john banks

Warning: long theoretical potentially boring thread....

If the wastegate was big enough that you don't get boost creep and the compressor is big enough that you don't overspeed it with your desired PR, then you could get an idea of EGBP for a given actuator tension if you bleed off all possible from the actuator's pneumatic feed or disconnected it.

If you use a larger wastegate then the exhaust pressure will result in a larger force acting through the lever attached to the wastegate hinge, hence the stronger actuators often supplied with them.

So assuming full bleed off the acutator, then EGBP at the point of boost taper =

[Actuator cracking pressure on turbo] * [Actuator diaphragm area]
-----------------------------------------------------------------
[Wastegate area]

A TD05's acutator can is about 1.9" diameter, so I guess the diaphragm might be 1.8" inside.

Standard wastegate diameter is about 1" diameter IIRC.

On my car when it had the TD05 with a cracking pressure of 8 PSI, at about 6000 RPM the boost tapers from about 21 PSI, so EGBP at this point could be estimated at:

8 * 1.9 * 1.9 / 1 = 29 PSI

With the TD04 (and different mods), with the same cracking pressure, at 5000 RPM the boost tapered from 18 PSI, so EGBP at this point could be estimated at the same 29 PSI with a similar wastegate and actuator size.

If you use a 1 bar actuator and a ported wastegate to 1.1" then the EGBP at boost taper point could be estimated as:

14.5 * 1.9 * 1.9 / (1.1 * 1.1) = 43 PSI

With 1.2 bar actuator and the same wastegate:

17.4 * 1.9 * 1.9 / (1.1 * 1.1) = 52 PSI

To double actuator strength but retain the same EGBP driven boost taper (assuming adeqaute sized compressor and all else being equal) you would need to double the area of the wastegate.

Clearly when modifying we end up running higher boost, and since we are not usually in the enviable position of MAP>EGBP because our turbos are too small then we will probably make this situation worse rather than better.

When boost levels are less than EGBP as is usually the case with our small turbos you get higher chance of detonation, loss of volumetric efficiency and polluting of the fresh charge with the old charge.

A larger AR or turbine for a given boost level should give lower EGBP, but there will be a clear spool up tradeoff.

With our small capacity engines and high power ambitions we make hybrid turbos with relatively capable compressors and arguably undersized turbines.

With this in mind can we protect our engines from excessive EGBP by making sure that if we use an x% stronger actuator that we also use an x% larger in area wastegate? Then no matter what you do with boost control you should not go down the slippery slope of excessive EGBP and reducing power and risk of damage?

Should be be aiming for a maximum EGBP? Do we risk quite high EGBPs with our lack of attention to the turbine sides of our hybrids? But the spool up and packaging are difficult for a big exhaust housing and wheel.

Although a lot of the extrapolations of possible EGBP are theoretical, the highest possible EGBP you allow should be able to be predicted from actuator cracking pressure multiplied by the ratio of areas of the actuator diaphragm and the wastegate.

So if we double the tension on our actuators without changing the wastegate size much or at all to get top end boost we are doing so by running much higher EGBP and possibly losing out on safety and efficiency, as well as power? It seems that it is not just the compressor side we can choke, and perhaps with asymmetrical hybrids biased to the compressor size we are more likely to be choking the exhaust side with excessive pressure and inadequate flow.

Little attention seems to be paid to turbine dynamics compared to compressor wheels and I struggle to find any maps anywhere, but they should also be able to be created.

With a 0.5 bar actuator on most Subaru turbos you seem to be relatively protected against silly EGBP almost whatever you do. Go to 1.0 bar + and great caution is required? Although to stop potential boost creep a 10% wider wastegate is good, say if you are running 18 PSI and wanted no more than double this EGBP you could argue that the actuator should be weaker than 1 bar.

[Edited by john banks - 4/9/2003 4:19:26 PM]

Andy.F

Some of the less well engineered Hybrids out there are running with very high EGBP. This can be identified by the high wastegate duty cycles they are running to hold boost.
IMO most of the VF turbine driven derivatives rely on high EGBP to produce the boost pressure. This makes for an innefficient engine with high internal pumping losses.

One further thing to consider in your calculations John, is the leverage the wastegate 'penny' has over the actuator arm link due to the longer lever arm inside the turbo.

john banks

Surely the wastegate would be shut if you were in max killer death duty cycle zones to keep the boost up, UNLESS the EGBP was pushing it open and you were fighting it? So it might have some relevance.

Wondered about the leverage effect but the fulcrum is at the wastegate hinge end so irrelevant if you are matching EGBP force exerted on the wastegate penny against the actuator force on the rod?

Trying to avoid another gauge which would be impractical to fit on a lot of cars as a one off or to get an idea of different turbos.

Although area = pi * r sqaured and I used diameter it doesn't matter as it cancels.

[Edited by john banks - 4/9/2003 4:27:58 PM]

Pavlo

but boost doesn't just taper because the wastegate opens.

If you were to microswitch the actuator so you would know for sure the external loads on the paddle at the time of opening, then the mesurement would be kinda valid for the point in the turbo where the wastegate was. This will not be representitive of the exhaust manifold as a whole.

Back pressure is to be avoided, as it leads to increased residual pressure in the cylinder which has to be overcome by the turbo, and leads to reduced VE (obviously) even if you can stuff the inlet charge in. (I realise nobody said EGBP was good).

Paul

Andy.F

From your own tests earlier on the td04 you found that the wastegate is indeed 'blown' open at high boost high rpm even with zero pressure on the diaphragm.

The leverage is only of relevance if working out actual pressures. IIRC the outer link is only approx 50% of the length of the inner arm to the centre of the penny therefore a 2:1 force ratio.

john banks

Just realised Andy there is that third lever I wasn't thinking about and the fulcrum is between the two short ones and nothing to do with the actuator arm hence your ratio. Does this mean the EGBP estimates would be even higher ?

If you keep it relative then and ignore the absolutes, it still suggests if you have to use a strong actuator with a standard or 10% wider wastegate (21% approx larger area) to get your top end boost then you are running quite high EGBP?

[Edited by john banks - 4/9/2003 4:41:57 PM]

Andy.F

Lower than if they were equal length, the penny has the longer lever arm therefore the mechanical advantage.

On the areas you need to square the radius, not use diameter (this will increase EGBP estimations)

john banks

Didn't realise the penny had such a long arm but just looking at it now the top is quite stubby indeed.

Think the radius/diameter thing does cancel:

(1.9*1.9) / (1.1*1.1) = (0.95*0.95) / (0.55*0.55)

Some of the Garrett GT blurb I am sure claimed MAP>EGBP potential. So do the VATN systems on about half the Euro diesel engines. Does this suddenly give you a really efficient engine? Is this why say a Clio 172 engine does its power on less than 1.3 CFM/BHP whereas the best we hear of a Scooby is about 1.42 WITH HEAD WORK (far less back pressure?) Interestingly although some passenger car NA engines produce far more specific outputs, the little Clio engine seemed to produce quite a lot for the low 6000s on only a 2.0.

Could a Scooby with the wonderful MAP>EGBP get to 1.3 CFM/BHP??

[Edited by john banks - 4/9/2003 4:54:07 PM]

Andy.F

Sounds like a nice hybrid John Wouldn't be surprised to see one appear shortly

David_Wallis

thought you were happy with the td05/06 John



David

john banks

I am happy with it but always looking for more. It has not done my 450/400 yet though. I wonder if the AR of 0.5 will cause it to hit a brick wall. I was also wondering if there was a Garrett compressor that could be popped in that would have a wider operational area. The GT series look as though they might, but there are no maps

pat

John,

I think you mean manifold *relative* pressure, since EGBP is a guage, not an absolute value. I can confirm that the Garret GT35/40 did indeed maintain MRP > EGBP up to 6500 RPM on the dyno with Mark's engine.

Cheers,

Pat.

Pavlo

disconnected actuator I was going to suggest, but thought only an idiot would be doing that to customer cars

Put it this way, I think it's a useful measure, but not a realistic measure of actual EGBP as we normally think of it.

Paul

john banks

Thanks Pat.

Paul I suppose rather than disconnecting the actuator we could try to avoid top end increasing duty cycles? They never seem to work very well anyway - ie 60% might get you 1.45 bar on a TD05 at say 6000 RPM, if you run 91% you only get 1.5 bar.

Pavlo

area of a circle is:

PI * radius squared

gut reaction is you can't use this unless you know the boost at point of wastegate opening, as once open you then have gas hitting the wastegate as it flows over it, which is effectively the stagnation pressure and not representative of the back pressure.

blah blah blah lots of assumptions blah blah blah

just plumb a pressure gauge into the up-pipe, job done.

having read your post again, the boost taper figures you speak of are they as measured with te actuator directly plumbed to the compressor cover?

Paul

[Edited by Pavlo - 4/9/2003 4:20:01 PM]

john banks

Boost taper figures are with the actuator pneumatically disconnected or effectively so with maximum duty cycle on a switching 3 port solenoid.

The question I suppose is if we are increasing actuator tension to get more top end boost then this is probably a giveaway that our EGBP is too high. I think I blew up a turbo and nearly my engine doing this whilst experimenting.

If you are going all out for top end boost and struggling, then the only thing opening that wastegate is EGBP if you are using a 3 port with a high duty cycle - therefore if the compressor is big enough and the boost is tapering it has to be the EGBP blowing it open?

Presumably there is a gradual pressure loss along the manifold from the ports to the turbine inlet and then a big drop across the turbine and then a fairly small drop to the tail pipe exit in most of our setups?

[Edited by john banks - 4/9/2003 4:36:05 PM]

Andy.F

Just checked an exhaust turbine pic and the leverage isn't that much. Estimate about 1.2:1

Having a big fan in the exhaust doesn't do much for VE, tuned length headers may help.

Ah, if you square the diameter then ok

[Edited by Andy.F - 4/9/2003 5:03:32 PM]

john banks

There seem to be 3 Garrett GT30 turbines to support a 55lb/min compressor (T04S 56 trim) (sounds like a nice compressor not sure if it is the one we have the map of or not I think they have rounded off the flow from 52.5 lb/min on the compressor map?)

http://www.turbofast.com.au/GTseries.html

You can get a core with a back and a front end CHRA, 84 trim exhaust wheel, with exducer/inducer diameters of 2.16/2.36". Listed turbine housing ARs for the complete unit are 0.63 for a 480 BHP rating, 0.82 for a 510 BHP rating, 1.06 for a 550 BHP rating.

TD05H turbine wheel exducer/inducer is 1.93/2.2" (77 trim). An estimate of a TD05H 7cm2 housing suggested an AR of 0.50 approx, and the P20 IHI housing is 0.71 and the P18 is 0.63. One source says the IHI VF turbine exducer/inducer is 1.875/2.065" (82 trim) the inducer seems a bit small (is this why they struggle to get over/much over 400 BHP vs the TD05H turbine which appears larger but in a smaller AR housing?) "For "families" of turbine wheels (those with the same inducer diameter), larger trim usually means better flow with less backpressure but with less energy recovered from the exhaust flow, and longer spool time." (hence the poor P20 spool up, high back pressure because of the small inducer which is about midway in size between the TD04 and TD05?) I wonder how much of the back pressure comes from the housing and how much from the wheel as the TD05H housing sounds restrictive but the wheel less so compared with the IHI VF turbine. But the P20 housing sounds great as a basis for mounting a competent exhaust wheel?

So with a P20 housing and suitable Garrett turbine this compressor might get to about 495 BHP and arguably any more and you'd need the custom up/down pipes?

[Edited by john banks - 4/9/2003 5:32:00 PM]