Asymetric Turbocharging

[RJetta8v]

The new 9-5 from saab features an "asymetrical" turbo. Some of you might have heard of it. Its a V6 turbocharged engine with the turbo running off one of the cylinder banks, while the other bank's exhaust does not go through the turbocharger. However, from what I understand, the turbocharger is connected to a common manifold which all the cylinders share.

How does this work? Wouldnt this put more stress on one side of the engine? If the turbo is only working off of three cylinders, how can it provide boost for 6? I know its a low preassure turbo, but how would it behave at high RPM's? Would there be any advantage to using a setup like this on a four-stroke, high RPM bike engine?

 

[HomoFaber]

the turbo is fed by gases from 3cylinders only, but charges all six - so it doesn`t put more stress on one side of the engine.

I think the main reason for this arrangements was to improve bsfc at partial loads and at medium speed rather then increase max. power; thus, max. boost pressure (reached at high speed) is of minor interest.

 

[RJetta8v]

I see...

What does BSFC mean?

 

[HomoFaber]

bsfc = brake specific fuel consuption

 

[NormPeterson]

You do get more exhaust backpressure on the bank that powers the turbo, so that bank might tend to run a little hotter, and it might prefer slightly different valve timing especially on the exhaust side.

Norm

 

[ruggedscot]

This engine was done for two reasons....

1. to increase torque

2. so that sabb had all its current vehicles turbocharged as this was an advertising slant.

was a good engine, made in ellsmere port down near liverpool.

 

[RJetta8v]

What if you were to use the asymetric turbo on a four cylinder engine? I asked someone about it today and they said that perhaps pulsing would be a problem. I thought about it for a while and remember that no matter what, the pulses are going to be the same frequency. So what do you guys think? The particular engine im thinking about doing this to would be a 600cc honda bike engine.

 

[NormPeterson]

There will be 50% more pulses at any given RPM with a six vs a four, as long as both are turbocharged either symmetrically or asymmetrically.

Most sixes would be easier to plumb for even pulsing into the turbo, as you'd connect all the cylinders in one bank (of a V6) or either the front 3 or the back 3 of an inline 6. An inline four would want either the middle two or the end two cylinders feeding the turbo.

Norm

 

[RJetta8v]

I dont understand why you would want the middle two or the outside two. In four cylinder engines, the inside pistons fire at exactly the same time. Same goes for the outside right? If you would use the outside two, or inside two, then you would only get 1 pulse per revolution of the engine. I dont know very much about turbocharging, but wouldn't you want more pulses/revolution?

If so, you could use cylinder one, and either cylinder two or three, since two and three fire at exactly the same time. I think?

 

[GregLocock]

"In four cylinder engines, the inside pistons fire at exactly the same time." No they don't.

"Same goes for the outside right?" Wrong

"If you would use the outside two, or inside two, then you would only get 1 pulse per revolution of the engine." True, but probably not for the reason you were thinking of.

"I dont know very much about turbocharging, but wouldn't you want more pulses/revolution?" You don't seem to know much about firing orders either...

Typical firing order is 1342. Each fires 180 degrees after the preceeding one. To get two evenly spaced pulses per 2 crankshaft revs you need to pick 1 and 4 or 3 and 2, as the previous poster suggested.

Cheers

Greg Locock

 

[RJetta8v]

Geez, im just trying to learn. Theres no need to be so harsh.

 

[GregLocock]

This is a site for professional engineers.



Cheers

Greg Locock

 

[Azmio]

Rjetta

The asymmetrical turbocharged Saab is unique because it has only one turbo instead of two. I just want to fill the gap by saying that they increased the exhaust backpressure on the other bank that does not have any turbo. They made a lot of iterations to ensure that the backpressure curve is as similar as possible to the backpressure caused by the turbocharged bank. Still, I dont think they get a good COV for all 6 cylinders especially at low rpm and idle. The variation between one cylinder and another is inevitable.

Anyhow, if you have a high performance bike, I dont recommend that you turbocharge it asymmetrically. If you still want to have it turbocharged, connect all 4 exhaust pipes and not just two. You will get less headache at low rpm, idle and part load engine operation. You will also get more power if you run your turbine from 4 cylinders instead of two.