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BOV Question
4

BOV Question

BOV Question

(OP)
    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 ?

RE: BOV Question

Ummm

What's a BOV or vent valve or inlet pressure relief valve or whatever someone chooses to call it got to do with a waste gate.

Regards

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RE: BOV Question

(OP)
I'm just trying to clear this up. Corkey Bells writing makes it sound as if you can throw the BOV away if you have a wastegate. Like their two seperate ways of doing the same job and not to be used in conjunction.

RE: BOV Question

A wastegate can't dump intake air that has already been pressurized by the compressor wheel when the throttle snaps shut...that's what the BOV is for.

RE: BOV Question

A wastegate can't dump intake air because it's not connected to the intake.

- Steve

RE: BOV Question

cyclecyko,

A "blow-off valve" is an intake manifold vent valve that bypasses air from the intake manifold to either the air duct upstream of the turbocharger, or sometimes overboard.


While the action of a BOV is very much effected by the exhaust  bypass valve
(wastegate), the purpose is not the same.

The wastegate is typically used for primary boost (or, preferably, manifold absolute pressure) control.

The BOV is typically intended to prevent compressor stage "surge" or backflow that occurs when the compressor wheel stalls aerodynamically and can no longer support the downstream static pressure.

Compressor surge can be violent enough to shorten the life of the turbo-compressor axial thrust bearing, hence the hot-rodders  and aftermarket turbo engine modifiers have employed a BOV valve.

RE: BOV Question

I think the need for a BOV is only an issue on throttled engines. Petrol spark ignition engines and anything else that requires stoichiometric air/fuel ratio achieved through use of a induction choke or throttle plate.

Unthrottled engines can probably easily accommodate the pressure already in the intake manifold before they've revved down significantly and as such only require a wastegate.

Why not have the engine management system leave the throttle plate fully or partially open and cut injection entirely instead of using a BOV?

RE: BOV Question

4
Surely its obvious that a BOV is only necessary on a throttled engine - isnt it the very action of closing the throttle that causes the over pressure in the intake system???

The same effect cannot possibly be replicated with a change in injected fuel mass on a CI engine.

Leaving the throttle open & killing injection might work but I think the software functionality would be very poor from a driveability point of view.

MS

RE: BOV Question

Because of weak engine braking?

RE: BOV Question

The radiator-cap idea was used as a primitive boost control device back in the day.  Of course any intake pressure that is released this way is a waste of the energy spent compressing it.  Also the turbo will not be regulated in terms of max speed and must be sized accordingly to run without a wastegate i.e. too large.  In effect you can use the pop-off valve idea to control boost, and I believe Corky Bell does suggest exactly that.

Blow off valves can also be set up to achieve this function if the spring is set to equal max desired boost pressure.  In fact some of the cheaper BOV's start to leak at higher boost levels, people thinking that they have max'ed the turbo instead when the boost curve goes flat.

RE: BOV Question

There are different designs of BOVs. A "pull type" BOV will not leak due to the fact that having + pressure in the charge pipe keeps the BOV closed, while a traditional "push type" BOV will have the + pressure from the charge pipe pushing against the piston towards the direction is opens.

i dont know why so many times ppl try to relate a BOV to a waste gate. they hve nothing to do with each other. One is used to control the amount of exhaust gas going into the turbine (wastegate), and the other is used to prolong the life of the compressor by limiting the amount of compressor stall (BOV)

RE: BOV Question

Also wanted to add, that many turbo chargers on the market today are build much much stronger than before and it may not even be necessary to use a BOV in attempt to gain throttle response. Some manufactures sell anti surge compressor housings.   

RE: BOV Question

The BOV is not for throttle response at all.  Its to vent pressure that would otherwise surge backwards through the compressor when the throttle is closed.  "Strength" has nothing to do with it - anti-surge (or map-enhancement) ports do, but all turbocharger applications require either a blow off valve or a bypass valve, and if you fail to use one you WILL surge the compressor and this will eventually damage your turbocharger which could lead to catastrophic engine damage if the turbo lets go.  Even with an anti-surge housing you still need a BOV - there are two types of surge: the kind that occurs when you shift and the throttle is shut and you hear the "ch-ch-ch" as your wheel spins backwards and forwards, and then the kind that happens when running a large compressor on an engine with low flow.  The latter is what an anti-surge port is for.

RE: BOV Question

"All turbocharger applications require either a blow off valve or a bypass valve"

Except of course draw-through arrangements and unthrottled engines, which couldn't use a valve of that type if you tried.

And, please inform Ford, Subaru, Isuzu, and probably more manufacturers that the thousands of thousands of turbo engines they produced in the 80's were faulty because they didn't have those devices.  At least, the Subaru turbo I had did not have one, the Isuzu turbo I had did not have one, and none of the Ford 2.3 turbo engines I have seen had one.  I understand that Mazda's first turbo rotary also did not have one, but the second one *did* have one purely for NVH reasons.

 

RE: BOV Question

They use an aluminum burst plate in top-fuel cars (non-cent supercharger) because a "BOV" can't react quick enough when an intake valve fails to close.  It's got little to do with normal blower operation.

RE: BOV Question

Quote:

... and then the kind that happens when running a large compressor on an engine with low flow.  The latter is what an anti-surge port is for.

I'm not sure if i understand that statement. Define low flow. Isn't the point of boosting an N/A motor to make it flow more: ie it had low flow, and now flows higher.

RE: BOV Question

I agree with izzmus. My turbodiesel gets along quite well with just a wastegate to prevent overboost.

RE: BOV Question

The anti surge ports/chambers/whathaveyou basically help extend the effective range of the turbo "left" on the map.  Low flow in this case means flow lower than the (normal) minimal amount of air flow that the turbo can handle at a given pressure ratio.



 

RE: BOV Question

I think that if the full paragraph from CBs book was quoted it would have saved a lot of trouble:

"Vent Valve:

A Rather sophisticated radiator cap can be used as a boost-control device. Generally, these types of controls will prove inaccurate and often noisy. While far superior to any form of restrictor (referred to earlier in the chapter), these valves probably have their greatest value as safety controls in the event of wastegate failure. They can be commonly found on production turbo cars as overboost safety features. The vent valve has no business being a primary boost control device."

This is 1970's technology, the ECU has been controlling the boost on engines since the mid to late Eighties....

These statements:

 "The BOV is typically intended to prevent compressor stage "surge" or backflow that occurs when the compressor wheel stalls aerodynamically and can no longer support the downstream static pressure."

"Compressor surge can be violent enough to shorten the life of the turbo-compressor axial thrust bearing, hence the hot-rodders  and aftermarket turbo engine modifiers have employed a BOV valve. "

"Even with an anti-surge housing you still need a BOV - there are two types of surge: the kind that occurs when you shift and the throttle is shut and you hear the "ch-ch-ch" as your wheel spins backwards and forwards, and then the kind that happens when running a large compressor on an engine with low flow.  The latter is what an anti-surge port is for. "

"And, please inform Ford, Subaru, Isuzu, and probably more manufacturers that the thousands of thousands of turbo engines they produced in the 80's were faulty because they didn't have those devices"

Are all untrue according to some schools of thought, that aside you will never ever see a compressor wheel spinning backwards and forwards!  All manufacturers use BOVs or dump valves (DV) as we call them in the UK and always have done, 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.

I have driven my own cars with no DV fitted, the turbo did not collapse, it did not slow down any further or speed up any quicker....

Do you think that a turbo WRC car uses a DV or BOV to release pressure on the gear changes? Nope.

RE: BOV Question

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....

RE: BOV Question

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

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: BOV Question

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

RE: BOV Question

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.

RE: BOV Question

Brian

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

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: BOV Question

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.

RE: BOV Question

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.

RE: BOV Question

"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
 

RE: BOV Question

Thank ou Matty for throwing some fact into this debate.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: BOV Question

(OP)
      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.

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