But ofc, wastegate opens flapper on exhaust part, so part of exhaust gasses goes around the impeller to reduce pressure turbo makes.
FYI: Was quite a lot into turbo's 5+ years ago, when I custom installed turbo on my Honda Civic + all changes (fuel, mapping) it needed. Hell, even understood how to read compressor maps for turbo's/calculate based on them
No, it's more of an opposite. Dump valve (blow off valve?) is in intake manifold, yes. But it opens up when pressure gets too high, not on negative pressure. And negative pressure in intake (on charged engines) happens when you flat the throttle and there is yet no pressure.. It happens when cylinder sucks more air than there is in intake. Not when you lift off.
Big turbine in big booster is quite heavy, which means it has quite a lot of inertia. When it rotates at it's full speed, it doesn't stop immediately when you lift off the throttle, but keeps spinning.
As long as it spins, it keeps building pressure (pushing air to intake). But when throttle is lifted off, the flap is closed and air has nowhere to go, it can't go to engine. So pressure in intake manifold very rapidly raises too high. It may even break the turbo, in case of very high boost. Boosters goes to state called surge, an unstable state between compressor and turbine, don't know the details of this.
And even with smaller boosters, it slows down the turbine dramatically meaning it takes a long time to build up pressure again after shift or quick lift off. With dump valve the pressure is released to atmosphere, which means the turbo can spin freely, without counterpressure, which means it slows down much slower, which means the pressure goes up much faster when you flat the throttle again.
Wastegate affects to pressure in a very different and much slower way, it simply adjust the pressure to wanted level by controlling the flow of exhaust gasses, if they bypass the booster or go thru it, so you don't blow up your engine/compressor with overpressure.
LFS has the most realistic Turbo simulation i've seen in a racing sim so far. GTR Evo/Netkar/rFactor don't even simulate boost.
Also iirc, turbo cars, when free revved, don't make a lot of boost, until you rev them flat out....need someone with a boosted motor + gauge to confirm that, was a while ago i was speaking to someone about it, something to do with the engine not being loaded? not 100% sure..
if you want to see bad turbo "simulation" watch a video/play NFS Shift *shudder*
pretty sure it sends the boost gauge up with the engine off *facepalm*
The pressures you mention are NOT in intake manifold but inbetween turbo compressor and flapper.
Blowoff valve/dump valve is connected via vacuum line to intake body - and to make clear, this is what open it. Blowof itself sits inbetween turbo and intake flapper.
When you release throttle, you close flapper -> at that point in intake manifold pressure goes to negative (vacuum) while turbo is still pushing air due to inertia. This air hits flapper and build still bust in intake pipe to manifold.
This is where blowoff comes to play. Due to vacuum in intake manifold it opens up BOV (remember, it's connected to intake manifold via vacuum line). While opening up, it releases boost either into air or back infront of turbo (VTA or recirtulating BOV) - air released is one inbetween turbo and intake flapper.
Reason for this is due to no gasses to spool turbo and pressure in intake turbo starts drastical deceleration which can damage bearings.
Yes you are and were right. There is negative pressure in the intake manifold, after the flapper, and too much pressure before it in tubes. Confused it..
I have always thought that dump valve is opened by the overpressure before flapper. But it's much better to use manifold pressure after the flap, i think it reacts much faster that way, less space to create negative pressure in intake manifold than there is in pressure tubes before the flapper? And you don't have to readjust it every time you change the pressure.
Well, BOV can also release air based on over-pressure, but that's like last-resort option, as it's not precise, and idea is to release any positive pressure out when flapper is closed.
It depends on spring stiffness in it.
This was turbo surge, due to wrongfully dimensioned turbo for engine.
This is why you have compressor maps for turbo's to size them appropriately, and why there are different A/R ratios, etc..
You can often heard that lift-off 'surge' sound, "dihdihdihdih", in very powerful track cars. +700hp audis and beasts like that.
Is it because the boosters and pressures are so big you can't just remove it completely, or is it intentional, a compromise where the pressure isn't released on small lift-offs? Or just bad design?
I get the surge when I don't boost more than 0,4-0,5Bar. Anything over will open the BOV properly. If I loosen the spring in the BOV it won't hold my full boost (1,7Bar).
Edit: Here's a little clip. Notice how at lower boost I get surge, higher boost it opens properly. This was on snow though, so there is not many times I get full load and boost, since the revs go up so fast. http://www.youtube.com/watch?v=1cwTerPgIIA
So does it mean that your BOV is not connected to intake manifold via vacuum line, and opened by negative pressure, but overpressure in intake tubes instead?
The more boost there is, the more far away it is from being negative pressure. Which means that more loosened BOV controller spring (if connected to manifold, opened by negative pressure) would only 'leak' on full throttle when there is yet not enough pressure and engine is vacuuming negative pressure to pressure tubes/hoses as well?
Can this even happen in boosted engines, negative pressure before flapper? Dunno, happens all the time on 'free breathing' (what's the english term?) engines.
My BOV is indeed connected to the intake via vacuum, but the vacuumline alone is not enough to overcome the spring and pull the BOV open. If it were, the spring would be too weak to hold the BOV closed at full boost. So in a lower boost config like the car is stock (0.65bar) there is no problem having the spring weak enough for it to open at even a fraction of positive pressure and still keep closed at full boost. My setting is a compromise.
Not entirely true... surge on BOV is situation you mention: when spring is too stiff and vacuum alone is not enough to open fully BOV.
Also, keep in mind vacuum alone >can< be enough to open BOV - it's pure physics.
Inside BOV is a piston which is pushed by spring so it's in closed position.
Area that's on pressurized side is for example 1 cm^2 (as example).
However on other side there's vacuum line and on other side of piston it's bigger surface area - example 4 cm^2.
This means if piston would move at 0.5bar negative pressure it is same as if it were pushed with 2 bar pressure on pressurized side (different surface size).
This way you can have stiff spring to hold high boost, yet have vacuum alone enough to open BOV.
What this also means - when there's pressure in intake it will also help keep BOV closed on boost, so it is not only the spring that is holding it closed.
if you have a diverter valve (aka recirculating BOV--or depending on what BOV you have) you can change its orientation around so the piston or diaphragm face away from the charge pipes and towards the intake side. this will effect pressure response, and can alleviate some forms of compressor surging.
that sounds like some really nasty surge you have happening. partial boost surge is caused from the turbo pumping out more air than the engine can suck in causing the air in the charge pipes to oscillate between the intake valves and compressor housing. over time this will destroy the turbo, and cause more wear+tear on the engine itself, since your intake is charged the BOV is being forced closed so the excess air cannot escape. my car did this when i installed less-restrictive turbo inlet pipe and using a few shims and a softer spring solved my issue.
your best bet would be to find an enthusiast forum for your car.
As for LFS, i hate how you don't loose boost when shifting, and the way the turbo spools on the TBO's. if you did more than a few shifts of pedal to the floor your clutch would be toast and your gear box would be taking a lot of damage. if we are going to have TBo's that can build max boost by just holding the pedal down, and not have to let off to shift we should at least have the benefit of shift lights so we don't have to do data logging to find our optimal shift points.
I guess i'm just used to my car, but in LFS the turbos seem to spool very slow for the boost and the power they generate. it seems like the turbo must be really lousy and/or is terribly mismatched for the application (too large of a turbo with the boost tuned down.)
Confused it..