Turbocharger Compressor Map 40K Speed Line

Question about the 40K speed line.

What is happening when it drops off to the right?

It starts around 1.2 pressure ratio  and 60 CFM and then drops to 1.0 pressure ratio and around 250 CFM.

Are they adjusting anything on the test stand to accomplish this?

Turbochargers compress air by accelerating it with the compressor wheel, and slinging it at a curved wall.  A certain compressor wheel speed makes for a certain amount of compression.

What is happening towards the right side of the map is the air is actually un-compressing in the housing, because the wheel can't quite keep up with flow at that speed.

Now look at the 110k line!  Lots of wheel speed, lots of compression happening in the snail... and then when it can't flow anymore, the pressure ratio just falls like a rock because the turbo simply can't move any more air.

This of course is really simplistic and ignores any volumetric efficiency (may not even be the right term!) problems happening at the inlet.

Thanks Pete

Are they keeping the turbocharger at a constant speed of 40K for all the points on the 40K speed line?

Thanks.

In reply to The Californian :

Yep.

I suggest finding a copy of Hugh MacInnes' "Turbocharging" book, which that scan was pulled from.  It was written in the 1980s but the basics are still relevant.

Thanks Pete

It is a good book.

What happens during the test at the 40K speed line stage?

I would think there are controls for the gas burner and blower to drive the turbocharger for instance.

Probably a flow rate and temp adjustment for that part supplying 'exhaust' to drive turbo.

So.How are they adjusting CFM for the 40K test?

If turbo was on an engine you could up engine RPM to draw more CFM while keeping turbo at 40K.

Providing you supplied turbo from it own source.

So.How do they vary CFM with turbo on a test/gas stand?

For the 40K speed line test?

The turbo manufacturer drives the compressor with an electric motor to produce these maps, the turbine wheel and housing (and the center housing) usually aren't attached. I think doing it with a turbo on an engine would be incredibly difficult.

As for how they vary the cfm during the test, IIRC there's a variable orifice on the output of the compressor housing to vary the pressue ratio/cfm.

Thank you Schmidlap

' variable orifice on the output of the compressor housing to vary the pressue ratio/cfm. '

Okay.Turbochargers are intended to be connected to running engines that flow CFM through the cylinder head

and then into the engine.

For testing they substitute a 'closing ring' type tube. On discharge side of comprssor. The 'feed' for the engine.

Then take measurements as pressure ratio drops and CFM rating gets larger or passes more air.

Almost seems like they are opening the ring as they go not closing it.

While keeping the turbocharger running at 40K RPM.

We will think on that.

The Californian said:

 

Thank you Schmidlap

' variable orifice on the output of the compressor housing to vary the pressue ratio/cfm. '

Okay.Turbochargers are intended to be connected to running engines that flow CFM through the cylinder head

and then into the engine.

For testing they substitute a 'closing ring' type tube. On discharge side of comprssor. The 'feed' for the engine.

Then take measurements as pressure ratio drops and CFM rating gets larger or passes more air.

Almost seems like they are opening the ring as they go not closing it.

While keeping the turbocharger running at 40K RPM.

We will think on that.

For each RPM step in the test, they spin it to the marked RPM, set the variable orifice for the CFM flow rate where the baseline crosses the chart, record the discharge pressure, then open the orifice to the next CFM measurement and do the same thing until they decide to stop. Connecting those dots makes the pressure ratio/CFM line for each RPM test.  For the 40k RPM curve you'll see that the pressure ratio is 1.2 at 100CFM, but by the time the orifice is opened to allow 250CFM, the compressor can't move enough air at that RPM to generate a pressure drop across the orifice.

Thank you djsilver

That seems pretty clear.

Thanks.