Design and Function of a Turbocharger: Turbine
The turbocharger's basic functions have not fundamentally changed since the times
of Alfred Büchi. A turbocharger consists of a compressor and a turbine connected
by a common shaft. The exhaust-gas-driven turbine supplies the drive energy for
the compressor.
Design and function
The turbocharger turbine, which consists of a turbine wheel and a turbine housing,
converts the engine exhaust gas into mechanical energy to drive the compressor.
The gas, which is restricted by the turbine's flow cross-sectional area, results
in a pressure and temperature drop between the inlet and outlet. This pressure drop
is converted by the turbine into kinetic energy to drive the turbine wheel.
There are two main turbine types: axial and radial flow. In the axial-flow type,
flow through the wheel is only in the axial direction. In radial-flow turbines,
gas inflow is centripetal, i.e. in a radial direction from the outside in, and gas
outflow in an axial direction.
Up to a wheel diameter of about 160 mm, only radial-flow turbines are used. This
corresponds to an engine power of approximately 1000 kW per turbocharger. From 300
mm onwards, only axial-flow turbines are used. Between these two values, both variants
are possible.
As the radial-flow turbine is the most popular type for automotive applications,
the following description is limited to the design and function of this turbine
type. In the volute of such radial or centripetal turbines, exhaust gas pressure
is converted into kinetic energy and the exhaust gas at the wheel circumference
is directed at constant velocity to the turbine wheel. Energy transfer from kinetic
energy into shaft power takes place in the turbine wheel, which is designed so that
nearly all the kinetic energy is converted by the time the gas reaches the wheel
outlet.
Operating characteristics
The turbine performance increases as the pressure drop between the inlet and outlet
increases, i.e. when more exhaust gas is dammed upstream of the turbine as a result
of a higher engine speed, or in the case of an exhaust gas temperature rise due
to higher exhaust gas energy.
The turbine's characteristic behaviour is determined by the specific flow cross-section,
the throat cross-section, in the transition area of the inlet channel to the volute.
By reducing this throat cross-section, more exhaust gas is dammed upstream of the
turbine and the turbine performance increases as a result of the higher pressure
ratio. A smaller flow cross-section therefore results in higher boost pressures.
The turbine's flow cross-sectional area can be easily varied by changing the turbine
housing.
Besides the turbine housing flow cross-sectional area, the exit area at the wheel
inlet also influences the turbine's mass flow capacity. The machining of a turbine
wheel cast contour allows the cross-sectional area and, therefore, the boost pressure,
to be adjusted. A contour enlargement results in a larger flow cross-sectional area
of the turbine.
Turbines with variable turbine geometry change the flow cross-section between volute
channel and wheel inlet. The exit area to the turbine wheel is changed by variable
guide vanes or a variable sliding ring covering a part of the cross-section.
In practice, the operating characteristics of exhaust gas turbocharger turbines
are described by maps showing the flow parameters plotted against the turbine pressure
ratio. The turbine map shows the mass flow curves and the turbine efficiency for
various speeds. To simplify the map, the mass flow curves, as well as the efficiency,
can be shown by a mean curve
For a high overall turbocharger efficiency, the co-ordination of compressor and
turbine wheel diameters is of vital importance. The position of the operating point
on the compressor map determines the turbocharger speed. The turbine wheel diameter
has to be such that the turbine efficiency is maximised in this operating range.
Twin-entry turbines
The turbine is rarely subjected to constant exhaust pressure. In pulse turbocharged
commercial diesel engines, twin-entry turbines allow exhaust gas pulsations to be
optimised, because a higher turbine pressure ratio is reached in a shorter time.
Thus, through the increasing pressure ratio, the efficiency rises, improving the
all-important time interval when a high, more efficient mass flow is passing through
the turbine. As a result of this improved exhaust gas energy utilisation, the engine's
boost pressure characteristics and, hence, torque behaviour is improved, particularly
at low engine speeds.
To prevent the various cylinders from interfering with each other during the charge
exchange cycles, three cylinders are connected into one exhaust gas manifold. Twin-entry
turbines then allow the exhaust gas flow to be fed separately through the turbine.
Turbocharger with twin-entry turbine
The turbine is rarely subjected to constant exhaust pressure. In pulse turbocharged
commercial diesel engines, twin-entry turbines allow exhaust gas pulsations to be
optimised, because a higher turbine pressure ratio is reached in a shorter time.
Thus, through the increasing pressure ratio, the efficiency rises, improving the
all-important time interval when a high, more efficient mass flow is passing through
the turbine. As a result of this improved exhaust gas energy utilisation, the engine's
boost pressure characteristics and, hence, torque behaviour is improved, particularly
at low engine speeds.
To prevent the various cylinders from interfering with each other during the charge
exchange cycles, three cylinders are connected into one exhaust gas manifold. Twin-entry
turbines then allow the exhaust gas flow to be fed separately through the turbine.
Water-cooled turbine housings
Turbocharger with water-cooled turbine housing for marine applications
Safety aspects also have to be taken into account in turbocharger design. In ship
engine rooms, for instance, hot surfaces have to be avoided because of fire risks.
Therefore, water-cooled turbocharger turbine housings or housings coated with insulating
material are used for marine applications.