Turbochargers, what's the diffrence between the turbine and compressor

[James125]

Hello everyone. I'm new to the forum. I'm an engineer...admittedly perhaps not a great one.

I wanted to know about a typical automotive fixed geometry turbocharger. Generally it seems that the compressor is larger than the turbine. Despite being more mass flow on the turbine side. Why exactly is this?

Also another part to my question - If you used the turbine as a compressor for atmospheric air, what would it's flow and pressure be like? Would it be very inefficient?

Thanks

 

[Enthalpy]

Hi James125!

Some photographs show a larger compressors, others not - comparison not obvious.

Gas speed determines the tip speed of the wheels, not the (marginally different) mass flow. On a centrifugal compressor, gas speed is a bit lower than metal speed, with the blades curved backwards, in order to stabilize the throughput: more flow means less pressure. But on all turbine designs I've seen for turbochargers, gas speed equals metal speed, with blade tips straight. That would be one reason for larger compressors wheels.

More reasons make the comparison difficult.
- Hot exhaust gas gets more easily speed than cold intake air (that's why a gas turbine works).
- The exhaust gas can expand further within the turbine wheel, not just upstream of it.
- The width of the wheels can differ, not just the diameter.

Use the turbine as a compressor: not so good...
- The straight blades give no stability to the mass flow.
- The outlet is a bad intake, with wrong diameter and blade curve
- The turbine's high-temperature material is more expensive!
It's more a question of optimization, but turbomachines need optimization to work.

 

[James125]

A very informative answer. I have observed that some turbines do have curved blades. If this is the case could the turbine be ran in reverse? So the outlet is the inlet and the blades spin in the opposite direction.

I ask because I am thinking of recycling a turbocharger and using the turbine impeller as a compressor, as I think the compressor impeller is too large for my application.

 

[RGNEM]

No. A turbine is designed to lower the energy of the air. A compressor increases it.
They are designed for two different things and are not interchangeable.

Just having a "curved blade" is meaningless.
There are critical parameters associated with the twist of the blade, as well as with everything else inside of it.