Compound turbo setup.

[pretzil]

Found this pic from a rotary forum. Obviously it would be 2 turbos feeding into the larger turbo.

How would that layout go if a fuel injection system was added between the small and large turbo, creating a turbojet engine power for the larger turbo


The guy who sold me my legnum claimed that some people in NZ had moved the battery to the boot and set up a third turbo sequentially to kick in at higher revs. Don't know if he was actually talking out his ass, I haven't found any evidence of it online.

 

[rockit]

Found this pic from a rotary forum. Obviously it would be 2 turbos feeding into the larger turbo.

I don't see the point of this... every single mechanical system you introduce will have under 100% efficiency (probably massively under 100%!!)

if the twin turbos can do enough work to power up the large turbo in this arrangement then surely the two small turbos are sufficient to do anything without the large one there!..

 

[TME_Steve]

Yeah that diagram makes no sense, dumb idea.... You have energy from exhaust gases turning a turbine which imparts energy to a compressor which loses some of the turbine energy in the process. then you spin another turbine with that new air losing more energy to then spin another compressor losing more energy. makes no sense. Here's how it should work working with std exhaust manifolds and TD03 turbo locations ignoring lots of little things like pipe pressure drops etc to keep it simple.

The inlet air is sucked into a large turbo, say a GT30XX ish sized thing and compressed to say 10psi, this started as say 25 degree air at 100kPa absolute (ambient) pressure.

When it's compressed it will be 170ish kPa absolute and probably warmer and the mass air flow will be the same with less volume.

Maybe you can then cool it it to 25 degrees again somehow (intercooler for example) and you'll then have the same amount of mass air flow still but only 0.588 times the original volume (1.7times less due to the density change). Then the TD03 compressors can compress this air from 170kPa to say 250kPa which volume wise is a 47% compression, like raising it from atmospheric to about 7psi, so not that hard and 250kPa absolute is roughly 22Psi of boot pressure. Cool, and the turbos are barely trying and moving 70%% more air than std at 7psi - Yay!. It's probably now something like 60 or 70 degrees but we can intercool it again to maybe 35 degrees or something, let's assume the world is awesome and we can intercool it back to 25 and we'd then have 150% additional air getting forced into the throttle body as it is at 250kPa absolute than if we didn't have any turbos and there was only 100kPa absolute pressure. This then goes into the engine and gets fuel added.

As the fuel is added and there is a bit of a bang, there are now more molecules in the exhaust gas than there was in the inlet gas (because we added fuel). On top of that the air is hotter from the explosion. So we may have double the mass of exhaust gas compared with intake (I can't remember the mass multiplier due to fuel and frankly cbf working it out now) but volume wise the temp has gone up to say 600 instead of 25. In terms of absolute temps this would be an increase from 298K to 873K so the volume goes up another 2.9times due to the heat alone. So assuming double the mass and some other incorrect assumptions about the gas makeup due to the fuel added, the total increase in volume compared with the air going into the engine would be 5.8times. the TD03 compressors compressed the air 47% as noted above so the turbine is flowing 5.8/1.47 about 4 times the flow than the compressor.

Then maybe 400degree C air is going into the big primary turbo and pushing that turbine but you can see from the TD03 side, the big volume change is due to the temp, and the added fuel.

In the diagram above, the exhaust gas exiting the engine and flowing through the first turbo's turbine would be far greater than the volume of air being pushed by the same turbos compressor. And with the same logic, the second turbo's compressor would move less air again. hence it being dumb. You'd be better off just putting the first turbos compressor air into the engine. in short, you don't usually feed turbines with compressors powered by turbines or you should just delete one of the steps. If we could 'create' energy to increase power like this we wouldn't need to be digging coal and oil out of the ground.

As for a remote turbo, it can still work, it's just laggy and less efficient. Look at an Sti vs an evo for lag. The Sti has half the exhaust gas running for miles to the turbo. the EVO has the turbo relatively close. and the STi's need 2.5L to make up for the their inherent laggy design vs the Evo 2L. This is also why heat wrap and ceramic coating on an STi can be of benefit, to keep the exhaust gas hot and the velocity up going through the turbo turbine.

Make sense now?

 

[trotty]

Yeah all makes sense when explained in numbers. The other thing my design would fail at is actually controlling the small turbo shaft speed, as it requires boost to open the wastegate it would never actually build and as there would be minimal resistance on the exhaust housing of the larger turbine. Therefore possibly over spinning and blowing the smaller turbo.

Thanks stave for the patience and time to explain.

Now to go the original compound turbo route what size would be best suited as the larger turbo. I was thinking xr6 gt 35/40 or would this be too big?

 

[smohekey]

I was thinking around the gt37 mark. I had tried to do all the calculations to pick the large turbo properly, but it seriously does my head in.

 

[TME_Steve]

That's make big power, I went through this a couple if years ago, I can't remember what size I came up with in this scenario, but it doesn't need much boost so a 30xx may be enough, I ended up concluding a pair of turbos would still have been the go for what I was thinking (270atw aim) at the time....

 

[smohekey]

Yeah, I was thinking something like 500hp at the wheels.

 

[trotty]

500 hp is definately a nice number. But in reality I would be happy with about 250kwatw. So what's that about 400hp. With stronger head bolts this should be reasonably reliable. As Steve said the rear diff would go first. So LSD the rear tear out the ayc.

 

[smohekey]

I have forged pistons and rods lined up, as well as lsd front and rear and a host of other mods (ported heads with cams etc). So 500hp was looking possible.

 

[trotty]

I have forged pistons and rods lined up, as well as lsd front and rear and a host of other mods (ported heads with cams etc). So 500hp was looking possible.

Shame ur not in Sydney. We could make something real special. Lol

 

[smohekey]

Hah, yeah. I have a tig welder, but I wouldn't call myself a good welder by any stretch.

 

[Guest]

Wouldnt 250kw be pretty easy to hit with a set of tdo4's, maybe ported manifolds (why not )
And a set of reground cams to keep the power and torque down low and mid range?

If you don't thrash it shouldn't our drive lines be safe at 250kw?
Evo drivelines aren't terribly different, my mate has a 300kw 7 that he thrashes all day (I mean limiter launching from lights, redline gear changes) every day on standard AYC diff and standard box and transfer and its all still healthy as a horse.


Sent from the fires of Mt Doom

 

[trotty]

Mr leg. The idea was to not have the expense of remaking 2 manifolds and buying 2 new turbos. Instead just plumbing a larger turbo into the system
For the high rpm boost that the 03's struggle with. I figured if we weren't over working the small turbos then the inlet charge wouldn't be so hot. Makin it more efficient.

 

[vr4cobra]

On clubvr4 the guys use td04-13 from wrx with the stock manifolds, they just weld new flanges on and havnt heard of any issues with them yet so might not be too much nodding to do if u go that path. And they are not much different flanges compared to ours either

 

[smohekey]

One of the problems with increasing the size of the stock turbos, is that the spool up time increases. Using a compound setup, you can retain the stock quick spool up, but still have a high flowing top end.

 

[trotty]

One of the problems with increasing the size of the stock turbos, is that the spool up time increases. Using a compound setup, you can retain the stock quick spool up, but still have a high flowing top end.

This is exactly what I wanted to achieve. Stock spool rates and a whacker of a top end.

 

[smohekey]

This is exactly what I wanted to achieve. Stock spool rates and a whacker of a top end.

Yeah me too.

 

[trotty]

How this for a compound compound setup. Holy boost batman!!!!

 

[smohekey]

That's a weird one! Seems like its back to front.

 

[Nash]

Actually you lose stock rates due to the thermal change you are creating buy using the Compressor end of your first "Larger" turbo. Running them in parallel staged (With shut offs) will give you a better outcome.