Why do EMD turbochargers have so many blades?

Question

On most turbochargers that I've seen, the compressor wheels have 6 full-length blades and 6 splitter blades, 7+7, 8+8, or 9+9. Even on large marine engine turbochargers this generally holds, like the MAN TCR which has 8+8 blades on the compressor.

Then you have newer billet compressor wheels with 11 full-length blades and no splitters. What benefit does this bring over the "conventional" arrangements that I described above?

MAN's axial-flow turbine turbos have 11+11 blade compressor wheels. Generally, on V engines, MAN will use an axial turbo with a dual outlet compressor housing as the low-pressure turbo, then each outlet will feed a conventional (radial turbine) turbo which has 8+8 compressor blades before going to the intercoolers and the cylinder banks.

Now something really struck me: the turbos on EMD 710 engines (which are two-stroke Diesel engines) have 17+17 blades on the compressor. Why do EMD use so many blades? Since two-stroke Diesels need forced induction for scavenging, my initial guess was that they did this to make the turbo more efficient at the higher pressure ratios needed to simultaneously boost and scavenge. At low speeds the turbo becomes a centrifugal supercharger as there is a clutch that allows the crankshaft to drive the compressor, but at high speeds the clutch disengages and the turbo takes over. Might the large number of blades have to do with this?

My question is, why does the low-pressure turbo have more blades than the high-pressure turbo? Is it generally true that, in a compound turbo system, you want more blades on the low-pressure turbo than the high-pressure turbo?

From what I've found, people seem to say that fewer blades flow better at high pressure ratios but are not as efficient at low pressure ratios, and that more blades leads to better low-end response but not as good of a top end. Another source that I've read claims that more blades flows better at high pressure ratios but not at low pressure ratios. The responses I've found were all over the place, so I'd like to gain more insight on what more vs. less blades does on turbochargers (and centrifugal compressors in general).

Answer 1

Think of blades as something pushing on the fluid. There is a pressure side and a suction side to each blade. Pressure varies from the low on suction side to the high on the pressure side.

Any work done to move the fluid where you need it to go (axially "forward" or radially outward at the compressor) is useful. Anything else (work moving it tangentially, against a wall, or backflow around your blades) is waste. Having a lot of space between blades or blades and shroud allows more of the dreaded but inevitable backflow around your compressor (from its "output" side to its "intake"). On the other hand, more blades can mean more to balance, accelerate, and spend energy deforming. Spinning a disk (100% blade and 0% fluid) does you no good. Extremely thin blades will themselves deform too much from stresses between pressure and suction sides.

In summary, optimization can be thought of as a matter of making the blades thick enough for the loads, sparse enough to have space for the flow you need, and plentiful enough to hold back the pressure you generate.

The contradictory information you are finding is likely because of the choice of control variables. There's a lot to choose from between the geometry (angle of attack, diameter, etc), rpm, fluid properties, flow rates and pressures. To just look for a trend between pressure ratio and number of blades is futile because it depends on so much more. You can come up with some custom metrics to be your constant control variables to make it go either way.

Answer 2

EMD 710's use hybrid turbochargers.

These are spun by a gear driven clutch at low load and by exhaust gas at high load. What this means is that they can optimize the blades without worrying about low load performance so much.