Quote:
Originally Posted by williamsvts
but you already have boost pressure built up in the intake. your starting with postive pressure as opposed to atmospheric.
|
Agreed. And that positive pressure is effectively trying to turn your compressor wheel the wrong way as the shut TB is forcing air to go back into the compressor housing it's just come out of.
Lets say the turbo is spinning at 50,000 rpm whilst making boost. In your example, the turbo is forced to slow to perhaps 5,000 rpm because of the back pressure on the compressor wheel trying to force the blades the wrong way as the air has to go somewhere - obviously it's losing inertia slowing down like this. With a dump valve, the boost is allowed to escape the intake system, meaning the compressor is NOT slowed down by the air being forced back on it, and it is allowed to continue spinning with it's own momentum, perhaps retaining 80% of it's inertia. The time taken to recover the RPM from this near stall is FAR higher than the time taken the recover from a 20% loss.
(I've plucked these numbers of of the air but just to highlight the examples).
If we look on wikipedia for example:
http://en.wikipedia.org/wiki/Turboch...low_off_valves
Quote:
Turbocharged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air has nowhere to go).
This causes a surge which can raise the pressure of the air to a level which can damage the turbo. If the pressure rises high enough, a compressor stall will occur, where the stored pressurized air decompresses backwards across the impeller and out the inlet. The reverse flow back across the turbocharger causes the turbine shaft to reduce in speed more quickly than it would naturally, possibly damaging the turbocharger. In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess air pressure. These are known as an anti-surge, diverter, bypass, blow-off valve (BOV) or dump valve. It is basically a pressure relief valve, and is normally operated by the vacuum in the intake manifold.
The primary use of this valve is to maintain the turbo spinning at a high speed.
|
Ever heard of Guy Croft? Engine building legend? Here's his personal take on it:
Quote:
The dump valve gets rid of surplus pressure when the throttle is closed thus preventing the compressor wheel from stalling. Repeated stall (or near stall) can fatigue the blades on the compressor wheel and a severe stall can burst it. Allowing the air to vent off keeps the compressor wheel spinning and thus enhances the turbocharger response when the throttle is re-opened.
I don't really recommend running a turbocharged unit without one, especially with boost pressures over 10psi or so.
|