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Let's imagine a concrete scenario. You have your foot on the throttle and you're flooring it. The throttle plate is wide open and the engine is trying to make the most powerful combustions it can and as such it's also producing a lot of exhaust gasses very rapidly. These exhaust gasses drive the turbine wheel which then spins the compressor wheel as they're connected together with a common shaft. The more exhaust gasses are being sent into the turbo the faster the compressor wheel spins and the more it compresses the air.

So when the throttle plate is fully open we have both high airflow and high air pressure. We have high air flow because the throttle plate is fully open and there is no restriction to airflow. We have high air pressure because the turbo is working hard to compress the air and stuff more of it into the same space.

So what happens when we release the throttle? What happens is that we transition from a situation of high airflow and high pressure into a situation of low air flow and even higher pressure. The rapidly flowing pressurized air coming from the turbo suddenly hits a dead end and has nowhere to go. At the same time the turbo is still rapidly spinning and trying to keep stuffing the air into the engine which it can't do because the throttle plate is closed and this blockage further increases the pressure in the piping.

Now the blades of the compressor wheel are designed to „grab“ the air and push it onto the engine. In a situation where we have enough air pressure and enough airflow followed by a sudden closing of the throttle plate the dramatic drop in airflow and subsequent pressure spike can actually overpower the aerodynamic capabilities of the compressor wheel blades which results in them no longer being able to „grab“ onto the air causing compressor surge aka turbo flutter.

If you look at a compressor map of any turbocharger the left-most line is your surge line. Everything left of that line is the surge zone.

In the simplest of terms when compressor surge or turbo flutter occurs it means that there is too much air pressure and too little air flow for the compressor wheel to do it's job. When this happens the air has nowhere left to go and it carries so much pressure that it can actually force its way back through the turbo, past the compressor blades and out the intake. This wrong way out actually becomes the only exit path for the high pressure air. Compressor surge in a car engine is almost never powerful enough to stop a turbo from spinning and it can never cause a turbo to start spinning in reverse. But what it can definitely do is slow the turbo down and shorten the lifespan off the turbo as it exposes it to increased stress.

The sound of compressor surge or turbo flutter and is often voiced as stutututu or chu chu chu chu.....in fact each of those stu or chu is a small chunk of air separating from the blades and escaping past the compressor wheel out the intake. The first stu is going to be the loudest because it has the most pressure behind it and each subsequent stu will be less loud until all the excess air pressure is gone.

So how do we get rid of compressor surge or turbo flutter that occurs when we release the throttle? The answer is simple – instead of relieving pressure back out past the compressor wheel we relieve the pressure somewhere else.
And that's exactly what a BOV or a blow-off valve does. It gets rid of the excess air pressure in the piping when you suddenly release the throttle. By getting rid of the excess air pressure at a different location we do not subject the turbocharger o the increased stresses associated with turbo flutter.

A blow-off valve has a piston inside it. When the throttle is wide open both the top and the bottom of the piston see the same air pressure because the top of the blow-off valve piston references pressure from inside the intake manifold. But when the throttle plate closes the top of the BOV sees vacuum and the bottom boost pressure. This means that the boost pressure can easily overpower the spring on top of the piston and open a path for the pressurized air either to be vented to atmosphere (blow-off) or recirculated in front of the turbo inlet (re-circulation or diverter valve)

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#d4a #boostschool #stutututu

00:00 What is turbo flutter (compressor surge)?
08:40 How a BOV prevents flutter
12:07 How a BOV works
13:55 Where to install a BOV
14:30 When a BOV can't help