Anyone seen this type of turbo system?

[Silverbullet86]

I've been reading up on different twin turbo setups, specifically sequential turbos where there is an electronic valve that controls exhaust flow from one turbo to the other. I was wondering if anyone has seen a setup where the dumped exhaust flow from the waste gate of the first turbo (smaller one) is used to spin the turbine of the second turbo (larger)? Without throwing in any numbers, i would predict is able to have a decent performance if the sizes of compressors and turbines and piping are matched properly.
This is something i would consider doing as a Senior Design class next semester if this system hasn't already been thought of and tried, or proven to be inefficient.

 

[globi5]

At least the Citroën C5 HDI with the sequential turbo mentioned in the article above has been sold two years ago already.

 

[TDIMeister]

In a paper presented by PSA that I have, they distinguish a serial sequential setup like BMW/BWTS' setup, and their parallel sequential setup. I personally think such a parallel setup as PSA has is better for a gasoline engine that operates at relatively modest boost pressures and over a wider RPM range compared to state-of-the-art Diesel engines. In a serial setup (going away from bypassed modes for a moment), the pressure ratios of each stage is multiplied, whereas in the parallel, there is no such multiplication, but the mass airflow contribution of each turbo is summed. I can see this setup beneficial in extending the mass flow range whilst maintaining a high compressor efficiency, but boost pressures will be limited to what can be put out by each stage. This is a function of the compressor A/R ratio and tip speed, and peaks at about at PR of 2.5 for a large number of turbos while operating at or above the 70% efficiency island.