How does the Wastegate work

[Paulm]

I am confused can any 1 explain.

The pipe going to the WG does the pressure in there increase to keep the WG shut longer via the WGFV

or

In fact is it the exact opposite

I know i have not explained myself very well

Tried to find a diagram and work it out but cant find 1




cheers

Paul

[Stick Insect]

Not being an expert on this, but MartinJS explained it to me.

The WGFV bleeds off pressure so that the wastegate doesn't pop off.

When the gas is booted the WGFV doesn't bleed off the boost and you get the real kick as the boost leaps in in real terms as seen by the engine.

on overrun the boost is high and the engine requirements are low. The wastegate leeds off the pressure and sends it through the exhaust turbine keeping it spinning ( positive feedback). This is meant to reduce turbo lag.

This is as I understand it!

HTH

David

[Mark Halligan]

When you nail the motor and the boost pressure increases and reaches a preset value (there are other variables too complicated for me to understand) stored within the ECU, the ECU diverts boost pressure from the compressor housing via the WGFV to the lower chamber of the WG, this pushes against the diaphram and spring and these in turn act upon the valve in the WG. This valve 'cracks' open and allows exhaust gases to divert away from the turbo's turbine into the downpipe, thus limiting/controlling the amount of energy available to the turbine from the exhaust gasses and this effectively controls boost pressure.

So the pipe to the WG carries boost pressure to open the WG valve, the spring is what keeps it shut, this is why putting in a stiffer spring can delay the WG 'cracking' open a little longer to give a better spool up as sometimes the ECU starts to open the WG before full boost preesure has been reached.
Also you dont hear the WG open (unless you have a seperate exhaust for the WG like I have), a lot of people get confused by the sound of the bypass valve/dumpvalve, this is what you hear on gearchange and not the WG.
Theres a picture here from SJM Autotechnik (scroll down).

This is the way I understand it .

Mark.

[Paulm]

thanks lads, good link Mark

Paul

[Greg_S]

yeah, under normal conditions without the WGFV using mechanical boost the wastegate spring is set so that it starts to divert exhaust gas throught it at 0.8 or 0.6bar cant remember which. Anyway this is diverting exhaust gas around past the turbo stopping it spinning too fast.

this is mechanical boost control. Because of the way the spring works you can get wastegate creep where it starts to open too early and bleeds some of the boost away when it should still be going thru the turbo.

By using electronic boost control (i.e ECU working the WGFV feeding boost pressure into the wastegate diaphargm to stop it bleeding the boost off as soon)
With this control it is possible to minimise the wastegate creep electronically. So the ECU can open the wastegate up in several increments minimising the creep and helping to get the boost sooner. If the wastegate was held closed and then suddenly opened it could stall the turbo somewhat which is why it is opened progressively.

But it isnt all about the wastegate because that only controls the hot side, on gear changes etc the DV comes into play to basically cut the turbo out of the loop (on the coldside) and helping to keep the turbo spinning round without loading it trying to compress the air. Once your have made your gear change and your back on the gas for example the DV closes and connects the cold side back into the loop and the turbo can compress the air again.

Greg

[Paulm]

Greg

Now things r getting mixed up, U r saying that boost pressure from the WGFV keeps the WG shut, but looking at the diagram boost pressure would open the WG

The way i see it now is the more pressure bled off and returned to the intake the more boost we will get.

If the WGFV was switched off the WG would see the same boost as the inlet side so boost would be low. If u switch on the WGFV bleed of boost giving less boost to the WG u will see more boost

or am I missing something

Paul

[Mark Halligan]

Paul,

that is the way I see it, boost pressure from the compressor housing fed to the lower chamber of the WG can ONLY open the WG, and does this by acting against the diaphram and spring.
You are correct when you say that if the WGFV is unplugged then the route from the compressor housing to the WG with the WGFV between them, is left open and the lower chamber of the WG will be open to boost pressure all of the time and boost will be governed only by the preload of the spring upon the diaphram and not the ECU.
When the WGFV is plugged in and powered up, it is essentially closed blocking this path from compressor housing to WG, boost then rises, until the ECU tells it to open by way of a duty cycle, and send boost pressure to the lower chamber of the WG in order to control boost, by opening the WG and diverting exhaust gasses away from the turbine and into the downpipe.

The DV/Bypass valve has NOTHING to do with boost control, its purpose is to relieve the pressure within the inlet tract that occurs when changing gear, this boost pressure 'slams' up against the closed throttle plate and 'pulses' back to the compressor effectively stalling it or in very extreme cases actually damaging it, by venting this pressure, either to atmosphere as with an atmospheric dump valve, or into the inlet before the compressor, as with a recirculating valve (what we use), this helps to keep the compressor spinning during gear changes.

Complicated stuff,

Mihnea. Marco.....anyone else?


Cheers,

Mark.

[S2central.net]

It does take a while to get this wastegate stuff in yor head. Mark & others have done a reasonable job explaining it. Hope I can add some pearls of wisdom - and yes - major slap - all of this should already be on S2 Central in glorious detail , but its not...

The function of the wastegate (WG) is to steer exhaust gasses away from the hot side of the turbo by means of a sodding great valve which works against a preloaded spring. The spring strength defines the mechanical element of boost control, but it does not work alone. In addition to the spring is a 'control' pressure chamber which is presented with pressurised air from the output of the wastegate frequency valve (WGFV). The amount of pressure in this control chamber helps or hinders the opening of the WG.

If is the ECU controlling the duty cycle on the WGFV (cycling between boosted air from a takeoff point on the turbo outlet and vacuum from a takeoff point on the MAF hose) that the control pressure is used for boost management.

If you ever get a wastegate on the bench you'll see how much finger strength is required to crack open the valve - quite amazing to think that exhaust gasses can do that. This is indeed wastegate 'creep'.

The throttle bypass valve (TBV) aka diverter valve (DV) has an entirely different function. This is to allow pressurised air in the intake system to keep moving over the turbo input vanes in between gear changes so as to prevent turbo stall. Try and think what would happen without one, when you build up to X amount of boost pressure in 1st gear - you go briefly into vacuum for a gearchange (with the throttle briefly closed) so all that excess pressure would slow the turbo unless it had a low pressure path to follow - thats what the TBV does.

Hope that helps - more info and pictures will eventually get to zee web site on this topic.


Cheers,

Paul