General > General Technical Chat
Turbocharger power input (consumption)
Circlotron:
Ages ago I did a rough calc for a 3 litre engine at 6000rpm with 1 bar boost and I got about 30kW needed to drive the turbo’s compressor. Many of the tricky words used in thermodynamics make my head spin, but I reckon the figure is within +/- 20% of correct.
Edit -> maybe half that figure. See my later post.
https://www.eevblog.com/forum/chat/turbocharger-power-input-(consumption)/msg2983598/#msg2983598
Mazo:
--- Quote from: Circlotron on March 25, 2020, 08:31:48 pm ---Ages ago I did a rough calc for a 3 litre engine at 6000rpm with 1 bar boost and I got about 30kW needed to drive the turbo’s compressor. Many of the tricky words used in thermodynamics make my head spin, but I reckon the figure is within +/- 20% of correct.
--- End quote ---
Your calculation closely matches mine.
1bar of boost on a pretty normal 3L petrol not really fancy engine gets you in the 350-420hp range easily,while the same not really fancy 3L engine has about 200-250HP that is say a 200hp gain for a say 60hp loss(for a supercharger and some driving loss added that's it).
Real life example->M112 the 3.2L NA vs the supercharged version(the engine from the W203 C32AMG).
NA-220hp vs 1bar boost coming from a supercharger for 354hp
So yes power in=power out only if you apply it correctly :)
Circlotron:
Also, I wouldn’t say the power to drive a turbo is free. If you have pressure at the compressor outlet then you are also going to have pressure at the exhaust turbine inlet. This exhaust back pressure is something the rising piston has to push against on the exhaust stroke, therefore stealing crankshaft power. Of course, this might be partially countered by the inlet boost pressure helping to force the piston down on the inlet stroke. To get it right is engineer’s stuff, not a shade tree mechanic.
langwadt:
--- Quote from: Circlotron on March 25, 2020, 08:47:57 pm ---Also, I wouldn’t say the power to drive a turbo is free. If you have pressure at the compressor outlet then you are also going to have pressure at the exhaust turbine inlet. This exhaust back pressure is something the rising piston has to push against on the exhaust stroke, therefore stealing crankshaft power. Of course, this might be partially countered by the inlet boost pressure helping to force the piston down on the inlet stroke. To get it right is engineer’s stuff, not a shade tree mechanic.
--- End quote ---
it is not quite so simple, the turbine is driven by the kinetic energy of the exhaust stream not just pressure
Circlotron:
--- Quote from: langwadt on March 25, 2020, 09:38:41 pm ---it is not quite so simple, the turbine is driven by the kinetic energy of the exhaust stream not just pressure
--- End quote ---
It sure isn't simple! I've often wondered about the effect of having a large plenum right at the inlet to the turbine similar to one commonly used at the compressor outlet where the inlet pipes branch from. That would spread and reduce the amplitude of the kinetic energy of the pulsating waves to more of a smooth flow, similar to the effect of a series choke with rectified DC current flow. The turbo might have to be resized to suit the new circumstances, but I wonder if there would be any benefit. Somewhat similar to a transformer not having to handle the peak currents of a capacitor input filter? Far fetched maybe, but sometimes physics has some interesting parallels.
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