“The BOV/dump valve debate will rumble on throughout the worlds forums for years to come and it
seems to me that no-one has the complete answer”
The reason for this is that some people read, and believe, too many posts written on too many forums by people (self proclaimed experts) that do not have the slightest idea about what they are talking about. To be honest, I am bored of reading over & over again every ‘Boy Racers’ take on the whys & wherefores of forced induction.
The basics
Firstly, a wastegate acts on the hot side of the turbocharger and serves to regulate the amount of energy recovered, from the exhaust gases, by the turbine. When the wastegate is open, exhaust gases effectively by-pass the turbine and there is (relatively) no energy transfer – with it fully closed then the maximum amount of transfer is achieved. Wastegates are used in all normal turbo applications except VGG/VNT
A dump valve acts on the boost circuit on, downstream of the compressor and pre throttle. Its purpose is to reduce the pressure ratio of the compressor by reducing the pressure within the boost circuit. They are not needed in unthrottled applications.
The physics
The fact that the compressor in question is part of a turbocharger, fitted to an engine seems to confuse people.
For any given compressor, if the pressure ratio is high enough and the mass flow low then surge will surely occur – it doesn’t matter if the compressor is part of a turbocharger, gas turbine or god knows what. The behaviour of the compressor remains the same.
In a boosted engine application, without a dumpvalve, when the throttle is closed the pressure ratio across the compressor increases yet the mass flow decreases – it is impossible for this not to be the case. If one then looks at the compressor map for said compressor, it is obvious that this then moves the operating point closer to the surge line. It is, however, dependant on the compressor in question if the surge line is ever met.
If the surge line is met or passed then surge will occur. Surge is not turbocharger shafts rotating backwards & forwards but is the cessation of the compressors ability to deliver fluid from the low pressure side to the high pressure side – with a corresponding, and unstable, release of compressed fluid back into the low pressure side.
In the same vane, a badly matched turbo that provides more massflow into the boost circuit than the engine is capable of using will also operate closer to the surge limit as the pressure ratio increases.
If a dump valve is not used in an application in which one is required then surge will occur – it will be very audible and it will reduced the durability of your turbocharger. If a dump valve is not used in an application that does not need one - then thank those clever engineering types for the reduced cost!
MS
