to dump excess air from the intake system when you close the throttle plate to stop the turbo stalling, but this is in the DW10 section and as they are pointless and don't work on diesels lets not go there
As Mark said.
Just to expand slightly..
Consider what a turbo does, it compresses the air entering the engine. In a petrol car when you lift off the throttle no air is needed for the engine, but the turbo charger is still spinning away, compressing the air. Dump valves in this case are used to provide an escape route for this excess air, otherwise it may flow backwards and force the turbo to spin in the opposite direction.
We do not suffer this problems with diesel car's, however, because the engine is constantly sucking in air anyway even when we lift off the throttle. It is still possible to fit a dump valve, but they require a micro switch operated solenoid to work so they're a bit naff really.
Diesels can still suffer from compressor stall also know as chatter. It is a rare event though seems common on bigger turbo xud's I know padges does it allot. Quite how it does it with out a throttle plate I don't know
Jenkosowls Wrote:Diesels can still suffer from compressor stall also know as chatter. It is a rare event though seems common on bigger turbo xud's I know padges does it allot. Quite how it does it with out a throttle plate I don't know
It's not just bigger turbos.
My T2 does it if I bog it down and lift off as it begins to come on full boost.
Probably not the best thong to do but like all things, moderation is key. :p
Except a Dump Valve won't alleviate those problems, since it's on a microswitch, it takes far too long to respond, and only at 0% throttle, not as the pressure builds infront of the throttle plate and the manifold goes into vacuum...
Mainly because there is no throttle plate.............
Also they just tend to make your car sound like it has a cold when fitted to diesels
aaron_one Wrote:What is it?
To mark out all the tit's on the road you should give a wide berth to.
People have nearly explained the whole theory here.
To maximise throttle response, the throttle plate will be positioned on the end of the inlet manifold, after the turbo (ie. it is sucking air in and forcing it down the pipework towards the throttle plate). This is fine when you're on wide-open-throttle. But when you take your foot off the throttle, the throttle plate closes. The turbo will still be spinning and dorcing the same quantity of air down the inlet however. This air has no route through, and therefore pressure will build up between the turbo and the throttle plate. When the pressure becomes too great, it will overcome the force applied by the compressor, and will reverse direction, causing a rapid and unbalanced deceleration or stalling of the compressor, which creates this chatter. Not to be confused with compressor surge which is a totally different kettle of fish altogether!
A dumpvalve is used to vent the pressure to atmosphere, and is actuated (ie the valve is pulled open) by the vacuum in the inlet manifold when the throttle plate is closed (fyi petrols have negative pressure (vacuum) in the inlet manfiold when the throttle plate is shut.)
A recirculating valve is very similar to a dump valve, but instead of venting to atmosphere, it recirculates the air back round into the inlet pipe just before the turbo, so in effect recyles the air again.
