Gas engine

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A gas engine or a gas engine is an old name for a heat engine . It partially converts internal energy of a heated and compressed gas into mechanical work. The theoretical basis of this process is the gas law (pV = nRT) with the three gas state variables pressure, temperature, volume and the universal gas constant . The name gas engine has nothing to do with gaseous fuel (e.g. natural gas).

There are machines that generate the gas pressure through "internal combustion" in an open process and there are machines that generate the gas pressure in a closed circuit using an external heat source.

Gas engines use right-hand (clockwise) cycle processes in the Ts diagram or pV diagram . Gas compressors use "counterclockwise" circular processes. The ideal comparison processes are used to assess the efficiency of circular processes .

Efficiency

Efficiency of gas engines

The efficiency of a gas engine, like that of all heat engines, is based on the Carnot factor .

The degree of expansion, sometimes also called the expansion efficiency, is an important parameter for the efficiency of piston engines. This is the volume difference between the compressed gas and the expanded gas at the end of the work cycle.

In the case of turbines, the pressure ratio or the temperature ratio of the working or fuel gas at the turbine inlet to the turbine outlet is decisive for the efficiency.

It is also important that the isentropic exponent of the working gas is as high as possible . This is a ratio of the heat capacity of a gas at the same pressure to that at the same volume. A free choice of the working gas is only available for machines with a closed circuit.

Working temperatures and comparison processes

Piston engines in the open process reach combustion temperatures of 2200 ° C to 2500 ° C and final work cycle temperatures of around 900 ° C to 800 ° C. The exhaust gas temperature is lower. The comparison process is called the Seiliger cycle . The efficiencies are between 35% and 50%.

Piston steam engines in a closed process work with steam temperatures of up to 350 ° C. The comparison process for these engines is the equal pressure process . The efficiency rarely exceeds 20%. Stirling engines can convert heat into power in an even lower temperature range (e.g. waste heat from the steam process). According to the Carnot equation, the efficiency then hardly reaches more than 10% to 20%.

Gas turbines work with turbine inlet temperatures from 1300 ° C to 1400 ° C and exhaust gas temperatures from 800 ° C to 600 ° C. The comparison process is the Joule process . Turbines do not achieve the efficiency of piston engines because of the lower maximum temperatures.

Steam turbines work with maximum temperatures of 600 ° C to 700 ° C and end temperatures of 130 ° C. The comparison process is the Joule process or the Clausius-Rankine process . Despite the low working temperature of the steam, the process achieves efficiencies of over 30% due to the good heat capacity and the good thermal conductivity of the steam in the heater and in the condenser.

Combined gas and steam plants ( combined cycle power plants ), i.e. the open and closed cycle processes, use the working temperature range between 1400 ° C and 130 ° C and achieve efficiencies of up to 60%. The combination of turbo-diesel engines with steam or helium turbines promises even higher levels of efficiency.

Internal combustion gas engines

As a rule, a mixture of air and hydrocarbons is ignited and burned after compression. The hydrocarbons can be mixed with the air and oxygen in liquid or gaseous form.

External combustion gas engines

Screw motor with steam drive
1 = superheated steam 2 = cold steam 3 = double screw 4 = sealing

As a rule, solid and cheap fuels are used to heat the working gas (coal, wood, garbage). Inferior liquid and gaseous fuels (crude oil, heavy oil, industrial and biogas) are also often used. A typical application is also the fuel rods of nuclear power plants. As working gases, helium and hydrogen are much more efficient than air (heat capacity, isentropic exponent, heat transfer coefficient ).

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