Heat Engines
	Heat engines Rankine cycle engines Stirling Cycle Engines Brayton Cycle Engines Kalina Cycle Engine More about engines	Heat engines convert heat energy into mechanical energy. Examples include steam engines, steam and gas turbines, spark-ignition and diesel engines, and the "external combustion" engine or Stirling engine. Such engines can provide motive power for transportation, to operate machinery, or to produce electricity. All heat engines operate in a cycle of repeated sequences of heating (or compressing) and pressurizing the working fluid, the performance of mechanical work, and rejecting unused or waste heat to a "sink." At the beginning of each cycle, energy is added to the fluid forcing it to expand under high pressure so that the fluid "performs" mechanical work. The thermal energy contained in the pressurized fluid is converted to kinetic energy. The fluid then looses pressure, and after unused energy (in the form of heat) is rejected, it must then be reheated or recompressed to restore it to high pressure. Heat engines cannot convert all the input energy to useful mechanical energy in the same cycle; some amount, in the form of heat, is always not available for the immediate performance of mechanical work. The fraction of thermal energy that is converted to net mechanical work is called the thermal efficiency of the heat engine. The maximum possible efficiency of a heat engine is that of a hypothetical (ideal) cycle, called the Carnot Cycle. Practical heat engines operate on less efficient cycles (such as the Rankine, Brayton, or Stirling) but in general, the highest thermal efficiency is achieved when the input temperature is as high as possible and the sink temperature is as low as possible. The "waste" or rejected heat (to the "sink") can be used for other purposes, including pressurizing a different working fluid, which operates a different heat-engine (vapor turbine) cycle, or simply for heating. Renewable sources of heat or fuels, such as solar or geothermal energy and biomass (as well as fossil fuels) can power heat engines. The following is a brief description of four types of heat engines, the Rankine, Stirling, Brayton, and the newly developed, and highly efficient, Kalina, that can be used or are being investigated for converting renewable sources of energy to useful energy.

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