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centrifugal s/c question (long sort of)

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by flexorcist, Jul 25, 2007.

  1. I don't know if this is in the right topic but I have a question, (at the bottom, the length of this OP is so i get the specific and correct answer.)



    I know that you can supercharge, or turbo charge a bike, but do they make ones that are centrifugal, the ones that kinda look like a turbo anyway??
    They're a fantastic way to go with car, using a blow through set up back on to the carby, because they spin so much faster than engine speed and blow compressed air into the original air/fuel mixture.

    However....

    What it is that confuses me, for instance, is that say you put one on a V8, that motor may redline @ 6500 RPM. At that speed, an average cf sc would have it's pully going at 1.5 times that speed, with it's internal gearbox spinning the inside bits at 5/5.5 times that speed.

    Essentially, the blower is spinning at 48-50,000 RPM, not an issue with the typical ones have a limit of about 65-70,000 RPM.

    So if you were to attach one of these to a bike motor, you need one thats got a 180 odd thousand RPM redline.

    Is this the reason turbo-charging is much more common with sports bikes? Or are the centrifugal blowers designed to hit 200k RPM?

    Thanks.
     
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  2. You can just alter the fan/blade pitch and run them at a slower rpm via the gearbox.

    You wouldn't want them to be spinning near 200k rpm on a bike!

    Turbos are much more practical for (most) bikes because they can be situated anywhere.
     
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  3. ah k, makes sense. adjust it so it's only 2x the output or something not 5.5? are there any out there that do run a blow through setup, or is output not worth the effort when turbo is so easy?

    edit* http://www.procharger.com/motorcycle.shtml there are ones for HD, but what do they rev to on avg?
     
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  4. You can always reduce drive them on the pulley.

    With any centrifugal pump what you are aiming at is a critical speed. Outside these speeds they are not very efficient and you will waste a lot of energy heating up the air and making noise.

    This is why turbo's work well on large low revving engines (contrary to popular belief). Hence their common fitment to trucks and industrial diesels.

    Any supercharge is a pain on a bike because you have to modify the casings to drive the thing and often the casings are filled with oil.

    also, last time I took and interest, these type of supercharges run old-style profile impellers which means they have an even smaller critical speed range than modern turbo chargers
     
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  5. this may help a little... copyied out of a datsun forum i used to frequent...


    When fitting a supercharger, you should match the swept volume of the supercharger to the size of the engine. If the choice is made carefully, problems from ovrboosting and the required paraphenalia to solve these problems will be minimised.
    The supercharger will also be kept in its safe operating speed with correct selection.

    To choose a setup you need to know :-
    1. Engine capacity
    2. Maximum engine speed you will be using.
    3. Boost level desired

    I am setting out the calcs needed for a 1200cc engine in the steps below.

    FIRST CALCULATION (Engine Litres/min @ 0 Psi )
    Multiply engine capacity (in litres) times maximum engine Rpm. E.g. 1.2 litres x 6000 rpm = 7200 litres/minute. Divide this figure by two as engine only fills every second stroke. (7200/2 = 3600 litres/min.
    This is the engines air requirements in litres/minute at 0 Psi boost.

    SECOND CALCULATION (boost ratio)
    Add the boost pressure desired (7 Psi) for the engine to 14.7 Psi (atmospheric pressure).
    (7 psi boost desired +14.7 =21.7 psi)
    Divide this answer by 14.7 and this gives the boost pressure ratio. (21.7/14.7=1.476) This is the boost pressure ratio above atmospheric pressure.

    THIRD CALCULATION (Actual air requirements @ desired boost)

    Multiply the boost ratio by the litres/minute obtained for 0 Psi and you get the actual air requirements in Litres/min for the engine at that boost. In our example this is 3600 litres/min X 1.476 = 5313.6 litres/min for 7Psi boost.

    To decide on the correct size of supercharger you need to know :-
    1. The swept volume per revolution of the supercharger. (Eaton M62 1 litre/rev, SC14 from a 1G-GZE 1.5 litres/rev)
    2. The maximum continuous safe operating speed for the supercharger. (Eaton M62 14000 rpm continuous, Toyota SC14 12000 rpm??)
    3. The maximum pressure that can be safely produced by the supercharger continuously. (Eaton M62 12 psi, SC14 10Psi??? teflon on rotors melts??)

    CALCULATION (Supercharger rotor speed)
    Divide the desired air flow (5313.6 L/min) by the swept volume of the supercharger (SC14 from the 1G-GZE is 1.5 litres per revolution). This will tell you the maximum speed the supercharger rotors must be run at to produce the volume required.
    5313.6/1.5 litres = 3542.4 rpm for the SC14.
    5313.6/1 litre = 5313.6 rpm Eaton M62

    CALCULATION (Pulley size ratio)
    Divide the rotor Rpm by maximum desired engine rpm to get the drive ratio of the pulleys. For an SC14 on a 1200 @ 7psi boost the desired supercharger pulley ratio is
    3542.4 /6000 rpm = 0.5904. With an 8" shaft pulley on the supercharger the crank pulley for this ratio would be 4.732".

    EXAMPLE OF CHANGE OF DRIVE RATIO
    If the 1200 cc engines supercharger pulley ratio is increased to 0.75 using the 1.5 ltr/rev SC14 charger, the volume of air produced when running to 6000 would be 6000 x 0.75 x 1.5 litres =6750 litres/min

    Boost produced would be 6750 litres/3600 litres (at 0 boost) =1.875 boost ratio For Psi boost ((1.875 x 14.7) - 14.7)=12.86 Psi.


    With that bit of theory i needed an amr300 supercharger with a reduction drive for my vfr400..... problem was teh spercharger would be impossible to mount on the bike!
     
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  6. Edit: Changed my response to be more specific and correct. ;)

    Turbochargers typically spin well over 100,000rpm, so it's not really a safety concern in that respect.

    Suspect the main reason superchargers are not used is because unlike car motors, there's no exposed crankshaft... Ie, no easy way to hook it up to the motor. Packaging issues too, because it has to be bolted to the side of the engine to run off pulleys, or to the front/back/top/underside in the case of a roots/twinscrew.

    Much easier to connect + package a turbocharger.

    ward_e's above comment refers to Roots-type/positive-displacement type superchargers, not Centrifugals, natch. You need to look at compressor charts and other fun things for sizing Centrifugals. :)
     
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  7. the theory for pressure ratios still applies but you will need to plot your required airflow vs vs rpm across the compressor map of the supercharger.

    Its a bit like turbo sizing...
     
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  8. so it's all possible, it would just require you to build a sports bike around a blown motor instead of the other way round huh??

    cheers for detailed info dudes!!
     
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