Sequential System - a discussion

[mikeb]

"...@ 1.1 bar both turbo's run 0.55bar boost on each beyond 4,000rpm. This is the most popular view at the moment in here."

 

This is the statement I have the biggest problem with here. The outputs of the two turbos are effectively joined together above 4000 rpm and so the pressure at the outputs of both the turbos will be the same (give or take some turbulence). The two turbos are not producing half the final *pressure* each, they are both working against the same pressure (the pressure in the output pipe). Its actually about air flow, what we should say is that each turbo is flowing half the required air mass per unit time, the final pressure is a result of the air being rammed into the output assembly by both turbos and is therefore higher when both are engaged as more air can be rammed into output pipe before the engine can use it, raising the equilibrium point of the final pressure in output pipe.

 

Spot on! I was just thinking the same. For anyone who understands electronics a simple analogy can be made with a pair of resistors. Imagine two resistors of equal reisistance, wired in parallel. The voltage drop across them (equiv. to pressure) is the total voltage drop. The current thorugh them (equiv. to flow) is half of the total flowing in the circuit. Reisistance is the electrical equivalent to the flow resistance of the turbos.

 

Both turbos are identical geometry, so both turbo's present the same resistance to the exhaust flow (ignoring any small pressure losses due to pipe work), the gas flow must divide 50/50.

 

You can't say each turbo makes .55 bar giving a total of 1.1, just like each resistor won't drop .55 volts to make the total 1.1V. Both turbo's must generate equal pressure (i.e. 1.1 bar). Each one will cause 50% of the flow, which is added to make the total flow.

[Markie]

Surely if thats the case then. When I am boosting at 1.3BAR, my first turbo is running at a 1.3BAR pressure? Thats not possible if the EGBV opens at around .7 .8 BAR.

 

If each is boosting at 1.1BAR and producing 50% each of the flow, then the total pressure is going to be (give or take a bit) 2.2BAR

 

What Ian says makes logical sence.

 

When the system is fully boosting both turbos there is no way of getting T2 to boost at a higher pressure, as the gasses exiting through the EGBV from T1 is then getting passed onto T2, therefore the same flow of gasses are reaching both the turbines.

 

If T2 had its own valve and this gate had a different pressure before it opened, THEN you would get your T2 running at a different pressure.

 

ie T1 boosting at 0.1BAR then EGBV opens at 0.1BAR exits gases hit another valve which opens at 0.5BAR then once it reaches that pressure the second one is online and producing 0.5BAR

 

Let me stress, I am still a newbi learning.. So if any of what I said makes sense, I will be a chuffed bunny.

 

Very interesting reading by the way guys.. Learing alot I think

[richardharmon]

The point is that the pressure on the high side of both turbo outlets will be approximately the same therefore you can't say that the turbos are contributing *pressure* x and *pressure* y, they are both working against the same pressure, the pressure in the outlet pipe (the pipe that then goes into the intercooler). You can say that each turbo *flows* a certain volume or mass of air per unit time and you can also say there may well be differences in the *flow rate* of each turbo. Even if you were to operate the turbos using separate waste gates then they would still be working against the same pressure (whatever that might be), just contributing more or less of the volume or mass of air flow (they could indeed have different *flow rates*). Of course, if the flow difference between the turbos is too great then one of the turbos (the slower) will stall. When both turbos are online the output pressure is controlled by the waste gate (on the no.1 turbo), not the bypass valve (as Ian pointed out). Finally, the pressure will not simply be the double of the value it is when only one turbo is online, primarily because the pressure is controlled by the waste gate and secondarily because the pumping efficiencies of the turbos fall off quickly beyond a given flow rate.

 

Edit: Nor will the pressure be the double of the pressure when both turbos are online; I am saying that no.1 and no.2 are working against the same high pressure (1.1 bar in the above example). Don’t misunderstand the difference between the output pressure being the same at both turbo outputs and the flow rate between the turbos, it is the flow that is divided between the two turbos, not the pressure, the pressure is *result* of trying to push air into the output assembly (and eventually the engine), the faster you can push the air through the higher the resultant pressure will be. Is should also be noted that the flow rate will be effected by the pressure generated at the output, like a feedback loop, so the higher the pressure gets the harder it will be to flow more air.

[tbourner]

Thanks Richard Paul and Mike, I've been thinking that since page 1 (wish I'd read this sooner!!).

 

The whole argument seems to have stemmed from differing opinions on air pressure and flow. The Chief is talking about air flow when it's nothing to do with that. Saying that the 2 flows meet in the Y so the lower 'pressure' produced by no1 meets the higher 'pressure' produced by no2........ that can't quite work because it's not pressure you're talking about it's air flow........ but that's still not right because the system can't really be described by the air flow it produces!!

 

 

 

 

Essentially if you forget that the engine is using the air, and just imagine that the whole inlet manifold is a box, then as the 2 turbos work their sequential system over the rev range the air PRESSURE in the box increases, there is no way to measure which turbo is supplying the pressure because they're both pointing into the box!! (I wouldn't want to buy your turbos after you've tried this magical measurement of the 1st turbo only BTW!!).

 

 

Also, read Hilliers Fundamentals of Motor Vehicle Technology. 2004 edition.

[Chiefgroover]

The Chief is talking about air flow when it's nothing to do with that. Saying that the 2 flows meet in the Y so the lower 'pressure' produced by no1 meets the higher 'pressure' produced by no2........ that can't quite work because it's not pressure you're talking about it's air flow

 

Tbourner - I suggest you read the above again, whatever it was you were meaning to say I dont think it came out right, as that contradicts itself. Trya again!.

 

Thanks Richard, you seem to have it pretty much sussed. I spoke at length with a very experienced motor engineer last night, who was only seeing the layout of the supra for the first time. I tried to make clear that my issue from the start has been - it is possible to have the system work with two slightly different flows from the turbo's, -which we have been unable to dismiss. Toyota most likely thought "what if one turbo is down on output?" allow for turbo wear, and designed the system so the car could still drive rather than die dog. Has anyone came across a Supra which had a slightly lame turbo and stoped the engine working above 4,000rpm?. I haven't, hence the point I have tried to make. A car with a worn low rpm turbo, down on output by about 20% that can still rev out is really all the proof I need to prove the point i was trying to make. Does that sound so unreasonable?

This discussion has come a long way, and most likely will continue for some time to come!.

[tbourner]

Why does it contradict itself?

 

The Chief is talking about air flow when it's nothing to do with that. Saying that the 2 flows meet in the Y so the lower 'pressure' produced by no1 meets the higher 'pressure' produced by no2........

 

This is what you say. You mean to describe airflow, but you use the word pressure.

 

that can't quite work because it's not pressure you're talking about it's air flow

 

This is what you mean. See.

 

 

Anyway enough of picking on my bad English.

Basically what I said still stands, you should be thinking of the pressure of air in the output of the turbos, not any flows that occur. You can't possibly split the air pressure in the single pipe from turbos to IC, and decide which turbo is producing which pressure - they're not seperate in any way over 4000 revs!!

[Chiefgroover]

they're not seperate in any way over 4000 revs!!

Only true post 'y' piece.

 

If you are unclear about what i mean, ask, dont dictate, your telling me what I am saying? I know what i am saying, I have said it so many times it unreal.

 

Take note "It is possible for the system to work with a slightly lesser output from the turbo that operates below 4,000rpm". The worn turbo clause proves this in itself.

 

Whatever may happen beyond the Y piece, thats another issue for which you may like to start a thread.

 

This thread went into debate mode after I had stated i believed one of my turbo's had run slightly less boost than the other due to the actuator on the higher RPM turbo have a bleed device fitted.

 

The "worn turbo" clause confirms that the engine will function in this state.

 

Now that satisfies me, I am happy enough, but if you guys want to debate forever go ahead, I will watch with interest and consider all opinions put forward.

[attilauk]

i would like to start by saying that i haven't got a clue how the switching on the sequential setup works but as a civil engineer i do know a fair bit about pressurised fluid flow through pipework systems. I can see exactly where Trev is coming from, you cannot have a flow of pressure, pressure causes the flow of air.

 

from what i understand Chief you are saying that (to use your terms) one turbo produces 0.8 bar and the other tops it up to the 1.1 bar as seen by your boost gauge.

 

if this were the case then there would be both a flow speed and pressure differential at the "Y" piece causing the higher pressure air to leak into the lower speed flow and a very turbulent flow. what i have just described is totally inacurate.

 

a faster flowing fluid is under less pressure than a slower one, this is called Bernoulli's principle and its what keeps moving planes in the sky.

 

so going back to the hydraulic principles of how the compressor in a turbo works, as the vanes spin in the housing they create a large pressure differential (due to the differing speeds of the air flow over the blades), Higher Pressure on the inlet side of the turbo and lower pressure on the outlet side. This large differential causes the air to flow through the turbo and on towards the "Y" piece at very high speed (and as such very low pressure) this flow is massively slowed by its passage through the twisting pipework, intercooler and finally its entrance into the engines cylinders. this massive reduction in speed forces the air molecules closer together and therefore increases the intake pressure, this is the pressure that you see on your boost gauge and is also the pressure that opens the Exhaust Gas Bypass Valve.

 

if, as you are suggesting, one turbo was producing a larger flow of air than the other then the slower, higher pressure, flow would be pushed into the higher speed, lower pressure, air flow, therefore changing its direction of travel and changing two relatively smooth (efficient) flows into one turbulent (very inefficient) one. a Turbulent fluid flow through a pipework system suffers much more resistance, and in the case of this type of air flow system the resistance would be seen as heat, giving higher inlet temperatures and therefore requiring a larger intercooler to keep things in check.

 

i find it very difficult to believe that toyota would spend that much money on designing a sequential turbo setup which would produce such an inefficient air flow.

[normore1]

Gents, I can't believe how long this argument has been going back and forth and the number of misconceptions put forth. Chief is obviously wrong . This is not meant to be a flame just a fact. Most of this is basic physics. Turbos take the kinetic energy contained in the exaust (flow velocity) and use it to increase the flow velocity of the intake air. Turbos technically do not generate pressure, they increase air flow velocity and thus volume. Pressure is generated when the flow is resticted (bernoulli effect) along with heat. Think of a garden hose: pressure is high behind the nozzle due to the restriction, open the nozzle and the pressure drops to zero in the atmosphere. BTW, pressure is not additive as someone suggested in a prior post (i.e. 1 bar + 1 bar = 1 bar not 2 bar). If you connect two tire valves togther with a hose and both tires are at 32 psi they will remain at 32 psi (as will pressure in the hose). They definitely do not suddenly increase to 64 psi.

 

So how does the sequential system work (best full description is at the USA MKIV FAQ site) http://www.mkiv.com/faq/faqtt.html#turbosystem1

but roughly: at about 3500-3700 RPM the system switches from single turbo (#1) to parallel. Essentially turbo 1 runs parallel with turbo 2 and the flow ouput of both is combined to feed the engine. Boost pressure is built because of the restriction through the plumbing (pipes and intercooler) and the volume of air being consumed by the pistons is less than what the turbos can supply. Close the intake manifold (throttle valve) and pressure increases (hence the need for a BOV). Above 4000 rpm both turbos see the same exhaust flow (input energy) and since both output sides have access to the same intake air source using the energy they boost the flow volume equally. As noted before pressure only builds as a function of restriction and inability of the engine to consume all of the air volume produced. Pressure is limited by reducing the input energy through use of an exhaust wastegate that bleeds off exhaust gas before the turbos intake impeller. Take out the wastegate and pressure will continue to build until either: the engine consumption matches volume produced (about 22 psi or 1.5 BAR for the UK Supra) at pressure or the turbo shaft twists off. Simple, eh!!!

[mikeb]

Take note "It is possible for the system to work with a slightly lesser output from the turbo that operates below 4,000rpm". The worn turbo clause proves this in itself.

 

The "worn turbo" clause confirms that the engine will function in this state.

 

Now that satisfies me, I am happy enough, but if you guys want to debate forever go ahead, I will watch with interest and consider all opinions put forward.

 

 

If one turbo is 'worn' more than the other, then it will present a slightly different resistance to the exhaust flow. It will still drop exactly the same pressure as the other turbo, but flow will be not be identical to the other one. The overall effect will be that combined pressure at the manifold will be slightly lower due to flow being slightly reduced.

 

In case you understand basic circuit theory, here is the electrical analogy again:

 

 

The voltage drop across both of the resistors is always 1.1v. If they are the same resistance then the current will divide equally. If they are different, for example R1 = 1 ohm, R2 = 2 ohm, then twice as much current will flow through R1 than R2. The voltage however, remains the same across both. It can't be different as each side of both resistors is connected together with wire. Just like both turbos are connected together with pipework.

 

I think everyone else on here agrees that when both turbos come online, the pressure across them is equal. There is nothing to debate, we are just trying to explain it to yourself.

[Ian C]

Jaysus, the big guns are out

 

I tried replying last night but a power cut put paid to that mid-type. All I have to add is a couple of quick notes - one, I wasn't factoring in system pressure drops/restrictions/losses as that would have complicated matters (too late!) and two, yeah, it's amount-of-air-flowed-per-second that really counts. I tried to get that across in my uber-long first post in that when the second turbo comes on song, the demand on it drops as there is another turbo there working away. The "0.55bar per turbo" is misleading (and plain wrong lol) when getting into the detailed view, I was trying to indicate that the demand drops in terminology that people would be OK with, without overcomplicating things by saying "actually, the pressure stays at 1.2bar but the turbo flows less air" haha So much for that!

 

So, when #2 comes on, to maintain the pressure in the system at 1.2bar, which the boost gauge reads, each turbo has to flow half as much air than the first turbo would if it was the only one online. If the flow is slightly imbalanced, it wouldn't have a perceptible effect. If it was a few psi out you'd get turbulence and a drop in overall pressure as system energy is being wasted by the air streams fighting amongst themselves. If the psi off one turbo was, I think, less than half that of the other, then flow reversal and stalling would occur.

 

-Ian

[tbourner]

BTW, pressure is not additive as someone suggested in a prior post (i.e. 1 bar + 1 bar = 1 bar not 2 bar).

 

 

I thought that too, at least I had the idea that it wouldn't be exactly additive and would be maybe a percentage of each turbo's pressure is shared, but didn't say for the same reason as Ian (also that I wouldn't be able to explain what I meant - as you can see from this paragraph!!).

[Chiefgroover]

The "0.55bar per turbo" is misleading (and plain wrong lol)

Could not agree more Ian, as you know.

 

here is the electrical analogy again:

Electric, air and liquid all have their own laws of flow. These are not revelant.

 

 

If you connect two tire valves togther with a hose and both tires are at 32 psi they will remain at 32 psi (as will pressure in the hose). They definitely do not suddenly increase to 64 psi.

I agree with this too.

 

Pressure is limited by reducing the input energy through use of an exhaust wastegate that bleeds off exhaust gas before the turbos intake impeller.

I agree, but you need to keep in mind one of my actuators has been fooled by a bleed device, hence a changed wastegate action

 

a faster flowing fluid is under less pressure than a slower one, this is called Bernoulli's principle and its what keeps moving planes in the sky.

I thought Bernoulli's principle had two different versions, one for non combustables, and one for combustables. We are also talking about air for which there is a slightly different theory, which i wish i could quote accurately, (cant remeber it that clearly) but maybe someone here has it fresh in their mind and call tell us. (or do i need to look it up? lol)

 

It really doesn't do not to toe the line of accepted theory, we have pages of theory posted here, and this has turned into a competiton to see who's theory gets shouted the loudest IMO.

On reflection for one to assume they are right (I include myself in this) when this question of whether combustable air flow can meet @ a Y piece and flow without problem occuring, is a theory based assumption, and not solid evidence in either direction. The only two things I need to clarify is this question, and the question of how the system manages to change the boost level of the first turbo (that has a spring acuator on it) to match the second one. I have had loads of theory, on how this happens but am still looking for the series of mechanical events that occur. I am well aware that both turbo's get equal feed from the engine, but i am also aware that in the particular setup to which we refer which is not stock, so conditions are slightly different.

I am tired of reading theorys, the only way forward in this is practical scientific measurement of these two points, buy an experienced engineer in this field. I cant guarantee i am right, but nor can you until such a thing is tested. So in the interests of not confusing the issue further, practical testing must begin.

[tbourner]

Is it possible to measure the rpm of a turbo fairly easily (I'm guessing no because of the oil seals etc.), and would this help? i.e.: the revs of turbo one should drop off pretty quick as no2 spins up? Or am I completely wrong!!

[TLicense]

Think of it like this:

If you have a piston in a cylinder. If one 1 side of the cylinder you've got 1 bar of air. On the other side you're adding 2 bar of air. What will happen to the piston?

For the 0.8 bar on one turbo and the extra 0.4 bar "topped up" with the other turbo people the piston would have to stay in the middle.

For the equal output turbo people what would happen is the piston will move to the 1 bar side of things, but as it does so it will compress the 1 bar up, untill there is equal pressure both sides, and then stay stationary. This is in fact what happens.

We can apply this to our turbo's:-

If we apply theory no.1 to our turbo's, turbo 2 would be stalled by the greater pressure from turbo 1. This clearly doesn't happen otherwise we wouldn't see any boost increase when turbo 2 came on-line.

If we apply theory no.2 to our turbo's, both turboo's produce an even amount of pressure (or flow if you want to go that way) and that presure is what is measurable at the y section (ignoring frictional losses of course)

 

Hope that helps.

[Chiefgroover]

tbourner, are those parking sensors in your rear bumper?

Thanks for offering something practical. If you do it, it cant hurt, all data put together will give the clearest final picture. I am so pleased someone is offering something practical I would almost fill you car with petrol for you. :-) You dont know what a relief this is, one small step in the right direction. You have made my day :-) . Again thank you.

 

Chiefgroover

[b_have]

Chief, the amount of time spent argueing about this you could have fitted a single and all this would become irrelevent. (runs for cover)

[Chiefgroover]

LMAO! @ b'have, how true! fitting hybrids next week, if i can spare the time! (though i think that a non starter due to recent events) This discussion couldnt have come at a worse time, Have 2 people in hospital, one dying, one recovering from a serious head injury, try to do a daily hospital run to 30 miles away, trying to work 5 1/2 days a week, looking after senior peoples interests for them, trying to do my supra hi-fi install, do house improvements, have one "half" night (11pm on for 3 hours) a week out on the town to keep myself sane, and this damm thread to answer every farts end cause I decided to sell turbo's and declare i ran extra boost to any potential new owner. I have no problem falling asleep when i finally make it to bed each night. So if you guys feel like slowing this thread down a bit (one post a day would do me) right now i could do with a bit less time on here, the pace of this wont effect the eventual outcome, whatever it may be, yawn................

[Supragal]

Guys can we please all play nicely? Sounds like it's run it's course.