Why run series turbos?

[Skaterkid]

Ok, so I was sitting in my fluid dynamics class today, trying to learn something, when we went into a discussion of series and parallel pumps. Now I love learning about pumps because its directly applicable to turbos, and the more I know about them, the happier I am (I'm a sick fuck like that).

So when you run parallel turbos, you gain flow, and when you run series turbos you gain head. So that comes to my point, why run a series turbo? Am I wrong in thinking that flow is the most important part of a turbo, that is, it is how you make more power, is throwing more air into an engine. That would make series turbos useless, since by conservation of mass (whatever one turbo flows, the other has to flow too) you can only flow what the smaller turbo would provide, so there would be no point to buy two turbos and run the complexity of trying to hook them up. The only reason I can think of why you'd want to do this is that head is a measure of energy, so the air that flows out of these two turbos will have more energy in it (I think), but does that do anything for the combustion process?

Basically, this whole rant comes down to, why would diesel engine compaines want series turbos? Why would you want to run 120 psi of boost into a deisel engine, when you could get the same flow from one turbo? Does the fact that deisel engines are based on controlled detonation get more power from higher psi and head (assuming the same flow)?

I busted out the SCC that had the article on series turbos to try and gain an answer, but it just left me with more questions. I'm probably making a poor assumption somewhere because I still haven't taken enough thermo yet.

Also, I'm assuming that for some reason you want to run a twin turbo system with this rant. I know singles are better for efficiency.

[stu]

What is head? Is that like pressure or something?

[Skaterkid]

Head is essentially how high a pump can pump a medium (water, air). Its a measure of energy cause if you pump up water 150ft, it has potential energy from being lifted that high. And in order to lift that medium that high, you have to put the same amount of energy into it as it has at the top. (Sorry I suck at explaining this, I'm a terrible teacher).

[Weston-work]

Why wouldn't you want to gain head? :D

[stu]

I know what you mean. you can't lift something until you can overcome it's weight plus it's potential energy kind of thing. That's not quite right, but I know what you mean.

[STIBungy]

Would you want to gain head from that fan of yours we saw at PFI? Didnt think so.

Why wouldn't you want to gain head? :D

[MirageZ31]

I'm ready to twin turbo my CRX now :rofl:

[Talus]

There could be a difference because with pumps you are talking liquid and turbos deal with air and air is compressable whereas a liquid is not. Was this dealing with a centrifugal pump? that would be most like a turbo.
I know with centrigugal pumps, you can run in series and the pressure from the first one is added to the second.

[Skaterkid]

The only pumps we cover are centrifugal. And all pumps have to abide by the conservation of mass though. What one pump flows, the other has to too. It just can't create flow where there is none. There might be something to do with air, and I'd like to ask my professor, but I think that the applications towards cars, he might not be too knowledgable about.

[myshtern]

Dont they do series for spool times?
More low end power?

[Skaterkid]

Any twin turbo system on a car today is a parallel system. And as far as I know only Semi's, planes and tractor pull tractors have them.

[rmcdaniels]

I've never seen a series turbo setup. I've seen sequential setups that take advantage of different compressor properties at different RPM's, but not identical turbos in series. In pumps it makes perfect sense, in the Navy we used to have to put pumps in series if we had to pump a significant vertical distance, like from the ocean to the upper decks.

[discopotato03]

Twin sequential turbos do exist but are not common on petrol road car's . The three examples that come to mind are some JDM Subaru Liberty's , B4's and some Mazda RX7's . The aim was to have the primary turbo small to spool up quickly and the second turbo to take up where the primary left off . Where it falls down is switching the exhaust over to excite the second turbo and bring it on line seamlessly .
Later single turbocharger technology works better on 4 cyl engines and the gradual increase in capacity helps as well . Probably the best known example of twins in parallel is the RB26 GTR Skyline engine . These were a homologation Group A engine and made for compact effective exhaust manifolding and low rotational innertia (ceramic turbine Garrett T28's) .The twins also had individual integral waste gates so they had good air flowing and exhaust flowing ability . 280hp was not too shabby from 2570cc in 1989 and in Group A the GTR Skyline with other technical attributes anihilated all comers .
Where the trucking industry is going is high pressure ratios (boost) . Making torque is what Diesels are good at and high pressure turbocharging can develop very high torque . Compared to older diesels todays trucks can have smaller lighter cleaner and more fuel efficient engines because of tighter electronic controls and high pressure turbocharging .

[Skaterkid]

I've only seen it on tractor pullers, and I just can't figure out why the trucking industry wants such high pressures (except for a series setup can provide big pressure at low shaft speeds) when a larger turbo could provide the same flow at a lower pressure and mild shaft speed.

[doctorstupid]

I think where you're getting confused is flow vs. pressure, which are two very different parts of a compressor's performance, although combined with effeciency they make up the overall compressor performance. You need enough air flow from the compressor to fill the engine, too much or too little flow and you can't form usable boost. As we know, boost pressure is how we make a 2.0L engine output comparable power to that of a larger displacement engine. Turbochargers increase pressure by multiplication, i.e. the pressure ratio. If turbo #1 in a series setup has a pressure ratio of 5.0:1 and at thatratio flows a max of 1,000cfm, and we assume atmospheric pressure to be 14psi, we would see 70psi (absolute) from turbo 1. Now as turbo 2 intakes that air and compresses it further with a pressure ratio of 3.0:1, we have 210psi absolute with air flow of 333cfm (give or take to account for effeciency in the compressors) since three times the pressure = one third the volume. If you're only revving the engine to 1,500rpm (large diesel), you don't need tons of flow compared to the boost pressure you could acheive in a series turbo setup. Of course I made these numbers up, but you get the idea :)

Make sense to anyone other than me? In short, you're worried too much about flow whereas pressure is what they're going for.

[chrisbarnett01]

Dr. Stupid is correct. A turbocharger will compress whatever you put into it.

Pressure ratios are all that matter when you talk turbochargers and boost level.

Let's say you have a turbo that is a 2:1 pressure ratio. Sea level psi is 14.7, or 1 bar. With a 2:1 pressure ratio, you have 2 bar, or 29.4 psi.

Let's say you have four 2:1 turbochargers in series. Atmospheric pressure into the first turbocharger, so 14.7 psi. Going into the second turbocharger, you have 29.4 psi. Going into the third turbocharger, 58.8 psi. Into the fourth turbocharger, 117.6 psi.

Coming out of the fourth turbo, into the intake manifold, you have 235.2 psi.

Ever watch tractor pulls? They frequently have 3 and 4 turbo diesel motors, with all of the turbochargers in series. 150+ psi boost is not uncommon.

To get the same amount of boost with a single turbocharger, well, would be more or less impossible.

[doctorstupid]

Dr. Stupid is correct. A turbocharger will compress whatever you put into it.

Pressure ratios are all that matter when you talk turbochargers and boost level.

Let's say you have a turbo that is a 2:1 pressure ratio. Sea level psi is 14.7, or 1 bar. With a 2:1 pressure ratio, you have 2 bar, or 29.4 psi.

Let's say you have four 2:1 turbochargers in series. Atmospheric pressure into the first turbocharger, so 14.7 psi. Going into the second turbocharger, you have 29.4 psi. Going into the third turbocharger, 58.8 psi. Into the fourth turbocharger, 117.6 psi.

Coming out of the fourth turbo, into the intake manifold, you have 235.2 psi.

Ever watch tractor pulls? They frequently have 3 and 4 turbo diesel motors, with all of the turbochargers in series. 150+ psi boost is not uncommon.

To get the same amount of boost with a single turbocharger, well, would be more or less impossible.
Not only near impossible, but the air flow out of a turbo that large would vastly outflow an engine that barely revs above what we would call idle speed, which would do exactly the opposite of what these people are trying to do since they wouldn't really form any usable boost in the manifold.

And I think it merits mention that with a 2:1 PR at sea level you have 29.4psi absolute pressure, but 14.7PSIg on te gauge. What you see on the gauge is PR times atmospheric pressure minus the incoming air pressure, which in a multiple series turbo setup is more math than I care to do to determine the net pressure in the manifold.

In contrast, a high revving engine breathes a lot of air up top, and we need a turbo(s) to meet that engine demand while supplying usable and non-destructive boost levels. Air flow is definately a bit overrated in general when it comes to general turbo discussions, far too many people think that flow is what's giving them power, when in fact flow is what's supporting the boost pressure which is what really nets power.

[discopotato03]

Um , my opinion only but I would state that flow is what makes it happen . Its possible to have a lot of boost and little flow eg too small a compressor but its difficult to have too much flow . The whole aim is too feed our humble piston pump with as great a weight of oxygen as possible in order to burn more fuel to develop more horsepower .
With turbocharging high boost pressures normally mean high charge temperatures so high flow at lower temperatures means higher charge density . This is why larger turbos do it better at low boost than smaller turbos at high boost . Where it all unravels is having an engine with a decent power range and some boost in the first quarter/half of the rev range . Most big Diesels have no waste gate so the turbo is effectivly free floating . They may have a useful power spread over 1000 rpm and useful boost for the last 700 of it . Petrol engines are in a different universe , they burn fuel with a much lower flash point and are throttled on the inlet side . With Diesels detonation is not an issue , if they run lean or high charge temps threy merely loose power . Petrol engines detonate themselves to death in no uncertain terms with lean mixtures or high charge temperatures . More later dad's on duty !

[doctorstupid]

I think I wasn't clear enough and mispoke a bit, but in my defense I did type that at 3:17am ;)

Here's an example I hope can clarify what I'm trying to say. Let's pretend we have an engine that can intake a mximum of 100cfm at redline. Now let's buy a turbo for it that flows a minimum 200cfm at 2.0PR (14.7PSIg, assuming sea level) before it falls to its stall speed. If you're familiar with aircraft, you know that stall speeed is where the wings (or compressor blades in this case) lose their "grip" on the air because it's moving too slow through the air and lift is lost. We're going to assume a few other things here for the sake of discussion, such as 100% adiabatic pumping effeciency and 100% engine volumetric effeciency, since we aren't debating the most effecient turbo set ups or engine design, but flow vs. pressure. This turbo would not be capable of forming usable boost in the manifold since the engine is not capable of intaking 200cfm, you can't force extra volume into the engine. We can however, force extra air mass into te engine by compressing it. Lesson learned? Our turbo is too large for our hypothetical engine, and performance will suffer greatly despite the fact the large turbo is realistically more efficient than a smaller unit.

Now back to the orignal discussion, if this is a diesel engine and we're looking to haul some heavy ass trailor loaded with fat bitches down a mud track, we need to make usable boost or else we'll lose and be subject to ridicule. By installing a second turbocharger comrpessor inline with the first, we can again double our pressure and output the correct volume of air for the engine. Again we're assuming 100% adiabaitc effeciency and all that, so as we double pressure we halve the volume air flow. The second turbo is intaking 29.6 absolute PSI (14.7PSIg), though as was pointed out with fluid pumps it cannot create flow that isn't there, i.e. it cannot flow more air volume than the first turbo without depressurizing the incoming air (the exact opposite of what we're going for). So since we're assuming the turbo is 100% effecient, it could output 14.7PSIg @ 200cfm minimum, but if we spin this sucker up and run it at 2.0PR we would see 29.6PSIg, or 40.1PSI absolute, at an airflow of 100cfm (2x pressure = 1/2 volume). Now this our engine can use, it's getting bent over and gang banged with three times the air mass it would intake on its own, thus three times the fuel and effectively three times the displacement. And the end result is three times the fat bitch hauling fun.

Now there are of course downsides to a series setup, of course, unlike our hypothetical engine we not only have the volumetric effeciency of the engine to worry about but now the effeciency of two turbochargers to worry about. As we know, each turbo will increase the air charge's temperature, thus decreasing it's density. Fortunately for us, there exist intercoolers, which make up for much of the turbos' effeciency (or lack there of). So while a series setup isn't the most effecient or even cost effective, it is a very good way of stuffing more air into an engine with limited volume air flow capabilites. Getting that type of pressure out of a single turbocharger while flowing very little is, well, impossible.

And yes, flow is what makes it happen, but no more so than air pressure. Divided, flow and pressure are useless, but as I said, combined with other variable of a turbocharger they make up the overall compressor performance. All I was really trying to say is that too much credit is given to flow when it's just a small piece of a much larger puzzle. And I am well aware I'm leaving a lot of variable out of these examples, but who really wants to sit here and nitpick?

And if my math was off in the above example, get over it :p

[doctorstupid]

Holy fucking hell I typed a lot, when you do it all in the quick reply box you lose perspective on length :p