External wastegate vs exhaust gas pressure discussion

[Kingtal0n]

I am trying to find information about how external gates are affected by exhaust gas pressure vs intake manifold pressure (there are disclaimers from manufacturers about this subject but no extra info)

for example, If my wastegate spring is a 7psi spring, and I have 14psi of pressure in the exhaust, will the wastegate open prematurely?

To continue, if I have a 15psi spring, and 30psi of exhaust pressure, (another 2:1 ratio) will it also limit boost at the intake manifold (prevent me from seeing 15psi there due to an early wastegate opening event)?

How does the spring rate or spring design take into account exhaust gas pressure vs intake manifold pressure?

[Kingtal0n]

I found some things,
can anyone find any flaws with this?


I found some good literature,
http://www.max-boost.co.uk/max-boost...st/exhaust.htm

This site is very detailed. They cover almost every nook and cranny about turbocharger setups, and even touch on the subject,

"Very roughly, for a stock(ish) turbo and a cat-less exhaust, the exh backpressure is about 2.5 times the boost pressure. So if we're running 1 bar boost, we've got 2.5bar pushing at the wastegate (against the actuator spring)
Then we calculate the area of the flap that the gases can see. Say it's 1 inch sq.
We've got (2.5)*14.7= 35.8psi (that's pounds per square inch!)
...so the gas force pushing the flap is 35.8 pounds. Simple.

That's why actuators that can hold high boost pressures need to have much stronger springs. "

In those cases they upgrade both the spring and diaphragm together, like here,

http://www.bankspower.com/techarticl...astegate-Works

" This happens because turbine inlet pressure also increases as boost pressure rises. The fix is to use a bigger spring in the wastegate actuator to hold it closed until the desired peak boost is achieved, however, that also requires a bigger actuator diaphragm to override the heavier spring when the desired boost level is reached. "


A high performance wastegate supplied by the manufacturer has this as part of it's design. The engineer knows the surface area of the valve, calculates resulting pressure vs the intended spring selection, that way the customer does not have to worry about the EGP except in extreme situations that crop up rarely.

The real question from this point: is it so rare? Do we often find ourselves buying a 15psi wastegate, and then seeing only 7 or 10psi at the intake manifold due to EGP ratio? I dont think so. But I'd like to hear from you guys.

[cotbu]

I don't ever see that, but I'm sure it could happen. I mean you're talking about a solid system without any leaks and the proper waste gate and bov setup, right?
You must be, if you're only including egp as the catalyst. Allow that question to escape the box!

Sent from my GT-P5110 using Tapatalk

[jdm>usdm]

This is why the surface area found inside the wastegate (i.e. the actuator) which is acted upon by the boost pressure created by the turbo is larger than the surface area of the flap valve experiencing the back pressure of the exhaust created by the turbo.

Take the example you provided of a flap with surface area of 1in^2. This means the diameter is around 1.12". If we say the diameter of the actuator is twice this, then the surface area is four times greater (neglecting the lost area taken up where the valve connects to the actuator), meaning the force from the pressure is also four times greater than the force on the flap valve (if back pressure was equal to boost pressure).

So yes, extremely high boost applications would require upgraded actuators of larger size (and therefore larger spring) to resist the back pressure created from the turbo, but it's unnecessary in most applications.

[Kingtal0n]

Thank you for the genuine response, if I understand correctly what you are saying, the manufacturer designs the wastegate with the surface areas in mind, to give a valve that opens when their rated spring says so, while preventing the EMAP(exhaust pressure) from prematurely opening the valve. While both egp and IMAP (intake pressure) work together to open it, the IMAP has a much greater influence, and this will vary as WG designs and surfaces also vary. Does it sound correct?

Still have some other questions to answer, so feel free to give more examples, thanks for the input. :D

[jdm>usdm]

Another thing to think about is that the turbine back pressure will not always be some set amount greater than boost pressure. I would imagine that at lower RPM, when the turbine is beginning to spool, you will have the most turbine back pressure, as you are having to overcome the inertial forces of turbine and compressor wheels, along with the fluid (air and exhaust) forces passing thru the wheels. When the turbo is up and spooling (given the turbo is properly sized for the engine), I believe you might have significantly less backpressure from the turbine. Someone else can chime in with their opinion on this.

But yes, you have the right idea on this. Both turbine back pressure and boost pressure act simultaneously to open the WG. The spring is sized large enough to prevent turbine back pressure from prematurely opening the valve, while the actuator is sized large enough to allow the boost pressure to open it. Like you said, the force created by the boost pressure (given the area it's acting on) will be respectably larger that the force created by the turbine back pressure, so it will dictate when the valve opens.

Also, WG spring preload is just as important as the spring itself, as it allows the WG to only begin to open at the designed boost pressure, as opposed to creeping open prematurely.