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.

 

[MechanicalChef]

I'm not aware of any system quite like that. I suppose you can do anything. What benefits do you think could be realized that would make this system superior to typical parallel/sequential systems. I'm not familiar with these elctronic proportioning valves, but it seems like feeding a large turbo using wastegate pressure would subject yourself to unnecessary flow-restrictions.

 

[TDIMeister]

BMW does exactly this with their biturbo Diesel engines (286 PS in the 3L six; 204 PS in the 2L four). Several other OEMs are developing biturbo Diesels based on the same principle. Borg Warner Turbo Systems supplies the BMW units and did a lot of the development work; their terminology is "R2S" (Regulated 2-Stage) turbocharging.

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

 

[TDIMeister]

Note to the above: the graphic that I linked to indeed calls the turbine routing device a wastegate. However, in other technical papers I have read on the subject, it is called a bypass. There is still a wastegate on the second-stage turbo.

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.greencarcongress.com/2007/12/borgwarner-comb.html

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.

 

[Lou Scannon]

TDImeister, you have correctly defined the term Turbocompound.
I don't see any problem with calling turbochargers in series "compound" or "compounded" turbos.

 

[Silverbullet86]

http://www.whnet.com/4x4/pix/R2S.jpg

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.

What benefits do you think could be realized that would make this system superior to typical parallel/sequential systems.

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.

 

[BrianPetersen]

I'm not 100% sure but I think BMW's system sends ALL of the downstream flow of the inner small turbo (and its bypass or wastegate or whatever you call it) to the inlet of the outer big turbo (and its wastegate). If all of the bypasses and wastegates are closed, all of the exhaust flow goes through first the inner small turbine and then the outer big turbine.

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?

 

[MikeHalloran]

Turbocompound originally referred to a centrifugal desupercharger mechanically connected to the crank of a piston engine... I think. There was a precise taxonomy to it all until turbochargers hit the mass market.

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

 

[Schatz65]

The VGT turbochargers now used on diesel engines offer excellent performance over a wide operating range, in terms of power output and RPM, specifically.

Perhaps you'd consider designing an independent control methodology that would be external to the CAN-BUS scheme?

 

[TDIMeister]

I don't see any problem with calling turbochargers in series "compound" or "compounded" turbos.

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.

 

[Lou Scannon]

Well, we're just getting into semantics now. In English, compound turbocharging is the analagous to compound interest and I think the meaning is pretty clear.

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?

 

[TDIMeister]

I'm quite aware about the metaphor of compound interest. I'm just relaying an industry terminology that specifically refers to anything related to "compound" as having a turbine with a direct mechanical coupling to the crankshaft.

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.

 

[BrianPetersen]

The system used by BMW (and soon Mercedes and possibly others) on diesel engines is indeed a "staged" system as described. At low revs, all the flow goes through both turbos, although the big outer turbo doesn't do much. At moderate revs, the bypass around the inner small turbo starts opening and the big outer turbo starts doing something. At high revs, the small inner turbo is completely bypassed on both the exhaust and intake sides, and the big outer turbo is supplying all the boost pressure.

 

[MikeHalloran]

Detroit Diesel and MTU have used 'staged' turbos on their V-type medium speed marine Diesels for some years. There are two similar turbos, one on each bank, cross- connected, more or less in parallel. One is shut off by valves in both hot and cold sides below ~1400 rpm. Both are active at higher speeds.

When the second turbo kicks in, the boat jumps, and the fuel flow meter goes to 'unbelievable'.



Mike Halloran
Pembroke Pines, FL, USA

 

[Silverbullet86]

'm not 100% sure but I think BMW's system sends ALL of the downstream flow of the inner small turbo (and its bypass or wastegate or whatever you call it) to the inlet of the outer big turbo (and its wastegate). If all of the bypasses and wastegates are closed, all of the exhaust flow goes through first the inner small turbine and then the outer big turbine.

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?

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.

 

[BrianPetersen]

You're forgetting that at higher revs, the inner small turbo is completely bypassed in the staged system that BMW/Mercedes (actually BorgWarner) is using. The bypass around the small turbo has MUCH higher flow capacity than the turbo itself. At higher revs, it operates as a single-turbo engine with a big single turbocharger.

 

[Warpspeed]

The suggested system has indeed been used before, as previous posters have indicated.

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.

 

[TDIMeister]

"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."
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://forums.tdiclub.com/showpost.php?p=2059848&postcount=58

http://forums.tdiclub.com/showthread.php?t=208045

 

[globi5]

Btw, Ford and PSA launched a diesel engine with a sequential turbo in 2005 already.

http://www.greencarcongress.com/2005/10/ford_and_psa_la.html

 

[TDIMeister]

But not in production. Opel/Vauxhaul had already shown a staged-biturbo Diesel in the prototype Eco-Speedster at the 2002 Paris Auto Show.