you have a wheel turning X RPM.. it is designed to move Y amount of air at this RPM.. now put a turbocharger in front of it that is supplying a much higher volume (CFM) of air.. the blades become inefficient anyway you look at it! they simply cannot move the same volume of air as the higher spinning turbo will at its torque peak (max boost) remember its not all about pressure as I see everyone preach here.. that is 100% true... so your telling me that the volume of air moved from the turbo being much higher then the lower speed supercharger at low RPM's which is crank shaft driven will be capable of moving the same amount of air.. negative ghost rider, you will have a pressure build up in front of the compressor wheel to the supercharger.. thats all there is too it.. volume and pressure are two different things here.. now once it is through the for it to be efficient you would need a MUCH larger compressor on the supercharger capable of moving more air then the turbo in order to be efficient in any way. don't believe me still.. here is another easier problem to solve.. yes it deals with fluid dynamics, but the principle will be similar.. put an A1000 fuel pump on a tank sump and run a smaller pump after it.. whats gonna happen? we are now dealing with volume... the smaller pmump is going to be a restriction.. now flip the situation.... put the smaller pump in front of the A1000.. now the smaller pump can run at a much higher efficiency because the larger pump is moving a greater volume of fuel.. so as the pressure is relieved in front of the smaller pump by the larger A1000, it can move a greater volume of fuel due to the fact that the pump will is capable of moving its greatest amount of fuel by volume at the least possible pressure!

also... you are saying that the supercharger will have greater parasitic loss by having the outlet dicharge to atmosphere.. NO WAY.. the movement of air will be at a higher volume same principle as the fuel pump scenario before, however, with a huge decrease in discharge pressure and the shaft speed of the compressor continuing to follow its predetermined crank driven shaft speed acceleration, the negative force on the compressor wheel is going to drop dramatically as its air movement efficiency increases... its like saying a larger downpipe on a turbo venting as close to the turbine discharge will make a turbo take longer to spool because you decreased the back pressure on the opposite side of the turbine blade... when in reality, it assists spool time by decreasing the amount of energy needed to turn the turbine wheel by removing the need to push air through a long and restrictive exhaust track. less force is needed to do the same job. still don't buy it? what happens to a fuel pump at higher pressures moving less volume? the amperage draw increases which means more power is needed to continue to run the motor. same same for the parasitic loss on the crankshaft. if this was not true... no one here would ever complain about belt slip at higher boost pressure levels....

Chris

 

I might be out of my league here but...

the example that you are giving with the fuel is not applicable, since fuel, being liquid, is incompressible. Turbos and superchargers are compressors.

The volume of air is not the question in this instance, it is the DENSITY of the air charge, and since air is compressible, the volume of air being moved is nowhere near as important as the density of the voume being moved.

If i am completely wrong, ignore what i said, but please do correct me.

 

Dave is absolutely correct.

I hate to go into the whole qualification spill, but I'm a Machinery Engineer for the largest company in the world. MY JOB IS TO WORK WITH MACHINERY (COMPRESSORS, TURBINES, PUMPS, ETC.) TO OPTIMIZE PERFORMANCE AND RELIABILITY. I've worked with every different kind of pump or compressor you can think of in any combination (series, parrallel, small with big, etc. . .).

Actually, a wheel is designed to move Y amount of air at X rpm, but will move varying amounts of air depending on Pressure Ratio. It will reach peak efficiency at the design point, but efficiency will fall off when you go either direction from there. Compressor peak efficiency can be adjusted by changing blade angle, volute taper, and specific speed. The blades become inefficient when you run them off design. HOWEVER, design point occurs at a given Pressure Ratio (here we are again). If the inlet pressure is increased using a turbocharger, the outlet pressure will increase by a larger amount, BUT THE PRESSURE RATIO STAYS THE SAME, SO THE BLADES ARE STILL AT THEIR PEAK EFFICIENCY. Also, a supercharger, even if it's spinning slowly compared to it's upstream turbocharger, WILL STILL INCREASE THE PRESSURE.

You are exactly opposite of correct. Optimally, you would actually install a very small supercharger downstream of the turbocharger. The air entering the supercharger is, say, twice as dense as it would normally be, so it would take half as much supercharger to increase the pressure the same amount. Once again, when we put centrifugal compressors in series (like a turbo feeding a supercharger), the downstream compressors are progressively smaller than the upstream wheels.

Comparing a positive displacement pump to a centrifugal compressor? About the only thing these two have in common is the fact that they both use power to create dP. One uses centrifugal force and aerodynamics to create pressure, while the other uses mechanical force to push it along. Even still. . . it DOES NOT MATTER which pump you put first and which you put second. The resulting pump curve will be the same. They ONLY time this may make a difference is when one pump may have better NPSHr (Net Positive Suction Head required) characteristics. You'd put that one upstream because it's less likely to cavitate. That's a whole different, and totally unrelate, issue.

Way, way wrong here. All centrifugal compressors (and pumps, for that matter) take more hp to turn when you move them to the right on their map (or curve, if you will). On the right side of the curve is very high flow and low/no pressure, AS IF THE DISCHARGE WERE OPEN TO THE ATMOSPHERE. As the specific speed (NOT rotative speed) increases (i.e. the machine becomes more "axial" than "centrifugal"), the hp required curve starts to flatten, then reverses. So, on a totally axial flow machine, you would be correct, but turbo's and superchargers are centrifugal flow, not axial.

And now we're comparing turbines (a device used to convert gas enthalpy into shaft hp) to compressors (a device used to convert shaft hp into gas pressure)??? About the only thing a turbine has in common with a compressor is that they're both on the same shaft in a turbocharger and they look similar, but I assure you, the principles are waaaay different. A turbine turns exhaust energy and mass flow into shaft hp. So. . . the lower the pressure downstream of the turbine, the greater the change in ENERGY across the wheel, the more hp is applied to the turbocharger shaft, the quicker it spools.

I won't regurgitate the whole fuel pump non-comparison again. Keep in mind that these compressor maps are 2 dimensional. High flow and pressure is not the same as high pressure/no flow, nor is it the same as low pressure/high flow. The hp it takes to drive a compressor depends on pressure AND flow. If both decrease, hp required will decrease. If both increase (high boost pressure car), hp required will increase, hence the belt slip. I couldn't find a good compressor map quickly, but I assure you that they are similar to this centrifugal pump map:

http://www.goulds.com/pdf/C36{2f}3742.pdf

Notice how the highest hp required is at 60 psi and 52 GPM (about 1.6 hp), NOT at 120 psi and Zero flow (about .75 hp).

I don't mean to offend anyone here, but I really do deal with this stuff EVERYDAY and have learned quite a bit over the last 6 years.

Mike

 

mike I'm not offended in the least I see alot of your points, but I need to point out that I am not unqualified myself.. I have a degree in mechanical design and have worked for a prominent engineering firm for some time.. in addition, I have been through schools at volvo penta, who has this setup and runs it precisely as stated above by me routing the charged air away from the mechanical compressor when the turbocharger reaches optimal boost. this is not a pissing contest, I was stating basically what I have put my eyes on in a functioning working system that is available on the market right now.. not theorized..

I stated that fluid dynamics were not the same as gaseous, fluid is non compressable due to its density..

compressed air has a greater density then atmospheric air.. I am pretty sure we have both made this point..

maybe I'm not seeing it.. but if the compressor turns the same RPM with pressure and the same without... because it is dependant on the crank shaft for turning...

you are saying the compressor requires a greater force to turn without a pressure load... instead of showing maps and graphs please explain it to me in terms of simple physics.. not saying I can't be wrong, but I have a hard time swallowing the engineers at volvo penta having designed and tested and refined a system that could have been built better before releasing it to market.. I mean maybe you are right... I have not sat down and crunched the numbers... but I have to go with the big company who has there money on the line on this one

as for the smaller compressor in front of the turbocharger.. I understand you work with them all the time.. and understand the ratio of inlet and discharge pressures and efficiencies.. and when a compressor is run at its max efficiency.. please explain exactly why that particular range is ideal in terms of pressure, volume and heat..

I also understand tubrine and compressor blade designs from working with gasoline turbine engines in aircraft, which use axial and centrifugal compressors... I understand that axial is linear flow and centrifugal requires redirection of the compressed charge using stators in a turbine engine or the volute in a turbocharger/supercharger.. I would like your professional input here.. if I am dead wrong.. I want to walk away knowing exactly why.. so please break it down for me!

thanks,
Chris

 

It's actually based on Thermodynamics. Power = mdot * dh. That is, power required to spin a compressor is proportional to mass flow rate times change in energy (enthalpy). When you take off the pressure load, the mass flow rate increases tremendously. Even though it's working against very low dP, since the mass flow rate is so high the hp required to drive it is high also. Try this at home. . . run a vacuum cleaner then block off the hose. You'll hear the motor rev higher. It revs higher because you just reduced the load by reducing the flow rate (to zero), even though the dP is more than normal operation.

Are you asking why a compressor is efficient in a certain range? The efficiency of a compressor is based almost completely on velocity. It's a complicated issue that takes up entire chapters in a book, but I'll try to summarize. When the air enters the compressor, the blades are traveling normal (90 deg) to air flow. If the blades were straight, this would cause low pressure areas behind the blades and high pressure in front of them. So, you angle the blade such that it matches the velocity of the incoming air. The problem is when you vary the flow, thus velocity, from this design flow and resulting blade angle. This causes efficiencies to drop at flows higher and lower than design. Also, the taper of the volute influences max efficiency point. Ideally, you want the air velocity to be the same all the way around the volute, or snail. Units designed for high flow will have alot of taper, while units designed for low flow can have little or no taper.

Sooooo. . . what does all this mean? Since design efficiency range is based on velocity only, you double the density (thus, mass flow rate) of the air and the velocity stays the same, so the efficiency stays the same.

Clear as mud?

Did I address all your questions?

Mike

 

i love lamp...i love lamp.

 

Very good stuff. Thanks for the explanations.

 

Mike, thanks for the good explanation of turbos and centrifugal blowers in series. I did not understand the process until reading your post.

 

My head hurts.