What Causes Turbo Flutter?
Turbo 'flutter' noise is present on many cars with aftermarket modifications, but why does it occur?
Ask about turbo flutter on the internet and you'll get 50 different answers. Some people love the noise, while others think it's a little too 'boy racer'. Mainly though, you'll get 50 different explanations of what causes it, and whether or not it's bad for the the turbo. Let's clear some of those up.
A few of the main parts of a turbo.
First up, the phenomenon is actually called 'compressor surge'. A turbo is broken into two primary parts; a compressor and a turbine, and these are mounted on a common shaft. The compressor spins and pressurises the air before the throttle body, creating boost which essentially allows the engine to burn more fuel and produce more power. The turbine is fed by the exhaust headers/collector, and harnesses the pressure and heat energy from the exhaust gasses. By connecting the two on the same shaft, you can use the turbine to power the compressor, and this is more or less how a turbo works.
A sample compressor map, courtesy of Engine Labs
Compressors on turbos have known efficiency ranges, and we can plot these on 'compressor maps'. Such a map is shown above. The near horizontal lines give an indication of performance at a given turbo speed, while the other, fainter lines are islands of compressor efficiency. The line we are interested in is the surge line. We can see that for a given pressure ratio, if we drop the mass flow sufficiently low, we end up moving to the left of the surge line.
Once we have done this, the airflow will start to undergo flow separation off the compressor blades, allowing that pressurised air to push back and flow through the blades. This is because once the separation has occurred, the compressor is not functioning effectively at all. The result is a drop in the post-compressor pressure, allowing the air to reattach on the compressor, and flow through, repeating the process. This is known as compressor surge.
Compressor surge can happen on aircraft too. It's a little different, but usually pretty bad.
We can see that slamming the throttle shut at medium to high RPM will push our mass flow towards zero, but the turbo is still spinning, and thus trying to pressurise the air, so we get compressor surge. The turbo doesn't stop spinning (no idea how that rumour started...), we just have surge. The surge will cause the turbo to slow down faster than if it was allowed to just vent out the pressurised air (think blow off valves, bypass valves etc.), and it will apply a degree of shock and vibration to the turbo as a result of the stall and pressure cycling.
The damage caused to the turbo depends on the turbo type and where the surge occurs. At low turbo speeds or low boost, the pressure waves and vibrations are much smaller, so for your casual low boost cruiser a bit of surge isn't going to cause a significant shortening of turbo lifespan. If you start to get full throttle or high load surge though, this can rapidly kill turbos. Turbos with large thrust bearings are often more resistant to this kind of abuse. Really though, turbo flutter is only a disadvantage from a performance standpoint.
The big flame is from the turbine outlet, the small one is from the wastegate.
You hear a few people around talking about 'wastegate chatter', but the characteristic flutter noise is not caused by the wastegate. The wastegate is used to regulate boost by venting exhaust gas around the turbine instead of through it, and is activated by the boost pressure at the compressor outlet (often moderated by an ECU controlled soleniod). While the pressure spikes created by off throttle surge can cause the wastegate to momentarily open or 'chatter', this is not what causes the noise, and doesn't even happen in most cases (think lower boost levels) especially if the ECU is already venting the wastegate actuator air.
For more details and a practical demonstration, check out the video below!
Kyle Forster is a qualified Aerodynamicist, race car engineer, and the man behind JKF Aero, a firm that offers a variety of aerodynamic consultancy services for racing purposes. If you have any questions for Kyle or have any suggestions for future videos, drop them in the comments below!