Una máquina de vapor es un motor de combustión externa que transforma la energía térmica de una cantidad de agua en energía mecánica. Este ciclo de trabajo se realiza en dos etapas: 1. \n* Se genera vapor de agua por el calentamiento en una caldera cerrada herméticamente, lo cual produce la expansión del volumen de un cilindro empujando un pistón. Mediante un mecanismo de biela-manivela, el movimiento lineal alternativo del pistón del cilindro se transforma en un movimiento de rotación que acciona, por ejemplo, las ruedas de una locomotora o el rotor de un generador eléctrico. Una vez alcanzado el final de carrera el émbolo retorna a su posición inicial y expulsa el vapor de agua utilizando la energía cinética de un volante de inercia. 2. \n* El vapor a presión se controla mediante una serie de válvulas de entrada y salida que regulan la renovación de la carga; es decir, los flujos del vapor hacia y desde el cilindro. El motor o máquina de vapor se utilizó extensamente durante la Revolución Industrial, en cuyo desarrollo tuvo un papel relevante para mover máquinas y aparatos tan diversos como bombas, locomotoras, motores marinos, entre otros. Las modernas máquinas de vapor utilizadas en la generación de energía eléctrica no son ya de émbolo o desplazamiento positivo como las descritas, sino que son turbomáquinas; es decir, son atravesadas por un flujo continuo de vapor y reciben la denominación genérica de turbinas de vapor. En la actualidad la máquina de vapor alternativa es un motor muy poco usado salvo para servicios auxiliares, ya que se ha visto desplazado especialmente por el motor eléctrico en la maquinaria industrial y por el motor de combustión interna en el transporte.

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A steam engine is a heat engine that performs mechanical work using steam as its working fluid. Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be used. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In the cycle, water is heated and transforms into steam within a boiler operating at a high pressure. When expanded through pistons or turbines, mechanical work is done. The reduced-pressure steam is then condensed and pumped back into the boiler. In general usage, the term steam engine can refer to either the integrated steam plants (including boilers etc.) such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine. Specialized devices such as steam hammers and steam pile drivers are dependent on the steam pressure supplied from a separate boiler. The use of boiling water to produce mechanical motion goes back over 2000 years, but early devices were not highly practical. The Spanish inventor Jerónimo de Ayanz y Beaumont obtained the first patent for a steam engine in 1606. In 1698 Thomas Savery patented a steam pump that used steam in direct contact with the water being pumped. Savery's steam pump used condensing steam to create a vacuum and draw water into a chamber, and then applied pressurized steam to further pump the water. Thomas Newcomen's atmospheric engine was the first commercial true steam engine using a piston, and was used in 1712 for pumping in a mine. In 1781 James Watt patented a steam engine that produced continuous rotary motion. Watt's ten-horsepower engines enabled a wide range of manufacturing machinery to be powered. The engines could be sited anywhere that water and coal or wood fuel could be obtained. By 1883, engines that could provide 10,000 hp had become feasible. The stationary steam engine was a key component of the Industrial Revolution, allowing factories to locate where water power was unavailable. The atmospheric engines of Newcomen and Watt were large compared to the amount of power they produced, but high pressure steam engines were light enough to be applied to vehicles such as traction engines and the railway locomotives. Reciprocating piston type steam engines remained the dominant source of power until the early 20th century, when advances in the design of electric motors and internal combustion engines gradually resulted in the replacement of reciprocating (piston) steam engines in commercial usage, and the ascendancy of steam turbines in power generation. Considering that the great majority of worldwide electric generation is produced by turbine type steam engines, the "steam age" is continuing with energy levels far beyond those of the turn of the 19th century.

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