(24-07-2014, 10:08 PM)DavidA Wrote: [ -> ]So why are they fitted to petrol turbos ? (Apologies if I'm being totally sense...)
Diesel engine speed is controlled purely by the amount of fuel that goes in, this is actively controlled by the throttle, (you push the throttle open, it simply pumps more fuel in, which as a consequence sucks more air into the engine as it revs faster etc), and in a diesel there is nothing to limit the air flow into the engine. in its most simple form the diesel engine could run with no manifolds what so ever attached, just the fuel pump and injectors in..
When a diesel engine throttle closes, it stops the fuel going in, hence the power reduces and the engine slows down (it uses less air as a consequence of this), there is no massive vaccuum as the engine still pulls air in and lets it out the other side, and there is no pressure buildup because there is no applied restriction in the air flow
The petrol engine speed however is controlled by limiting the air available to burn the fuel, in a carb engine the air going past the venturi in the needle pulled the fuel in passively, more air flowing past/faster flow, meant more fuel, this is a little more complicated in modern fuel injected petrols but the principle is still there as modern petrols still adjust the fueling based on the amount of air going into the engine at its most basic (MAP, MAFs etc).. so to limit the amount of air that goes into the engine there is a valve or flap (the butterfly valve) that when open allows air in, and when closed stops this (and its a pretty damn good seal too).
basically when a petrol throttle closes on a turbo 2 things happen..
1. everything between the throttle's butterfly valve and the actual cylinder becomes a vacuum (the engine is still trying to pull air in but cant pull it past the valve)
2. the turbo side of the butterfly valve becomes pressurised, and quickly. Especially if you have been ragging the twat off your car, this causes compresser stall (and makes that chooo choo chooo sound you hear on really big turbo's).
it is the vacuum generated post throttle (1) that pulls the wastegate open pre throttle (2).,
For the most part compressor stall is a bad thing, (if for no other reason you have to respool the turbo again) and releasing this pressure that is likely to cause stall means the compressor can slow down at its own pace (much more slowely than if the pressure was there) and during things like quick gear changes is likely to repressurise the area much faster cause it hasnt lost so much speed, and prevent stresses of compresser stall breaking shafts etc on turbos..
petrol turbo dump valves however do have drawbacks if not set up correctly they actually reduce throttle response when you are quick with the throttle (like during gear changes) as they can release most of the pressure in the system meaning with a big piping setup and FMIC you have a lot of gas to pressurise again.
So to sumarise above.. on a diesel there is no residual pressure buildup when you come off throttle so no reason to vent it as there wont be any compressor stall, there is no manifold vacuum to actuate it in the first place..
if you want to have a look at this in action, look at a petrol with a boost gauge, (i have a petrol boost gauge in my diesel truck), on a petrol when you close the throttle quickly you will acheive a vaccuum, the gauge will swing backwards below 0 and the gauge usually has 2 parts to accomodate this movement (and show readings). on my truck if i come off throttle it simply sits back at 0 and doesnt move.
