wastegating facts

J

JWT

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TURBOCHARGER WASTEGATES

Waste gated turbine housing is designed to give faster low end response and stabilize boost pressure when the gate is opened.
To understand waste gating we must remember that a turbocharger is a turbine driven centrifugal supercharger, exhaust gasses leaving the engine are pumped out by the piston on its exhaust stroke; this creates pulses of superheated exhaust gas that drives the turbine.
The turbine must develop sufficient power to drive the compressor wheel as pressures and flow increase, on a well matched turbocharger turbine pressure will be lower by 5% -10% than the compressor side.
Some turbochargers are waste gated on one volute and some on both, Opening
The wastegate diverts some of this exhaust energy around the turbine so that the compressor boost pressure can be stabilized at a constant pressure.
During acceleration as engine rpm’s climb the compressor speed and flow must also increase to supply the engine demand; (Flow is load) even when maintaining a constant pressure. so turbine pressure must also increase to drive the compressor, When the wastegate is opened too early a portion of the exhaust energy is diverted; and the volume of exhaust gases to compressor ratio is now lower, therefore the remaining usable turbine exhaust pressure must increase to supply the compressors power demand, and this will cause a negative delta pressure too develop.
(Negative delta pressure (-d/p) turbine higher than compressor)
If you are using a turbocharger with a single volute waste gate, the none gated volute will have take over a larger portion of the work load when the waste-gate is opened.
When the wastegate is opened too early extremely high pressures can develop on the none waste gated side. In the case of an in line six cylinder engine the load would be on the front 3 cylinders;
On a turbocharger that is gated on both volutes the drive pressure will be divided equally on both; but a negative pressure will still develop if the wastegate is opened early before reaching the desired max pressure.

(THE WASTE GATE SHOULD NOT BE OPENED UNTILL TARGETED PRESSURES ARE REACHED)
 
hmmm interesting opening post


But.....

in order to limit boost at the target value, the gate MUST open early. This is called "cracking pressure" Afterall, it's just a diaphragm operated valve.

oh, and if it opens early, it is not possible for drive pressure to exceed boost untill such conditions exists which causes the compressor to overspeed...ie...load on the engine has exceeded the capacity of both the gate and turbine housing. This means the engines power level and displacment are more than the turbo can handle....the engine needs a bigger turbo...
 
Diesel Freak,

you should do a little More research on wastegating effects. turbine pressure will always increase when wastegate is opened.
 
JWT said:
Diesel Freak,

you should do a little More research on wastegating effects. turbine pressure will always increase when wastegate is opened.
Click to expand...

I doubt there are many more on this board who have as much "research" into wastegating as much as DF.

That being said:
:pop: x3
Chris
 
JWT said:
Diesel Freak,

you should do a little More research on wastegating effects. turbine pressure will always increase when wastegate is opened.
Click to expand...
HURRAH!! A thermodynamics debate!
 
JWT said:
Diesel Freak,

you should do a little More research on wastegating effects. turbine pressure will always increase when wastegate is opened.
Click to expand...

Offering a second path for exhaust gas to flow through that does not pass thru the turbine wheel increases turbine pressure??? Elaborate, please!
 
I am sorry that i made an attempt to inform some of the members of this forum about something that I do know. if you feel that what i have said in this forum is not correct then talk to an engineer from Garrett or BorgWarner;
or any other turbocharger engineer. untill then i rest my case.
JWT
 
JWT said:
I am sorry that i made an attempt to inform some of the members of this forum about something that I do know. if you feel that what i have said in this forum is not correct then talk to an engineer from Garrett or BorgWarner;
or any other turbocharger engineer. untill then i rest my case.
JWT
Click to expand...


Explain it, don't just come on here and regurgitate what someone told you. If you can explain it then I will believe you.

I will buy the fact that the delta can change, but you said that opening the wastegate always increases turbine pressure. You can't make a blanket statement like that. There maybe some cases when that is true when cracking the wastegate allows the delta pressure to increase at a faster rate.
 
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Justice4all said:
No kidding. First post? Talk about coming out swinging!!!
Click to expand...




Step aside kids.... let me show you how a home run is done. :hehe:

If turbine pressure is increased when the wastegate is opened, how does the exhaust pressure drop when it is opened? Those two don't jive. Usually the more drive pressure you have, the turbine is then subjected to more pressure. This is what happens when the turbine wheel is too small or the wastegate is not big enough.
 
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hmmmm??? on my single turbo gasser the drive pressure drops when the waste gate opens??? or my datalogger is retarded?
 
Think about this the exhaust gasses drive the compressor wheel, the compressor boost is controlled by opening the wastegate. The wastegate is not opened to lower the exhaust pressure; it is opened to stabilize the boost pressure.
Opening the wastegate bypasses a portion of the exhaust gasses around the turbine wheel that is driving the compressor, the remaining exhaust gas pressure must rise to maintain the compressor horse power demand, and as engine RPMs increase the diminishing exhaust volume caused by opening the wastegate must increase pressure to maintain the desired boost level, eventually rising to a higher pressure than the compressor boost.
Opening the wastegate on a duel passage turbocharger with a single passage wastegate will lower the pressure on the wastegated side; but increase the pressure on the none wastegated side; if this did not happen the boost pressure would drop with the exhaust pressure,
 
JWT, you are precisely right. When the wastegate opens, it bypasses enough exhaust to stabilize=maintain boost pressure. If boost drops, wastegate closes so target boost is maintained. What are you trying to teach besides what I just stated.

Most guys on here understand what a wastegate does.


The flaw to your argument: As boost rises, compressor efficiency does not remain constant. The higher the boost, the hotter in temperature the boosted air. 99% of the trucks on this site spin their turbos way past the mapped out 115-144K shaft RPM. When your shaft is turning way into the inefficient range=off the map, you can slow down shaft speed, maintain shaft torque=leverage to compress air but lower shaft HP and preserve mass flow of air. In simpler terms, you can slow down the shaft and still have the same airflow when you are way off the map= 150-200K shaft rpm. Look at this turbo map, notice how at 117K shaft rpm, you can compress anywhere from 35 to 58 lbs of air per minute depending on pressure ratio. This has to do with compressor efficiency.

I am no expert so please point out deficiencies in my points as well.
 
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I gotcha, you are refering to a twin scroll/divided entry turbine housing with a single side wastegate port where the wastegate only dumps half the cylinders exhaust pressure. When that half is dumped, the other half has to compensate thus increasing the pressure in that side of the exhaust.

theory sounds good to me but most of the upgraded turbo's use a modified wastegate that dumps from both halves of the exhaust almost eliminating that situation.

definitely a doosey of a first post :)
 
JWT, I do agree that on an HX35 that only gates the 3 rear cylinders, the front 3 cylinders must pick up the slack and run higher drive pressure to a certain extent. The fact that the back 3 are bypassing means that they have lower pressure in the common area of the turbine which is the biggest exhaust restriction in the system. So if the back 3 have lower pressure in the common turbine area, the front 3 will gain some benefit although you are absolutely right, the front 3 will have higher drive pressure but not as much difference as you think.

The back 3 cylinders also run hotter to begin with due to the head plumbing and the location of the air horn. The front three cylinders can handle more drive pressure before engine damage because they run cooler.
 
I hope that reading this will inspire some of you to read a book on turbochargers, I feel that no matter how much I try to explain turbocharging facts some of you will still not believe what I am saying, I can appreciate that you don’t know who I am so why should you. But I will repeat again the exhaust must produce sufficient horse power to meet the compressor demand, a turbocharger is a turbine driven supercharger.
 
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