69gaugeman

Kimmie's in the north ;-)

69gaugeman

I've started making fixed diameter sleeves just like the factory does. Just a piece of mild steel bored out .050mm smaller than the bore. Polished a bit and tapered at the top. Slide the piston in until the skirt is poking through, place on the cylinder bore and it just slides in. Works fantastic every time.

Of course I have free access to a machine shop, which makes it that much easier for me....

AO

As far as you know... He's a huge K-POP fan!



That's the way to go, for sure. I assume there's a taper so the rings get compressed as you slide it in the bore. Would have loved something like that!

ptuomov

I agree, it just has to be consistent in that the amount of air in the cylinder per the relevant stroke is consistent given the MAF load and RPM readings. It's a lot easier to understand and tune if it's symmetric, but thinking about it as a black box, it'll be enough for it just be consistent.

I don't get the need for the bypass valve. This is a positive displacement supercharger, right? Twin screw? So if I close the inlet throttle, it'll such a vacuum to the compressor inlet and air is very low density. It'll compress the low density air, but so what? The air mass is limited by the throttle position and the inlet vacuum. There isn't even much parasitic loss given that the compression work is done regardless.

In contrast, if you recirculate the compressed air to the inlet side, it'll expand, turn the potential energy into heat, and the compressor has to redo the work of compressing it. So why is this desirable?

I understand that you might want to bleed off some boost to control knock at some rpms, for example. But I don't see the need for a bypass valve for throttle closings if the throttle is on the suction side of the positive displacement compressor.

This is an honest question, I don't understand the exact functioning of these bypass valves on positive displacement supercharger systems with draw-thru throttles.

AO

When a gas expands, it cools.

Here's the most succinct explanation I could find.

ptuomov

Gas does cool when it expands.

But consider the following system. First you compress the air. It heats up. Next you relieve the air into the inlet side of the compressor. It cools. However, the result is hotter gas than what was initially ingested, because the potential energy stored in the pressurized gas is partially converted into heat when the gas is allowed to expand. So the work that the positive displacement compressor does in a closed system turns into heat. Right?

If you have an intercooler, the heating problem from recirculation is less severe, but the you're heat soaking your intercooler, unless it's a boat.

"Positive displacement superchargers (Twin Screw and Roots) are positioned downstream of the throttle body (suck through). If the throttle body is closed at high rpm, there is no surge problem as with centrifugals (blow through)."

I am in this camp, I am thinking that with a positive displacement supercharger that has a draw-thru throttle, there's no surge problem.

There's also no heating problem, as far as I understand. As I commented above, I believe that the heating of the charge in a closed system is determined by the work that the compressor does. One way to reduce that work is to reduce the inlet pressure. You do that by closing the throttle and not bypassing anything into the inlet. Another way to do that is to reduce the compressor outlet pressure. In theory, you might be able to get a small relief there if you'd let the air out thru a bypass valve immediately, but the engine itself will pull the compressor outlet to high vacuum very quickly, so I don't see there being a benefit there either.

I haven't figured out how the bypass valve from the outlet reduces the heating of the charge on a positive-displacement draw-thru-throttle supercharger system. Pretty much the only way would be if the compressor can be bled from the compression chambers such that when the valve is open it doesn't actually compress the air, just "blows" it. I think the twin screw bypass valves I've seen in photos of factory cars are internal to the compressor and are also used to bleed out oil if any collects in the compression chamber, that from memory.

Any education on the topic would be greatly appreciated.

AO

Not at all. If you have a properly sized heat exchanger, any heat will be transferred to atmosphere. Your IC medium should always be at ambient temp. And while you are correct in that under cruise there is some NET heat generated above the original inlet temperature, it's not that much higher. I believe Tony, had his SCer thermocoupled pre and post SCer. It's somewhere in the Twin-screw thread. The point is, that even at cruise, it's not like the temps are going to continue to climb. They will stabilize at some delta above normal inlet teps. That's why there are some TS setups without an ICer.

With the addition of an IC, however, your post ICer air temps (at cruise/idle) will be close to normal inlet temps with no ill effects of heat soaking the ICer medium (again, as long as HE is properly sized).

Getting back the the bypass valve, if you didn't have it, you'd always be making boost. The net effect of the bypass is to allow the engine to operate in "suck mode" instead of "boost mode." It serves the same purpose as a BOV. Often, BOVs are vented back to the inlet for noise considerations. In that context, it is essentially a large circuit bypass valve.

ThetaTau87

This is correct. The bypass valve (BPV) is required because at any RPM the SC pumps more air than the engine can draw in at WOT. This is how it makes boost, by forcing air into the engine. With the throttle closed the SC would still be trying to pump in more air than the engine requires creating a pressure differential between the intake and outlet of the SC. This creates a load on the SC. By opening a BPV to recirculate the excess air flow from the outlet to the inlet side the pressure is equalized and the SC is allowed to rotate with minimal load. In the case of a twinscrew there is still the load of the internal compression between the screws, but there is no way around that. With a roots style SC that doesn't internally compress the air the drag from the idling SC is just from the friction of the bearings and other rotating parts.

Your statement about a BOV is not correct. A BOV always vents to atmosphere while a BPV or diverter valve recirculates excess airflow. They both perform the same basic function, to dump excess boost, but they are not interchangeable terms since they dump to different locations. BPVs should always be used with MAF systems because any metered air dumped to atmosphere will cause a rich condition the same as a boost leak. BOVs should only be used with MAP systems that measure airflow based on manifold pressure.

ptuomov

As an approximation, suppose that the supercharger compresses 2:1. Ignoring the ideal gas laws a bit and just saying that the pressure doubles and temperature stays the same, at full throttle you're going from 14.7 psia at the compressor inlet to 29.4 psia in the compressor outlet. At closed throttle, your in principle going from 0 psia to 2*0 psia = 0 psia. It's not going to be literally 0 psia because there are leaks and there's some cavity volume, but it's going to be a very, very low absolute pressure. And 2x a very low number is still a very low number. If you define boost as pressure differential, you aren't making any meaningful boost when the throttle is closed. I can repeat the computation with ideal gas laws, but the result will be essentially the same. Am I misunderstanding something?

BOVs are required in supercharger systems that blow thru the throttle, that is, have the throttle between the intake valve and the compressor. But your system doesn't have that.

Imo000

Don't positive displacement compressors have a bypass so that they don't rob power from the engine when they don't need to make boost?

69gaugeman

Yes. Tapered to guide the rings into the grooves. I use 3 degrees or so. I've thrown away my ring compressors (figuratively).

ptuomov

Just for clarity, I'm defining a positive displacement compressor as a compressor that would break if you'd let it suck water. Often they do seem have all sorts of valves, I just don't understand why. If you have a throttle closed in the inlet, it's going to compress near perfect vacuum into near perfect vacuum and consume very little power.

ThetaTau87

Yes, the BPV equalizes the pressure on either side of the SC to minimize the load on the engine.

ThetaTau87

The level of vacuum here is no where near a perfect vacuum. Even with the throttle closed the ISV is open to allow air into the system for the engine to maintain a stable idle. We are talking about different levels of vacuum pressure none of which is even approaching an absolute vacuum.

The SC is by definition a larger pump than the engine itself. With the throttle closed on a normally aspirated engine the engine will pull a partial vacuum in the intake. With a SC you introduce a larger pump between the throttle plate and the engine. The SC will pull a higher vacuum against the throttle plate than the engine would and lower the vacuum on the engine side. This pressure differential causes a load on the SC. By equalizing the pressure on either side of the SC you remove the load and the vacuum in the intake returns to the vacuum pulled by the engine alone.

ptuomov

I don't think that's how a twin-screw compressor works. I think it works as a fully internal compressor. That is, if inlet sees 15 psia, it'll compress that to say 30 psia. If inlet sees 5 psia, it'll compress that to 10 psia. (Ignoring the ideal gas law stuff). Whether or not the inlet and outlet pressures are equal, the compression work has to be done. Right? Increasing the inlet pressure is going to increase the compression work that the compressor has to do. What am I missing?