Hey all, need some help for a uni project. I need some method of limiting boost, possibly at 1psi upto 7psi (though i doubt we'll use it more thtn 4psi max.). Its using a supercharger so we cant use a waste gate. What options does this leave me with? Bleed values? Boost controllers? I know little about these and was wondering if anyone could shed some light? we're going to be using a centrafugal type compressor (same as a trubo) which wont boost till ~20k rpm, but then will run all the way upto probably 100k. this means that the boost isnt going to be constant and so it cant just be leaked off at a fixed rate. We need to be able to change the amount of boost it holds easily as we'll be running it probably at 3 different levels throughout the event it will be in.

Anyone got any ideas?

Cheers
JAmes.

 

Dont know for sure matey, but I was under the impression that the amount of boost that a supercharger made was dependant on the size of the pulley wheel driving it? If so, you will need diff pulleys for each setting.

I have to be honest - this is a complete guess!

JJ

 

The amount of boost run on a supercharger can be changed through the size of the pulleys involved (think of gearing on a mountain bike...).

Now, to make it easily changed is less easy...

The one thought that immediately springs to mind is to have a quick release pulley on the crank, a quick release pulley on the charger and a tensioner in the middle. To change the boost through a pulley ratio, all you'd then need to do is take the tension off, release the pulleys and the belt and change them. If designed properly i see no reason why that couldn't be done in under a minute.

The other, slightly more tricky option is to develop a wastegate type setup before the throttle body. An external wastegate would probably be your best option but you'd need to change the spring rates inside and if time is going to be an issue, you'd need 3 wastegates all with different spring rates and then be able to mount them on V clamps so you'd be able to unclip the clamp and change the wastegate.

The rough as hell way to do it would be to introduce a controlled boost leak into the equation - fitting either a mechanical or electronic bleed valve into the boost control would allow you to 'leak' unwanted pressure - but i'd not advise this as a primary choice...


Thats the beer induced thoughts off the top of my head - hope it helps.

Oh, and having just re-read your first post - i should point out that you would be using bleed valves/wastegates etc in a much different sense than their normal application.

External wastegates normally vent exhaust gases to prevent the gases reaching the turbine wheel of the turbocharger, i'm suggesting you use them to vent unwanted boost pressure.

Bleed valves are normally used to fool the actuator into seeing a lower pressure than is actually being achieved, again, i'm suggesting you use it to vent unwanted boost to atmosphere.

 

You just need a PWM valve (an idle valve for example) correctly sized and controlled by an ecu or simple circuit to give enough flow bypassed to atmosphere to lower the boost to the required ammount. No need for wastegates or anything complicated.

I'm assuming your plumbing will be

Air filter - charger - intercooler - throttle - inlet manifold

if so, just fit the vent valve between the charger and the throttle.

You wouldn't be able to get away with this on something like the eaton which is positive displacement.

An alternative would be to use a clutched charger and switch the clutch on and off rapidly to give the required boost (PWm again). Most charger clutches would burn out quickly doing this tho.

 

Thanks guys, i'll speak to the crew at uni about it, though its not technically my issue

Because its not a positve dispalacment type changing the pulley wont really do. Its also being put on a small engine (600cc, though possibly going down to 500) so not having it at boost untill the last few rpm isnt going to be a useful option. I had though that the use of an external wastegate might work and just to put it on the inlet.

I like the idea of using some kind of valve like a ISCV, and i'm sure someone on the project would be able to do the electronics required. making it optimised to produce the max required boost and leaking off wat isnt used would be the easiest option for us. I think we've got a company willing to give us 2 compressors but much has to be done on the matching side at the moment. Im assuming that a pulse type input for the valve would be needed. Does anyone know of any ISCV specifications for such control? We might be getting an ECU off one of the sposors, i guess this could all be taken care of by the ECU if it is complex enough.

As we're running low boost we wont be using an intercooler though the setup needs to be

Restrictor - throttle - charger - engine

Even if we run as little as 1psi of boost it may prove to be adaqute. The current engine is reduced from 100 to 70ish HP because of the restrictor and so the idea behind supercharging is to negate the effects of the restrictor.

Does anyone know how big a hole you need to loose a significant amount of boost? I cant imagine it needs to be very large.

Keep the ideas coming!

Cheers
JAmes.

 

Why cant you utilise a nice simple blow off valve?

Get a mechanical valve and fit a spring that is overcome by exactly the pressure you want, that should get the job done.

The valve should be positioned somewhere prior to any electronic metering and the management would like it most if mapped speed density as oppossed to mass air flow.

 

Even simpler would be an extra throttle flap such as fitted to my BMW (used for traction control).
What this does is restrict volumetric efficiency of the pump (supercharger)
thereby limiting boost.
Those flaps you see poking out the top of dragsters are used for that purpose

 

As centrgfuial chargers are dogshit IMO and dont produce full boost till high rpm, the ONLY way people get em giving good boost at sensible rpm and not hugely overboost at the top end is to do as what people have already touched on, run a simple wastegate or blowoff valve type thing on the inlet piping.

Same as they use on many race cars to stop teams cheating by raising the boost higher than the rules.

Only problem here is your overspeeding it causing the efficiency to drop like a sack of shit, so you will need a good intercooler and/or water injection to keep the charge temps and efficiency in check.

So to sum up, throw it in the bin and use a proper positive dispacment charger