Controllling the turbo wastegate from the manifold
Discussion
Can anyone give me a good reason why controlling the wastegate using manifold pressure is a bad idea... if noboday can, why don't car manufactures employ this idea (or do they?). Surely it would give you much more accurate boost controll. I understand that a wastegate set to 4.5 bar (for example, massive inacuracy in figures no doubt!) will controll boost to 1 bar at the manifold. Going on this assumption, having a wastegate made to be fully open at 1 bar of boost pressure and feeding it from the manifold, would give you less "boost spike" and better control in achieving 1 bar positive pressure at the manifold.
Anyone?
Anyone?
It already is controlled by manifold pressure. On the impellor side of the turbo is the wastegate. On the Compressor side of the turbo is a diaphram. They are linked by an adjustable shaft, (adjustable to set the boost pressure, can be replaced by an electronic system, but princible is the same.) When the pressure in the compressor is high enough to move the diapham it will push the wastegate open. With the wastegate open, some of the exhaust gasses bypass the impellor, causing the turbo to spin slower, thus building up less pressure in the compressor.
However, the air that has already passed through the compressor will still have to go through the engine. If the car is fitted withy a BOV then any pressure above the BOV's limit will cause it to open and release the air so it won't have to go through the engine. But... BOV's aren't very accurate and will normally only open if the throttle is released. If the throttle is released then the compressor will be forcing air into the closed intake, so the BOV will sense a high pressure, open and let the air out. This also prevents the turbo from stalling and having to spin up again.
Tim
However, the air that has already passed through the compressor will still have to go through the engine. If the car is fitted withy a BOV then any pressure above the BOV's limit will cause it to open and release the air so it won't have to go through the engine. But... BOV's aren't very accurate and will normally only open if the throttle is released. If the throttle is released then the compressor will be forcing air into the closed intake, so the BOV will sense a high pressure, open and let the air out. This also prevents the turbo from stalling and having to spin up again.
Tim
timsta said:
It already is controlled by manifold pressure.
Tim
Actually its not, as you described its controlled via the positively charged side of the inlet system, but before the throttle butterfly not after as in the manifold.
IF the actuator was controlled at the manifold the actuator would recieve negative pressure when the throttle is closed effectivly holding the wastegate shut. if the system didnt have a dump valve you would get serious compressor stall! the DV needs both positive and negative pressure so that should be manifold controlled.
When the actuator is controlled before the throttle butterfly, it recieves a positive spike when throttle closed causing the actuator to twitch and open the wastegate, this causes the 'chatter' you sometimes hear on a turbo car
True, sorry, I was oversimplifying it. It would hold near true, except for closed throttle situations.
Your best bet is to have a look on http:///www.howstuffworks.com for a good explanation
Tim
Your best bet is to have a look on http:///www.howstuffworks.com for a good explanation
Tim
Cheers lads. Really bloody obvious. For thoes that have not understood the conversation so far:
The wastegate is opperated by an air pressure acctuated diaphram. The typical location for the source of this pressure is at the compressor housing of the turbo. My beef with this was that the turbo achieves less pressure than is actually at the manifold. I would hazzard a guess at this being because of the difference in air speed in the two locations. When the compressor housing is a 4.5 bar of boost, the manifold is at 1 bar(these figures are make believe). For this reason the acctuation of the wastegate is dependant on vaugely accurate air pressure. Without going down the "plug and play" electronics route, I wanted to know if you could set your wastegate up to run 1 bar and have the source of the air pressure to acctuate it from the manifold. This would work wonders I'm sure, barring when the manifold is in vacume(the throttle flap is closed). At this point the wastegate would snap shut and the boost woud head for the skies, along with any pipe work between your turbo and your manifold, unless you had a monster of a Dump Valve, to cope with the enourmous surge in bost. Short of destroying your inlet tract and no doubt your turbo, I reckon you'd have some serious throttle response!
The wastegate is opperated by an air pressure acctuated diaphram. The typical location for the source of this pressure is at the compressor housing of the turbo. My beef with this was that the turbo achieves less pressure than is actually at the manifold. I would hazzard a guess at this being because of the difference in air speed in the two locations. When the compressor housing is a 4.5 bar of boost, the manifold is at 1 bar(these figures are make believe). For this reason the acctuation of the wastegate is dependant on vaugely accurate air pressure. Without going down the "plug and play" electronics route, I wanted to know if you could set your wastegate up to run 1 bar and have the source of the air pressure to acctuate it from the manifold. This would work wonders I'm sure, barring when the manifold is in vacume(the throttle flap is closed). At this point the wastegate would snap shut and the boost woud head for the skies, along with any pipe work between your turbo and your manifold, unless you had a monster of a Dump Valve, to cope with the enourmous surge in bost. Short of destroying your inlet tract and no doubt your turbo, I reckon you'd have some serious throttle response!
The boost would not head for the skys when the throttle is shut. The throttle is shut !! there is no longer air passing through the engine to drive the turbo. The inertia the turbo would have already had would be small, and not enough to create any boost any more.
Using a boost signal from the manifold would be of no benefit. If you want more boost from the normal way, add some sort of bleed device, or a stronger wastegate.
You are trying to complicate a rather simple thing.
Using a boost signal from the manifold would be of no benefit. If you want more boost from the normal way, add some sort of bleed device, or a stronger wastegate.
You are trying to complicate a rather simple thing.
This all came from a car I helped build last year. We were having real trouble controlling the boost. We were getting spikes and then a gradual blow past the watsegate and subsiquent overboost. We were trying to get away with not having to change the exhaust housing of the turbo to run a bigger wastegate/fit an external wastegate to get rid of the over boost. We were bleeding a lot of air to produce 1.7 bar of boost. Have to agree with you on the "blow your inlet tract to bits" bit being a little over exajerated but with the very short run of pipe work and existing turbo stall during gear changes, it was one of the concerns with the set up. I'm still convinced that the turbo would produce excesive boost during gear changes compared to a "stock" actuator set up, despite the obvious lack of load and expoding gas etc.
Just wondered if anyone had tried it before or a cheep and easy to fiddle with alternative to blowing £100 on a controller. I'm not trying to reinvent the wheel, I'm just curious.
Just wondered if anyone had tried it before or a cheep and easy to fiddle with alternative to blowing £100 on a controller. I'm not trying to reinvent the wheel, I'm just curious.
If you are having to bleed too much, then there are problems within your installation. whether the turbo is too small, inlet restrictive, wastegate too soft etc etc.
As for a bleed to turn the boost up...cheapest are fish tank bleed valves. Can be bought for a couple of quid.
If one doesnt bleed enough, place a restrictor in the line feeding them, from the compressor.
If you dont like that, there are plenty of mechanical type controllers, and none cost £100. about £30 would be more like it.
As for a bleed to turn the boost up...cheapest are fish tank bleed valves. Can be bought for a couple of quid.
If one doesnt bleed enough, place a restrictor in the line feeding them, from the compressor.
If you dont like that, there are plenty of mechanical type controllers, and none cost £100. about £30 would be more like it.
crash10bandit said:
When the compressor housing is a 4.5 bar of boost, the manifold is at 1 bar(these figures are make believe).
This would only be true just after the throttle had snapped shut. During steady state WOT operation, any pressure difference between compressor outlet and manifold would be minimal.
It should be minimal, that doesnt mean it will be.
Some cars intercoolers and pipework, and throttle etc can pose more of a restirction that you think.
Pressure differences of over 5psi are very possible on some standard cars with restricitive intercoolers etc.
If you increase the boost, then this can get even worse. Tha can also be combined with a standard restrictive cat exhaust on the other side, it doesnt help matters at all.
Some cars intercoolers and pipework, and throttle etc can pose more of a restirction that you think.
Pressure differences of over 5psi are very possible on some standard cars with restricitive intercoolers etc.
If you increase the boost, then this can get even worse. Tha can also be combined with a standard restrictive cat exhaust on the other side, it doesnt help matters at all.
stevieturbo said:
It should be minimal, that doesnt mean it will be.
Some cars intercoolers and pipework, and throttle etc can pose more of a restirction that you think.
Pressure differences of over 5psi are very possible on some standard cars with restricitive intercoolers etc.
If you increase the boost, then this can get even worse. Tha can also be combined with a standard restrictive cat exhaust on the other side, it doesnt help matters at all.
Agreed, but 5PSI is what I'd call minimal compared to the 3.5 BAR difference quoted above!
stevieturbo said:
The boost would not head for the skys when the throttle is shut. The throttle is shut !! there is no longer air passing through the engine to drive the turbo. The inertia the turbo would have already had would be small, and not enough to create any boost any more.
I had an interesting situation a few years back on a Rover V8. It ran fine with boost going from the turbo's straight into the intake manifold ie no intercoolers. Later, I modified the inlet manifold to accept 2 copper chargecoolers. The turbo's fed air through those and down the trumpets
All was fine until I booted the engine and lifted off the gas! It would make a sonic boom! Boost pressure would peak at 10 psi on hard gearchanges as per normal. I figured the turbo's were overspooling maybe stalling or surging. Nothing got damaged and after a while I had to fit an irritating dump valve which cured the problem. I never worked out why the coolers caused the problem though. I think it was because they caused a restriction between the compressor outlet and the inlet manifold.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff