Since the days of Alfred Büchi, the essential duties of the turbocharger have not altered substantially. A compressor and a turbine linked by a shaft are a turbocharger. The exhaust gas-powered turbine provides the compressor with driving energy.
Design and Function
The turbocharger turbine consists of a turbine wheel and a turbine housing and translates the exhaust gas of the engine into mechanical power to power the compressor. The gas that is limited by its cross-sectional flow area causes the intake and exit to reduce pressures and temperatures. The turbine converts this pressure decrease into kinetic energies to drive the wheel of the turbine.
The axial and radial flow are the two major types of turbine. The wheel is flowing exclusively in the axial direction in the axial flow type. Gas intake is centripetal, that is, from outside in a radial direction and gas out-flux in an axial direction, in radial flux turbines.
Only radial-flow turbines are utilized up to a wheel diameter of around 160 mm. This equates to a turbocharged engine power of around 1000 kW. Only axial flow turbines are employed from a distance of 300 mm onwards. Both possibilities are available between the two values.
The following explanation is confined to the design and function of this type of turbine, being the most prevalent type for automotive applications. The exhaust gas pressure is transformed into kinetic energy in the volute of such radial or centripetal turbines, with the exhaust gas at the wheel rim at a constant speed to the wheel. The transmission energy from kinetic energy to shaft power is carried out on the turbine wheel, which has been engineered to convert almost all of the kinetic energy when the gas reaches the wheel exit.
Operating characteristics
The turbine performance improves when the pressure drop between input and output increases, i.e. when a greater engine speed causes more exhaust gas to be dammed upstream, or when the exhaust gas temperature is increased owing to increased exhaust gassing energy.
The distinctive behaviour, at the transition zone of the intake channel to the volute, is governed by the particular cross-section of the flow, the throat cross-section. This narrowing of the throat cross-section leads to further downstream damage to the exhaust gas and a higher pressure ratio enhances the turbine efficiency. This causes higher boost pressure due to a smaller cross-section flow.
By altering turbine housing, the transverse flow zone of the turbine may be readily changed.
The outlet area at the wheel intake affects also the mass flow capability of the turbine, in addition to the cross-sectional size for turbine housing. The workmanship of a turbine wheel cast contour permits adjustment of the cross-sectional area and thus the boost pressure. A contour expansion leads to a bigger cross-sectional flow area.
The flow cross-section from the volute channel to the rotor intake changes with the changing rotor geometry. Changeable guiding valves or a variable sliding ring covering a piece of the cross-section will vary the exit zone of the turbine wheel.
In practice, maps depicting flow parameters plotted against turbine pressure have been used to outline the functional features of exhaust gas turbine turbines. The turbine map displays the mass flow curves for different speeds and the turbine efficiency. The mass flow curves and efficiency can be displayed using a medium curve to simplify the maps.
The coordination of compressor and turbine wheel sizes is of essential relevance for a high overall turbocharger efficiency. Determines the turbot speed by locating the operating point on the compressor map. In this operating range, turbine wheel diameter must be so as to maximize the turbine efficiency.
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