blau928

Surge occurs when the compressor outlet charge is greater than the engine can ingest after the throttle is closed. Without the bypass this can occur during rapid throttle closure.

The rear case bearings on screw compressors are usually sealed, not oiled.

Oiling is usually to the drive gears within a closed chamber. If open chamber oiling is desired, then the chamber must have a drain and the oiling feed to the gear case must be metered. (Similar to a turbo)

On another note, popping can be controlled with material thickness, and bypass valves can be optimally placed to be seamless if internal or external based on system design.

Belt slip occurs unless the belt is cogged, and clutches introduce another drive link weakness into the system.

Best regards,

ptuomov

Is the mechanism here that the volume between the throttle blade and the compressor inlet is large? After the throttle is closed, the engine has to first consume much of that air before slowing down. If that's the mechanism, then a bypass from the compressor outlet to the compressor inlet might make the engine respond to throttle closure more gradually, the initial response being a tiny bit faster but the full slowdown tiny bit later.

Or is there some other mechanism for "surge" at throttle closure? I am asking because I'm thinking that the above described mechanism is by my intuition going to be trivially insignificant.

blau928

The volume between the throttle blade and compressor inlet varies depending on system design.

In as much as it may appear to be trivial on surge, one is not able to solve the bearing case pressure equalization to prevent damage to the the bearing seals without valving.

Some manufacturers have gone to liquid cooling the front bearings and also equalizing the pressure on the front seals to decrease failure rates.

Bypass valves help to significantly address the pressure equalization issue in the system that damages the rear bearing cases and seals.

In another note, if you have excessive volume in the chamber between the throttle plate and the compressor inlet, you will have tip in as well as surge issues...

Best regards,

ptuomov

Can you explain me the disease and the cure like you would explain it to a smart fifth grader?

My understanding is that some superchargers have a combined oil and air seal. Others have an inner air seal, a double wall cavity, then oil seal, and then oiled gears. In the latter arrangement, the middle cavity is vented to the air filter or something and it equalizes the pressure on both sides of the oil seal. The air seal will still see a pressure differential. Is this correct?

Whether or not the above is correct, how does an external bypass circuit circuit help solving the bearing case pressure equalization problem?

That makes sense, because one end of the screw compressor is always cold and the other end is always hot. This is because of the internal compression. Higher the compression ratio, higher the temperature differential. Cooling the hot end of the case seems like a good idea to prevent the tilting of the rotors and the gear timing becoming imprecise. But how would this problem be solved with an external bypass circuit?

How? Can you please point me to something that shows how such a bypass valve system is plumbed so I can try to understand how it works? Or just explain to me verbally (/in writing) how the system is plumbed and what are its operating modes?

blau928

I will try to get you a pdf of the plumbing tomorrow. However, if you happen to have access to Corky Bell's book on supercharging, it has some good diagrams.

See my blue typing above if you are able to visualize #1, #2, & #3. In essence, the bypass connects the intake plenum post compressor outlet to the compressor inlet post throttle body. (#1 & #2)

The way the valve functions, it equalizes the differential pressure on the inlet side of the compressor and the outlet side of the compressor, and allows the compressor to be in recirculate mode until the throttle is open. The bypass valve is open under vacuum. Once there is less vacuum and more pressure, the valve closes and allows all compressor outlet charge to go into the engine.

Do not confuse the way this valve works with a Blow Off Valve. BOV releases air charge/opens under set spring or electronic valve target pressure; A Bypass or diverter valve releases/opens when system or target is vacuum. It's logic is inverse to that of a BOV.

Hope that helps.. Sorry, at the office, got a business to run..

Best Regards,

ThetaTau87

The first component of the work due to internal compression can not be increased. It is fixed. The work done during internal compression is defined by the internal compression ratio, not the inlet pressure relative to the outlet pressure.

Take a look at the twin screw energy efficiency PDF that you linked. Fig. 4 shows the two components of work that you describe. The work to the right of the Vs/Vi line is fixed. If you raise p1, pi increases by the same amount and the work done right of the Vs/Vi line remains the same.

When pi > pe as is the case with the bypass valve open you get over compression as seen in Fig. 6. The wasted work of over compression is due to the work related to internal compression being fixed. As you stated in another post the only way to reduce this work would be with a valve that could change the internal compression ratio and equalize the pressure inside the compressor with the inlet pressure, but a bypass valve does not change the internal compression ratio. It equalizes p1 and pe, but the internal compression ratio ramains fixed.

Now look at the work to the left of the Vs/Vi line. This is the second component of the work done by external compression. It is dependent on the pressure differential between the inlet and outlet of the compressor (pe - p1). With the bypass open this pressure differential goes to zero becasue pe=p1 and the work required on the left side of the Vs/Vi line goes to zero. This reduces the overall work done by the compressor to only the work required for internal compression when the bypass valve is open.

As I've already demonstrated the work for internal compression can not be increased or decreased by raising the inlet pressure relative to the outlet pressure. I also showed that a bypass valve eliminates the work done by external compression. That makes the net effect of having the bypass open a reduction of the work done vs. having it closed.

A twin screw compressor does not do all of its work by internal compression. Even if it is perfectly balanced and neither over or under compresses. As shown in the pV diagrams of the PDF you linked there is always a component of work done by external compression against the pressure differential between the inlet and outlet. This is shown by the rectangular blue area of the pV diagram. The amount of work done by internal compression relative to the total work is determined by the internal compression ratio. As the internal compression ratio goes up more work is done due to internal compression, but it can never be 100%. Even if the internal compression ratio were 100:1 that would mean that 99% of the volume reduction was done by internal compression, but 1% is still done by external compression. An infinite compression ratio would be needed to have no work done by external compression and this is only possible in theory.

ptuomov

That's simply not true. As a thought experiment, think of sending the twin screw compressor into the outer space with a space shuttle. (Also as a thought experiment, suppose that the space shuttle Russian rockets work, the fuel tanks doesn't explode, etc.) When you turn the twin screw compressor in outer space, in near perfect vacuum, how much work does it take to turn the compressor? Almost none. Think about it.

I don't think that's true. The work done is proportional to the area, that's true. The Vs/Vi side is true as well. However, where you're wrong is that increasing p1 by one unit does not lead to an increase of pi by one unit. The increase is closer to proportional. Again, just as a thought experiment, think what happens to the internal compression work as the inlet pressure and density approach zero.

That's not true. There is another way to reduce the internal compression work, which is to reduce the compressor inlet pressure. The problem with bypassing a high pressure from the intake manifold to the lower pressure of the compressor inlet (supposing those are the conditions) is that it increases the inlet pressure and therefore also increases the internal compression work.

What I don't know is whether this increase compares in size to the reduction in pumping losses.

I agree with that. That's the benefit. If p1 would not change, it would be all benefit. But connecting p1 and pe with a bypass will in the usual situation result in p1 increasing as well pe declining. Increasing p1 will increase the internal compression work.

If you look at the third mathematical expression on page 5 of the pdf document which is the theoretical work done by the compressor, you'll see that every term in the expression is multiplied by the inlet pressure p1.

Furthermore, if you take the case of a bypass that completely eliminates the system pressure differential, that is, causes p1=pe, then only the second term inside the parentheses remains. The work done by a fully bypassed compressor equals p1 * Vs * {[k/(k-1)]*[(1/k)*(vi^k) -1]}, where Vs is the fixed displacement, vi is the internal compression ratio of the compressor, and k for air is 1.4. In other words, the theoretical work done by a fully bypassed compressor is proportional to the inlet pressure.

It's simply the case beyond any shadow of doubt that higher inlet pressure increases the internal compression work done by a twin screw blower.

This I believe is correct.

blau928

Please see the PDF attached. It has bypass as well as oiling information. I will try to attach the bearing seals and issues tomorrow as time permits.

Best Regards,

DKWalser

Richard -- Thank you. The PDF was very informative. What is the source?

blau928

You're welcome...

It's from Corky Bell's book on supercharging. It is filled with information and computational tools to explain and design forced induction engines. He also wrote a turbocharged book that has many overlapping items with the supercharged book.

Best Regards,

ptuomov

Thanks for the explanations.

I've been slow to respond because it's taking me a little time understanding this.

Bell writes: "Cruise conditions and other normally aspirated operational modes can suffer from undesired pumping losses. When cruising at approximately 15 inches of vacuum, the system without a bypass valve will create about 20 inches of vacuum between the throttle and blower, because the Roots is pulling from the throttle plate. this "boosting" from 20 inches to 15 is a constant and unnecessary waste of power and also produces a small amount of heat."

I tend to agree with the above in the case of a roots blower. However, I'd say that whatever issues one creates there when running without a bypass valve are roughly analogous to running a larger displacement normally aspirated engine. I can see why a bypass valve will create a small efficiency gain in a roots supercharger system.

In the noise section, Bell writes about twin-screw superchargers: "At low-speed boost, say 8 psi or less, [the noise] is not a problem. At 10+ psi it becomes piercing, and some precautions may need to be taken, similar to those taken on the exhaust side of an an engine. The features that suppress noise are thick material sections, heavy-wall tubing, overlapping tube joints, and thick rubber flex joints."

I'd want to clarify one thing there. It's my understanding that the popping noise comes from the intake manifold pressure being different from the compressor outlet pressure. That is, the exit pressure after the internal compression is different from the intake manifold pressure. If the exit pressure is higher, then the air burst out of the supercharger at each compressor cycle. If the intake manifold pressure is higher, then the air bursts into the supercharger at each cycle. By my logic the popping noise is minimized by making the compressor exit pressure naturally equal to the intake manifold pressure. This I believe can be by either correctly matching the compressor to the rest of the engine or by oversizing the compressor in terms of flow / undersizing it in terms of internal compression ratio and then bypassing some of the flow from the intake manifold into the compressor inlet.

I can't agree with everything that Bell writes about the bypass valve on page 92 for the twin-screw compressor. I don't think he's sufficiently thought thru, or at least explained, the differences between the roots and twin-screw superchargers. The roots logic only applies to the twin-screw superchargers in modes in which the compressor grossly undercompresses and therefore much of the compression happens externally, like in a roots blower. It's only in those modes in which I can see a benefit from an external bypass valve. (An internal bypass valve that reduces the internal compression ratio of the twin-screw compressor is a different story, and there we've got real benefits.)

Because of this, if I were to design a twin-screw system, I'd set up the bypass valve to only bypass from the intake manifold into the compressor inlet if the pressure ratio is larger than the internal compression ratio of the twin-screw and only to equate the intake manifold / compressor inlet pressure ratio with the internal compression ratio of the twin-screw. Bypassing beyond that isn't going to help with heat or noise etc. Do you agree?

Still have to think about the oiling.

ThetaTau87

You are correct about the internal compression work being relative to the inlet pressure. By increasing the inlet pressure with the bypass the work done during internal compression will increase. I was thinking that the relationship of p1 to pi was linear, but it is proportional to p1

However, I don't think this negates my original point that the bypass valve reduces the total load on the compressor by removing the work done by external compression. The load from external compression is completely removed, but the load from internal compression is only increased by an amount proportional to the change in inlet pressure. The increase in inlet pressure caused by opening the bypass will be 1/2 the delta between the inlet and outlet. This doesn't seem like a large enough increase for the added work required for internal compression to be greater than completely eliminating the work from external compression.

Even when balanced a twin screw can only do a portion of the work required internally as shown in Fig. 4. This portion is determined by the internal compression ratio. For all the work do be done internally the compression ratio would need to be infinite. Regardless if the twin screw is under or over compressing or perfectly balanced opening the bypass will remove the work being done by external compression.

In this application a balanced internal compression ratio would be a ratio around 2:1. That would leave a significant proportion of the work required to be done externally.

blau928

With that said, I think it would be interesting to try a newer liquid cooled, seal pressure equalization SC from Kenne Bell. (I think these are HPS units, but am not sure). Not sure a 3.6 or 4.2L SC would fit in a 928 engine bay without cutting the bonnet open though... They are large pieces of kit...

PS See the PDF for the bearing seal issue.

ptuomov

"It would be interesting if you can find a compressor that is a production item that had both the desired internal pressure ratio as well as the flow requirement for a specific size of engine. In my experience, it has always been a compromise of parts to achieve a system output of desirable results. as such, i think it's easier said than done, and easier to have a $100 valve plumbed appropriately."

I think this could be done as follows. First, choose the boost that you want to run in the operating rpm. Say it's 10 psi. Next, estimate the throttle, inlet, filter, etc. loss. Say it's 0.7 psi. Now, the compressor inlet sees 14 psia and the desired outlet pressure is 24 psia. This gives a system compression ratio of 1.71. Pick a twin screw compressor with that internal compression ratio. Then, adjust the pulley size such that the running engine actually shows 24 psia in the intake manifold in the operating rpm range. There's two degrees of freedom in the compressor selection (pulley size and the internal compression ratio) and you can use those to match the desired parameter (mass flow rate and the manifold pressure). I think that the ability to independently select the pulley size is what makes this feasible in practice.

Now that I am thinking about this more, do you guys run your cams retarded when using twin-screw supercharger? Compared to a larger, normally aspirated motor, the supercharged motor is less efficient because you have to lower the compression ratio to avoid knock and that will give you a lower expansion ratio. But can you get that efficiency back by retarding the cams significantly? That is, open the exhaust later than usual to extract the maximum amount of power from the charge. This means that you'll have to keep the exhaust open later to evacuate the combustion chamber, but that's ok because the intake manifold has a higher pressure than the exhaust manifold and you can use the intake charge to blow the exhaust out of the combustion chamber in the end, much like in a two stroke. Now, you can also open and close the intake valve later than usual. The intake charge is very dense, so the pressure wave near the IVC should be pretty powerful and allow for a later IVC. And even if it doesn't, the positive displacement supercharger will cram the air mass in there by increasing boost. Seems like this would get you a pretty efficient setup. This is the first I thought of it, so retarding the cams may be a stupid idea for some reason that I haven't thought of.

""Because of this, if I were to design a twin-screw system, I'd set up the bypass valve to only bypass from the intake manifold into the compressor inlet if the pressure ratio is larger than the internal compression ratio of the twin-screw and only to equate the intake manifold / compressor inlet pressure ratio with the internal compression ratio of the twin-screw. Bypassing beyond that isn't going to help with heat or noise etc. Do you agree?""

"I have to give that some thought, and figure out how the valve trigger would work. Would it be a real time monitoring of MAP tied to a lookup table for ratios to only open the valve at certain ratios? This seems like it would make a simple valve issue be difficult at best, as it would involve much more than is needed for the small gain in performance."

I was thinking simply two diaphragms on a rod. The ratio of areas of the diaphragms would equal to the internal compression ratio of the twin screw compressor. The large area diaphragm port would be connected to the compressor inlet. The small area diaphragm port would be connected to the compressor outlet. The valve would be normally closed with a spring that would just offset the desired target boost level. The rod would keep the valve closed as long as the intake manifold pressure less than or equal to the compressor inlet pressure times the diaphragm area ratio. Since the diaphragm area ratio is set to equal the internal compression ratio, the valve only bypasses when the system compression ratio becomes larger than compressor internal compression ratio. No popping because of overcompression. That's simple enough mechanical valving, no?

ptuomov

I agree, with the minor tweak of saying that if the compressor is perfectly balanced (pi=pe) the external work is merely pushing and not external compression. But I agree that its still work.