Compound turbochatging / wastegates

[Rat5]

Hello,
I came across this video of THailand drag trucks. They use compound turbo systems. However, it seems that they dont reintroduce HP wastegates outlet before LP stage. (wastegate on collector, before small HP turbo dumps to open air)

Is there a strategic reason behind this ? To me this is wasted energy not going to spool the LP turbo. But maybe the HP turbo runs more efficiently because of the greater pressure delta /expansion ratio ?

At 1:40 minute mark

https://youtu.be/LnRv8HJFoD8

 

[TugboatEng]

Turbochargers have windows they need to work in. Stall, surge, and speed are all limitations. You can't just recover everything or else why have a wastegate to begin with?

 

[Rat5]

Very true, but most compound systems have wastegate to bypass the HP turbo usually, they never dump them to open air. Hence why i asked what made it different case here

 

[Lou Scannon]

Anybody's guess. You have to be the one doing the layout to know the boundary conditions and constraints.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[BrianPetersen]

Wastegates work by spilling excess energy in the exhaust. The engine only cares about the total pressure ratio start to finish of both turbochargers, it doesn't care that the low-pressure made a little more while the high-pressure made a little less or vice versa. If the total system is making too much boost, spilling the excess from the high-pressure side cuts down the available exhaust energy to both turbochargers simultaneously, so that they both don't overspeed.

Compound turbochargers in this layout are designed to both work together all the time. In this application, they're not designed to have user-friendly driveability and low-end torque for puttering around town. They only care about max power output, and probably only in a relatively narrow engine RPM range right at the top of the engine's useable rev range.

Independent wastegate control for each stage is only if you're trying to sequentially control them, i.e. a little turbo to take care of the bottom of the rev range (big one spins lazily without doing much while the little turbo does most of the work) and a big turbo to take care of the top of the rev range (little one is bypassed). Those are way more complicated to get to work right.