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MODELING
Stirling engine model. Tips for a modeler
Directory / Radio control equipment The Stirling engine is an external combustion engine in which thermal energy is supplied to the working fluid (in our case, to air) from the outside - through the cylinder wall. Its principle of operation is based on a well-known physical law - the expansion and contraction of air during heating and cooling. Therefore, Stirling is also called an air-thermal engine. To understand the operation of the engine, which Stirling developed back in 1816, we will be helped by the model described in S. Baranov's book "Working Models of Thermal Engines" (published in 1936). First, about how the stirling model works. It is assembled from four main parts: two communicating cylinders - heat exchange 6 and working 3, a heating chamber - let's call it firebox 4 - and a cold water tank (it is not shown in diagrams I-III, see it on the general view of the engine).
In the upper part of the heat exchange cylinder 6, a chamber 7 for water is hermetically soldered. Its task is to cool the heated air. The piston-displacer rod 5 passes through this chamber. The displacer is installed in the cylinder 6 with a gap, without touching the walls. The working piston 2, on the contrary, is tightly fitted to the cylinder 3 and moves along it practically without a gap. Between themselves, the displacer 5 and the working piston 2 are connected through a crank mechanism, and the crank and the eccentric are installed relative to each other with a phase shift of 90°. The cylinders are interconnected by a tube, and therefore air can easily pass from the heat exchanger to the working cylinder, and vice versa. The crank mechanism consists of a crank with a connecting rod and an axis (node 8), an eccentric 1 and a flywheel 9. The flywheel diameter is 80 mm, and the distance from the axis to the eccentric pin is 14 mm. So, suppose that we put the spirit lamp in furnace 4 and began to heat the bottom of cylinder 6. After a while, the air under the displacer piston will heat up (and therefore expand) and rush upwards (recall: there is a gap between the displacer and the cylinder wall). Let's move the flywheel 9 off the dead center, and the piston-displacer 5 will start to rise, while displacing cold air from top to bottom. The working piston 2 will also slowly start moving. The cold air, in contact with the hot bottom of the cylinder 6, will heat up, the pressure will increase, and the air will go through the tube into the working cylinder 3. Piston 2 will begin its working stroke under its influence. The piston moves up, and in the meantime the displacer has already begun to go down, because, as already mentioned, their phases are shifted by 90 °. The piston has taken the upper position and, under the influence of the inertia of the flywheel 9, begins to descend, displacing the exhausted air that has lost its original heat into the cylinder 6. Once in the upper part of the heat exchange cylinder, it cools even more and decreases in volume. The displacer, on the reverse stroke of the working piston, begins to rise again and again distills cold air from top to bottom. In contact with the hot bottom of the cylinder 6, the cold air heats up, expands, and the cycle repeats. The main thing in the operation of such an engine is air cooling. In our model, this is done by water coming from a tank installed next to the engine. As soon as the water in chamber 7 is heated by hot air, it rushes up the pipe and enters the tank. And in its place, already through the lower pipe, cold water comes from the tank. In physics, this phenomenon is called thermal convection. Now about how to make a model of the engine.
Both cylinders 3 and 6, the firebox 4 are easiest to solder from tin. First, cut a blank for cylinder 6 (its width is approximately 223 mm), drill holes in it with a diameter of 4,2 mm for the axis, and then bend it on a round blank. Solder the cylinder. From the outer sides of its ears, solder bushings with an inner diameter of at least 4,2 mm - they act as bearings. Then proceed to the manufacture of the water chamber 7. According to the diameter of the resulting cylinder, cut out two circles from the tin. In the center of them, drill holes for a tube with an inner diameter of approximately 3 mm (its length is 32 mm). Solder the tube into circles so that the distance between them is 30 mm. Fix the resulting part by soldering inside the cylinder, departing from its lower edge by 35 mm. Try to perform this operation as carefully as possible, chamber 7 must be airtight, and water must not seep through the walls. The displacer 5 is assembled from a light wooden cylinder, the diameter of which is about 2,5 mm less than the inner diameter of the cylinder 6 (its height is selected experimentally) and a rod made of a spoke with a diameter of 2,8 mm. Cover the cylinder with tin circles on both sides. Drill a hole in the center of the cylinder according to the diameter of the rod and firmly insert the rod into it. And so that it does not jump out from heating, solder it to tin circles. The rod must move freely along the chamber tube 7, without excessive friction. Drill a hole for the connecting rod pin in the top of the stem. Pay special attention to cylinder 3 and piston 2. The operation of the entire model depends on their quality. The cylinder can be made from a piece of copper tube 40 mm long and 18-20 mm in diameter, soldering it from below with a brass circle. In the finished cylinder, do not forget to drill a hole to communicate it with a large cylinder. on a lathe. The rod is fixed in the upper part of the piston pivotally. The workpiece of the furnace 4 also needs to be bent on a round blank, having previously made holes in it for air and fixing screws. It is desirable to solder it directly on the finished cylinder 6. Now you need to assemble the model: solder cylinder 3, fit piston 2 to it, solder a tube into the cylinders to communicate with each other, mount the crank mechanism, solder the bottom of the cylinder 6. Install the finished engine housing on the furnace 4 and fix by soldering. The water cooling tank is a tin can with pipes soldered at the bottom and top, on which rubber hoses are put on. The tank is fixed next to the engine on a wooden stand. Summing up, we note that the Stirling engine operates on such a physical phenomenon: the work done by hot air during expansion is greater than the work that must be spent on its compression. Therefore, try to better debug the kinematics of the model in order to minimize friction in moving nodes. A few words about modern stirlings. External combustion engines are being built even now, and in some respects they are ahead of other engines. Today they are no longer as bulky as they were in the last century. They use a light gas as a working fluid: helium or hydrogen (Robert Stirling used air). The work of a modern stirling is not affected by the external environment: the gas pumped into the pressure vessel is in a closed volume. Therefore, modern stirlings can be used almost everywhere: in water, underground, and in outer space, that is, where conventional engines cannot work. Author: V. Gorstkov
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