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 operating point.

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 the system.

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.