BOV Question 4

[howboycat]

I was reading more into my Corkey Bell book and noticed there was no mention of a blow off valve. The only means of venting boost from the pressure side of the system he mentions is a "vent valve". I assume this "vent valve" is meant to be the BOV.
About the "vent valve" he states. "A Rather sophisticated radiator cap can be used as a boost-control device. Generally, these types of controls will prove inaccurate and often noisey."
Now am I mistaken or is the primary need for a BOV to release boost pressure when the throttle is snapped shut.
And is it not a necesity because the wastegate alone cannot reduce the pressure fast enough during this particular driver action ?

 

[Deltona]

Damnit, I hate it when i can't edit my my own posts on here!

"The reason? Because the engine makes a weird embarrassing noise when they don't have one fitted, that's the only reason - it's noise control."

Should read:
"The reasons are down to emmission controls and noise suppression"

If I remember rightly....

 

[patprimmer]

According to some schools of thought the earth is flat.

I would take the word of a few of these guys over Corky Bell any time.

I temporarily removed mine from a low boost OEM application as the valve was leaking. It was automatic transmission so gear change throttle off was not an issue. It made no noise on closing the throttle at high speed. Many after market systems with very large BOV vented direct to atmosphere are very noisy and it would seem that often the reason for fitting it was to make the "rally car" noise.

Regards

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[SonyAD]

You see euroricers fitting them to turbodiesels. Like VW's tdi engines don't smoke enough as is.

 

[BrianPetersen]

Yeah, but that's typical aftermarket stuff. Just because something's available in the aftermarket, doesn't mean it actually accomplishes anything. All it means is that someone's willing to pay for it.

 

[patprimmer]

Brian

OEM models often come with wings or spoilers on the sporty models, so your comment is not strictly limited to aftermarket.

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[Deltona]

I think it's pretty conclusive that the earth is not flat and I have never heard a current school of thought suggest otherwise.
There are some terrible mistakes in the CB book, in one diagram it states that the further the air gets from the compressor housing (through the intercooler) the hotter it gets....

Maybe your turbo car was too well insulated for you to hear the strange noise it makes with no valve on it? My Euro box isn't...

People do fit dump-to-air valves to make a noise, but not that of a rally car, they never whoosh on the gearchange, all you can hear is the skitter of the air hitting the turbo blades and the explosions of the ALS. Another reason for turbo competition cars not to utilise any form of boost release valve is that they use the boosted air to keep the turbo spinning via the Antilag system.

To throw some more scenarios at you:

My diesel van which is bog standard and of 2001 vintage makes a quiet whoosh when dumping the throttle, so what is all that about?

My car which has the OE recirculating DV sited a little differently now skitters when backing off the throttle...

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.

 

[SonyAD]

A wastegate is enough for diesel engines. No BOV present or required. Only ppl. who fit BOVs to diesel turbos are ricers. Seems really silly to me.

http://www.peugeotclubromania.ro/forum/showpost.php?p=167682&postcount=16

Older engine turbo. Note the simple pneumatic only control of the wastegate as a function of compressor housing pressure. Only difference to diesel turbos currently being used by PSA in anything but the 1.4 hdi is it doesn't have variable geometry and the wastegate is not ECU controlled, though I'm not sure there's even a need for a wastegate on VG turbos.

Diesel engines, at least the ones I know of - HDis, are unthrottled except from Euro 3 up and only between idle~1500 with NO LOAD, for emissions concerns. Namely, aiding EGR by creating a slight intake depression via an ECU actuated plate.

 

[mattsooty]

“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

 

[patprimmer]

Thank ou Matty for throwing some fact into this debate.

Regards

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[howboycat]

Yeah, thanks Matt. just what I was looking for. A different way of saying what I was thinking.

Thanks for all the responses guy's but I think this thread is about done.