roots vs centrifugal superchargers
 

Wondering if someone could help me understand why a belt-driven roots blower can provide higher boost at low RPMs than a belt-driven centrifugal?

The both spin linear to crank speed.

Do roots blowers have a bigger bypass?

thx

no it is to do with how they compress the air:

the roots blower is a positive displacement pump so it moves a volume of air per rotation from the inlet to the outlet and a part from a very small gap between the rotors and the casing the air cannot flow back. even when it is stopped entirely it is quite difficult to get air to flow backwards through it.

the centrifugal spins the air by the blades on the impeller thus causing it to be flung to the outside of the housing where the outlet is. at low speed the air is not moving fast enough so that as the pressure rises in the outlet the air can flow back through the relatively large gaps between the blades. when it is stopped air can freely flow backwards through it.

the bypass is more when you have more boost than you need at part throttle and allowing the air to flow back to the inlet of the charger to reduce the power required.

Centrifugal supercharger is a belt driven turbo.

maybe in the future they will put variable speed supercharger woud be that bed or good?
http://www.greencarcongress.com/2012...echnology.html

Variable boost is a very good thing, Porsche has developed it on some of there turbos... After decades of turboing motors I dont see subaru changing the plan in the future.

The next big thing will be combining an electric motor onto the turbo, at low revs or for fast response the motor will spin the turbo up and at times where there is excess energy from the turbine (ie when the wastegate is open) the motor will act as a generator to recharge the electrical system.

There have been a few demos (including by Subaru and BMW) but the systems are a few years away from production yet but they offer response that could previously only be gotten with a roots/twin screw supercharger and the the boost control of a turbo system. Also the electricity generated (far more than is used by the spinning up of the turbo) if combine with a hybrid drive will effectively be thermal energy recovery hopefully giving a boost to fuel consumption.

^^^ yup, and there are rumors that Subaru has this coming sometime in the future, and BMW has created working prototypes or something as well. It's not exactly thermal energy recovery, it's more recovering the blowdown loss that is what makes engines so loud and require muffling.

The way it would look is a much bigger turbine and freer flowing turbine housing for the same compressor size, and the electric motor will spin up the compressor and turbine quickly to provide boost, and then depending on circumstances, start generating electricity from the excess turbine power available.

Thanks. Nicely explained

I'm not an engineer or anything but wouldn't hooking up a centrifugal supercharged to the exhaust to flow in the outlet and then connecting an alternator to the pully generate electricity much in the same way I don't understand the need for new more expensive technology...

It just seems they could have added this to every hybrid and added a more powerful electric motor to get better performance and fuel economy.

Thread necro skillz are OVER 9000!

Engine exhaust transmission vs forced induction thread

So lost... lol besides the necro part I get that

We didn't have an FI specific forum when this was created.