Design and Function of a Turbocharger: Control System
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.
Target and function
The driveability of passenger car turbo engines must meet the same high requirements
as naturally aspirated engines of the same power output. That means, full boost
pressure must be available at low engine speeds. This can only be achieved with
a boost pressure control system on the turbine side.
Control by turbine-side bypass
The turbine-side bypass is the simplest form of boost pressure control. The turbine
size is chosen such that torque characteristic requirements at low engine speeds
can be met and good vehicle driveability achieved. With this design, more exhaust
gas than required to produce the necessary boost pressure is supplied to the turbine
shortly before the maximum torque is reached. Therefore, once a specific boost pressure
is achieved, part of the exhaust gas flow is fed around the turbine via a bypass.
The wastegate which opens or closes the bypass is usually operated by a spring-loaded
diaphragm in response to the boost pressure.
Today, electronic boost pressure control systems are increasingly used in modern
passenger car diesel and petrol engines. When compared with purely pneumatic control,
which can only function as a full-load pressure limiter, a flexible boost pressure
control allows an optimal part-load boost pressure setting. This operates in accordance
with various parameters such as charge air temperature, degree of timing advance
and fuel quality. The operation of the flap corresponds to that of the previously
described actuator. The actuator diaphragm is subjected to a modulated control pressure
instead of full boost pressure.
Boost pressure control of a turbocharged petrol engine by proportional control pressure
This control pressure is lower than the boost pressure and generated by a proportional
valve. This ensures that the diaphragm is subjected to the boost pressure and the
pressure at the compressor inlet in varying proportions. The proportional valve
is controlled by the engine electronics. For diesel engines, a vacuum-regulated
actuator is used for electronic boost pressure control.
Variable turbine geometry
The variable turbine geometry allows the turbine flow cross-section to be varied
in accordance with the engine operating point. This allows the entire exhaust gas
energy to be utilised and the turbine flow cross-section to be set optimally for
each operating point. As a result, the efficiency of the turbocharger and hence
that of the engine is higher than that achieved with the bypass control.
Turbocharger for truck applications with variable turbine geometry (VTG)
Flow cross-section control through variable guide vanes: VTG
Variable guide vanes between the volute housing and the turbine wheel have an effect
on the pressure build-up behaviour and, therefore, on the turbine power output.
At low engine speeds, the flow cross-section is reduced by closing the guide vanes.
The boost pressure and hence the engine torque rise as a result of the higher pressure
drop between turbine inlet and outlet. At high engine speeds, the guide vanes gradually
open. The required boost pressure is achieved at a low turbine pressure ratio and
the engine's fuel consumption reduced. During vehicle acceleration from low speeds
the guide vanes close to gain maximum energy of the exhaust gas. With increasing
speed, the vanes open and adapt to the corresponding operating point.
Today, the exhaust gas temperature of modern high-output diesel engines amounts
to up to 830 °C. The precise and reliable guide vane movement in the hot exhaust
gas flow puts high demands on materials and requires tolerances within the turbine
to be exactly defined. Irrespective of the turbocharger frame size, the guide vanes
need a minmum clearance to ensure reliable operation over the whole vehicle lifetime.