Explain recirculation valves vs external dump valves

[Frimley111R]

Saw a post on this in relation to a RenaultSport Megane RS265 but I have no idea what these things do.

[nitrodave]

most turbo cars have a recirc valve as standard. When you lift off the throttle the boost pressure gets recirculated back to the inlet manifold. it makes the car run quieter and can help keep the turbo spooled between gear changes. Typically a turbo car runs best with one of these, does not over fuel as the metered air is not lost and is smoother between gear changes.

A vent to atmosphere dump valve releases the turbo boost pressure to the atmosphere when the throttle is lifted. This creates the chavvy woosh sound on gear changes. It can cause over fuelling, rough running, flames, pops and bangs from the exhaust and also reduces performance as the turbo has to spool back up again.

Alternatively, you can just remove a dump valve and block it off. This creates the pigeon flutter sound, but has a similar effect to vented vales which causes over fueling, pops, bangs, and rough running.

Best option is to go uprate recirc vale IMO

[Frimley111R]

Thanks, will now attempt to get my simple brain to understand that.

[andygtt]

In its simplest terms:

A recirculation dump valve exits the air into the air flowing into the turbo.

An External Dump valve exits it to atmosphere.


The benefit of the recirculation dump valve is it is much quieter, the draw back (and it can be a big one) is that the air you are dumping has been heated by the turbo and therefore you are returning that additional heat into your engine package and making it work to cool it again... not great and is why external dump valves are more popular in the aftermarket (deliberately ignoring that most fit external ones just for the noise).

Im not a fan of Dump valves and would prefer not to run one, but the size of the turbo i run means I need to for reliability, I have deliberately used one that will not dump all of the pressure in the system, just enough to prevent damage so I still get the 'flutter' as some of the air goes back through the turbo vanes.

[Huff]

There's another hidden 'gotcha' to dump valves, and that is - depending on manifold vacuum - they can be held open allowing unfiltered air into the intake. Not good for MAF/MAP accuracy, not good for engine longevity.

No use on a road car IMHO

[anonymous-user]

andygtt said:

that the air you are dumping has been heated by the turbo and therefore you are returning that additional heat into your engine package and making it work to cool it again.

Not really true. When the valve opens, the compressor pressure ratio falls effectively to zero, and as such the temperature increase across the compressor is also effectively zero.

[Luther Blisset]

andygtt said:

In its simplest terms:

A recirculation dump valve exits the air into the air flowing into the turbo.

An External Dump valve exits it to atmosphere.


The benefit of the recirculation dump valve is it is much quieter, the draw back (and it can be a big one) is that the air you are dumping has been heated by the turbo and therefore you are returning that additional heat into your engine package and making it work to cool it again... not great and is why external dump valves are more popular in the aftermarket (deliberately ignoring that most fit external ones just for the noise).

Im not a fan of Dump valves and would prefer not to run one, but the size of the turbo i run means I need to for reliability, I have deliberately used one that will not dump all of the pressure in the system, just enough to prevent damage so I still get the 'flutter' as some of the air goes back through the turbo vanes.

What dump valve are you using Andy? What can I tweak in one to make it not release too much boost when it opens?

[andygtt]

Max_Torque said:

andygtt said:

that the air you are dumping has been heated by the turbo and therefore you are returning that additional heat into your engine package and making it work to cool it again.

Not really true. When the valve opens, the compressor pressure ratio falls effectively to zero, and as such the temperature increase across the compressor is also effectively zero.

Not quite following, the air thats being sent back to the turbo inlet is hotter than the air going in the air filter... are you saying the act of decompression effectively cools the air to ambient again?

[andygtt]

Luther Blisset said:

What dump valve are you using Andy? What can I tweak in one to make it not release too much boost when it opens?

I use a Tial one, you can fit different spring rates and I have a very strong one so it only opens at very high boost... I have found this works very well.
Im not entirely sure if when it does open it dumps all the pressure or not, Id prefer it not to however I suspect it all goes.
I welded mine on the cool side of the intercooler.

I know you can buy some that are adjustable springs.

[nitrodave]

andygtt said:

I welded mine on the cool side of the intercooler.

Sounds like you got a pretty serious setup. What car is this on?

Don't mean to be pedantic, but if your valve is on the cold pipe after the intercooler, then surely it's near enough ambient temperature and won't make much difference if it goes through the turbo or not? It will only get intercooled again.

I see your point if running the valve on the hot pipe though, but even then it will be negligible as it's passing through a super hot turbo only to be intercooled.

[Luther Blisset]

andygtt said:

I use a Tial one, you can fit different spring rates and I have a very strong one so it only opens at very high boost... I have found this works very well.
Im not entirely sure if when it does open it dumps all the pressure or not, Id prefer it not to however I suspect it all goes.
I welded mine on the cool side of the intercooler.

I know you can buy some that are adjustable springs.

So with the stiff spring it won't crack open at lower boost levels?
I wonder what effect a restrictor would have on the outlet side of a recirc type.
Like a rubber bung with a small hole in it.

[andygtt]

nitrodave said:

andygtt said:

I welded mine on the cool side of the intercooler.

Sounds like you got a pretty serious setup. What car is this on?

Don't mean to be pedantic, but if your valve is on the cold pipe after the intercooler, then surely it's near enough ambient temperature and won't make much difference if it goes through the turbo or not? It will only get intercooled again.

I see your point if running the valve on the hot pipe though, but even then it will be negligible as it's passing through a super hot turbo only to be intercooled.

I would agree to a certain extent but for a different reason to what you mention... No cooler gets charge temps to ambient, mine is efficient but on full chat on track this will be 10deg or more above whats entering the air filter.
On my car I do agree with you as my intercooler works well and sheds heat (different story if the intercooler was marginal imo), plus the hot air going in will be a lot less than the engine will consume and will be gone very quickly.... its a bit like the argument that the air filter MUST only have cold air rather than drawing from the engine bay which to a reasonable extent I don't agree with
Mine is a noble M12 running over 700bhp... I designed and made the engine package myself and its a very different package to the other nobles in the UK being a single turbo rather than twin.

Im waiting for Max to expand on his comments above, my experience tells me he know a whole load more about this kind of stuff than me so I have a feeling I might learn something

[Undirection]

Watched a video on YT about this which helped a lot, the crux is that shutting the throttle stops air going into the engine and so that air from the turbo has nowhere to go and so the turbo gets slowed by this and once the throttle is opened it has to speed up again causing a lag.

[anonymous-user]

Undirection said:

Watched a video on YT about this which helped a lot, the crux is that shutting the throttle stops air going into the engine and so that air from the turbo has nowhere to go and so the turbo gets slowed by this and once the throttle is opened it has to speed up again causing a lag.

Not strictily true. Without a bypass, shutting the throttle reduces the compressor mass flow to zero (in fact, it reverses it temporarily) as such, despite the high pressure ratio across the compressor, the work done my that compressor is effectively zero, which means the shaft speed is actually maintained.

A bypass value reduces the pressure ratio to near zero, but the mass flow to max, so this actually slows the compressor more!


BUT, as the airflow across the compressor stalls, the loading on that compressor is highly dynamic and variable. This means axial and radial loads on the blades, and the bearing system can be chaotic. For a production car, running over a high mileage this is generally a bad thing, and can shorten the turbochargers life, and lead to catastrophic failure. It also causes a significant noise issue, with the characteristic "chirping" sound, that most drivers would find annoying and distracting. Hence, for a tiny lag penalty, OEMs fit closed circuit bypass valves, giving the compressor and easier life, and having little NVH issue (there is still some gas rush noise to content with as the valve operates, even for a closed circuit valve)

[anonymous-user]

andygtt said:

Im waiting for Max to expand on his comments above, my experience tells me he know a whole load more about this kind of stuff than me so I have a feeling I might learn something

sorry, missed that ^^^ initially



When you compress air, it gets hot.
When you expand air it cools.


If you vent your bypass valve from AFTER the intercooler, you have removed heat from the air in the system at a higher energy level, so when you release the pressure and re-expand the air, it cools to BELOW the ambient temp it started! (assuming you do that quickly enough to the heat has to come, by and large, from the air itself and not from it's surroundings)

Also as, when the bypass valve opens, the pressure ratio across the compressor falls to effectively zero, and so the only upheat is conductive from the material of the compressor(and pipework) itself. So, whilst the bypass is open, and the air is going round in circles, there is a relatively small increase in its average temp, certianly nothing to cause any degredation in performance. (certainly many many times less than the performance "lost" by closing the throttle!)


As an interesting aside, intercooling at a higher state of Enthalpy (energy) is a technique used to increase the performance of "air limited" racing engines.

For example, on the inlet restricted WRC engines we developed in the early 2000's, with there 34mm inlet restrictor effectively capping the max airflow of the system, we made more power by deliberately over compressing the air charge (using more turbine work) and then adiabaticaly (as much as possble) expanding it after intercooling. As this intercooling occurs at a higher average temperature over ambient, the heat rejected is also higher. The subsiquent expansion process then leverages that heat loss to reduce engine inlet charge temperature. Although this didn't increase air flow (inlet restricted) is did improve the engines knock limit, and for a highly knock limited, high boost, high CR, inlet restricted engine (with high internal EGR as a result) this made more power!

Here's a photo:



I'll leave it to the reader to link the physics i have just explained to the hardware in that photo..... ;-)

[stevieturbo]

Does the expansion chamber really make that much difference ?

What sort of air temps would they be seeing ?

[anonymous-user]

stevieturbo said:

Does the expansion chamber really make that much difference ?

What sort of air temps would they be seeing ?

It wouldn't have been fitted to the car if it hadn't!

Of course, in wrc, we are talking relatively small percentage improvements making the difference. The 34mm restrictor, even fully "optimised" means they only have around 0.22kg/s of air to play with, so combustion efficiency becomes crucial. There is no such thing as a "std" air temp (inlet or manifold) on a wrc car because they run under such different conditions, but iirc, the system was good for a roughly 5% drop in IMT.