Hi,
Sorry if sounds like a stupid question but do twin turbo charged cars (if they have installed) have two dump valves?, and two wastegates? etc... The same with quad turbos etc... Can anyone enlighten me.

Cheers

Stiver3

 

yes. if they are two turbos for two banks of cylinders it's essentially two turbocharged engines that share one crank.

i tried to think about sequential turbos for you, but i couldn't. my brain isn't working - next question?

 

Two types of twin turbo.

Twin equal size small turbos on perhaps a V8. Each turbo serves half the V8 so each is the same as the turbo on a 4 cylinder engine, so has wastegate, dump valve etc etc. Just like two turbo'd 4 cyl engines under one bonnet, two of everything.

Twin turbos with one big and one small. In this case the small turbo kicks in first and gives some boost early on at low revs to reduce lag etc. Then at high revs the big turbo kicks as the small one is running out of steam. If you just had the small one you'd have no top end, if you just had the big one you'd have a laggy engine with no power below 5000 rpm. In this case just one wastegate, one dump valve.

 

wouldn't you need two wastegates with sequential turbos? you'd want gasses to bypass the bigger turbo at low revs. in fact, wouldn't you need three? in fact, how the **** do they work? the big turbo must have a wastegate that channels the gas into the smaller one and is wide open at low revs? what?

 

My guess is that the smaller turbo has a wastegate that dumps into the biger turbo to stop it over-speeding at high revs then the big turbo has it's own wastegate. Probably only one dump valve though.

 

A sequential turbo system will work along the lines of:

At low speed, all exhaust flow is to the primary turbo (which will have it's wastegate closed). Gasses can back up to the secondary turbo, but is not allowed to flow through the turbo due to a valve (similar to a wastegate) blocking the exit pipe. Usually, on the inlet tract, a strangler valve stops charge flow reaching the compressor. As the engine speed and exhaust flow increases, the primary turbo's compressor flow and boost will being to peak. Usually, the wastegate would be actuated at this time, but in a sequential set-up, the ECU will signal the secondary turbo's exit valve to begin opening. This allows flow through the bigger turbo, allowing it to spool. However, as the strangler valve is still closed, and the compressor blow-off valve is open, the turbo does not supply any air to the engine, all flow is essentially closed-loop from the compressor.
As the rpm's and exhaust flow increase further, the ECU signals to fully open the secondary turbo's blocker valve, which simultaneously closes the inlet vent valve. The strangler valve now opens as boost pressure rises, and the secondary turbo's compressor begins to supply air flow to the engine. As flow from this compressor continues to rise, boost will again peak, the ECU pulses the wastegate to avoid an overboost scenario!

Edited to change bigger/smaller to primrary/secondary!

 

thank you

i was thinking along those lines but the hour was too late for little swirling air diagrams in my head...

 

Don't thank me, thank the book I kinda plagiarised from (although changed significantly into "normal" speak)

 

Early Legacy TTs had wastegates on both turbos, later ones had one (on the primary).

 

What book was that Tony? Thats a good well detailed description.

Its a myth that sequential systems have one small and one large turbo. They could either be of any size. In the case of the leggy TT they are both near enough equally tiny.

 

forgive me if i'm wrong but the legacy is a flat four?

it's just a 180deg V-engine (so there's two banks of cylinders). So each tiny turbo feeds one bank?

 

http://www.amazon.co.uk/exec/obidos/...427248-2486849

Excellent book - very concisive - although needs to be read a few times for most of it to be "absorbed" (and is fantastic for pretending to be knowledgable by plagiarising and not admitting to it!)

 

Sin Bin for you m8 Not knowin yer Scoobies... All Subarus are boxers.

The Legacy twin turbo is a sequential twin turbo design: primary feeds at low engine speeds, providing fast spool up and good response. At hgher speeds the secondary is introduced - working with the primary - to push high outputs.

HTH

 

I think tath was just saying 'essentially' a boxer engine is a flat (180deg. ie FLAT) v engine.

You can't parallel T/C a four cylinder anyhow it would have to be sequential..!