Anyone seen this type of turbo system?
Anyone seen this type of turbo system?
(OP)
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.
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.





RE: Anyone seen this type of turbo system?
RE: Anyone seen this type of turbo system?
Only their terminology is different; what you call the wastegate on the small turbo BMW calls a bypass.
http://www.whnet.com/4x4/pix/R2S.jpg
RE: Anyone seen this type of turbo system?
International now has a new Powerstroke with 2 turbochargers in a staged-arrangement; however, at least one of the turbos is a variable geometry type.
http://www
P.S.: Many people erroneously refer to these type of turbocharger arrangements as "compounds". They are nothing of the sort. By definition, turbo-compounds pass the exhaust gases though a turbine, that is true, but the shaft power from the turbo is coupled directly to the crankshaft through reduction gears and supplements the crank brake work.
RE: Anyone seen this type of turbo system?
I don't see any problem with calling turbochargers in series "compound" or "compounded" turbos.
RE: Anyone seen this type of turbo system?
damn it. And i thought i was a genius lol. I guess it was too simple of a concept to have been not thought of.
I think this system is superior over the sequential setups RX-7s and Supras have from the factory because it can be an easy bolt on product for aftermarket performance. OEM cars with sequential setups have programs integrated into the ECU. Perhaps im just ignorant in electronics, but I find it a hassle to have to splice additional piggy back boxes into the ECU harness to make an aftermarket sequential set up to work (hypothetically if there were such products). With this, it would be a straight bolt on with some air-fuel and timing tuning.
And the advantage of having sequential turbos is to reduce turbo lag since the smaller turbo would spool quickly in lower RPM or daily driving, and the larger turbo would spool for high HP numbers at higher RPMs.
RE: Anyone seen this type of turbo system?
I do not know why you would want to send only the "bypass" flow through the big turbo. I suppose it could work, but it would seem to me that the big turbo would be completely stopped and its compressor would be just sitting there being a flow restriction. Sending everything through both (except where bypassed or wastegated) keeps them both spinning. Less lag?
RE: Anyone seen this type of turbo system?
Compounded turbos are possible, and it's been done:
I worked on a proposal for a boat that was to use a pair of MAN B&W Paxman 18VP185 Diesels, 5.27l/cylinder, 18 cylinders, 4000kW @ 1950 rpm.
The engines have nine turbos each, compounded, three working at low pressures, six working at higher pressures.
My part was the boat's exhaust system, which was relatively simple because there were only three exhaust flanges per engine, at the low pressure turbos.
It was intended to be a fast boat; think Cigarette, but more than twice as big.
Mike Halloran
Pembroke Pines, FL, USA
RE: Anyone seen this type of turbo system?
Perhaps you'd consider designing an independent control methodology that would be external to the CAN-BUS scheme?
RE: Anyone seen this type of turbo system?
I do, because they are two different and specific things. "Compound" from some texts refer to an arrangement that has a turbine coupled directly to the crankshaft. "Turbo-compound" has a conventional turbocharger upstream of the turbine proving shaft work. Confused yet? :)
The proper terminology should be "staged" turbocharging.
RE: Anyone seen this type of turbo system?
To me, using the term "staged" in the case of turbochargers has ambiguity, because does "staged" mean they're in series, or because the first stage is used for low flow rates and the flow is progressively diverted to the second stage for higher flow rates; i.e. a "sequential" setup like the OP?
RE: Anyone seen this type of turbo system?
The BMW setup is not a pure "sequential" setup, nor is it strictly a series arrangement. At the bottom load- and RPM range, the small turbo is doing most of the boosting work; the large turbo just goes along for the ride. At the top-end, the roles are reversed, and the small-turbo is just freewheeling from the small pressure drop across turbine as most of the flow is being bypassed. Only in a limited operating range between both extremes are both compressors working in tandem, that is to say, in series. The combination of bypass valve and wastegate in the turbine side of both turbos can effectively make the flow either in series or in parallel.
RE: Anyone seen this type of turbo system?
RE: Anyone seen this type of turbo system?
When the second turbo kicks in, the boat jumps, and the fuel flow meter goes to 'unbelievable'.
Mike Halloran
Pembroke Pines, FL, USA
RE: Anyone seen this type of turbo system?
i see a problem with that. Parallel twin turbos work like that, but not sequential. The purpose is to have a larger turbo and a smaller turbo. The amount of exhaust flow to spool the larger turbo in its efficiency range to make big HP numbers would DESTROY the smaller turbo if it had to go through the smaller turbo first.
I have a Nissan track car with a 2.0L turboed engine. It is a fairly small GT25 turbo that can make about 270HP at the crank. I can hit full boost around only 2500RPM. The amount of exhaust flow at 2500RPM isn't a whole lot compared the the amount at 4500RPM which is where my peak torque happens. So baring in mind that anything after 2500RPM, the turbo has enough flow and the rest of the exhaust gas is being dumped through the waste gate. So why waste that energy? By using that dumped gas, you can decently spool a much larger turbo. With having two different sized turbos, the spool time would decrease compared to if you had just one large turbo.
RE: Anyone seen this type of turbo system?
RE: Anyone seen this type of turbo system?
There are two problems that I am aware of. The first is higher than ideal total exhaust back pressure. Both turbines will be effectively operating in series, and the total exhaust back pressure will be additive.
The second problem has to do with transient response. The two turbos will be vastly different in size, inertia and dynamic characteristics. A system like this would be more suitable for something that required a steady wide operating rpm range under full load, but where there was not a lot of very fast throttle openings and closings.
In practice, a much better and more responsive solution is to compound a large turbocharger with a positive displacement supercharger. You get the benefits of low rpm boost, high rpm airflow, and excellent transient throttle response.
RE: Anyone seen this type of turbo system?
Would be true if the BMW R2S system were a true series arrangement, which it's not. When the small turbo operates off its efficiency peak and begins to choke at higher demanded airflows, it is completely bypassed and the large turbo works alone at the highest RPM range. The total back pressure here is not NOT additive, although granted there could be a pressure drop across the exhaust bypass flap if not sufficiently dimensioned and doesn't have a clean flow path.
Büchi's Law of Turbocharging states that, choked flow excepted, the turbine pressure ratio in relation to the compressor pressure ratio is a function of the total turbocharger efficiency and the turbine inlet temperature.
"The second problem has to do with transient response."
Again, true if it were truly series. At low engine loads and speeds, only the small turbo is active. There is a slight pressure drop upstream of the small compressor inlet because it's sucking air through the large compressor, and also a higher backpressure as it's pushing exhaust gases through the turbine of the large turbo; at both points the large turbo is just freewheeling along and acting as a slight restriction. However, the small turbo responds very quickly at low speeds (dampened a little by the down- and upstream restrictions), and at higher speeds the control system regulates between a series arrangement and one where the small turbo is completely bypassed and the large turbo works alone. The rate of the transient response and the smoothness of the transition places the onus on the control system. I have papers that show the staged arrangements by BMW provides better transient response than the single-VNT setup.
The following may also be good reads into turbocharging math and matching series turbochargers:
http:
http://forums.tdiclub.com/showthread.php?t=208045
RE: Anyone seen this type of turbo system?
http://www
RE: Anyone seen this type of turbo system?
RE: Anyone seen this type of turbo system?
RE: Anyone seen this type of turbo system?