Centrifugal Twincharge Idea
Centrifugal Twincharge Idea
(OP)
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!
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!





RE: Centrifugal Twincharge Idea
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
RE: Centrifugal Twincharge Idea
Making it work at any fixed point in your speed/load plot won't be all that difficult; rubber meets road during transients, especially when you're switching from supercharged to turbocharged operation.
There's really two ways to skin that cat- you either make the transition as immediate as possible. Meaning that the turbo will be pressurizing the inlet of the supercharger (the interaction which is most complicated) for only a very short period, and the transition between intake air through the supercharger and intake air through the bypass is as fast as you can make it.
End result of this is a system which WILL have a little 'blip' in driveability during this transition; depending on your goals, this may or may not be OK.
The alternative is, of course, to optimize and make the transition as smooth as possible. This requires more time, better controls, and more mechanical ingenuity; the end result will be a car that is better for daily driving.
RE: Centrifugal Twincharge Idea
Your idea should be fairly simple to implement and calibrate since the supercharger and turbo aren't compounding, merely bypassing each other. In theory it should give the best of both worlds. In reality it will be expensive to install/maintain and fill already crowded engine bays to the limit. Whether or not its worth the cost and effort depends on who you ask. ;)
RE: Centrifugal Twincharge Idea
The scheme used an electrically driven supercharger. Drive an alternator with an exhaust driven turbine.
The motor may be speed controlled to give the level of boost that you require.
At low power levels the motor will be driven by the battery. At high power output the exhaust driven alternator powers the motor.
No need to worry about transitions from supercharger to turbocharger mode. The motor does not know or need to know whether the power is coming from the battery or from the exhaust driven alternator.
Has anyone seen this done? Does it work in the real world?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Centrifugal Twincharge Idea
What is the bottom line objective of what you are trying to accomplish?
At a certain point you just have to tell the driver (or, more commonly these days, the automatic transmission) to downshift instead of trying to make stupendous (detonation-inducing and main-bearing-shattering and clutch-vibration-damper-smashing) torque right down off idle.
I own a production vehicle with a mechanically driven centrifugal supercharger (Kawasaki H2), and it's driven using a plain ordinary fixed drive ratio from the crankshaft, and it's in combination with very mild bottom-end-torque-oriented cam timing on the engine itself. It will pass everything except a gas station. It has a bypass valve in the intake system, but it only operates under near-shut-throttle conditions so that the compressor is not operating in surge.
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
Regarding the transition control: "The trick is when to use either or both compressors, and that’s left to some complicated software developed by a staff of 10 in-house engineers, according to Vollrath. The result is a steady supply of forced air with virtually no gaps from idle to redline, generating forced-air power and torque with improved efficiency and no lag." Sounds easy, LOL.
Ref: http://autoweek.com/article/technology/why-mercede...
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
If you divert its discharge to atmosphere it will consume a lot of power doing nothing - the power consumption of a centrifugal blower increases with increasing mass flow rate through it, and diverting discharge to atmosphere means the mass flow rate will be the maximum possible.
If you shut the flow off completely it will be operating in the surge regime.
If you clutch it, it will require time to speed up.
Study the production Kawasaki H2/H2R engine, please.
RE: Centrifugal Twincharge Idea
Remember that the second stage compressor will see inlet conditions where the air is much denser than atmospheric, so you can pass more mass flow through the small compressor than you could if the inlet conditions were STP.
Corrected flow through a turbo compressor is m-dot x sqrt(T1/Tref) / (P1/Pref).
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
Compressed Air Supercharging
http://casupercharging.com/#cas-system
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
I have no idea how many CFM you need, but you can get a 250 CFM blower for $208 at https://www.mscdirect.com/browse/tn/?searchterm=ja.... The blower is 180W continuous, so you'd want a 180W electric motor/generator, and one such motor is available for $12 at https://hobbyking.com/en_us/turnigy-l2210a-1650-br.... 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 https://www.digikey.com/products/en/development-bo....
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 https://hobbyking.com/en_us/turnigy-plush-25amp-sp....
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
RE: Centrifugal Twincharge Idea
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
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 ...
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
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!
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
https://en.m.wikipedia.org/wiki/Electric_superchar...
RE: Centrifugal Twincharge Idea
http://www.superstreetonline.com/how-to/engine/040...
RE: Centrifugal Twincharge Idea
A real supercharging blower needs to generate enough boost pressure that you can't neglect it any more.
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
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
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
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.
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
- turbo operated well below its boost limit (to avoid overspeeding)
- modified compressor seal arrangement (two piston ring seals with atm' venting between)
- ball bearing or modified thrust bearing to handle greater thrust loads
- water-air intercooler (or no intercooler) to minimise intake volume
je suis charlie
RE: Centrifugal Twincharge Idea
A super charger discharging into and through the turbo.
You size the supercharger to give the boost that you want.
Blow through the turbo.
As the turbo spools up, it will be be sucking air away from the supercharger. At the point where the turbo mass delivery equals the mass delivery of the supercharger, the pressure between them will equal atmospheric pressure. As the turbo increases the mass delivery further a check valve will open and allow more air to enter the turbo inlet.
At this point the pressure drop across the supercharger will be close to zero and it will be doing very little work. It can be left spinning without wasting a lot of energy and be ready for a smooth transition when the turbo contribution falls off.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
In the case of the PD blower, by component sizing I mean the volumetric displacement per 2 crank revolutions (assuming 4-stroke engine), as a ratio of the engine displacement.
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
RE: Centrifugal Twincharge Idea
A turbocharger gets its energy from the heat of the exhaust gases, energy that otherwise would be wasted.
Thus the turbocharger is more efficient than a supercharger.
However, the heat of the exhaust increases with the loading on the engine and so the boost from the turbocharger varies from little or none at low loading to more than enough at high loading.
The low boost at low loading produces throttle lag until the turbo spools up.
The plan is for a smooth and simple transition from constant boost at low loading to avoid throttle lag and then a transition to turbo boost, with the supercharger basically moving air against zero back pressure, at which point the power demand of the supercharger will be almost nil.
The best of both worlds with a check valve allowing the turbo to overtake and unload the supercharger when the turbo comes into its working range.
Note: A two cycle GM diesel does not have a supercharger, it has a blower. The purpose of the blower is to scaveng the exhaust gases out of the cylinder during the time when both the intake valves and the exhaust ports are open.
Yes, the GM blower does make an effective supercharger when fitted to a high performance engine, but it does not increase the charge volume.
Add a turbo ahead of the blower and it works against the back pressure of the exhaust impeller of the turbo to produce boost.
There is an interesting range of GM blower/supercharger sizes.
A 2-71 blower from 2 cylinder engines.
A 3-71 blower from 3 cylinder engines.
A 4-71 blower from 4 cylinder engines.
A 6-71 blower from 6 cylinder engines and 12 cylinder engines.
An 8-71 blower from 8 cylinder engines and 16 cylinder engines.
Then there is the 10-71 supercharger. GM blowers/superchargers became available from independent manufacturers when they were either too expensive or no longer available fro GM. I response to requests for a larger supercharger than the 8-71 the 10-71 supercharger was developed for the high performance after market, but there never was a 10-71 engine.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Centrifugal Twincharge Idea
- higher top end power thanks to maintaining or rising boost. Also exhaust energy takes over the role of driving the compressor from a (probably inefficient) positive displacement blower.
- or (in the case where waross's check valve/bypass is eliminated) better efficiency by using exhaust energy to take over the compression role AND return some work to the crankshaft by "driving" the blower with a PR less than unity.
je suis charlie
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
RE: Centrifugal Twincharge Idea
je suis charlie