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I don't know how to put this nicely, so I will say that you are misinformed.
While on boost, a typical OEM turbo will create a backpressure of around 2x's the boost pressure. So for a system with 15 psi of boost, the pressure in the exhaust manifold will 30 psi or more. That means that the engine is having to pump the exhaust gases againt that pressure. That reduce the shaft HP output of the engine. These gases also expand during the intake stroke and take up volumne which reduces the amount of AF charge. So voumentric efficienty goes down, again a reduction in HP.
Getting energy out of the exhaust gases with a turbine... is not part of the combustion process, but is part of the overall thermodynamic cycle of a spark ignition engines running within the cycle of a turbo.
When not on boost, many of the on boost issues disappear. However, the turbine will continue to be an exhaust flow restriction which will reduce the engines efficiency when off boost. So the fule efficiency goes down. A turbo engine wull typically have lower compression ratios which also hurts mpg's and power at lauch before the onset of boost.
A low pressure turbo can have higher compression ratios and be in some wayes more responsive amnd will be more efficient.
With aysymetric boost on a V6 engine, one bank of cylinders will have less power because of the back pressure while on boost. But the V6 is very smooth and this might noit be noticed. All cylinders will get the same boost pressures. These setups, if low boost pressure, will simply deminish such effects.
Now back to turbo turbines. A flow of hot air will never make a turbine work. There has to be a pressure difference. The trim in the turbine scroll uses this pressure difference to create high velocity flows with lots of kinetic enery. The turbine converts a fraction of that to the turbo shaft HP to power the compressor. The process is inefficient. And when you see compressor maps if efficiency, thats for the compressor stage only.
When the boost pressure taget is reached, the wastegate starts to open. This does not reduce the turbine back pressure in the manifold. What it does mean that lots of energy in the 30pis or so exhaust gases is simply been dumped by the wastegase. So if 1/2 of the gasses were going though the wastegate, the turbine will only be able to sork against 1/2 of the energy in the exhaust flow, so the efficiency will be the efficiency of the turbine x's 1/2.
With a VTN or variable turbine trim unit, the VTN controller will open the exhaust vanes instead of opening up a wastegate, there is no wastegate! So to reduce the power from the turbine to keep boost from getting above the control point, the vanes open, and the gasses flow through the turbine with less of a pressure drop and less kinetic energy for the turbine to work with. So the turbine section can be very much more efficient, as the varible turbine trim allows the turbine geometry to be varied. Compare that to a wastegate's turbine where the pressures are high and this high pressure flow is spilt, to down regulate the power from the turbine wheel. In a few rare VTN applications where things are highly optimized, the turbine back pressure can actually fall below the boost pressure! This can occur as hot exhaust gases will much greater volume compared to the charge air. A VTN turbo will be very much more responsive. It can provide boost at lower engine speeds and can be more efficient at high RPM's.
posted by 65.68.10...
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