other engine cycles is that the sterling cycle is a closed cycle. A closed cycle means that the gas used in the engine (or the working fluid) remains contained within the engine. • A sterling engine makes use of an external heat source which could be an open flame or even solar or nuclear power. On the cold side, the engine makes use of a heat sink in order to cool the working fluid. • There are four processes for the sterling engine, heating, expansion, cooling and compression. The sterling engine works when the working fluid moves between the hot and cold sides of the engine. • Key to the sterling engine is the temperature difference between the hot and cold sides of the cycle. With a greater temperature difference, there is more power created and a greater efficiency. The Workings of a Sterling Engine • The Sterling Engine cycle consists of four thermodynamic processes. That is shown in the P vs. V graph and explained below. All the processes follow the Ideal Gas law: PV=nRT.   • The process starts from point 4, and from 4 to 1 in a isometric process where heat is added at a constant volume and the pressure increases. -In this illustration all the gas is on the hot side being heated and the pressure builds up. http://en.wikipedia.org/wiki/Image:Stirling_Cycle_1.png • From points 1to 2, there is an Isothermal expansion which means that the temperature is constant, but the gas expands causing a decreased pressure and an increased volume. -The hot gas expands and the pressure decreases. In the process, the gas does work moving the piston on the cold side. http://en.wikipedia.org/wiki/Image:Stirling_Cycle_2.png -Now, the engine has reached point 2 where all the gas in on the cold side. http://en.wikipedia.org/wiki/Image:Stirling_Cycle_2a.png • Point 2 to 3 is an other isometric process where the heat is removed and the pressure decreases. -The gas has cooled down at a constant volume and as pressure decreases to bring the engine cycle to part 3. http://en.wikipedia.org/wiki/Image:Stirling_Cycle_3.png • From point 3 an isothermal compression returns to the original position 4 where the volume is decreased and the pressure is increased again. -As the gas cools down, the volume decreases, and the piston does work on the gas to compress it and begins moving towards part 4. http://en.wikipedia.org/wiki/Image:Stirling_Cycle_4.png -The piston on the cold side has forced the cool gas to the hot side to begin the process of heating it up again (step 4). http://en.wikipedia.org/wiki/Image:Stirling_Cycle_4a.png • It is very important to mention that what have been stated above are the idealized workings of a sterling engine. Because of this, an actual P vs. V graph does not have as sharp of corners and that from the Second Law of Thermodynamics; some energy is lost in the process so not all heat is transformed into mechanical energy. Thus, there can never be an engine with 100% efficiency.