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In this engine, the heated cylinder is heated by an external flame. The cooled cylinder is air-cooled, and has fins on it to aid in the cooling process. A rod stemming from each piston is connected to a small disc, which is in turn connected to a larger flywheel. This keeps the pistons moving when no power is being generated by the engine.
The flame continually heats the bottom cylinder.
- In the first part of the cycle, pressure builds, forcing the piston to move to the left, doing work. The cooled piston stays approximately stationary because it is at the point in its revolution where it changes direction.
- In the next stage, both pistons move. The heated piston moves to the right and the cooled piston moves up. This moves most of the gas through the regenerator and into the cooled piston. The regenerator is a device that can temporarily store heat -- it might be a mesh of wire that the heated gasses pass through. The large surface area of the wire mesh quickly absorbs most of the heat. This leaves less heat to be removed by the cooling fins.
- Next, the piston in the cooled cylinder starts to compress the gas. Heat generated by this compression is removed by the cooling fins.
- In the last phase of the cycle, both pistons move -- the cooled piston moves down while the heated piston moves to the left. This forces the gas across the regenerator (where it picks up the heat that was stored there during the previous cycle) and into the heated cylinder. At this point, the cycle begins again.
You might be wondering why there are no mass-market applications of Stirling engines yet. In the next section, we'll take a look at some of the reasons for this.