BorgWarner to Supply Regulated Two-Stage Turbocharging Technology to BMW

"BorgWarner Turbo Systems will supply its market-leading regulated two-stage turbocharging (R2S(TM)) systems to BMW for its new, 3.0-liter, 6-cylinder diesel engine for the BMW 5-series. The engine debuted at the Geneva Motorshow in March. With about 67kW/I, the engine has the highest power density of any passenger-car diesel engine."

stolen from: http://www.germancarfans.com/news.cf...013/bmw/1.html







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Here's a pic on boost developement graph with two-stage charger setup, when comparing over to single turbocharger. I detached this pic from a Borg Warner study on 'Regulated Two Stage' turbocharging. It does not say to which engine this graph relates to. It's my guessingwork that we're talking about a truck engine, since graph stops at 2100rpm. Nice graph.

 

Hmmm look like they are going in the same direction as the Cummins with the cat muff system with a differential sensor on the inlet and outlet and a thermocouple.... regeneration system using the engine injection system to keep it hot and clean, of course means worse fuel economy. definately not a simple turbo system, I notice no turbo speed sensors or electronic controllers, looks to be all mechanical.

 

I bet each of those three actuators are driven by ECU controlled PWM modulated solenoid valve.
I have one of those suckers driving my wastegate actuator. The technology in my car dates back to around 1995-1997, or may even be older.

 

whats up with the big check valve between the outlet of the big turbo and inlet of the small one? that looks like a good idea, any downfall to this? it seems like the small turbo would be bypassed completely some of the time.

 

Unlike tuned 6BT Cummins, smaller passenger car diesel engines are not capable of withstanding ungodly boost pressures. They need that check valve to keep boost under control.
They use two-stage boost buildup when it's needed. When the big low pressure spinny thingy get's going, the smaller high pressure turbo is 'dropped out' by that very check valve.

I could not find a proper web site to explain how this all works. I need to borrow another forum here.

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stolen from: http://www.ohiovw.com/forums/topic.asp?TOPIC_ID=18802


THE DAILY UPDATE ON AUTOMOTIVE TECHNOLOGIES
autotech
daily™
THURSDAY
OCTOBER 7, 2004

BorgWarner Inc.’s Turbo Systems unit has developed a dual-stage turbocharging system that could help engines achieve up to 40% higher torque than is possible with single-stage variable turbine geometry designs. With the new design, it says, low-end torque no
longer has to be sacrificed to boost horsepower. The system, dubbed R2S, made its passenger-car debut on the new BMW 535d, which is powered by a 3.0-liter six-cylinder diesel engine. It also is being used in commercial vehicles in conjunction with DAF’s MX 410 engine. The technology is applicable to gasoline engines as well. Other European and North American applications are expected to be announced in the next several months. Consisting of two different-sized turbochargers connected in series, the two-stage system allows continuously variable adaptation of the turbine and compressor for all engine operating parameters. Bypass, or wastegate, regulation is used to ensure a constant increase in power. A small, high-pressure turbocharger and a larger low-pressure unit work independently at engine speeds under 1500 rpm and over 4400 rpm, respectively. The two work in tandem at mid-range speeds. For passenger vehicle applications, an additional bypass on the low-pressure turbine and an extra bypass for the compressor are used. This allows expansion of the entire exhaust gas flow via the high-pressure turbine or a partial flow diverted downstream to the low-pressure turbine. BorgWarner says the system can allow an engine to achieve a specific output of more than 94 hp per liter at a maximum torque of nearly 148 lb-ft per liter. In the BMW application, the engine’s performance is boosted to 272 hp, while maintaining 413 lb-ft of torque. In addition to increased power, the new turbocharger also promises an improved charge-pressure curve and fewer oxides of nitrogen emissions. Performance attributes vary depending on engine and vehicle application. Another plus: The R2S can withstand operating temperatures as high as 1,922°F compared with current turbochargers that are rated up to 1,562°F. The dual-stage system weighs slightly more than a typical turbo system. Cost also is higher but is less than two single variable turbine geometry turbochargers.

 

Simulation of turbocharged SI-engines
- with focus on the turbine

Doctoral thesis
KTH School of Industrial Engineering
and Management TRITA – MMK 2005:05
Royal Institute of Technology ISSN 1400-1179
SE-100 44 Stockholm ISRN/KTH/MMK/R-05/05-SE
Fredrik Westin
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1.3.2 Sequential systems
A more extreme case of variable geometry is sequential turbocharging. Instead
of varying the geometry of one charger the number of chargers is changed.
The reason for using sequential turbocharging is to widen the flow range for
the boost pressure required. Since still only one compressor stage is used it
does not necessarily increase the boost pressure.
Volvo tested sequential systems presented in an SAE paper in 1991 [1.26] as a part of a methanol engine program. The engine was a six-cylinder unit and the boost pressure target was as usual, slightly below 2 Bar abs. They state that a parallel turbo system has around 30% lower inertia than a single turbo with the same top end flow characteristics, and thus should have a benefit in terms of response. To further increase the response sequential systems was investigated.
They conclude that a series-sequential system was beneficial over a parallelsequential system. The reason for that was the difficulty of achieving a smooth transition from one-turbo operation to two-turbo operation for the latter.
However, the series-sequential system has a narrower flow range because the
entire mass flow has to go through both compressors, on the other hand, the
two-stage compression would allow for higher boost pressures with maintained
reasonable flow range. Fig. 1.14 shows Volvo’s series-sequential turbo-system.




1.3.3 Two-stage systems
No publications have been found covering two-stage turbocharging of Ottoengines.
Traditionally it has been used for boost pressures far above 2 Bar abs [1.27] but 3K have presented a two-stage system for 3 bar abs of boost
pressure, but for diesel engines, see Pflüger et. al. [1.28]. Figure 1.15 shows an example of the division of pressure ratio between the HP and LP compressors.
The system consists of two unequal sized turbochargers with wastegate on the HP stage and intercooling after each stage. The system is optimised for a 12-liter diesel engine and the HP and LP stage’s sizes are 85% and 112%
respectively of the size of a single turbo optimised for the same engine. No
transients were tested but in the text it is stated that two-stage is better in
transients than a sequential turbo system.
At the low speed end the 2 Bar abs boost pressure point is moved from 1000
rpm for the single turbo system to 700 rpm for the two-stage system.
In 2004 BMW released the new 535d-model equipped with a 3K-developed 2-
stage boosting system [1.29]. The system is similar to the truck-sized system
with the addition of a wastegate for the LP-stage. On paper the system, and the engine, looks very promising with impressive performance. 3K described the differences in matching between the two systems in a very informative paper at the 9. Aufladetechnische Konferenz in Dresden [1.30]. As a literature on radial compressor design constraints it is very informative reading. They conclude that if both the boost pressure and range should be increased simultaneously with at least maintained efficiency a single stage compressor must be fitted with variable guide vanes probably both at inlet and in the diffuser, or a two-stage system must be employed. Since the two-stage system consists of known technology, they think it is a safer route.
Cantemir [1.31] also describes a twin-turbo, sequential system where the
compressors is run in series (two-stage) operation for the low speed range of
the engine and switched to parallel mode for the higher speed range. He argues that with turbochargers of equal size the shape of the compressor maps are very suitable for this kind of operation. The turbines are always run in parallel.
Unfortunately no performance figures of the system are presented.

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I can email the whole pdf file if anyone is interested reading the whole thesis, just drop me a private message with email addy.