Originally Posted by Tony

I have so much fun driving my car.

Originally Posted by Ninespub

You want more twin-screw tales? My ex-wife got the house AND I've been paying alimony for 10 years and will for at least 6 more!

Originally Posted by Chris Lockhart

I want more twin screw tales!

Originally Posted by DR

Hi Chris,

How about Twin Screwing my heart with PtCr alloy a few weeks ago, does that count?



Cheers,

Dave

Originally Posted by Chris Lockhart

Platinum and Chromium? Stent? That definitely counts!! BTDT, got 7 of 'em.

I hope you and Jeannie are doing well my friend. We think of you both often.

Originally Posted by DR

...

Back to the Bypass Valve topic, the fact that most (or all) factory OEM TS/Roots systems have them, plus Opcon/Autorotor states warranty is void without the use of a bypass valve has always been good enough for me. Life is short and I have better things to worry about :-)

...

Originally Posted by DR

Hi Chris,

I knew you would get it! Jeannie and I were trying to remember how many you had. I only have 3 and hope I never have to get more! Went in with massive STEMI HA with 2 arteries blocked (90%/85%), luckily I live only 3 miles from The Heart Institute of Bayonet Point, one of the top 10 Heart Hospitals in the country!

Hope we get to see you guys soon at SITM!

Back to the Bypass Valve topic, the fact that most (or all) factory OEM TS/Roots systems have them, plus Opcon/Autorotor states warranty is void without the use of a bypass valve has always been good enough for me. Life is short and I have better things to worry about :-)

Cheers,

Dave

Originally Posted by DKWalser

Dave -- What is the current availability of your supercharger kit?

Originally Posted by 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. Others may differ, but I would never design/size a system with "boost" as the primary consideration.. ever...
I would look at mass flow required for desired power. Boost is pressure, and that is a resultant of flow restriction, and this equals heat, which is bad. (I look at all this as a giant thermodynamic equation)
As most of the heat in the air charge is caused by compressing the air molecules, the lower the pressure (boost), the more dense the air charge, and more power you can make with less heat and less pressure spiking (knock) in the cylinder. The mass flow logic says X kg of air required at Y BSFC or Z Lambda to make H hp in the engine being discussed. Reduce the inlet restriction to "0" if possible, as the Twin Screw compressor will produce more heat in a log scale relationship to the inlet restriction, as the relationship is not linear. Throttle to be sized for air flow (CFM/KgMin etc.) based on total mass flow requirements. (rule of thumb is as big as needed for the flow with no intake restriction based of mass flow for desired power) Intake restriction also extends to the manifold pre and post intercoolers, as again, pressure equals heat equals bad.

Say it's 10 psi. Next, estimate the throttle, inlet, filter, etc. loss. See above in blue on sizing the system

Say it's 0.7 psi. Now, the compressor inlet sees 14 psia and the desired outlet pressure is 24 psia. Not necessarily due to relational pressures between inlet restriction and external pressure and temperature. Delta P, PVnRt etc..

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. Not a good way to pick your operating parameters per above reasons. Simply put, you can make more power with less pressure flowing the same volume (mass) of air due to density.. Bernoulli at play here.

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.Pulley size in a twin screw determines only compressor speed and mass flow rate. System design pre and post throttle determines pressure in the manifold based on the engine's ability to consume the air in the manifold.

Now that I am thinking about this more, do you guys run your cams retarded when using twin-screw supercharger? Some might, and some might not, it depends on the dynamic compression goals of the system

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. I have to disagree with this entirely, as when properly designed, a supercharged engine will be more efficient than an identically built N/A motor. Knock is caused by heat and resultant pressure spikes. A properly designed system will flow more mass air at less pressure and have a lower intake temp charge that is more dense than an N/A engine every stroke that the engine makes.

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 This is key, see further below.. by increasing boost. Boost no, Mass, yes, see below. 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.Your thoughts are on the cusp of a "Miller Cycle" system. You would time the exhaust cams to scavenge at optimum system efficiency. The intake cam is where the magic is happening. You delay the closing of the intake valve to lower the dynamic compression, where it allows you to continue filling the cylinder as the piston is on the way up on the compression stroke. The delay discussed in several areas is a delay of about 35 degrees. With a Twin Screw this is extremely effective, as the compressor has the ability to flow more air at lower rpm due to the design, as well as the system flow is mostly governed by throttle position when the system is appropriately sized for the engine. As such, any loss in low rpm operation from delaying the intake closing can be made up for by the compressor's ability to put more air in the cylinder and have a "bigger bang' than an N/A engine with a retarded intake cam. The best of both worlds would be to have a variable intake and exhaust cam setup that can be adjusted for compressor/engine speed and flow or manifold pressure (in essence load). This would allow the the optimizing of the load on the engine as a function of throttle position and engine speed. (Of course ignition timing would be mapped to these variables as well).

With that said, this is one of the reasons I bought the only set of Variocam 928 heads ever made. I intend to use the Motec M800 to be able to match ignition and fuel to compressor speed and engine/cam position under variable load scenarios allowed by the variocam heads using a twin screw system.

""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? It sounds simple enough. However, again, I question the desire to integrate a variable valve into a billet case or cast metal twin screw case, or even connect such a valve externally with hoses. I am not sure it would not work, but for the purposes of theory, it might.

For the purposes of system design, unless you match mass flow and power desired with target Lambda and have as few restrictions in the system, I think the gains are minute enough to make such a valve a moot point.

For example, if we have two identically prepared engines, one with 0.7psi restriction in the compressor intake, and your ratio valve, and another with a simple bypass, and 0.0psi restriction in the compressor intake, the one with the 0.0 psi intake restriction engine will make more power all day every day.


From a design and engineering perspective, if the diaphragm fails and gets into the compressor, it will destroy the rotors and possibly get into the cylinders. On a butterfly and or plunger valve, very little can go wrong if the valve fails.Yes, a screw could come loose on the butterfly shaft, but if sealed in that is minimal. In any case, this is why I prefer metal diverter valves with plungers (Preferably billet ones). Nothing to come loose and get into the engine....

Originally Posted by blau928

With all that said, everyone can obviously do as they wish. I have had countless discussions on these subjects over the years on Rennlist, other forums, and with various engineers. It's a free country, so everyone should do what they like and are able. My $0.02 is just that, and I am aware I see the world through very different lenses. Best Regards,

Originally Posted by ptuomov

Of course its the air mass (and not boost per se) that matters, and you want the engine to breathe freely upstream and downstream of the supercharger. I was just showing as an example how to match the internal compression ratio to the system compression ratio by changing the pulley, holding the rest of the engine the same. It's a thought experiment, not a design recipe for a complete engine.