Stirling Engine

Dedicated to all those wise and illustrious deans and students.

Foreword.

The Stirling engine is a heat engine with low noise and toxic emissions, this engine can use any external power source coming to have zero emissions when using solar energy. In the near future, these applications are likely to have engines, reaching even replace the current internal combustion engines in some industrial applications.
During the investigation a historical overview of the Stirling engine is the principle of operation of the engine basics of thermodynamics says. The various configurations of these engines are also mentioned, and the thermodynamic cycle of this is explained. It also discusses the design and construction of the beta type Stirling engine. The purpose of this study is to provide the basis for the design and construction of a Stirling engine as well as an incentive to anyone interested in further research.

Introduction.

In 1816, Scotsman Robert Stirling patented an engine that ran on hot air, which he called Stirling. The patent for this engine was the successful end of a series of attempts to simplify the steam engines, considered it impractical heat water in a boiler to produce steam expand in an engine, condense and by introducing water pump again in the boiler, so decided to develop a new system that performs the same processes but in simplest form.

Throughout history, people have created various machines that appeared in the industrial revolution to help man in more efficient industrial processes and this has caused a rise in demand for fossil fuels, because the world is in an environmental crisis , we have to see how to replace the way we work with conventional methods of burning fossil fuels. Stirling engine in the same processes of heating and cooling of a gas a steam engine, but all within the motor and the gas was air instead of steam, so that the engine does not need boiler are conducted.. Finally, although it was much simpler and efficient (at least in theory) than a classic steam engine, Stirling engines were never well known and its application in the real world was not over, as internal combustion engines replaced them.

The Stirling engine operation is based on the use of volumetric changes of the working fluid as a result of changes in temperature that this affected. These volumetric changes are due to the displacement of working fluid between the hot zone and the cold zone in a closed cylinder.

1. If air is enclosed in a cylinder and then heated, it is observed that the pressure within the cylinder increases. It is assumed that one of the covers of the cylinder is a piston and this is tight, there will be an expansion of gas and increase the volume inside the cylinder until some final position of the plunger.

2. If the same cylinder in its expanded state is quenched pressure decreases, then the volume is contracted and the piston position returns to the initial state.

3. If the process from state 1 is repeated, but now joining the piston to a crankshaft and in turn a wheel, the increased pressure will force the piston move causing the crankshaft and this at the wheel too, with this achieved that the volumetric change is transformed into motion.

4. When the process from state 2 is repeated, the piston return cooling rapidly by movement of the wheel and occurs the pressure decrease and volume.

5. If the processes 3 and 4 meet is a single cylinder engine displacer movement will occur due to expansion of the gas during compression and the piston return to its position due to the energy of the flywheel.

With this principle of operation of the Stirling engine are explained.

Hot Zone

This part of the engine where heat is delivered, the materials used for construction must be resistant materials CREEP (creep deformation is plastic-type material that can undergo when subjected to elevated temperature)

Cold zone

where heat is extracted. Heat extraction may be performed by free or forced convection. In this case be placed aluminum fins to quickly dissipate heat, because this type of cooling is inefficient decided to use materials of high thermal conductivity in the cold zone, materials such as copper and aluminum. This is a very important part of the engine, because it must be able to evacuate at least 50% of the heat supplied to the engine, and it must do so at the lowest possible temperature to improve the thermal efficiency of the engine.

Regenerator

The regenerator absorbs and gives off heat to the working fluid offsetting some of the heat lost by the engine, causing the engine power and speed increase, this is because when you work the regenerator needs to absorb less heat per cycle, which makes the cycle time required to perform less and less fuel is also consumed. Regenerator material must have a high capacity to store thermal energy for its temperature is stable. Should also have a low thermal conductivity in the direction of flow, to generate a temperature gradient. The volumetric heat capacity of a material is measured with the product ρ x Cp (J / m3 * K), the higher the value the material can absorb more heat. The regenerator operates to the next way, assuming that the gas in the hot zone is 150 ° C and the cold zone to 30 ° C, when the gas passes from the cold zone to the hot zone, an ideal regenerator temperature rise gas at 60 ° C therefore has to deliver heater least amount of heat to raise the gas temperature 60 to 150 ° C, in the same way, when the gas passes from the hot zone to the cold zone heat absorbed by the regenerative gas would in the cold zone to 60 ° C so it will have to cool slightly to spend 60 to 30 ° C. This assures in both cases reducing the time of heating and cooling the gas which develops faster cycle.

First is the state 1. The elements are: cylinder, piston, fluid displacer. All the gas is in the cold zone, and the piston is in the lower position.

• Process 1-2.- When the piston moves from state 1 to 2 an isometric compression is performed at the lowest temperature. The process is represented in the previous pressure volume diagram. Work consumed in this process is equal to the heat rejected in the cycle.

• Process 2-3.- If remains fixed piston and displacer moves, is passed around the fluid into the hot zone, obtaining an isometric process that increases the pressure without changing the volume. Here the regenerator delivers heat to the working fluid, raising its temperature Tmin to Tmax.

• Process 3-4.- Right now you can get an isometric expansion at the higher temperature by lowering the piston and displacer. In this process, external heat is supplied to the working fluid.

• Process 4-1.- Moving the slider to the initial state, another isometric finalize the process represented by the thermodynamic cycle process 1-4 will be obtained. Here the regenerator absorbs heat.

With this the ideal Stirling cycle has the same efficiency as the Carnot cycle, which is the maximum efficiency that can reach a heat engine considering that all losses are zero. The Carnot cycle processes used isotropic, non-regenerative heat exchange processes, assuming that the specific heat of the regenerator is infinitely large, as the Stirling cycle. Because there is no mechanism to make the ideal movement of the piston and displacer for completion of the cycle and the difficulty of obtaining purely isothermal cycles due to the mechanisms of heat transfer associated with the speed with which it is intended to make the cycle, power and efficiency is lost, the end result is an ellipse shaped cycle.

General outline of a Stirling engine

TR is the temperature of the heat source, TO is the sump temperature, TH is the temperature of the surface in contact with the working fluid in the hot zone, TC is the temperature of the surface in contact with the working fluid in the cold zone.

Well, basically I designed this engine in solidworks and made drawing for after give this drawing to the center machining.

You need solidworks software if you want manipulate the ensemble, I use 2013 version.

The materials I bought was in standard measure, so I adapted to metric.

The position 3 say "join with silver weld" but I had problems because the copper do not resist the high temperature as well as stainless. The solution to this problem I made position 3.1 and 3.3 more big in the diameter ø23.3 for enter in pressure with position 3.2. another solution is modific the drawing make thread to this positions but need increase the diameter extern to the cylinder and dissipators. (positions 3, 4 and 5). Or make all position 3 with stainless in one piece.

Is extremely necessary the cylinder do not have air leakage because the engine works with this pressure. critical positions 3, 6 and 14.

when you assemble parts be careful to alignments from the positions 20 (vertical) and 30 (horizontal) another way the engine have problems with positions 2, 6 and 8.

Do not despair if the engine do not work in the first time, because this need adjustments and some oil for pieces in moving, I made many attempts for it work.

I spend a lot of time searching in the internet for some drawings but I did not find, many people sell these planes on the internet and only show machines running so I decided to share these planes, I hope you serve these drawings.

If write bad please correct, I'm from mexico and not very well write in English. And if you have any questions you can ask.

-Actualización acerca de dudas sobre el rendimiento-

En la pagina 2 del documento de Word "Estudio Termico" viene --La definición de rendimiento para una máquina térmica es n=W(neto) / Qabsorvido este valor es de 0 a 1 por que se supone que la energía que suministras al motor es la que transforma y si es 1 quiere decir que pudiste transformar toda la energía calorífica en mecánica, después al final de la hoja dice --En la medida que el funcionamiento del regenerador se acerca al caso ideal, el rendimiento del ciclo se aproxima al del ciclo de Carnot-- que es n=(1-(Tf / Tc) este numero nos dice que el rendimiento también es igual a la resta de 1 entre la division de la temperatura en la zona fría (Tf) entre la temperatura en la zona caliente (Tc), el ejemplo se encuentra en el vídeo cuando tomo la temperatura del motor cuando esta funcionando (ver el video "Stirling Engine en funcionamiento" al minuto 1:20) las temperaturas son Tc= 188.7 °C y Tf=27.8 °C entonces tenemos que nuestro rendimiento de la maquina es de
n=1-(27.8/188.7)
n=0.8526
n=85.26%