Compound boost not as expected

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craigory
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Re: Compound boost not as expected

Post by craigory »

If my wastegate bypass valve was routed back to between the turbo and charger and not to the turbo inlet... That would keep boost made within the system and not be venting to the unboosted air at the air filter.
It would also be pressurising the other side of wastegate helping it stay shut essentially pressing the valve onto its seat...

Make sense??
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ptuomov
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Re: Compound boost not as expected

Post by ptuomov »

I second the need for a diagram!

If I understand this correctly, there's two kinds of setups:

1: air filter -> turbo's compressor -> throttle body -> belt-driven supercharger -> intake port
2: air filter -> turbo's compressor -> belt-driven supercharger -> throttle body/bodies -> intake port

Which one is this? I think they have different needs for intake side pressure control.
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craigory
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Re: Compound boost not as expected

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ptuomov wrote: Wed Jun 20, 2018 1:33 pm I second the need for a diagram!

If I understand this correctly, there's two kinds of setups:

1: air filter -> turbo's compressor -> throttle body -> belt-driven supercharger -> intake port
2: air filter -> turbo's compressor -> belt-driven supercharger -> throttle body/bodies -> intake port

Which one is this? I think they have different needs for intake side pressure control.
Option 2.
I wanted the trottle body as close to the engine as possible and adding a fmic after the supercharger would mean a delayed throttle response if the throttle body was before supercharger... Been there before, closed throttle at high revs and engine would remain at revs for a second or so before it could vacuum the fmic and all the associated extra pipe work after the throttle body.. AND this requires solid pipe work as it subjected to vacuum...

Make sense?
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ptuomov
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Re: Compound boost not as expected

Post by ptuomov »

craigory wrote: Wed Jun 20, 2018 2:13 pm
ptuomov wrote: Wed Jun 20, 2018 1:33 pm I second the need for a diagram!

If I understand this correctly, there's two kinds of setups:

1: air filter -> turbo's compressor -> throttle body -> belt-driven supercharger -> intake port
2: air filter -> turbo's compressor -> belt-driven supercharger -> throttle body/bodies -> intake port

Which one is this? I think they have different needs for intake side pressure control.
Option 2.
I wanted the trottle body as close to the engine as possible and adding a fmic after the supercharger would mean a delayed throttle response if the throttle body was before supercharger... Been there before, closed throttle at high revs and engine would remain at revs for a second or so before it could vacuum the fmic and all the associated extra pipe work after the throttle body.. AND this requires solid pipe work as it subjected to vacuum...

Make sense?
Thanks. If this is the case, my understanding is that you should still use the turbocharger exhaust wastegate as the main boost control method. The engine should be built such that the belt-driven supercharger is safe to run absent the turbo without additional boost control, and that the bypass valve from between the throttle body and the intake port is only used to relieve pressure under throttle closing scenarios.

So in other words, here's what I'd do:

air filter -> bypass valve outlet -> turbo's compressor -> belt-driven supercharger -> bypass-valve inlet -> throttle body/bodies -> bypass-valve vacuum signal --> intake port

That is, the bypass valve should be springed to only open when the throttle closes and sucks it open with vacuum. With the throttle position beyond certain percentage, the bypass valve should stay closed. In other words, you'd need a big diaphragm and a stiff spring ...or electronic boost controller that can apply pressure and vacuum to the different sides of the external bypass valve.

This based on logic alone, haven't ever done anything like this in practice, so don't believe me!
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craigory
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Re: Compound boost not as expected

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ptuomov wrote: Wed Jun 20, 2018 2:59 pm
craigory wrote: Wed Jun 20, 2018 2:13 pm
ptuomov wrote: Wed Jun 20, 2018 1:33 pm I second the need for a diagram!

If I understand this correctly, there's two kinds of setups:

1: air filter -> turbo's compressor -> throttle body -> belt-driven supercharger -> intake port
2: air filter -> turbo's compressor -> belt-driven supercharger -> throttle body/bodies -> intake port

Which one is this? I think they have different needs for intake side pressure control.
Option 2.
I wanted the trottle body as close to the engine as possible and adding a fmic after the supercharger would mean a delayed throttle response if the throttle body was before supercharger... Been there before, closed throttle at high revs and engine would remain at revs for a second or so before it could vacuum the fmic and all the associated extra pipe work after the throttle body.. AND this requires solid pipe work as it subjected to vacuum...

Make sense?
Thanks. If this is the case, my understanding is that you should still use the turbocharger exhaust wastegate as the main boost control method. The engine should be built such that the belt-driven supercharger is safe to run absent the turbo without additional boost control, and that the bypass valve from between the throttle body and the intake port is only used to relieve pressure under throttle closing scenarios.

So in other words, here's what I'd do:

air filter -> bypass valve outlet -> turbo's compressor -> belt-driven supercharger -> bypass-valve inlet -> throttle body/bodies -> bypass-valve vacuum signal --> intake port

That is, the bypass valve should be springed to only open when the throttle closes and sucks it open with vacuum. With the throttle position beyond certain percentage, the bypass valve should stay closed. In other words, you'd need a big diaphragm and a stiff spring ...or electronic boost controller that can apply pressure and vacuum to the different sides of the external bypass valve.

This based on logic alone, haven't ever done anything like this in practice, so don't believe me!
That's exacly as I have it set up atm.

But my bypass must have an issue.. Spring is weak so that it it closes in a linear motion to changing engine vacuum, I like how it behaves but need to try a stronger spring.
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Re: Compound boost not as expected

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Ok so I had my external wastegate bypass plumbed with the top side if diaphragm plumbed to the engine (hot) side of throttle body and the lower side plumbed to the other (cold) side of throttle body. I found this to work well at gliding onto boost with charger only installs.. But I have never run this amount of boost or something as complex before, so somethng was going wrong and the wastegate was bleeding air. I assumed that once the top and bottom were receiving the same amount of boost at WOT the valve wouldve stayed shut by the spring pressure alone.

I removed the bottom cold side hose from wastegate. holy sh1t! Its dangerous! Once the engine vacuum reaches 100kpa it flicks onto full boost in an instant.
The turbo is already spooling as the charger is pulling air through it so ot hits about 15psi in seconds!

I have my rev limit set at a modest 5k as the engine is pretty fresh and ive hit that with low boost with no issues.
Now with the extra boost it it hits a brick wall at around 3700rpm.?
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Re: Compound boost not as expected

Post by gruntguru »

Yes. If the bypass valve is open under WOT conditions charge air is flowing through the bypass away from the engine - even when the turbo is making a lot of boost.
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Re: Compound boost not as expected

Post by TimIacobucci »

Hi, sorry if I'm late to the party and you already figured this out. I am in the planning stages of a twincharge system myself and have been reading everything I could find that warpspeed has written about it online. Which is far more than anyone else and spread across multiple forums. I honestly think he could write an excellent book on the subject. 90% of what he shares I don't believe to be in print anywhere.

Anyways I came across this,
Getting it working properly depends entirely on fitting a suitable spring to the wastegate, and that requires a great deal of thought and attention to get right.

The first requirement is that the spring be stiff enough to hold the wastegate closed against full boost pressure without leakage. But it also has to be weak enough to open the wastegate against plenum vacuum at part throttle.

The problem is the ratio between poppet valve area and control diaphragm area. A ratio of at least 2 x diameter (4 x area) would be a minimum requirement, and many cheaper wastegates do not have sufficient difference in areas to work properly in this application.
credit warpspeed from this discussion https://www.eng-tips.com/viewthread.cfm?qid=174690

So my thought is you probably don't have enough spring pressure for the combined boost you are running and it's just blowing it open safety pop off style. At WOT the 2 sides of the diagram are equalized so the only thing holding the gate shut is the spring. If you have a way to boost leak test the system with the throttle held open you can check if it's leaking.

Then if you get it to hold boost but it's not responding properly to throttle closure it may be that the diaphragm is not large enough to counteract the higher spring pressure.

Another idea that I had along these lines was vacuum signal manipulation akin to boost control systems for a standard turbo wastegate. Like a manual or electronic solenoid cut off of the signal line to either side of the diaphragm. If you have a check valve in the line going to the bottom side of the gate, the one before the throttle, to stop that signal past a certain boost pressure where it begins to leak on a weaker spring the top signal from the manifold alone should be able to hold the valve closed with no opposing force left on the underside of the diaphragm. This could also be done with electronic control of a solenoid on that line.

I actually thought intentionally using a light spring and this control method would be an interesting way to to play with the blower / turbo boost ratio in a controlled manner on a dyno to see where and how how it affects output, especially with respect to exhaust backpressure.

Obviously the turbo compressor will likely have a higher efficiency if it's big enough but if you lean on it more it will drive up exhaust backpressure. More supercharger will reduce backpressure but drain power directly from the crank to do so and usually with a lower compressor efficiency at higher flow rates. If anyone has done testing on where this balance is for a given setup I am not aware of it.
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Re: Compound boost not as expected

Post by craigory »

Hi.

Credit to warpspeed indeed.

I have now got this pretty well sorted.. Doing pretty much what you have mentioned.

I introduced a small bleed hole into the line supplying the underside of the bypass wastegate diaghphram. This adds a slight differential to help it stay shut.. (Basically like having a stronger soring. I'm having a hard time finding suitable springs and having a spring that can hold 30+psi is a bit of a problem when it comes to being opened with vacuum)

I also plumbed in a 3 port mac valve that shuts off boost to the underside and vents underside to atmo at >75% tps via ECU. This way at WOT there is nothing stopping that valve from staying shut and it cuts the bleed hole out of the loop too so no boost being lost at WOT. It works really well now.
I had a thought that if my throttle body becomes a restriction at some point my cold side reference will be higher in boost than the hot side.. Having more boost under the diaphragm is a definate no no. This 3 port system elimiates any issue i may run into down the line.

Im liking my ratio results, I have a massive turbine housing and back pressure is way below total boost.

I was using total boost made to control the turbo wastegate but i can't get enough spring in it to keep it shut beyond 18ish psi. So im now referencing turbo only boost to control the wastegate. Getting more psi and waiting on another 3 port mac valve to pwm boost to the upperside of turbo wastegate for ecu controlled boost. I think the lower boost from turbo only will be easier to pwm...

I also took the ref off the turbo wastegate for fun, set my boost limit protection on the ecu to 24psi and i can hit that pretty quick!
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Re: Compound boost not as expected

Post by craigory »

https://www.flickr.com/gp/157274311@N07/Sm3FuC

Top left total compound boost
Bottom left turbo only boost
Top right exhaust back pressure.

Sorry for the crap vid.!
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Re: Compound boost not as expected

Post by TimIacobucci »

That is awesome to hear, they say great minds think alike right?

I'm glad both that your setup is working and my theory was validated :D

The video looks like total 20, turbo 15, backpressure 15? What size turbine?

Are you running the bypass mac valve on/off or have you experimented with pwm it at all?
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Re: Compound boost not as expected

Post by Big Al »

Get a 4 port mac valve. Then you can switch side of which side of the diaghphram where the boost goes. Then you can have weaker spring.
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Re: Compound boost not as expected

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TimIacobucci wrote: Sun Jul 08, 2018 5:46 am That is awesome to hear, they say great minds think alike right?

I'm glad both that your setup is working and my theory was validated :D

The video looks like total 20, turbo 15, backpressure 15? What size turbine?

Are you running the bypass mac valve on/off or have you experimented with pwm it at all?
I'm getting 22.5 psi on the datalog.. Btw glowshift gauges are not very good quality. My main boost gauge is 3-4 psi out. It sits at -3inhg powered off.

Ill get another vid of sustained (lower) boost so we can see better.

I'm just running the mac on/off. I don't see any real benefit to pwming it. Also its way easier just to trigger an on/off

ETA- my turbine housing is 18cm.
Last edited by craigory on Sun Jul 08, 2018 2:09 pm, edited 1 time in total.
craigory
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Re: Compound boost not as expected

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Big Al wrote: Sun Jul 08, 2018 8:15 am Get a 4 port mac valve. Then you can switch side of which side of the diaghphram where the boost goes. Then you can have weaker spring.
A 4 port might be over kill. I really dont need to interrupt the flow to the top of the diaphragm.
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Re: Compound boost not as expected

Post by naukkis79 »

craigory wrote: Wed Jun 20, 2018 8:14 am I'm trying to compound boost beyond exhaust turbine back pressure. Have more pressure at the inlet valve than back pressure at the exhaust valve.. most turbo engines will create way more back pressure than boost..
Turbo only creates more backpressure than boost with really small turbines. Holset 1C with huge turbine is make for all-time full throttle use with diesels, so it's exhaust backpressure is nowhere near boost pressure. You can't operate at full throttle long periods with small exhaust turbine with more back pressure than boost, it will melt engine soon.

And roots blower is only a fixed-volume air pump, it isn't effective as second phase charging. Usually when using roots blower combined with turbo it is made so that roots blows engine at low rpm range when big-turbine turbo isn't spooled up yet, making boosted power before turbo and making that turbo come to live at much lower rpm range than without blower assistance.

Those Holset diesel turbos are usually tuned to operate at high boost pressures so they might not be in best operating range with 2.3litre engine with 8.5 psi boost, if that compressor has split intake it probably is still reverse-flowing until near 1bar boost(14psi or so) to keep turbine from surging - small roots blower between it and engine probably just makes that situation worse.
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