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A turbo for the turbo
With two-stage regulated charging, the performance of an internal combustion engine can be increased with lower consumption and lower emissions. Walter Müllberger from the Turbo Twins explains the complex technology using the new BORGWARNER CHARGING PROCESS.
A turbo for the turbo
The constantly lowering emission limits and the growing competition from alternative drive systems are encouraging the developers of combustion engines to achieve top performance. With turbocharging you have a proven recipe at hand to get more horsepower with less fuel consumption and pollutants from gasoline or diesel engines. “The design of such a charging system leads to a conflict of objectives during development,” explains Walter Müllberger, who, together with his brother Manfred, runs the Turbo Twins company in Vienna-Simmering, which has specialized in trading and repairing all brands of turbochargers since 1993. “For the nominal power point, you need a relatively large exhaust gas turbocharger,” says Müllberger. However, the desire for a very high boost pressure even at low engine speeds means that the turbine and compressor have to be made significantly smaller. “Ideally, you would need a combination of both,” explains the turbo expert.
“The two-stage regulated charging enables continuous adjustment on the turbine and compressor side to the requirements of engine operation.”WALTER MÜLLBERGER, TURBO TWINS
THE IDEAL TURBO DESIGN
To solve this conflict of objectives, BorgWarner Turbo Systems has now developed the two-stage regulated turbocharging “R2S”. This combines both of the requirements mentioned and therefore corresponds to the ideal structure of a turbocharger. The system provides continuously variable adjustment of the turbine and compressor sides for each engine operating point. Conclusion: With this newly developed charging process, new generations of combustion engines can be developed that meet the strictest requirements in terms of power delivery, consumption and environmental compatibility. The technology in detail: The two-stage regulated charging R2S is a series connection of two turbochargers of different sizes with a so-called bypass control. The exhaust gas mass flow coming from the cylinders first flows into the exhaust gas collection pipe. From here it is possible to either expand the entire exhaust gas mass flow via the high-pressure turbine (HP) or to divert a partial mass flow via the bypass line to the low-pressure turbine (LP). The entire exhaust gas mass flow is then used again by the downstream low-pressure turbine (LP). The entire fresh air mass flow is first pre-compressed by the low-pressure stage. Further compression and intercooling then take place in the high-pressure stage. Due to pre-compression, the relatively small HP compressor works at a higher pressure level so that it can achieve the required air mass flow.
COMPACT CHARGING SYSTEM
When driving, at low engine speeds, i.e. small exhaust gas mass flows, the bypass remains completely closed and the entire exhaust gas mass flow expands via the HP turbine. This results in a very quick and high boost pressure build-up. As the engine speed increases, the expansion work is continuously shifted to the LP turbine by increasing the bypass cross section accordingly. “The two-stage regulated charging enables continuous adjustment on the turbine and compressor side to the requirements of engine operation,” explains Walter Müllberger. The system is controlled by pneumatic actuators that act on bypass flaps, such as those used in large-scale production of turbochargers with wastegate flaps. Thanks to the use of proven components, the compact charging system meets the highest requirements for torque, response, exhaust behavior and performance. The new Borg-Warner R2S charging system can be installed in both cars and commercial vehicles. In vehicles with larger displacements, an electrical control unit is used instead of the vacuum control box for exhaust gas bypass control.