Regulated 2-stage turbocharging (R2S®)
The basic development goals for future combustion engines for automobile and commercial
vehicle applications make more refined charging systems necessary. The design of
such a charging system leads to conflicting goals in terms of the rated output of
the engine on one hand and the transient response and the range of maximum torque
on the other hand. You need a relatively large exhaust turbocharger to attain the
nominal output point. The desire for a very high boost pressure even at low engine
speeds means, however, that the turbine and the compressor need to be made much
smaller. A combination of the two would be ideal.
To resolve this conflict, BorgWarner Turbo Systems has developed regulated 2-stage
turbocharging. It meets the demands of an optimal design and allows for the continuously
variable adaptation of the turbine and compressor sides of the system for each engine
With this newly developed charging system, BorgWarner Turbo Systems offers the engine
manufacturer an additional extremely high-performing charging system for future
engine generations that fulfills the highest requirements in terms of power, fuel
consumption and emissions.
The regulated 2-stage turbocharger consists of two turbochargers of different sizes
connected in series that utilize bypass regulation. The exhaust mass flow coming
from the cylinder flows into the exhaust manifold first. Here it is possible to
expand the entire exhaust mass flow using the high pressure turbine (HP) or to redirect
some of the mass flow through a bypass to the low pressure turbine (LP). The entire
exhaust mass flow is then utilized again by the low pressure turbine (LP).
The entire fresh air flow is first compressed by the low pressure stage. In the
high pressure stage, it is compressed further and then the charging air is cooled.
Due to the precompression process, the relatively small HP compressor can reach
a high pressure level so that it can force the required amount of air to flow through
At low engine speeds, i.e. when the exhaust mass flow rate is low, the bypass remains
completely closed and the entire exhaust mass flow is expanded by the HP turbine.
This results in a very quick and high boost pressure rise. As the engine speed increases,
the job of expansion is continuously shifted to the LP turbine by increasing the
cross-sectional area of the bypass accordingly.
Regulated two-stage turbocharging therefore allows for continuous adaptation on
the turbine and compressor sides to the actual requirements of the operating engine.
The system can be regulated via pneumatic actuators that control the bypass valve
in the same manner as when used in mass-produced turbochargers with swing valves.
This makes it possible to model a compact charging system (when detailed knowledge
of the complex system response is available) that fulfills the highest torque, response
and power requirements while utilizing proven components.