The variable vane turbocharger,
fitted to the LH exhaust manifold of the TdV6 engine, makes it possible to vary
the exhaust gas flow of the turbine, dependent on engine operation. This
improves the power transfer to the turbine wheel and compressor, particularly at
low engine speeds, thus increasing the boost pressure. The guide vanes are
opened progressively as the engine speed increases so that the power transfer
always remains in balance with the required charger speed and the required boost
pressure level. Variable vanes facilitate better use of the exhaust gas energy
so as to further improve the efficiency of the turbocharger and thus of the
engine, compared to the more conventional 'wastegate
control'.
Advantages:
High torque at both high and low engine speeds
Continuous and optimum adjustment for all engine speeds
No wastegate valve required, exhaust energy is better utilised, less
back-pressure in conjunction with same compressor work
Low thermal and mechanical load improves engine power output
Low emissions
Optimised fuel consumption over the entire engine speed range
The
turbochargers construction is similar to the unit fitted to the Freelander Td4
engine. However, a stepper motor instead of a vacuum diaphragm electronically
controls the variable vanes.
The DC rotary actuator motor operates a
drive shaft. The drive shaft is connected to the vanes by an actuating lever.
Adjustment of the vanes is achieved by moving the actuating lever. When the
drive shaft is turned, a signal is created at the end of the drive shaft; this
feedback signal is used to determine the angular position of the vanes. This
information is transmitted to the Engine Control Module (ECM).
There is a
temperature sensor in the control unit, which drives the stepper motor to a safe
position (vanes fully opened) if the maximum temperature is exceeded. The ECM
detects any malfunctions in the stepper motor and generates Diagnostic Trouble
Codes (DTC).
A
-
Closed (fast)
B
-
Open (slow), default position
1
-
Turbine
2
-
Vanes
3
Actuator lever
At low engine speeds, because of
the flat setting of the vanes, the relatively low flow of exhaust gases are
accelerated and routed in such a way that they meet the vanes of the turbine
wheel at the outermost edge. Consequently, a high torque is produced at the
turbine wheel, and a high turbine speed is therefore achieved.
At high
engine speeds the vanes are progressively opened, so that a large flow of
exhaust gas is delayed and is directed more and more towards the centre of the
turbine wheel.
The torque at the turbine wheel is artificially lowered.
In this way the turbine speed, and therefore the air quantity required by the
engine, are matched to engine speed. Consequently, the boost pressure is
approximately constant over the whole engine speed range.
The operating
parameters are controlled electronically by the ECM using powertrain sensors and
driver inputs.
Electronic Engine Controls (303-14C Electronic Engine
Controls - 2.7L Diesel)
The turbocharger is designed for fail safe
operation. If a fault occurs regarding the control of the unit, the vanes
default to the fully open position so as to produce minimum boost.