Centrifugal Twincharge Idea 2

[Boostedbimmer]

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!

 

[gruntguru]

Thanks for clarifying. The main cause of poor turbo engine throttle response is the downstream throttle plate. Move it upstream. Trailing throttle through corners maintains reasonable turbo rpm and throttle response is vastly improved. The formula goes something like this:
- 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

 

[waross]

Has this setup been mentioned yet?
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

 

[BigClive]

Waross - seems like a practical idea.

 

[Lou Scannon]

I assume you mean a positive displacement supercharger. If it is sized to provide a useful amount of boost, I'm failing to see what is the probable net gain of adding a turbocharger to the setup. I suggest you do a back of the envelope calculation of the component inlet and outlet conditions and component sizing at the crossover point, and then another one at the design point of the turbocharger with the same component sizing.
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

 

[waross]

A supercharger normally gets its energy from the engine that it is supporting. It provides a constant boost ratio throughout the load range.
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

 

[gruntguru]

The net gain would be either:
- 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

 

[Boostedbimmer]

This is essentially my design only with the turbo feeding the supercharger. I'm starting to realize nobody shares my vision and I'm getting little advice that is benefitting my progress. This project is mostly for the fun of it however I have spent a lot of time researching and am reaching a point where free information is less available And more trade secret. I appreciate all the input guys. I'll let you know how it goes. If I fail at it feel free to laugh. I won't have any regrets.

 

[SwinnyGG]

No one is going to laugh at you- this is a community of engineers. This isn't bimmerforums.

 

[gruntguru]

With the turbo feeding the supercharger you are going to end up with a lot of boost. If you include a means for de-clutching the supercharger you will experience a sudden drop in boost and a pause waiting for the turbo to catch up.

je suis charlie

 

[johandelivera]

I am doing something similar. I have a 2JZ engine swapped into a BMW E46. The 2JZ as everyone probably knows is a 3 liter straight 6. Mine has the stock sequential twin turbo (0.5 bar boost when turbo 1 kicks in at 2500 RPM, and total boost of 0.8 bar with both running at 4500 RPM). I recently acquired a used HKS centrifugal supercharger GTS8550 quite cheaply, pressure ratio 2.6 and max flow 26m3/min. The SC has a traction drive that is supposed to reduce parasitic loss at high RPM.

My questions:
I plan to feed the turbos into the SC to compound boost.
1) What boost can I expect to get total? I have read the threads here on twincharging using roots blowers, but was unclear if the same principle of compound boost multiple applies since this SC is not a positive displacement pump.

2) Should I install a wastegate after the SC to control total boost?

Thanks!

 

[BrianPetersen]

A quick google search suggests that a GTS8550 has a fixed 9.368:1 drive ratio. Got any information on that CVT?

Don't throw away pressurized air in an attempt to regulate boost pressure. If your supercharger indeed has a built-in CVT, use that to alter the drive ratio. Otherwise ... All the statements earlier in this thread indicating why this is a bad idea, are still in effect.

 

[johandelivera]

See attachment about the CVT

 

[BrianPetersen]

That's not a CVT. It's a traction-drive planetary "gear" set with a fixed drive ratio. (Rollers instead of gear teeth, pressure applied to get enough traction between the "meshes" for them to stay "engaged" without slipping)

 

[RodRico]

Boostedbimmer, there is no failure as long as we learn.

 

[johandelivera]

Any idea on what total boost I would get feeding 0.8 bar of boost from the turbo into the HKS supercharger? Is it approximately 0.8 x 2.6 pressure ratio?

 

[BrianPetersen]

"Boost" of 0.8 bar (at normal atmospheric conditions) is 1.8 pressure ratio. Your total pressure ratio is going to be approximately 1.8 x 2.6 if the operating conditions coincide, which they might not, and intercooling or lack thereof is going to have some influence.

Still, you're going to have an engine operating with somewhere near 4.6 atmospheres of intake manifold pressure when everything is operating at "design conditions" ... and not much over 1 atmosphere intake manifold pressure just off idle. That doesn't sound very driver-friendly.

I have a long story about a stock Mazda 3 beating a heavily turbocharged VW at the drag strip. No idea what was in the VW, I'm assuming the usual too-big turbo plus too-big cams, thus leading to (let's say) 400 horsepower at 1 rpm short of rev limit, the problem being that if the revs weren't high enough to come "on the cam" then it also didn't produce enough exhaust flow to "spool the turbo" so now you had an engine running below the camshaft's operating RPM range and a turbo that is not doing anything, thus giving approximately as much useful power output as a lawnmower engine.

 

[BrianPetersen]

I found the compressor map for the HKS8550.

Max impeller RPM is 110,000 and that's when the max pressure ratio is just shy of 2.6.

At half of rated RPM the pressure ratio is around 1.4, maybe less.

At three-quarters of rated RPM, let's say a little over 80,000 rpm, the pressure ratio is going to be around 2.

This, on its own, leads to a very top-end-biased torque curve.

Earlier in this thread, the topic of the Kawasaki H2 (motorcycle) came up. This is a production engine with a production fixed-ratio centrifugal supercharger. It so happens, that I have one of these. In order to counteract the natural tendency of centrifugal supercharging to make a ton of power on top but nothing down low:
- The cam timing is very conservative, with the intent that the engine itself naturally has a lot of low end torque. That also naturally leads to the engine itself not breathing so well at higher revs ... but that's when the supercharger is ramming air down its throat. One tends to offset the other.
- The supercharger is "undersized". It is operating quite close to choked conditions high in the engine RPM range. This naturally offsets the tendency of a centrifugal blower to make boost that is exponentially higher with revs.
- The engine is fully drive by wire. The tendency to have bizarre driveability characteristics because of the exponentially-higher-with-revs boost is offset by the drive-by-wire throttling it down.
- Traction control is standard equipment ...