Turbo Wheel Tech: Basics

Hello Interwebs,

This is Sam from SteamSpeed, and we are briefly going to talk about modern turbo wheels.

For a quick refresher, here are the super basics concept of how a turbocharger works:

  • Hot exhaust gasses coming out of an engine flow  through the turbine housing scroll (painted red in the picture)
  • The scroll focuses the gasses to a point and aims it onto the turbine wheel blades
  • This causes the turbine to spin on its shaft
  • The other end of the shaft is connected to a compressor wheel
  • The faster the turbine spins, the faster the compressor wheel spins.  To give you an idea of wheel speeds, the turbine and compressor wheels can spin up to 250,000 RPM.
  • As the compressor wheel spins, it compresses the air coming into the engine from the intake path
  • In super basic terms, the speed of the spinning compressor wheel will make boost and flow a certain quantity of air
  • As we compress air, it will become hot.  Ex. an 18G at will take 75 F ambient air compress it to 19 PSI, and the exit temperature will be about 300 F.  Normally intercoolers are used to bring down the charge temperature closer to the ambient temperature.
  • The boosted intercooled air is forced into the engine

Turbocharger Cutout

If there interest, we can do deeper dives in the details if anyone is interested.

Compressor Maps Explained

Waning: Physics Content 

Now with the quick primer out of the way, you can see that the aerodynamics of the wheels are super important.   It determines how much force is applied to the wheels.  Torque (moment of force) is applied to the turbocharger’s shaft.  Torque is:

\boldsymbol \tau = \mathbf{r}\times \mathbf{F}\,\!

Next, you might think that you can also increase the radius “r” of the wheels to increase the torque.  This is true, but it also increases the mass moment of inertia which in layman’s terms means the wheels resistance to start or stop spinning, so the bigger the wheel (“r”) and mass (“m”, the harder it is get the turbo spun up.  Inertia is:

 I = m r^2

End: Physics Content

How do you make higher performing turbos?

These are the basic laws of physics which cannot be cheated.  How do turbo manufactures then make high performance turbo chargers?

  • One way is to increase the wheels’ size.  The side effects are increased flow, but also more lag by increasing both “m” and “r”.
  • Anther way is keeping the wheel sizes the same, but increasing the efficiencies of the wheels.  More on this later.
  • Another option is to decrease mass (“m”).  This will reduce the inertia
  • You can also decrease parasitic friction on the shaft by using ball bearings instead of journal bearing for example.

The fact of the matter, is that you try to do all of these things in balance.  We will follow up with some more posts on these points.

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