wave tuning & turbo

General engine tech -- Drag Racing to Circle Track

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Post by FastFourierTransportation »

Horsewidower wrote:My point, in raw layman's terms, is that you have a turbine spinning at 80.000+ rpm which creates its own set of waves. That set of waves will, and I'm making an assumption, wreak havoc with the wave tuning solutions that exist for NA applications.
The exhaust does not hit one blade at a time, so the waves generated are not so prominent. Moreover, for there to be a significant pressure differential across the turbine, the speed of the gas relative to the tips is just a bit below Mach 1, so little, if any, of the fluctuations will travel back up-stream.

Horsewidower wrote: I agree with the idea, however, that there may be an advantage to header design that makes use of a pulse methodology.


In most turbo applications, it's more important just to make them short, high flowing, and equal-length. Remember that turbo exhaust gas temperature is MUCH higher, and so thus is the local speed of the exhaust pulses. That means that the header tuned length must either be MUCH longer, or tuned for the second reversion wave (which is no easy task to accomplish). Sometimes, even getting them equal length is asking more effort than is really practical, and it's easier to achieve similar gains by separating adjacently-firing cylinders to remove residual back-pressure interference.
Horsewidower wrote:The only time that an NA type wave tuning solution might be applicable would be in a wastegate priority system wherein a majority of the exhaust is being "wasted" after the engine reaches a preset psi. I.E. the former Cosworth CART engines.
As mentioned, it's not turbo back-pressure which makes it tough: it's the temperature. Typical turbo tuning length would have to be about THREE FEET. Talk about laggy, eh?

-Adrian
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Post by Horsewidower »

Thanks for your reply.

I read an interview of the head of AER in Racecar Engineering. AER designed the "Mazda" turbo 4 used in the LeMans series LMP2 category. He made an interesting point that there is no benefit to tuned exhaust on a turbo app for pressures above 2.5 bar. What's unfortunate is that you don't know if he is talking about absolute or gauge.
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Post by FastFourierTransportation »

Horsewidower wrote:Thanks for your reply.

I read an interview of the head of AER in Racecar Engineering. AER designed the "Mazda" turbo 4 used in the LeMans series LMP2 category. He made an interesting point that there is no benefit to tuned exhaust on a turbo app for pressures above 2.5 bar. What's unfortunate is that you don't know if he is talking about absolute or gauge.
You're welcome! Also: considering I've SEEN benefits on examples above absoluge2.5 bar, I would assume he means gauge.

Also, above about a 2.5:1 ~~~3:1 pressure ratio across the turbo's turbine, the flow is completely chocked, which means that the scavening pulses meet unusual "reflected" pressure pulses, only slightly out of phase.

NORMALLY, the pressure pulse exits the primary header tube, goes through the collector, and out the other side. With a turbine in choke condition, that high pressure pulse goes out the primary tube, hits the turbine wheel, and gets reflected back up at an EVEN HIGHER pressure shock wave, which can easily catch up-to, or outrun, the scavenging wave. (high pressure waves travel faster)

So, yes, once the turbine reaches choke flow, say bye-bye to scavenging all together ... however, unless you've got an engine revving to 18,000 RPM with an EGT lower than 1,400*F in the first place, you're not really going to have much scavenging anyway. :-k

-Adrian
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Post by Erland Cox »

You can not use N/A tuning as a N/A engine will reflect with opposite sign from an open end while a turbo engine will reflect with the same sign from a closed end. This is why you must use a much shorter header. Erland.
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Post by FastFourierTransportation »

Erland Cox wrote:You can not use N/A tuning as a N/A engine will reflect with opposite sign from an open end while a turbo engine will reflect with the same sign from a closed end. This is why you must use a much shorter header. Erland.
Which end? Turbos still reflect opposite sign at the tube into the collector, but progressively stronger and stronger same sign reflections from the turbine wheel until choke flow, at which point any following reflection is a shock wave, until a sufficiently high balance temperature is reached so flow is no longer at a choke condition, or the mass-flow is reduced until the flow is no longer at a choke condition.

-Adrian
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Post by Erland Cox »

You do not want to use a collector with 4 cylinders into a turbo. You do not want to use a collector with a bigger area with 3 cylinders either. There is no use in going up in area just before you go down in area int the turbine housing. A bigger area only trades kinetic velocity for pressure. When you have a collector you trade the kinetic velocity against pressure and the turbo is pressure driven instead of kinetic driven. This is not very effective. Erland.
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Post by Warpspeed »

Erland Cox wrote:You do not want to use a collector with 4 cylinders into a turbo. You do not want to use a collector with a bigger area with 3 cylinders either. There is no use in going up in area just before you go down in area int the turbine housing. A bigger area only trades kinetic velocity for pressure. When you have a collector you trade the kinetic velocity against pressure and the turbo is pressure driven instead of kinetic driven. This is not very effective. Erland.
Yup.
Agree with Erland 100%.
Cheers, Tony.
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Post by Horsewidower »

Erland: Then are you suggesting a 4-2-into a divided scroll turbine housing? On a 4 cylinder then you would pair 1-4, 2-3, in order to maintain the pulse.

If not can you show an example of what you are recommending?

Thanks

Bob
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Post by FastFourierTransportation »

Erland Cox wrote:You do not want to use a collector with 4 cylinders into a turbo. You do not want to use a collector with a bigger area with 3 cylinders either. There is no use in going up in area just before you go down in area int the turbine housing. A bigger area only trades kinetic velocity for pressure. When you have a collector you trade the kinetic velocity against pressure and the turbo is pressure driven instead of kinetic driven. This is not very effective. Erland.
That's fine with me. When did this turn into two different designs options? Was just discussing the effects of one of them ... if you prefer the other ... uhhh ... great? I think? :?
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Post by Erland Cox »

Horsewidower wrote:Erland: Then are you suggesting a 4-2-into a divided scroll turbine housing? On a 4 cylinder then you would pair 1-4, 2-3, in order to maintain the pulse.


Thanks

Bob
Yes. Erland.
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Post by Erland Cox »

FastFourierTransportation wrote:
Erland Cox wrote:You do not want to use a collector with 4 cylinders into a turbo. You do not want to use a collector with a bigger area with 3 cylinders either. There is no use in going up in area just before you go down in area int the turbine housing. A bigger area only trades kinetic velocity for pressure. When you have a collector you trade the kinetic velocity against pressure and the turbo is pressure driven instead of kinetic driven. This is not very effective. Erland.
That's fine with me. When did this turn into two different designs options? Was just discussing the effects of one of them ... if you prefer the other ... uhhh ... great? I think? :?
There is a big debate regarding the design options. I believe it is better to use the kinetic energy and pulsing than to convert them to pressure.
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Post by Horsewidower »

Thanks to you both for replying.

As Erland states there is a debate concerning the "brute force" pressure driven method and the pulse "low pressure" method. It is an interesting debate, at least to me.

This is the best summary of the debate that I've found, over at gofastnews.

"There are two Basic Approaches to Turbocharging

What you describe with log manifolds or manifolds with short, stubby runners are prime examples of the traditional Approach to boosting: Brute Force Turbocharging.

With Brute Force, the exhaust gases are forced into restrictive and constrictive exhaust manifolds. And from there, those compressed gases are forced to pass through small turbine housings.

The rationale behind Brute Force is: in order the make the turbine spin, the exhaust gases HAVE to be compressed and forced through. Those exhaust gases need to be as hot as possible also. And the turbocharger turbine housing needs to be small enough so as to force the turbine wheel to spin up faster. [that's the only way to reduce lag, don't cha know!]

Well, that Approach does work. It has been working for years. It is the Approach used on nearly all OEM boosted motor cars for decades.

Brute Force has its drawbacks: greatly elevated operating temps for things like exhaust valves; tremendously increased exhaust reversion back up past the intake valve during overlap; and substantially increased levels of negative work that the pistons have to deal with.

To address those drawbacks, a 'new' Approach to Turbocharging has been showing up. First, in competition motors. Now, a lot more on the street.

Low EBR Turbocharging

With Low EBR Boosting, the exhaust gases are NOT constrained and constricted in tiny little manifolds. Headers are used to let the gases FLOW. Turbine housings are not tiny little snail shells either.

With Brute Force boosting, the EBR....Exhaust Backpressure Ratio...the ratio of exhaust pressure to the intake boost pressure....can exceed 3:1.
[i.e., for 10 lbs of boost on the boost gauge, there is possibly 30 PSI (or more) of exhaust pressure resisting that boost pressure.]

With Low EBR Boosting, the exhaust backpressure may remain less than the boost pressure reading.

[there are some guys in Sweden (the savarturbo boys) running EBRs less than 1:1 on motors that are running over 30 PSI of boost. And the output is in the neighborhood of 6 hp per cubic inch of displacement.]

The rationale of Low EBR Boosting is: the energy to drive the turbine comes from the velocity of the exhaust gases. Let the exhaust flow its best. Gather and direct those gases into a turbo that can take the velocity and use it to spin the turbine wheel. Keep the exhaust flow flowing well, so that the motor itself does not have to fight the exhaust gases during the exhaust stroke, or during overlap.

Or, to put it another way: Low EBR Turbocharging is an Approach that uses free exhaust gases to achieve, or exceed, the power output levels of motors with mechanically driven superchargers, but without the parasitic drag of the mechanically driven huffer.

In my own experience, every time I have done something that lowers the EBR, the power output has gone up without raising the boost level.

good luck with your further research."

Thanks again.

Bob
ndaglis

Post by ndaglis »

How far does the blow down pulse travel along a collector before kinetic energy is converted to pressure?



> You do not want to use a collector with 4 cylinders into a turbo.

Hmm, I'm almost at the point of concluding that two cylinders feeding the turbine and two cylinders venting to the atmosphere might be a better solution.

I think I'll do some more research into pulse converter setups.

Nick.
Last edited by ndaglis on Wed May 27, 2009 11:34 pm, edited 1 time in total.
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Post by Warpspeed »

There is nothing really new here, it just seems that way.

Sure you can get the turbine inlet pressure well below boost pressure by using an insanely large turbine and housing. If you are happy to climb up to full boost less than a thousand rpm below redline, that approach works just fine.

But what if you wish to reach full boost at or below mid rpm ?

You are forced to downsize both turbine housing and the wheel, and then fit an exhaust wastegate to vent at lest half the total exhaust mass flow at full power. Doing that you throw away at least half your potential turbine power out through the wastegate.

It is not that race engine builders are utilizing some mysterious secret hidden science, and the engineers of multinational car companies know nothing of all this. It is just that the applications are totally different.

Why can hot rodders get 600 Hp out of a 150 Hp factory engine ? Simply because they make compromises that the factory engineers are not prepared to make in order to do it.

It may well be that a long branch individual runner tube exhaust manifold performs better than the cast iron lump we are all familiar with. But cost, durability, and engine bay temperatures may or may not be an important issue depending if it is your own car, or you are planning to sell a million of them, have few warranty failures, and make a profit.
Cheers, Tony.
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Post by Erland Cox »

The idea with pulse and kinetic energy driven is not to use tiny manifolds. On the contrary you can use larger manifold diameters and a larger turbine because you use the energy available better. If you have a collector that is significantly bigger than the primary pipes you convert the kinetic energy and pulse energy to pressure. Then the gases have to speed up again in the turbine housing. OEM with log manifolds is not a proper manifold. Short tubing with as few bends as possible and the right size is the way I would do it. This gives less heat losses also. 4 pipes into the same turbine housing makes it impossible to not get interference between the cylinders, twin scroll is much better in this case. Erland.
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