How does an external wastegate work?

[drk mrk iv]

I am a little confused on how an external wastegate works. To my understanding all one needs is a turbo, a manifold with a place for an external wastegate and an actual wastegate. As in you don't need to hook up any lines from the wastegate to the turbo. Is that correct? Does an externally wastegated turbo sound different than an internally gated one? Do they affect spool up times or boost creep?

 

[peregrine]

*sigh* this took me all of 3 seconds to find on google.


The wastegate (WG) is part of a feedback loop used to limit boost. The boost controller, be it manual or electronic, the turbo, and the wastgate make up the elements of the loop. As boost builds, the controller monitors the pressure. When the pressure exceeds the controller's set point, it supplies air pressure to the WG actuator (the thing that moves the WG). The WG opens and diverts some of the exhaust flow away from the turbo. The turbo slows a little and the boost drops to below the controller's set point. When this happens, the controller stops supplying air to the actuator and the WG closes. The cycle repeats over and over until you stop nailing the accelerator!

How much the boost varies about the set point is a function of the "gain" of the loop and how much it is "dampened." It's more than you probably want to know.

There is a spring in the WG actuator that has a lot to do with how well the WG remains closed while boost builds. The WG actuator has to overcome the spring force to open the WG. The exhaust gas pressure isn't very significant so the actuator has to do most of the work. The spring force has an affect on the "gain" of the loop, so it can affect the loop response time.

Small turbos imply low boost levels so they are able to employ internal WGs. A small turbo running high boost levels (not recommended since turbo efficiency falls off dramatically) can overwhelm its WG and boost creep will result (the gate can't open enough to divert the huge amounts of exhaust gasses away from the turbo). External WGs are often larger and can more effectively divert larger amounts of exhaust gasses away from the turbo when they open.

Boost creep is what happens when the boost climbs beyond the set point limit established by the controller. Boost creep is caused by the inability of the WG to divert enough exhaust gas away from the turbo. There are several reasons this can happen but the cheap solution is to lower the set point!

WG designers expected a significant pressure difference on either side of the WG valve. It is required so the gasses rush through the valve when it opens. If the pressures on both sides of the WG are equal, nothing happens when the WG opens (so no boost regulation can happen). What can make the pressures equal? A restrictive exhaust system!

Bear in mind that when your turbo is making HUGE amounts of boost, your engine is generating HUGE amounts of exhaust gasses. If the exhaust system is not capable of flowing huge amounts of exhaust gasses, significant backpressure will develop and can manifest itself in ways such as an engine performance dropoff or poor boost regulation.

Turbo lag is the time it takes for the turbo to build boost. Big turbos take longer because they have big parts and just take longer to get rolling. They are large because they are designed to compress air efficiently. Small turbos compress small amounts of air efficiently but they become inefficient when operated beyond their design limits and heat the air more than they should. It may be impressive to some to get large amounts of boost from their tiny turbo, but they should realize there is no free lunch! The hot air entering the engine has less oxygen per unit volume, so the the gas cannot be burned efficently. The result is that you lose races!

The turbo is properly-sized to the engine when it begins making boost at about 2200 rpm. Some turbos are so large they start making boost at 5000 rpm! These are not considered "streetable." Bigger is not always better.

People use big turbos because big turbos can efficiently fill an engine with large amounts of air at high rpm. At high rpm the engine uses a lot of air and a small turbo just can supply enough at high boost levels. If your engine doesn't keep you pushed back in the seat with the same, or increasing, force all the way to the rev limiter, your turbo is probably too small! In a super-fast car, the acceleration should not build at low rpm and fall off before the rev limit.

 

[Defiant]

I am a little confused on how an external wastegate works.

A wastegate's a wastegate. It opens to leak out exhaust pressure to keep it from driving the turbo. As a manufacturer, it's tidier to us an internal all bundled-up in one package from a turbo supplier than to go to the bother of assembling an external.
What part of how it works doesn't make sense to you?

 

[drk mrk iv]

A wastegate's a wastegate. It opens to leak out exhaust pressure to keep it from driving the turbo. As a manufacturer, it's tidier to us an internal all bundled-up in one package from a turbo supplier than to go to the bother of assembling an external.
What part of how it works doesn't make sense to you?

I just didn't know how the external wastegate knew when to release the pressure, but then saw a picture of a hose going from the turbo to the wastegate... Guess that answered my question.

 

[CanadianTSi]

I just didn't know how the external wastegate knew when to release the pressure, but then saw a picture of a hose going from the turbo to the wastegate... Guess that answered my question.

The boost controller tells the Wastegate when to open, exactly like an Internal...