The drivability 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 (wastegate)
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 drivability 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.
Boost Controller
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.
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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 utilized 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.
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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 behavior 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 minimum clearance to ensure
reliable operation over the whole vehicle lifetime.
