Centrifugal compressor (turbocharger) question... Help ??

[Rat5]

Hello,

Here 's my problem :

Turbocharged engines are fitted with a blow off valve, a valve which opens when throttle is slammed shut, in order to keep the air moving, and prevent the turbo from slowing down...

Yet, i read an article and now i have 2 theories, if someone could tell me which is right :

1st : The turbo is moving air, if you slam throttle shut and there is NO blow off valve, air pressure will build up and slow the centrifugal compressor ? Yet, if there is a blow off valve the air will just move freely outside, letting the turbo spin.

2nd : There is no blow off valve, throttle is slammed shut, since there is zero flow possible, turbo will not move air, hence will spin freely, just like when you clog your vacuum cleaner, it speeds up.

Which is right ? 1st or 2nd ?

 

[itsmoked]

Since the turbo is a mass-flow device that is doing work only when it is moving air it would likely accelerate wildly, possibly flying apart if it suddenly has no mass-flow (air) to move. Opening of the valve is to keep the turbo loaded so it doesn't over-speed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianPetersen]

2nd. Sorta. But there's more to the issue.

The shaft power that a centrifugal blower absorbs has a component that is dependent on the mass flow rate through the blower. It's not exactly "proportional" because of complex flow relationships, but the general trend is that the blower will absorb less shaft power when there's less mass flow through it (shut throttle).

But the reason for the blow-off valve is nothing to do with "... prevent the turbo from slowing down". Turbocompressor blades are each like miniature airplane wings. If you fly an airplane too slowly (think: insufficient mass flow across the wings relative to the load being demanded of the wings) the wings stall and the airplane falls out of the sky. If you ask a turbocompressor to deliver too much pressure ratio with insufficient mass flow through it, the blades stall. This can lead to an unstable operating condition called "surge". The performance map (pressure ratio on the vertical axis, mass flow rate on the horizontal axis) of a turbocompressor has the "surge line" on the left side of that map - establishing the minimum recommended mass flow rate for a given pressure ratio. The blow-off valve opens under shut-throttle high-boost conditions to establish an alternate flow path for just enough of the flow through the machine to relieve surge.

 

[EdStainless]

The pop off also comes into play at the other side of the performance curve.
In order to get good low end response turbos spin up quickly. This often means that at high speed they will produce more flow than the engine can handle. So to prevent stalling at high flow some air is bled off.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[gruntguru]

If you want to prevent the turbo from slowing down on closed throttle, move the throttle to the upstream side of the compressor. Requires a compressor with shaft seal. Can also create instability and response issues if there is a large volume (eg intercooler) between throttle and engine.

je suis charlie