Sound suppression for Lysholm compressor with blow-thru throttles 2

[stimpee]

I am considering putting a twin-screw supercharger on an engine which has individual throttle bodies mounted directly to the cylinder head. I plan to attempt to make this work in a blow-thru configuration, so I am not looking for people to dissuade me from that. I have done enough research, and seen enough setups to be confident that it can be made to work, and work quite well.

However I do have concern over the noise output from the compressor. Blow thru setups are typically quite noisy. I don't mind the sound under heavy acceleration, but I don't really want it to be screaming under normal cruise conditions. The removal of the throttle plate in front of the compressor obviously has a negative impact on the noise transmitted when running with the bypass open.

So, anyone have any good pointers to inlet noise suppression on a twin-screw Lysholm type compressor? I have looked a little bit at resonators (tuned), but the tuned resonator might be limited to being effective in a relatively narrow rpm range.

I would be interested in any good ideas, links, pointers, etc!!

Thanks,
Steve

 

[patprimmer]

I initially said I thought this approach was stupid and dangerous and promised not to help. I should have kept my word. More than enough said

Regards

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

 

[stimpee]

Pat, with all due respect, you need to thicken your skin a bit and relax.

Also, saying this is a stupid idea is rather closed minded, and short sighted for an engineer. Do a bit of research. As I said, it can, and has been done, by automotive experts, engineers, and manufacturers who most would strongly agree "know what they are doing".

A lot of things we as engineers design and develop, have inherent risks and in many cases dire consequences if we are wrong. That is why there are engineers. To identify the risks, solve problems, develop solutions, and hence MITIGATE those risks.

However, thanks for calling me stupid. I will try to apply that as motivation. I love being told something can't be done, and then going ahead and doing it, and doing it well. Proving prople wrong can be quite satisfying...

If there was a "rolleyes" smiley available on this forum, now is when I would choose to apply it.

 

[Warmington]

Steve,

I was thinking that you could possibly tune out the noise in the transitionary period using a tuned length of pipe running up to the intercooler if it is close enough. May I ask what is the exact model of SC that you are using?

Tmoose
The noise from this SC is produced at the exhaust when the gas expands again. Not at the inlet, not due to it "taking gulps of air". Without internal compression there would not be a pulsing at exhaust either. And how do you know from those cases that you posted that the unacceptable noise is generated by the 'pulsing' and not the drive or re-expansion anyway?

Greg, I agree with the prnciple completely, but imagine the loudspeaker that chops the air never completely covers the hole.. that each blade leaves a gap either end. It wouldnt be so loud. That is what manufacturers have managed to do to make these SC's very quiet. The problem with the twin screw is the re-expansion component generating noise as Stimpee has qualifed. This is not the same as the 'stopping and starting of airstreams with inertia' problem.

Greg

 

[berkshire]

Warmington (Automotive)
Yes there is an edit function in the preview post.
After you have submitted it you cannot take it back.
B.E.

 

[stimpee]

Greg,

I am not certain I understand what you are suggesting.

I am using a Lysholm 2300AX compressor.

I am still a bit unsure whether I plan to use an air-air intercooler with the associated piping, or whether I will integrate an air-water core into the manifold plenum similar to my previous setup. The air-air has some advantages, and in some ways might damp the likely brutal throttle response that this setup will probably have with the instant boost hit of the twinscrew combined with the ITBs. However using the air-liquid setup will ultimately result in a more compact setup, and will also allow me to later integrate it into a twin-charged setup if I decide to really go nuts, where the twin-charged setup would have air-air cooling after the turbo, and air-liquid in the manifold after the compressor.

In the case of a twin-charged setup, I would remove the 2300AX and switch to the 1600AX for better low-end boost response, while the turbo would maintain the upper end while keeping the compressor revs reasonable.

 

[Warmington]

I saw a couple of videos of the VW G60 engine fitted with the Lysholm SC. Those things are REALLY loud. How loud was the previous setup with the twinscrew SC post TB Steve?

I think you could tune out the noise in the transitionary period with careful sizing of the pipe to the intercooler for a certain frequency, whether using an expansion chamber or a tuned resonator, but it will be realively narrow spectrum. Maybe with careful tuning of the bypass valve and this pipe size/length, you can get rid of most of the noise during the period where it is uncomfortable at cruising speeds, but its still going to be really noisy as soon as the SC is fully hooked up. Best bet is just old fashioned sound deadening measures on the SC casing and inlet tract I guess. Have a look at this little setup, not suggested but its one way of doing it with a not so happy ending. Straight lobe SC though.

http://autospeed.com/cms/A_2570/article.html

Out of interest regarding the "positive displacement SC before a closed valve is suicide/stupid/ignorant" comments, Nissan didnt think so:

http://i17.tinypic.com/6prihdv.jpg

The TB is attached to the inlet manifold AFTER the charging system. There are no other valves used other than the bypass pictured.

Greg

 

[stimpee]

The draw thru system i did previously is actually barely noticable at idle and normal highway cruise, etc.

It is only moderately loud at WOT, and not unpleasant at all.

Not sure what I will be willing to live with on this setup. Interesting the link you posted. However that is a very very quiet car to start with, and they chose probably the worst case supercharger on the planet (straight lobed roots) to create the noise.

I guess we will see how bad it is, and then figure out what I need to do. The last ditch option will be to stake open the ITB's or remove the throttle plates, and put a big TB at the SC inlet.

However I will only be able to do this (I think) if I build the system so the SC discharges directly into an inlet plenum which then contains the air-water intercooler. Otherwise the throttle response effects will probably be undesirable.

 

[Warpspeed]

I have run roots blowers with a down stream throttle, but never a screw blower, at least not yet.

A blower air bypass system will work fine, the most successful home made bypasses I have used have been based on a modified external turbocharger wastegate,

Your concerns about light throttle rotor noise are very valid with this system. I can only make two suggestions. Use rubber hose (radiator hose) to route the air from the bypass back to the blower intake. That will tend to damp any high frequency resonances. An fiberglass absorbtion type muffler might work, but I have never been able to find room for anything like that.

If all else fails, fit a second large throttle body immediately after the air filter. The idea here is not to throttle the air, as much as reduce the air intake opening size, and keep the noise in. This sounds rather suspect I know, but it does in fact work rather well.

Arrange it so at idle or light throttle there is minimal pressure drop across this additional throttle body, but it only needs to be open just a fraction. Make sure the linkage opens this faster than the main throttle, so it is never restrictive. You will find that it can give a very worthwhile silencing effect without any real disadvantage.

 

[stimpee]

Warpspeed,

My system has a drive-by-wire throttle setup where there is an existing throttle control motor, with a linkage to the ITBs. I could possibly slave off this linkage somehow to open an aux throttle, however I am not certain that the existing drive system has adequate "horsepower" to open a big throttle body.

I think if I was going to go to the trouble of doing that, it might make sense to just dispense with the ITBs altogether? I am not sure what the advantage would be to keeping them at that point, other than slightly improved throttle response perhaps. In this case, with the free revving nature of the engine, coupled with the instant boost hit of the twin screw, I don't think I would really lose much in responsiveness though.

In my case, due to the layout, and the "bling" factor of having the ITB's I would probably keep them in place and either fix them in an open position, or likely just remove the throttle plates altogether.

Anyone have any thoughts or insight to share? This would ultimately meet my needs in that I would get the power delivery I desire, keep the ITBs in place for "looks" and simplicity, get the improvements in noise abatement and safety of using a draw thru system.

The only drawback would be in the fundamental view of "giving up" on the attempt to do something a bit more challenging than the norm!

 

[Warmington]

Maybe you could slave another throttle actuator from the same control signal? (assuming the ECU can handle the extra load).

You could use a very lightweight plastic moulded 'throttlebody' at the airfilter controlled by a second throttle actuator, it would only have to be slightly bigger than the total area of all the individual bodies and open at the same rate in order that it prevents any restriction.

In the back of my mind I can picture something just like this fitted to a VW... Can't seem to remember what that was, probaly some resonator that opened at a certain rpm. It was definately cable controlled though.

I for one would be very interested to see how the system performed in the blow-through setup before you ditched the ideas and comformed!

 

[patprimmer]

You could attach a conventional cable to the throttle pedal and use it in parallel to the fly by wire system. If there is different degree open vs pedal position issue between the two systems, make sure the suck through throttle opens fastest under all circumstances.

Regards

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

 

[Warpspeed]

The way I did it, I ran a second throttle cable direct from the accelerator pedal as Pat suggests.

My car has a manual gearbox, but there was provision for a kick down cable if an automatic transmission was fitted. This made fitting the second throttle cable dead easy, as everything was already there at the pedal.

This is reasonably simple to set up, usually a cable operated throttle body has a "snail" cam. The secret is to use the same snail cam on both throttles to give identical opening angles, but make the first throttle body sufficiently large in diameter never to be restrictive.

Wedge this first throttle body wide open, and set the warmed up engine idle speed. Then allow the first spring loaded throttle body to close against it's idle stop. Because of the extra size, the first throttle body can then be adjusted so there is no change in idle speed, or no significant airflow restriction.

Opening and closing this first throttle with your fingers at idle, will quickly demonstrate the significant induction noise reduction this system offers at small throttle.

 

[stimpee]

Interesting ideas. I think adding a cable to the throttle would be problematic, but I am not certain. The DBW throttle assembly is quite complex, and quite expensive on the BMW, and quite likely also quite quirky! I think I would be better off slaving either an electronic throttle off the existing command signal, or mechanically slaving it off of the ITB actuator.

I can say that my biggest concern over running in blow thru (once the appropriate bypass redundancies are in place per Pat's warnings), is the noise.

If I could abate the noise via resonators, airbox, etc, it would be simple. HOwever I fear that it will not be adequate.

Not sure if I should just design to accomodate a pre-compressor throttle "just in case". Or whether I should design it into the system from the get-go, and then simply remove it if I determine it is not required.

Decisions, decisions...

In any case, I thank those of you who have been sticking with this (especially Pat, who is obviously compelled to chime in, even though he doesn't want to!!) I appreciate having some qualified minds to help me think thru the ideas.

Thanks!
Steve

 

[Warpspeed]

I have no idea what a screw blower sounds like with an open air bypass, but a roots blower at idle sounds like a big angry bumble bee droning away. At cruising speed it rises to a fairly intrusive hum, but the strength dies away with further throttle opening (as the bypass progressively closes).

Full throttle with the bypass fully closed, there is no problem, until you change gear in a manual, then it really roars.

Either music to your ears, or a bloody awful racket, depending on your age, social class, and general attitude to the gentle art of motoring.

 

[stimpee]

In no case will my system be put together with an open air bypass. The bypass will be a true bypass, and plumbed back in.

If I do a blow-thru setup, I will have an open-air pop-off or blow-off valve, as a fail safe, and also it will likely activate during brief throttle closures such as shifting similar to a turbo, but that I can deal with.

I just will not be able to deal with a constant SCREAM from the blower under normal cruise conditions, etc.

 

[Warpspeed]

Even completely plumbed back, the blower intake will still effectively have an acoustic path back out through the air filter. Pipework is great for transmitting sound.

At the very least, some sort of rudimentary muffling will be required. My own problems with this, led to the second throttle body idea, which does provide one fairly effective solution.

The only other idea I have come up with to solve this noise problem, might be to vent all the unused air down the exhaust pipe (after the cat converter). That should be practical if a MAP sensor is used. Not possible with a mass airflow meter though. I have not yet attempted this myself, but may possibly try it one day on a future project.

 

[stimpee]

On my current setup, there is a path around the throttle from the compressor inlet housing to right after the MAF and air filter, for idle air bypass. Granted that is a smallish 1" diameter rubber hose, but it doesn't seem to transmit any noise.

Let me get this straight though, if I read what you are saying, you have done or attempted a blow-thru setup in the past, and ended up adding a throttle on the inlet just due to the excessive noise from the blower (in this case a roots)?


Steve

 

[Warpspeed]

You say "compressor inlet" that suggests a turbo ?

Roots and screw compressors have a pulsing output which creates an entirely different, and more severe noise problem. Bypassing some relatively smooth flowing air around a centrifugal compressor can be made virtually noise free.

Yes Steve, I have run roots blowers with a down stream throttle, and a modulated air bypass. It worked exceptionally well from every aspect except the noise problem from the bypass air. The second throttle was a very crude but effective solution.

 

[stimpee]

When I say compressor inlet, I am still talking screw compressor, not turbo or centrifugal. Since the Lysholm unit is a true compressor, I didn';t consider that an issue!

It is looking like I should plan to include the auxiliary draw thru throttle, since it is likely that I will find continuous noise to be "problematic".

 

[jcd06]

Hi,
Some people here have arguments against the idea of the blow-through and by that pointed out some of the potential difficulties/dangers of that system. Thanks for that because that's the way I was planning to go a I still am actually but now more conscious of these issues.
As Stimpee told in the beginning of this thread, some production cars are setup like that. The supercharger I was planning to use comes from a Mercedes M271 engine which is used in a number of different forms. It is an Eaton M45 equipped with an ECU controlled throttle bypass valve and installed in that famous blow-through together with an electronic throttle. It has 3 noise dampers, first between air filter and charger, and two others -one of them called "broadband"- between charger and intercooler.
I have a MB booklet here presenting the engine and its ancillaries containing nice pics and tech specs like the different torque and power curves, charge pressures, pulley ratios but hey it's copyrighted material. I won't drop it on the internet but I could e-mail scans to those interested in this matter for purpose of technical study…
I have the first damper after the charger too, could take some pics.
Regards,

Jean