I dont get a quote?
Seperately, turbo motors typically require mid to high compression in the engines, to help increase low/mid range torque. This is also true for camshaft selection, unless you have varible cam timing, most require smaller cams that help increase cylinder pressure at lower speeds. A positive displacement, with high efficiency (screw) can run far lower static compression ratio's and much larger camshafts. Lower static compression allows the engine to have cooler temps/lower cylinder pressures while in 90% of operation. This means longer engine life.
To note, VW is set to release a new vehicle that has "compound boost" which has both a positive displacement and turbo. The turbo feeds the SC. By force feeding the PD, efficiency levels far exceed 100%, but when the turbo can't keep up, the increased vacuum and airflow from the SC helps fulfill that void.
so they are working out pretty good. although, I think for the $1800 I probaly could have hired some one to sweep my shop floor.
I think you have to answer the question to get quoted.
So with your education in the turbo world and everything you have learned good and bad.............why do you feel a Turbo engine is safer or will last longer than a blower engine of equal power in excess of 1000 H.P. ?
"If some is good, more is better.
And to much is just enough."
Typicaly we built turbo engines with 10 lbs of boost and around 6 to 7.5 compression. One I remember was a 460 69 tbird single turbo. Pulled 800 lbs of torq if I remember correctly perty much from 2400 rpm to 5200. Turned 11.1 in the quarter mile weighed 5300 lbs.
Big thing with making them last was keeping the RPM low
hard core rotating mass parts as boost increases.
Have one friend that runs a 2000 cc lotus at 40 lbs of boost has busrted everthing in the car but the engine. He went as far as water cooled brakes and machining a billet transmission case for the car.
With the variable geometry stuff I did with the electronic controler boost was available at idle. First over the road truck we put one on when it took off all the drive wheels jumped off the ground at once. We then strapped it to the chassis dyno to dial in the power curve.
Been meaning to jump into this thread for a while, I've spent quite a bit of time with turbos!
The VW system quoted by Dustin above is the VW Twin Charger but you appear to have the operation the wrong way round: The SC feeds the turbo, not the other way around.
The supercharger is very highly gearing and is clutched. It is only there to fill in the gap BEFORE the turbo spools up to ensure good drivability. Since the turbo size no longer has to be compromised to give good response, the match can be a lot larger and so the performance ratio (remember this is a 170bhp 1.4l engine) can be a lot higher.
The SC is only there for response at the low end and has a very large drive ratio as it is clutched. It HAS to declutch before 3000rpm or it's exceed it's allowable max speed. This engine has already been released in the European market.
The SC does not take over when the turbo runs out of breath as you seem to suggest. This would not make sense as quite apart from the parasitic losses associated with an SC (as has already been discussed) as even a screw charger efficiency (isentropic effy) is nearly ten points lower than a typical centrifugal compressor eg the front stage of a turbo. So at max power where the losses are highest, the turbo takes over as it can provide the boost for the lowest cost.
Hope that helps!
Link to schematic: http://bioage.typepad.com/.shared/im...si_airflow.png
Last edited by Ruaraidh; 11-29-2007 at 11:34 AM. Reason: Added link
Don't yell at me because I don't post but this thread is up my alley. I was AMT Diesel, we developed all kinds of things for racing diesels and our designs made it into Cummins & Navistar truck engines - turbo and supercharged(roots)
We also ran the racing engines for Super Hero/Buzzi and my own little diesel boats here in the US years ago. We developed the small hi-po diesels that were to be marketed as small Seateks/Super Hero based on Cummins B series engines (also did C's and L-10's to compete with the Seatek)
I pride myself on the shear number of engines we have blown up on the dyno - in the name of developement & testing you have to break them.
Please ask Craig or Bud from Cig/Hawk who I am, no bull.
There are so many things being said here, if you have a question ask me and I'll answer it as best I can based on our cubic dollars of experience.
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