Elaborate twin-scroll turbocharger technology avoiding the usual
turbo “lag”.
For the first time in this class, the direct injection engine in the new family
comes with a twin-scroll turbocharger. Featuring this technology, the ducts of
each two cylinders in the exhaust manifolds and turbochargers are different
from one another in their design. Reducing exhaust gas counterpressure at
low engine speeds, twin-scroll charger technology capitalises on the dynamic
effect of the pulsing gas columns in the exhaust manifolds. The result is
additional power and thrust on even less fuel, enabling the turbocharger to
boost engine output from an earlier point. This effect is clearly noticeable,
with the charger building up extra power from roughly 1,400 rpm, almost
completely avoiding the “turbo lag” often criticised on turbocharged engines,
and generating torque almost as fast as in a compressor engine.
The flow of exhaust gas accelerates the turbine wheel to a speed of up to
220,000 rpm. And at the same time the compressor running on the same
shaft compresses the fresh air fed into the system. A wastegate complete with
a check valve monitors the maximum turbocharger pressure of 0.8 bar.
In addition, overpressure in the system is controlled by a dump valve activated
when coasting with the intake manifold closed. To increase the charge level,
the pre-compressed fresh air is cooled down in an intercooler before flowing
into the combustion chamber. The intercooler itself is fitted in the car
at a predetermined point meeting all the requirements of this particular
configuration.
Maximum exhaust gas temperature is monitored by the electronic engine
“brain” and is limited to 950 °C (1,742 °F). To prevent excessive build-up of
heat in the oil- and water-cooled turbocharger after the engine has been
stopped, an additional electrical water pump starts automatically as soon as
the car comes to a standstill, dissipating any excess thermal energy from
the system.