Heat engine

Heat engine: energy balance

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A heat engine is a machine that converts heat into mechanical energy (work). It takes advantage of the tendency of the heat to flow from areas with higher to those with lower temperatures. Examples are steam engine , steam turbine and all internal combustion engines .

In contrast, a machine that uses mechanical energy to transport thermal energy from a lower temperature level to a higher one is referred to as a combined heat and power machine , heat pump or cooling machine .

Heat engines use “clockwise” cycle processes in which the closed curve in the TS or pv diagram in the sense of “top to right, bottom to left” is run through. Heat pumps use "counter-clockwise" cycle processes.

Efficiency

The efficiency of a heat engine is the proportion of the thermal energy flowing away from the upper temperature level that is converted into the mechanical energy used. An upper limit for him by the efficiency of the Carnot cycle , where, in the heat absorption and release at defined temperature levels and caused to take place, and no friction, Wärmeabfluss- and heat transfer losses. For him applies: ${\displaystyle \eta _{\text{Carnot}}}$ ${\displaystyle {T_{\mathrm {max} }}}$ ${\displaystyle {T_{\mathrm {min} }}}$

${\displaystyle \eta _{\text{Carnot}}={\frac {T_{\mathrm {max} }-T_{\mathrm {min} }}{T_{\mathrm {max} }}}=1-{\frac {T_{\mathrm {min} }}{T_{\mathrm {max} }}}}$

A prerequisite for achieving Carnot's degree of efficiency is that all sub-processes of the cycle are designed to be reversible . This is synonymous with the fact that a quantity called entropy S of the overall system consisting of the heat engine and the environment does not grow. (According to the second law of thermodynamics , it cannot decrease, so it must remain constant.)

${\displaystyle \Delta S=\oint dS=\oint {\frac {dQ}{T}}=0}$

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

The Carnot efficiency is never achieved in practice because

• the heat absorption even at lower temperatures than and the heat emission also at higher temperatures than takes place (e.g. in the Stirling process ), ${\displaystyle T_{\mathrm {max} }}$ ${\displaystyle {T_{\mathrm {min} }}}$
• Despite the insulation, there is always heat transport without exchanging work,
• every machine has frictional losses that also worsen the heat to work flow ratio, and finally
• in fast-running processes, the heat flow requires a temperature difference due to the non-vanishing thermal resistance, which is lost for conversion into work (see heat conduction ).

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

Examples

Internal combustion engine

Internal combustion engines have combustion temperatures of up to 2500 ° C (2773 K) and final working gas temperatures of around 1000 ° C (1273 K). The theoretically maximum achievable efficiency would be

${\displaystyle \eta _{\text{Carnot}}=1-{\frac {1273\,\mathrm {K} }{2773\,\mathrm {K} }}=0{,}54=54\,\%}$

In practice, under optimal conditions, gasoline engines achieve 38% efficiency, diesel engines 45% and low-speed marine diesel engines 50%. In passenger cars, under real driving conditions with a high proportion of partial load operation, gasoline engines typically achieve a time-averaged efficiency of less than 25% and diesel engines less than 30%.

Combined cycle power plant

A heat engine can be composed of various cycle processes (eg. Combined cycle power plant : Combination of the gas turbine process with a steam power plant ):

1. Utilization of the working capacity of a process in the temperature range from 1500 to 700  ° C in the gas turbine, then with the exhaust gases from the gas turbine process
2. Utilization of the working capacity of a process in the temperature range from 700 to 100 ° C in the steam power plant,

whereby 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 efficiencies of up to 60% in practice.

Classification (typology)

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

According to the generation of thermal energy

• Internal combustion engines (thermal fluid energy machines with internal combustion)
  • Combustion engine (petrol λ = 1 max. Consumption temp. 2500K. Diesel λ = 1.4 max. Consumption temp. 2200K)
  • Gas turbine (kerosene λ = 4-6 max. Consumption temp. 1500K. Turbo exhaust gas temp. Max. 1300K)

• Thermal gas energy machines with external combustion
  • Steam turbine (max.steam temp. 800K - 1000K)
  • Old steam engine (lower steam temp. 500K, upper steam temp. 800K)
  • Stirling engine (lower helium temperature 400K)

After pressure build-up

• Turbo machines
  • Turbo machines
    • Steam turbine
    • Gas turbine
    • Jam jet engine

• Piston engines
  • Combustion engine (internal combustion)
  • Steam engine (external combustion)
  • Stirling engine (external combustion)

See also

• Power machine
• Micro-CHP
• Combined heat and power
• Schukey motor

Web links

• Further details at Physikon Physik-Lexikon