Yet even more Supercharger questions

[nesikachad]

T or F

If building a conventional two lobe rotor system for a Roots Style positive displacement blower the two rotors should contact one another along a tangent the entire time right? (minus of course the minimal clearance needed for the thing to run.)

What I mean is the rotors should not have any appreciable gap at say 45* from vertical? I've been studying this epicycloid thing and I thought I was doing it right but my animation is proving otherwise in CAD. (Grrrrrrrrr)

Thanks.

C

 

[Ken I]

I might be wrong - but here goes.

The original Roots type blowers used simple radii rotors - as you found with your CAD simulation this has considerable clearance in various places so early roots blowers had poor adiabatic efficiency in the order of 60%.
From what I can divine from my own CAD simulation there is no way to get two identical rotors to mesh with minimal clearance (as in a gear pump) because of interference.

Also a two lobe design has serious pulsation issues. Modern Roots blowers use 3 or 4 lobes as well as a helical twist to improve efficiency and reduce pulsation (the interference problems diminish with the number of lobes until eventually you are back to an involute faced gear pump)

The 700HP Shelby Super Snake uses a 4 lobe Roots type blower (My son got to drive Shelby's #1 serial car as principal stunt driver for Death Race II - coming soon - a direct to DVD cheapie).

I believe that modern blowers have moderated the profiles to reduce interference clearances - here the two rotors are not the same but to all intents and purposes look like they are.

If you are trying to build a scale blower - good luck - the leakage problems and inefficiencies get worse as you scale down.

 

[aussie bruce]

Hmmm this is a very interesting topic that i have been watching for some time nesikachad please don't get me wrong but i fear your scaled down twin rotor design is going to be very inefficient unless you can control the interference lines points to extremely tight tolerances.

May be a better design might be a shorrocks style rotary vane type maybe better able to be scaled lets face it they are the power source for our air grinders etc and are available off the shelf relatively cheaply and cheap to play with. there was a design for a supercharger in strictly ic designed for a radial 3 cylinder .22 ci as there was now crankcase pumping do the supercharger was added to the design August/September 1993 edition

1.625 OD 1.0 id x 1.2 wide at 7252 rpm it gave 2.010 cu feet per minute also for its size if its performance you want not scale appearance its a better way to go.

A good site i have followed for some years is the fang http://www.thefang.co.uk/superchargers.htm

Great discussion points and lots of build info will soon flow maybe even sticky this as discussion topic only ???

Sorry i mucked up this bit should have been at the top I work with rotary lobe pumps and geometry of progressive cavity pumps. and yes to get efficiency in the design the seal line must be theoretically in contact at all times to minimise pumping losses

Then you get temperature throwing in the mix and in some cases for the change in every additional 20 deg of process means taking off 0.1 mm of material off the seal line ie rate the pumps at 20 deg / 40 / 60/ 80 / 99 deg if they drop temperature back process efficiency drops like a stone
All the Best mate Bruce