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3.2 The combustion chamber
An acceptable motor temperature of our car is not only guaranteed by the cooling water. Also the inlet air and the fuel act as coolant before they combust in the cylinders. The hot exhaust gasis heat expecially the outlet valves. During start and shut down of the motor, these valve discs are subject of a temperature cyclus. Are the temperatures too high, heat expansions can lead to a crack formation. To the hot parts of a motor belongs also the exhaust, especially the pipe, the silencer and the catalytic converter. They also experience thermo cycles, crack formation and thermal abrasion by oxidation. Hotgas corrosion from the inside and outside as well as vibrations stress also. Therefore some parts must be exchanged once in a while.
Different to a car motor, in a gas turbine a continous combustion takes place ( "Ill. 3.2-1"). Therefore the component temperatures are here considerable higher and require costly, highly heat resisting materials. Also the today required life time of a gas turbine is far higher than for a car motor. All the more we have to look out that the hot parts are not abnormally „stressed“. Instructions and specifications and the state of the art allow an optimal operation.
Fuel is mixed with the compressed air supplied from the compressor and burned in the combustion chamber ( "Ill. 3.2.1-1"). A stable combustion, however, is only possible within certain limits of the fuelair mixture. ( "Ill. 3.2.1-3"). Gas turbines are able to burn very different types of fuel. The use of natural gas, as opposed to that of other more solid, more liquid and gaseous fuels, has a number of advantages relative to pollutants, coke formation ( "Ill. 3.2.3-1") and instable combustion ( "Ill. 3.2.2-5") . This refers to emissions, contaminants likewise lifetime and repair costs of the engine (see Chapter 4). Of particular importance with regard to the durability of the combustion chamber is the low heat radiation of the typical ‘colorless’ gas flame . It should be pointed out that the usage of these advantages requires certain measures of design for engines with gaseous fuels. As such, the susceptibility of the engine to oscillations of gas in the combustion chamber during a low NOx operation has to be considered ( "Ill. 3.2.2-5"). The insertion of mostly fluid energy at the start necessitates a self -contained fuel system.
"Illustration 3.2-1": (Lit. 3.2-2): both, piston engine and gas turbine ingest air. It is compressed, fuel is added and the mix is ignited. Than this thermodynamic energy of the hot gas is as far as possible converted in mechanical energy by means of an expansion process . In a gas turbine this conversion takes place in a continious process. In contrast the piston engine uses the expansion of the gas in an intermittent process. The picture shows comparable phases of the energy conversion and the involved components. As example a single shaft gas turbine was choosen. The motor is equipped with a direct injection as it is used today in modern gasoline motors.