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Centrifugal Twincharge Idea 2

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Boostedbimmer

Boostedbimmer

Mechanical
Nov 5, 2017
11
I have no industrial experience outside of my own garage. I consider myself humble about my current knowledge and am seeking advice on an idea I've had difficulty finding information about after extensive research.

What I'm exploring is the feasibility of a twin charged engine using a turbo feeding a clutched centrifugal supercharger that is oversized and designed for operational rpm before the turbo has "spun up." At this point the supercharger will disengage to prevent exceeding the max allowable impeller speed and a bypass around the supercharger will gradually open to remove the flow restriction. Pressure relief valves where necessary after closed throttle will be used. The thought process was to gain the benefits of a conventional twincharged (positive displacement) system with more flexibility in engine bay arrangement and the removal of any supercharger flow restriction during the turbocharger's maximum output.

As I said before I am a novice although I do have quite a bit of time under the hood and online for my short years on this planet. I'm looking for your guys' thoughts and predictions on this. There very well may be a major design flaw and I need help finding it or any advice for success. Thanks guys!
 
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Just a little update on the project. I have begun construction on the supercharger using a Borg Warner S475 compressor cover and wheel with a custom gear housing and shafts. Bear in mind the compressor in the picture is not Borg Warner it is only for demonstration. The final design will have a Precision 6769 turbocharger feeding the centrifugal supercharger with a mechanical vacuum operated bypass valve similar to those used on factory sequential turbo setups like the V6 TDI biturbo motor. Of all the possible configurations this was chosen due to simplicity. An electronic valve may later be implemented for further improvement. The bypass will open when delta p for the supercharger becomes close to zero and attempt to hold the supercharger in zero duty while the turbo is at maximum boost. Theoretically this will reduce supercharger power consumption to only friction losses and the pressure drop in the housing due to viscous forces. I will update when the supercharger has been completed and passed testing phase. I will be running simulations of all transients with ANSYS CFX to determine optimal bypass operation and if a mechanical valve is capable of meeting such requirements. More to follow.

20180130_222101_dhnkmc.jpg
20180130_184721_n84crb.jpg
 
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The idea of attaching an alternator/generator to the turbo to recover waste heat (and disabling the main alternator until loads exceed what the turbo generator covers) then using an electric blower as a supercharger is intriguing to me... an electric blower won't have lag and allows precise control. What are the pros and cons of this approach versus augmenting a turbo with a mechanical blower?
 
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Cost. Obviously that system would be far superior. Perhaps a future concept.
 
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Bear in mind I am constructing this supercharger for only $600 not including my time. The first endeavor of the project was to build a proprietary supercharger using a turbo compressor and wheel as a possible drive system for a cheap diy supercharger kit. A series of compressor adapters may make it possible to sell the housing with gears/shafts for $500-$700 and the customer selects their own housing and wheel as a cheaper alternative to the big dog companies for grassroots and entry level cars.
 
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Boostedbimmer,

I have no idea how many CFM you need, but you can get a 250 CFM blower for $208 at The blower is 180W continuous, so you'd want a 180W electric motor/generator, and one such motor is available for $12 at The rest of the parts would be common off-the-shelf diode bridges, regulators, etc. You'd also need a bit of software running on a $10-$20 Arduino-like board like those at
Thinking a bit more about it, I'd consider buying one blower, copying the fan/housing design (just make a mold), and using another 180W hobby motor like the one above coupled with a speed controller costing $14 at
I'm not sure I have the CFM or motor/generator wattages as high as you would need, but those I selected give you a clue of just how cheap this could be; two $12 motors, $14 speed controller, a blower fan and housing (very cheap if you can make it by pouring high temp plastic into a mold of the commercial fan), a $20 Arduino board, and $100 miscellaneous parts totaling $158. I also think this kind of design is lower risk than a new design of an impeller blower unless you own CFD software and are proficient in its use. You *would* have to learn some electrical and software skills if you don't already have them, however. Those skills are more in my wheelhouse than impeller design and CFD, so I'd be comfortable. Fortunately for anyone who is not, there are tons of online tutorials and help forums for electrical and software design.

Rod
 
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A few calculations show a need for 310 CFM in a 2.5L four stroke at 7,000 RPM. The $150 leaf blower at claims to produce 430 CFM. It’s variable speed and brushless, so it should be easy to control and convert to a generator. These blowers (and the one I posted earlier) produce high pressure; they’re usually made of centrifugal fans of some sort, typically impellers. These type blowers are used in vacuum cleans specifically because they can generate high vacuum on one side (or high pressue on the other).
 
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"High" by what standard. "High" by vacuum cleaner standards is "peanuts" by forced-induction engine standards.

The mechanically driven centrifugal supercharger in my beast of a motorcycle spins at 120,000 rpm at engine redline and develops something like 1.2 bar of pressure. I don't know how much crankshaft power it takes in that condition but I'm sure it's several kW.

Using a leaf-blower as a supercharger is rather widely regarded as a joke ...
 
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If someone can find an electric supercharger for less than $600 that can do 40 lb/min at 25 psi reliably I'm all game. To my knowledge this does not exist.
 
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Per Wikipedia, air density is .0765 lb/cubic foot, so 40 lbs per min would be 522 CFM and 25 psi is only 1.7 Atm. That’s entirely feasible. There’s a 600 CDM leaf blower for $58 at , and I’m pretty sure based on comments it produces at least 1.7 atm. It’s AC, however, so the the electronics would be more difficult.
 
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Nowhere in that ad does it say anything about the pressure that it makes.

It can somewhat be inferred from the "110 mph" claim. If it can blow at 110 mph then we can deduce the velocity pressure from the Bernoulli equation with the assumption that the nozzle is fully effective at translating static pressure into velocity pressure. It won't be dead-nuts accurate but it won't be a million miles away from the truth, either.

So what velocity pressure corresponds to "110 mph" flow velocity under the assumption of starting with standard atmospheric conditions ...

I have to do these calculations in units that make sense. 110 mph = 180 km/h = 50 metres per second.

Standard air density is 1.2 kg/m3.

So the velocity pressure is then 0.5 x density x velocitysquared = 0.5 x 1.2 x 50^2 = 1500 pascals.

Standard atmospheric pressure is 101325 pascals.

So this thing is capable of developing a "boost pressure" of about 1.5% of atmospheric pressure. If you prefer old English units, 1500 pascals is 0.2 psi.

Now, I doubt if the "nozzle" is converting ALL of the static pressure to velocity pressure. We don't really know the basis of the "110 mph" claim. (Is it at the tip of the nozzle, is it some standard distance away from the end of the nozzle, etc.) But it's not out by two orders of magnitude compared to what's needed!
 
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Compressibility of air at 1500 Pa "boost" pressure is insignificant and can be neglected.

A real supercharging blower needs to generate enough boost pressure that you can't neglect it any more.
 
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This sounds like a bad idea to me. Brian summed it up in post #6 - the centrifugal blower is not the ideal machine for low rpm grunt. BTW to make it do that, it needs to be UNDERSIZED for the engine to keep it out of the surge region under low-rpm/high-boost conditions. On changeover to the turbo you will need to uncouple the drive to the supercharger to keep it from over-speeding (with the drive ratio needed to produce significant low rpm boost). An air con clutch will probably do.

The bypass will open when delta p for the supercharger becomes close to zero and attempt to hold the supercharger in zero duty while the turbo is at maximum boost.The delta p will increase not reduce as you raise the inlet pressure. The PR of the centrifugal supercharger remains the same.

Use a positive displacement blower or staged turbochargers if you must twincharge.

Easiest route is not to twincharge at all - just use a positive displacement supercharger. If you must have the outright power of a turbo there are ways to fill in the bottom end - careful exhaust design with divided housing for blowdown, advance exhaust valve timing, larger exhaust valves, a shot of nitrous brings on boost nicely.

je suis charlie
 
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I guess I didn't clarify this will not be for low end grunt. Merely throttle response as the car will live in high rpm's on the track. If it was a street car you would be absolutely right about the positive displacement being better suited. The car will stay above 5000 rpm's on track with my gearing and rev limit. The purpose is to get instant throttle response out of the corner without antilag or nitrous. A pd blower will be more parasitic and difficult to fit in my engine bay.
 
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Adding complexity to your system makes fast throttle response LESS likely, not more.

I either missed it, or that last post was the first time you mentioned the application for this design; i.e. this isn't a street car. As you move from a street driving profile toward a race car (i.e. much more time @ WOT, much higher mean RPM levels, etc) the advantages of a supercharger over a turbocharger fade rapidly.

This sounds like a heavier, more expensive, less reliable version of a well tuned turbocharged race engine.

If this engine really will spend all of its time in anger above 5000 RPM, and unless we're talking about a motorcycle or a very expensive/advanced car engine, you're trying to provide peak pressure and maximize throttle response over a range of maybe 4000 RPM.

With current variable vane turbo technology and even remotely competent tuning, achieving both of these goals simultaneously is not hard.

If this is for a trackday toy and your goal is just to have fun and do something crazy, by all means soldier on. If your goal is to build the fastest car possible for X amount of money- this is not it, pretty much regardless of what X is.

There are a lot of problems to solve which you've glazed over- namely what you're going to do with the high volume of high pressure air coming out of the supercharger discharge when you shut it off. Clutching the supercharger doesn't solve this problem, because the supercharger has a moment of inertia greater than zero; when you declutch, it will not stop, potentially for several seconds. Using giant gears to drive the compressor makes this problem worse, not better.

You also have to do all this in a way that doesn't cause the compressor to operate in a state across the surge line- unless you want to destroy a lot of compressor wheels and rebuild a lot of engines.
 
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Once again, I implore study of the Kawasaki H2 and its mechanically-driven centrifugal supercharger, and the bypass valve that is built into the design to address the near-shut-throttle situation.
 
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