Heat engine

A heat engine is a machine that heat energy (short and heat ) into mechanical energy. In this case, it takes advantage of the tendency of the heat to flow from areas of higher to such lower temperatures. A machine which is transported using mechanical energy, thermal energy from a lower temperature to a higher level is referred to as combined heat and power engine, a heat pump or refrigerating machine.

Use heat engines " right-handed " cycles, in which the closed curve is traversed in about TS or pv diagram in the sense of " top right, bottom left ". Heat pumps use " left-handed " cycles.

  • 3.1 Depending on the source of thermal energy
  • 3.2 After pressure build-up

Efficiency

As the efficiency of a heat engine, the proportion of the effluent from the upper temperature level heat energy is called, which is converted into the mechanical energy used. An upper limit for it is given by the efficiency of the Carnot cycle used when generating the heat absorption and release at defined temperature levels and take place and no friction, heat loss and heat transfer losses. For him, the following applies:

Is a prerequisite for the achievement of the Carnot efficiency that all sub-processes of the cycle are designed reversible. This is equivalent to saying that a size S called entropy of the entire system from the heat engine and the surrounding area is not growing. (According to the second law of thermodynamics can not be less, so they must remain constant. )

(dQ is the amount of heat exchanged an infinitesimally small process step, T is the corresponding temperature):

The Carnot efficiency is never achieved in practice, since

  • The heat absorption at lower temperatures, and the heat emission at higher temperatures to take place (for example, in the Stirling process)
  • Despite isolation always heat transport takes place without exchange of labor,
  • Having each machine friction losses, which also worsen the ratio of heat to work flow, and finally
  • In fast processes of heat flow due to the non-vanishing thermal resistance requires a temperature difference that is lost conversion work (see heat conduction ).

For heat pumps, the characteristic size used is the coefficient of performance.

Examples

Internal combustion engine

Internal combustion engines have combustion temperatures up to 2500 ° C ( 2773 K) and the working gas - end temperatures of about 1000 ° C ( 1273 K ). The attainable efficiency would be

In practice, under optimum conditions, gasoline engines achieve 38%, 45% and diesel engines, slow speed marine diesel engines 50 % efficiency. In Automobiles achieve under real driving conditions with a high proportion of part-load operation gasoline engines typically have an efficiency of less than 25% and diesel engines less than 30 %.

CCPP

A heat engine may be composed of different cyclic processes (eg combined cycle power plant: combination of gas turbine cycle with a steam power plant ):

Thus theoretically the efficiency of a ( comparative) cycle process in the temperature range from 1500 to 100 ° C can be achieved. Combined-cycle power plants achieve up to 60% in practice efficiencies.

Classification (typology )

Since a gas is used as the working medium, are heat engines to the thermal fluid energy machines.

Depending on the source of thermal energy

  • Internal combustion engines ( Thermal fluid energy machines with internal combustion ) internal combustion engine
  • Gas turbine
  • Thermal fluid energy machine with external combustion steam engine
  • Steam turbine
  • Stirling engine

After pressure build-up

  • Turbomachines turbomachinery steam turbine
  • Gas turbine
  • Ramjet
  • Piston engines Internal combustion engine (internal combustion )
  • Steam engine ( external combustion )
  • Stirling engine ( external combustion )
Temperature–entropy diagram Steam-electric power station
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