Help with the theory of Compound Turbocharging?

[goatnipples2002]

I understand that this increases pressure at the expense of flow...does this matter much? From what I have read this is so you can achieve big PSI numbers out of smaller faster spooling turbos. So I guess big power at low AND higher rpms. Have I read wrong or is there more to this? Wouldn't this setup increase efficiency?

What are the differences between psi and cfm when it comes to producing power?

I have never heard of this until I was searching the other day. Seems good because I can get a pair of smaller turbos and run them at like 6 psi and end up with like 20ish psi.If it is how I think then that means an insane amount of torque down low and no restrictions in the top end.

http://en.wikipedia.org/wiki/Twin-turbo ... bocharging

Is there a theory/formula for the ending PSI? Is it just triple whatever the 1st turbo puts out?

[The Dark Side of Will]

Go read a compressor map. Some have CFM across the bottom, some have lb/min (either set of units makes some assumptions about the air the compressor is moving). Along the side they should have pressure ratio. This is the ratio of inlet pressure to outlet pressure.

In a staged turbo setup, the inlet pressure for the low pressure turbo is atmospheric. The inlet pressure for the high pressure turbo is the outlet pressure of the low pressure turbo (less intercooling losses).

So if you run both turbos at a pressure ratio of 2.0, you'll have 15 psi at the inlet of the low pressure turbo and 30 psi absolute (15 psi boost) at the outlet. This air will then go to the high pressure turbo and be compressed to 60 psi absolute (45 psi boost). The MASS flow stays the same at every point in the system, but the CFM drops as pressure goes up.

Staged turbo systems are bulky and heavy. They require TWO intercoolers and twice the plumbing of a single stage turbo system. They're only really useful on applications where space is easy to come by and weight barely matters... Big trucks, ships, stationary powerplants, etc.

[teamlseep13]

Will is right, they are only used in applications where weight doesn't matter, the use of one turbo isn't enough and you can support the large amount of cylinder pressure/heat that accompanies large pressure ratios.

Only way I would want to use a compound turbocharging system is if I had less than 2L of displacement, was able to use a fuel with an octane above 100 and I needed to make over 500hp.

You basically mutiply the pressure ratios together to get the final pressure ratio in absolute pressure.

[bryson]

I've been looking at doing sequential turbocharging something like this:




Except that I might have my "main" wastegate (the one that vents to the atmosphere) before the collector splits. I have always ruled out sequential setups because I couldn't think of a way to make a decent "flapper valve." I thought about using a diesel exhaust brake, but for some reason I never considered a wastegate. I think using a pair of small turbochargers (maybe two mitsu 16g or 14b turbos) would be slick. Actuate the first internal wastegate of the primary turbo at 7psi or so to "pre-spool" the larger turbo, then open the intermediate wastegate at around 9psi to let full flow to the secondary turbo. Then, at 14psi (or whatever you plan on running), open the main wastegate just like normal.

The turbos in that focus are really large, but I think this could be a pretty slick setup without a whole lot of extra weight/cost if you planned it well. I have been looking into it because I plan on building up the 1.6L 20v in my Corolla. I wouldn't use it to get big hp numbers, but so that I could get the power I want at a lower boost. Also, using 14b turbos would be pretty cheap -- you could rebuild one and have a total of about 150$ in each turbo. I would do several things different than in that picture, but I liked the idea of using a wastegate to actuate the second turbo, and I think that the idea of using the internal wastegate from the first turbo to help "pre-spool" the second would work well.

[The Dark Side of Will]

sequential =/= staged



WTF kind of car is in that picture?

[bryson]

What's the difference between sequential and staged? Is staging similar to compounding? I know that what I was describing is different from compound turbocharging, but I don't know what staged (or the real differential of sequential) means. The car in the picture is a focus, but I think that type of turbocharging would work much better with a longitudinal engine setup, which is why I'm considering it for the Corolla and not so much for the Fiero. Some of the other pictures I've seen were on some crazy volvo (C30?).

[The Dark Side of Will]

Staged = Compound (outlet of one turbo goes to inlet of next).

I haven't heard it called compound turbocharging until this thread came up.

The problem with using a wastegate in a sequential turbo setup is that it makes the transition from small turbo to large turbo a function of BOOST rather than a function of airflow (engine RPM), which is what it should be.

I'm rather enamored of the concept of twin sequential turbocharging, which is simpler than asymmetric sequential (depicted above) in that it uses two identical turbos, one at low airflow, both at high airflow. There's no need to protect the first turbo from higher levels of pressure than it would generate on its own.

[bryson]

I agree -- I was weighing my options between two 14b turbochargers and a single 16g. I could make about 350 crank hp with either, but it would probably take close to 18psi. Based on stock output of a DSM, I'm guessing that it would take less than 10psi with two 14b turbos, and spool would probably be similar. The 16g is a larger turbocharger, but it builds more boost at a lower impeller speed. I've heard that these will actually build low boost faster because of this, but I'm not sure. I don't know how accurate that is, but it doesn't matter too much. I figure that I could pay $50 each for the 14b turbos, then about $100 each for the rebuild kit. That, plus about $100 for an ebay wastegate, which I can't imagine being significantly worse than a quality wastegate, and I'm at $400, which is less than a new 16g and about the same as a used one (which I would want to rebuild anyway). So, I'm not looking at too much more expense to do the twin setup. The extra exhaust and intake piping, as well as the coolant/oil lines would add to the time spend designing and fabricating the system, but wouldn't cost significantly more than for a single turbocharger.

As far as the transistion being based on boost, isn't that how many sequential setups are? I think that the RX7's twin setup works that way. I'm not sure about the Supra's. What are the advantages to basing the transition from airflow? It seems that manifold pressure (not compressor outlet pressure) would be a pretty good approximation of the engine's airflow. What would be the problem with having boost control the transition? I don't think that the engine would flow more than the first turbocharger can support. At least, not before the turbocharger builds enough boost to make the transition, which should be ~2800rpm. Also, if the engine doesn't injest enough to keep up with the turbocharger, then intake manifold pressure will increase and the wastegate would open, allowing the second turbocharger to supply air to the engine.

I never thought about the risk to the turbocharger on an asymmetric setup. I was only planning on using the twin setup to simplify things and keep costs down. I had considered a 14b/16g setup, but it would probably increase the cost of the sequential setup enough to where it wouldn't be worth it.

[goatnipples2002]

The only reason i refered to it as compound charging was because that's what it said in a couple encyclopedia sites. My brother has a warmed over 350 in his fiero and another friend has a 3000gt vr4 and he runs about a second and a half faster at the track and that is with one of his turbos going out. They come stock with twin 13Cs on a 3L motor.

So for down low and high end would twin turbos be better?

[The Dark Side of Will]

The idea behind twin turbo setups is to make the engine more responsive, not more powerful. The pair of turbos has a smaller MOI than a single turbo of equivalent airflow capacity, so a given amount of exhaust energy will spin them up sooner and quicker than it will a single large turbo.

As far as the transistion being based on boost, isn't that how many sequential setups are? I think that the RX7's twin setup works that way. I'm not sure about the Supra's. What are the advantages to basing the transition from airflow? It seems that manifold pressure (not compressor outlet pressure) would be a pretty good approximation of the engine's airflow. What would be the problem with having boost control the transition? I don't think that the engine would flow more than the first turbocharger can support. At least, not before the turbocharger builds enough boost to make the transition, which should be ~2800rpm. Also, if the engine doesn't injest enough to keep up with the turbocharger, then intake manifold pressure will increase and the wastegate would open, allowing the second turbocharger to supply air to the engine.

I thought you had a good grasp on the idea behind a sequential system, but maybe not.

The idea of using turbos sequentially is to broaden the engine's powerband. A large single turbo can make X psi of boost and supply Y CFM, but spools at 4K RPM. A twin turbo setup with each having the same efficiency as the large one, but at X psi and Y/2 CFM might spool at 3K RPM. A sequential setup in which all of the engine's exhaust gas is pushed through 1 turbo at low engine speed might provide full boost by 2K RPM, since it's being pushed by twice as much energy relative to its size as either the single large turbo OR either of the straight twins.

So when a sequential turbo setup spools, the small turbo by itself hits set boost at 2K RPM. This means that by itself, this turbo would be severely undersized for the engine and might be done by 4K RPM. By bringing in parallel a second turbo at 4K, the airflow demand on each is half and together they can supply the engine all the way to 8K RPM. Notice that here's no change in boost when the 2nd turbo is brought in.

Remember that all this fancy technology is to idealize the bahaviour of the engine. What does an ideal engine do? It ingests the same amount of air and produces the same amount of torque at ANY operating speed.

Making the transition from single to twin or from small to large based on boost is thus inadviseable.

So airflow is the determining factor when switching from single turbo operation to twin turbo operation. Pure RPM isn't a good determining factor either, as the engine can be at high RPM and light load. In that case, it would only want one turbo in operation for fastest spool and best response and only bring the second turbo in when the throttle was open far enough to demand enough airflow to require it.

The algorithm is pretty complicated, but you can probably simplify it by using the product of MAP and RPM as the independent variable for mapping wastegate control for the secondary turbo.

Sequential turbo operation also obviously results in uneven turbo use and uneven turbo wear. When the primary turbo wears out, just put the old secondary turbo in the primary location and put a new turbo in the old secondary location.

[bryson]

I may be missing crucial facts of sequential turbocharging :scratch: I really haven't dealt with it before other than taking apart the system on a FD RX7, which may be vastly different than many other systems out there.

I was designing the system based on the assumption that the single turbo wouldn't be significantly out of it's efficiency range at moderate boost levels for any realistic engine speed. The system that I had in mind would retain spool characteristics of a stock car while providing the benefits of a larger turbocharger once the engine is flowing enough air to keep them both spooled. My motivation is to make the engine more responsive.

For example, say I have a T2 and a T4 (made-up turbos, t4 flows twice as much as t2). If the T4 makes 300hp at 15psi, and the T2 will make 150hp, then I could use the T2's in a sequential setup and get the same power at the same boost with better response. However, there is a middle ground. If I run twin T3 turbochargers, I will make my desired power at less boost than the twin T2's, and the T3 will still spool faster than the T4 would. I guess that's the best compromise -- in my case, a 14b would build boost quickly without running out of steam until much higher RPM, and two would satisfy my power goals without running a lot of boost.

I think that I was just having trouble explaining my plans for the system, because I agree with everything you said and I don't think that I've missed any of the points. My error might be in assuming that intake manifold pressure is indicative of airflow. I know how the turbochargers will supply the airflow, but in what situation would boost-controlled operation be detrimental? If the turbochargers flow more air than is needed, then the main wastegate will control the flow. If they don't flow enough air, then manifold pressure will drop and cause only one turbocharger to supply air. It seems that controlling the transition based on boost would effectively do the same thing as controlling it on airflow because manifold pressure is a result of the difference between the air produced by the turbos and the air injested by the engine. If one exceeds the other, boost will rise or fall and the actuator for the second turbocharger will act accordingly.

[goatnipples2002]

So are two turbos better than 1 for drag racing and top end racing. I found a site that sells used turbos for pretty cheap, but not sure which ones to get. I thought about running 1 massive T4 off a diesel and run nitrous so it would spool faster so I could run 10-15psi at a cooler temp and more flow, but then I started reading on twins and got to wondering.....this is all theoretical because I need to build my motor 1st, but never hurts to plan ahead.

http://www.spooledmotorsports.com/
click on the turbo top left

[The Dark Side of Will]

There are SEVERAL topics in this section about proper selection of turbochargers. Search.

[The Dark Side of Will]

My error might be in assuming that intake manifold pressure is indicative of airflow. I know how the turbochargers will supply the airflow, but in what situation would boost-controlled operation be detrimental? If the turbochargers flow more air than is needed, then the main wastegate will control the flow. If they don't flow enough air, then manifold pressure will drop and cause only one turbocharger to supply air. It seems that controlling the transition based on boost would effectively do the same thing as controlling it on airflow because manifold pressure is a result of the difference between the air produced by the turbos and the air injested by the engine. If one exceeds the other, boost will rise or fall and the actuator for the second turbocharger will act accordingly.

At a given RPM, MAP is indicative of airflow. But 30 psi MAP at 2K RPM is half the airflow of 30 psi MAP at 4K RPM.

So what you want to do is spool a single turbo when you kick the throttle, then once that turbo supplies full boost, transfer the load to split evenly between the two turbos, using previously wastegated gasses to spool the 2nd turbo so that there's no undue load on the engine. When the throttle snaps closed, both spool down and when you get back on it, you spool one, then the other as before. That's an interesting idea, but the problem I see is that if you're in a higher speed corner (significant, but not maximum, power required) at the upper end of your RPM range for whatever gear you're in, you may want more airflow at partial boost than the single turbo can supply.

[bryson]

That's an interesting idea, but the problem I see is that if you're in a higher speed corner (significant, but not maximum, power required) at the upper end of your RPM range for whatever gear you're in, you may want more airflow at partial boost than the single turbo can supply.

That's a good point -- I was trying to think of a scenario in which boost actuated transition would be a problem, but I didn't even think about partial throttle operation. Also, if the throttle is suddenly opened at a high RPM, the engine may consume more air than the first turbo can provide, and full boost won't ever be seen and the second turbocharger won't ever be actuated. Can you think of a way to actuate the second turbocharger based off of airflow mechanically? How about measuring vacuum before the turbocharger inlet? If both turbochargers drew from a common inlet, would pressure at that inlet correspond well to airflow? Maybe operating the second turbocharger at a set vacuum would be as effective as using a solenoid controlled by the ECU.

[The Dark Side of Will]

If you want to get real ghetto about it, put a frequency output MAF in front of the throttle and use it to operate an RPM switch that controls a solenoid valve that regulates the secondary turbo's wastegate signal.

Yeah, if the primary turbo is significantly undersized for the engine (which it would be... that's kinda the point), then going to WOT at redline (or above whatever RPM at which the single can no longer sustain full boost) will mean the secondary never comes on. Of course you can then use an RPM switch in parallel with your pressure switch to ALWAYS spool both turbos above a certain RPM...

But at that point you're adding a lot of switches that the ECM doesn't "know" about. It would almost be as easy to write a routine to control the setup using some extra space in your ECM's program.