ThetaTau87

Yes, there is always a load due to the internal compression. When you spin a twinscrew compressor by hand on a bench you feel the resistance of the internal compression and hear the pops when each pulse is released. That is with zero pressure differential across it and both inlet and outlet open to atmosphere. I'm talking about the load from a pressure differential between the inlet and outlet.

When creating boost any compressor (turbo, centrifugal SC, roots SC or twinscrew SC) must work against the positive pressure it is creating in the engine. The higher the pressure differential the more power it takes to force additional air in. For a draw through SC system the reverse is true. When the throttle is closed the compressor becomes a vacuum pump that is working to draw a vacuum behind the throttle plate which causes a load on adition to the load from the internal compression. When the BPV is open the pressure differential is equalized and the is no more load from trying to pull a vacuum behind the throttle plate.

AO

What he said.

ptuomov

I have a couple of problems with this explanation. First, any time one bypasses anything to between the throttle plate and the compressor inlet, one increases the work that the compressor needs to do to internally compress the air at each cycle. This is because the inlet pressure increases. Second, a perfect vacuum in the compressor inlet isn't applying any force to the compressor, it's the absence of force. Third, under which conditions is there a high pressure at the compressor outlet when the throttle is closed, isn't the base engine pulling a high vacuum to the compressor outlet, especially at high rpms?

The bypass valve that makes sense to me in a twin screw compressor is the integrated flappy valve in the bottom of the screw compressor that prevents the internal compression when the outlet pressure becomes lower than the pressure inside compression chamber. But I don't think that's the type of bypass valve this system has.

Still confused...

Thoomas

This will be really helpful for me. Many thanks!

/Thomas

Thoomas

+1 on that!

blau928

A bypass helps with the following on screw compressors:

You need to equalize the pressure differential between the rear bearing seals and the inner and outer compressor case when the throttle is closing quickly and MAP falls rapidly.

It also prevents surging when the throttle is slammed shut when you let off the gas pedal.

As a screw compressor needs the throttle to be places before the compressor inlet, this causes distinct dynamics on the system via a vis a centrifugal or turbo compressor which have the throttle placed after the compressor. The heat loss in recirculating the air is minimal. The bypass is not used to bleed off charge air, as it is vacuum controlled by engine MAP. Bypass will not open under positive manifold pressure. To control manifold pressure and or air mass/flow in a screw compressor setup, one calculates compressor rpm and uses the output measurement of the compressor. And rpm geared with the crankshaft pulley for system flow at rpm. It is linear compared to a centrifugal or turbo, which are non linear.

Hope that helps,

ThetaTau87

The bypass does not increase the work that the compressor needs to do internally to compress the air. It removes the pressure differential which lowers the load on the compressor. Moving air from lower pressure to higher pressure takes more work than moving air from two spaces at equal pressure.

This statement is completely wrong "Second, a perfect vacuum in the compressor inlet isn't applying any force to the compressor, it's the absence of force." There is no perfect vacuum anywhere in the system at any time. There is always a load on the compressor if there is a pressure differential across it. The only time there is no pressure differential is when the bypass valve is open to equalize the pressure on both sides.

You seem to be getting confused by thinking that a compressor works differently in a partial vacuum than when creating boost. The only difference is that boost is higher than atmospheric pressure and partial vacuum is lower. A compressor works the same regardless if there the inlet pressure is above, below or at atmospheric pressure. There is no situation when there is no force on the compressor because it is working in a partial or even a theoretical perfect vacuum.

If there were no bypass valve the pressure would be lower at the compressor inlet than the outlet under all conditions, vacuum or boost. This is because the SC moves more air than the engine. Under boost the SC takes air at atmospheric pressure and by moving more air than the engine can consume it increases the pressure in the intake manifold creating boost. When the throttle is closed the SC would still be moving more air than the engine and pull a higher vacuum pressure on the inlet side of the compressor than the engine is on the outlet side. This pressure differential creates a load on the SC. The bypass valve eliminates the pressure differential and the load caused by it.

It would be exactly the same if the bypass valve were opened when the SC was making boost. All the boost pressure would be released and there would no longer be a load on the SC due to the pressure differential from atmospheric pressure to the boost in the intake. The only load on the SC would be from the internal compression ratio.

A twin screw does not only make boost from its internal compression ratio. It makes boost because it pumps more air than the engine than the engine can consume. A 2.0L twinscrew compressor will make more boost than a 1.8L twinscrew compressor with the same internal compression ratio. This is because the 2.0L unit pumps more air per revolution not because of the internal compression ratio.

ptuomov

Undercompression and over compression point is a good one.

This is the best explanation of the physics of over and under compression point I've read:
http://www.plantservices.com/assets/Media/1402/white-paper/twin-screw-blowers.pdf

ptuomov

AO - if this discussion is cluttering your thread too much, ask the mods to kick it to another thread.

Did some thinking and reading about twin screw compressors over the weekend. I was in Las Vegas with my friends and since I am happily married man with only modest interest in college basketball and no gambling addiction, I had a lot of time to scribble stuff on scrap paper. Here's what I concluded.

A perfectly matched twin screw compressor has an internal compression ratio that matches the system compression ratio. That is, the compressor doesn't pop, because the manifold pressure is the same as the compressor outlet pressure. Since the intake runners are short and the cams are conservative, in the 928 twin screw systems with a correctly sized supercharger and pulley should get you close to ideal. Thus, I believe it's possible to put together a combo that will not need or benefit from a bypass valve.

If the compressor is not matched correctly, then it either over or under compresses. The compressor may also be overall correctly matched but slightly over compress at some rpms and under compress at do a other rpms. In those cases, there may be a benefit from a bypass circuit. The main benefit is noise reduction, but there may be other benefits as well.

The idea of the bypass is to match the internal compression ratio with the system compression ratio. The best bypass is a bypass built into the compressor casing. This sort of internal bypass can reduce the internal compression ratio when the intake manifold pressure is lower than the natural outlet pressure if the compressor. This is amounts to a real efficiency gain, because the internal compression ratio is variable.

An external bypass can also be used, but it doesn't produce the full benefit of the internal bypass. An external bypass is such that it modulates the pressure ratio between the compressor inlet and the intake manifold to match the internal compression ratio of the compressor. Because the bypass equates the intake manifold pressure with the compressor outlet pressure, it eliminates the popping sound of air rushing either back into the compressor.

The bypass valve system that one sees in roots blower systems, such as Eaton blowers, is fundamentally different from the external twin screw bypass systems. This is because roots blower is an external compressor and the correctly matched twin screw blower is a pure internal compressor.

The above assumes that the compressor is not clutched. A clutched compressor would need a large bypass circuit.

So based on my logic, an external bypass valve circuit for a twin screw blower should be designed as follows. We need a valve that has two signal ports, one connected to the compressor inlet (downstream of the throttle) and the other to the intake manifold. The valve is such that it opens if the pressure ratio of intake manifold to compressor inlet exceeds the internal compression ratio of the compressor. Then it relieves some of the intake manifold pressure to the compressor inlet, reducing any popping sound.

Does this make sense to you?

AO

Naw.. it's all good. I never really took time to think it all through - I'm more of a macro guy. I understand the basic concepts, so a more detailed discussion is good for all.

The way I'm thinking about it is this:

The TS SCer will always try to compress air, that's it's function. It will always provide more air than is needed by the engine (based on relative pulley ratios) when at something less than 100% boost. But you need a way to divert that "extra" air away from the engine at times, otherwise you'd be at max boost all the time - even at idle.

I like to think of it as a fluid pump that will supply a volume usually greater than what I need. Imagine you have a reservoir of fluid. There is a valve that can replenish the reservoir as needed. Downstream I have a machine that depends on this fluid. There is a natural flow of fluid to the machine, but I've also fitted a pump to help when I have high-demand situations.

Let's give these some values to represent them. Let's say the natural flow can be represented by X, the pump supplies Y. The total system provide X + Y. The system will always consume at least the natural flow - X, and occasionally as much as the entire system, X + Y, can provide. But the pump is always on - that's an important concept here.

So I need some way to return the excess volume back to the reservoir. So at it's nominal state I use a bypass valve to return the excess (Y) back to the reservoir and it basically recirculates that volume. The net delivery to my machine is X.

Now if i increase my needs, X', the pump will supply me with X' + Y', and return Y' back to the reservoir. I can choose to divert slightly less than Y' back to the reservoir by modulating my bypass valve. This is a akin to running partial boost.

Now, if I request full demand, the bypass valve closes. Now the full amount of X' + y' is being delivered and consumed by the system. This is full boost.

When i no longer need all this fluid, it's much faster and easier for me to control the flow into the system by modulating the bypass valve than controlling the amount going into the reservoir.

ANyway, that's how I think of it. Maybe it's over simplified, but I find thinking of it in simple terms often helps with the underlying concepts.

ptuomov

Can you please explain to me in detail how this oiling issue works? I know that draw thru throttle turbo system needs different seals than a blow through throttle turbo system. However, the drive mechanism and oiling system of twin screw are completely different from those of a turbo. So how does the twin screw oiling work in different pressure scenarios?

Also, why would anything surge in a twin screw system when the throttle is closed? I've been assuming that the belts is not intended to slip. If the clutch is not depressed, the system will run at the driveline speed, i.e., engine braking. If the clutch us depressed, then the engine by my logic should surge or not surge the same way that a normally aspirated engine would or wouldn't. What's the underlying cause of this additional surging with a supercharger installed?

DKWalser

Please allow a rank amateur to demonstrate his lack of knowledge by offering an opinion: When the throttle closes it cuts off air from the intake. It doesn't cut off all air, allowing just enough for the motor to idle. However, if there is a supercharger after the throttle, the supercharger will suck more air past the throttle. This will cause the supercharged motor to idle at a higher RPM and/or take longer to slow down when you come off the throttle. It might even cause the motor to surge.

A bypass valve avoids these problems by shunting excess air back to the inlet side. This prevents more air from reaching the cylinders than what would happen with a naturally aspirated motor. An alternative approach would be to have the supercharger disengage whenever you come off the throttle.

ptuomov

Like with naturally aspirated motor, there's an idle air circuit (or a screw) that leaves an orifice open for idle air even if the throttle is closed. How much air goes into the engine, whether it is naturally aspirated engine with a plenum manifold or a twin-screw supercharged engine with a draw-thru throttle, depends entirely on the pressures on both sides of the throttle plate and the flow rating (area and the discharge coefficient) of the idle circuit. The idle circuit doesn't know or care whether its feeding a supercharger or not.

If the compressor inlet pulls a similar vacuum as the naturally aspirated motor would, then the same size idle circuit should produce similar idle speed. If the compressor inlet pulls more vacuum than the naturally aspirated motor, then one can restore the idle speed by reducing the idle circuit flow rating by, for example, making the size of the hole smaller. I don't see any reason here for a bypass valve from the compressor inlet to compressor outlet, do you?

ptuomov

Suppose that without the bypass, the compressor would neither over or undercompress. There are two components in the work done. The first is the work done by the internal compression. The second is the work done pushing the compressed volume of air against the intake manifold pressure minus the work that compressor inlet pressure does to push the uncompressed volume of air into the compressor. If the compressor would neither overcompress nor undercompress in the absence of the bypass circuit, then the bypass circuit flows air in the direction of from intake manifold to the compressor inlet. This kind of bypassing from inlet to the outlet increases the first component of work and reduces the second component of work. How do we know that the net change is lower overall work?

The statement happens to be exactly correct. Fluids can't pull, they can only push. There is no way that positive (but lower than ambient) absolute pressure exerts any sort of "pull force" on the inlet. There is no uncertainty about that statement.

Higher inlet pressure helps push the air into the compressor, reducing the power needed to run the compressor. Higher outlet pressure makes it harder to push air out of the compressor, increasing the power needed to run the compressor. If this were a roots blower that does external compression and blows into a large intake manifold, then we'd be done -- bypass would reduce the power draw of the compressor.

However, this is not a roots blower. It's a twin screw compressor. The difference is that the twin screw compressor does internal compression.

But it increases the work needed for internal compression because it increases the pressure at the inlet. Right? What's the net effect of the two?

This is a good point. Before I read that, I was only considering the case of correctly matched twin screw compressor. If it undercompresses or overcompresses, the situation is a bit more complicated. One can think of a roots blower as a limit of a twin screw compressor as the internal compression ratio approaches zero and the compressor becomes infinitely undercompressing.

ptuomov

Isn't it the other way around? The intake manifold pressure is a measure of restriction in the base engine. High restriction, high boost. Low restriction low boost. By that logic, the intake manifold boost becomes high, not low, when the supercharger attempts to deliver more air than what the base engine can consume. The higher boost increases the air density, and then helps the base engine ingest the mass of air that the positive displacement supercharger provides.

If you measure boost as the difference between the intake manifold pressure and the ambient pressure, then when the draw-thru throttle is closed, you'll get no boost after you close the draw-thru throttle. Suppose you have correctly matched screw compressor with compression ratio of say 1.5. Closing the throttle will draw a big vacuum in the compressor inlet, say 5 psia. Then (roughly approximating) the compressor outlet pressure and intake manifold pressure will be something like 7.5 psia. If ambient pressure is about 15 psi, you'll have a negative boost of -7.5 psia in the intake manifold with the throttle closed.

Since the air is compressible, I don't think this intuition really works that well.

Since in our case all the air goes thru the pump, I don't think you can just add the two capacities. Furthermore, since the capacities are in volume and not mass, the compressibility of air gets you. So the engine will build pressure in the intake manifold, which will then cause the base engine to consume all of the air no matter what.