How much shaft torque can a turbocharger's turbine produce?

[Kozy]

If therotically, you throw away the compressor side of a turbocharger and measured the torque at the shaft required to produce say, 1 bar of boost, what sort of reading would one expect to get at an average speed of around 80,000rpm?

I had a search but it's not the easiest topic to find answers for, all I get is pages upon pages of how turbocharges increase engine torque, not how much torque the turbocharger itself produces from the exhaust gases flowing through it...

Anyone got any ideas?

[cptsideways]

You should be able to calculate it with airflow in 1bs v pressure less efficiency I do beleive??

[GreenV8S]

It'll vary, of course. But to give a rough starting point, 2 lb-ft at 80,000 rpm equals roughly 30 bhp, which is somewhere around the sort of power that I'd expect a typical turbo to absorb.

[Kozy]

GreenV8S said:

It'll vary, of course. But to give a rough starting point, 2 lb-ft at 80,000 rpm equals roughly 30 bhp, which is somewhere around the sort of power that I'd expect a typical turbo to absorb.

Excellent, thats a ballpark figure to work from cheers!

[chuntington101]

just after WW2 a US company build a 18cylinder radial engine that had 3 turboines that put the drive back into the engine. i think it netted them about 300bhp!

also might be worth looking at centrifugal superchargers as you can obviopusly messure the amount of torque required to give xxxcf/m. you should then be able to scale up or down to get the figure your after.

Can i ask why you want to know??

Chris.

[Pumaracing]

You'll find your answer to this question much easier if you study superchargers first where the drive power required to operate the supercharger is more readily studied and measured. With early Rootes type blowers which only had an efficiency of 40% to 50% it generally took about 35% to 40% of the total developed power to drive the supercharger itself. In other words if you achieved a 400 bhp gross power gain it would take 150 bhp of that to drive the SC and you'd see a net 250 extra bhp at the crank.

Centrifugal and twin screw SCs operate at much higher efficiencies, rivalling that of turbochargers, of about 70% to 80%. The supercharger absorbs more like 20% to 25% of the total developed power so with a 400 bhp power gain it would take 80 to 100 bhp to drive the supercharger and you'd see 300 to 320 extra bhp at the crank.

I'd hazard a guess that an average turbocharger installation absorbs 15% to 20% of the gross developed power so in other words 17.5% to 25% of the net extra crank power.

If we take an engine running at 1 bar boost (14.5 psi) we'd expect just under twice the crank bhp of the base engine running no boost at all - crank bhp is closely proportional to inlet manifold pressure. Let's take a 2 litre engine producing 100 bhp at zero boost and 200 bhp at 14.7 psi boost with an average sized turbo like a Garrett T3. Maybe a Sierra Cosworth.

We could estimate that the turbo is absorbing 17.5 to 25 bhp, call it 21 bhp. If as you say the turbo is spinning at 80k rpm, which is probably a bit low for a modern turbo, then the torque on the shaft is 21 x 5252 / 80,000 = 1.38 ft lbs.

Postulate a bigger engine and bigger turbo and the numbers change in proportion. They are still very small numbers though. About the same torque as required to tighten a 2mm to 4mm bolt.