Quote:
Originally Posted by pete c
This is a tricky question. The turbo is generating shaft hp, but, it is using that hp to stuff more air into the intake. If something else is done with that power, the engine will have a substantially reduced power output, which means that turbo won't spin as hard.
The bottom line here is that spinning a turbo to do anything other than stuff air into the intake is kind of dumb. Perhaps taking a small amount of the power off and further reducing the exhaust restriction to compensate may make some sense, but, I kind of doubt it.
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At very light loads maybe. Current turbochargers use a waste gate to dump excess exhaust gas around the turbocharger once the desired boost (compressor work) is achieved. There's still plenty of energy available in the exhaust that is being routed around the turbine.
Since about 1/3 of the energy in the fuel goes out the exhaust and ~1/3 ends up available for useful work, in theory there's as much power in the exhaust energy as the engine achieves at the flywheel. At least some of that energy could be used to drive an electrical generator connected to an exhaust gas driven turbine.
Quote:
Originally Posted by ChazInMT
Yeah, it is true. There is power required to spin the turbo charger, that has to come from the engine. People always think it's just "Waste Gas", but it isn't. There is a load placed on the engine to make the turbo charger spin, if you just let the air blow into the open by not hooking it up to your engines air induction, it would really slow you down. But since it is packing in a lot of air into your engine, the net gain is that it does help, you're MPG increase is due to the turbo allowing you to operate in a better region of the BSFC chart for your engine. This is basic turbo charger stuff.
You don't get something for nothing, but the benefits outweigh the losses in this case. It is understandable for you to think as you do, but it is not accurate.
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If you replaced the engine with a combustion chamber that generated exhaust gas with the same temperature and pressure as that past the engine's exhaust valve, the gas would still drive the turbine. The engine isn't driving the turbine by acting solely as a compressor on the exhaust gas.
There is also that the exhaust valve(s) are operating in choked flow at least part of the time. When that is occurring you can do whatever you like downstream of the exhaust valve, even pull it down to a perfect vacuum, and not increase the mass flow rate of the air past the exhaust valve(s).
That means it is possible to increase the exhaust gas pressure between the exhaust valve and turbine, via the restriction caused by the turbine, and not necessarily reduce the flow past the exhaust valve(s).