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Don't beleive anything that talks about a turbo capturing free energy!
When on boost at 15 PSI, the exhaust back pressure in the exhaust manifold will be 30 PSI or more for a typical OEM turbo configuration. All the more when the turbo is undersized to make the boost come on faster.
That 30 PSI backpressure reduces shaft HP from the engine! There is nothing free about that. The turbine extracts power from the exhaust flow. The gas is expanded as it goes through the turbine, and that expansion also reduces the temperature of the exhaust gases.
The turbo compressor uses a lot of power to compress the air, and the air heats up from being compressed. Because of inefficiencies of the compressor, the air is even hotter. The IC cools the air and that makes it denser. But the work performed, P*V, by the compressor is not deminished by what goes on in the IC. At high flow rates there can be a significant pressure drop in the IC and the turbo has to work to overcome that as well and there is more work and more exhaust back pressure.
The amount of boosted air that can get into the cylinders is limited by the amount of remaining exhaust gases that did not get pushed out. This is determined mostly by the volume of gas when the piston is at top dead center. But if the gas there is at 30 PSI, then its volume will be greater and less combustion air and fuel can get in. This can be even worse because of valve overlap. So the exhast back pressure reduces the volumentic efficiency of the engine! The amount of exhaust gas volume left in the cylinders is probably 3 times what you would have in a NA engine at WOT. But if equal power, you might have a 6 or 8 cyliner NA engine.
So those are two negative terms. But when a piston is doing its intake stroke, the 15 PSI of intake air will push down on the piston and this contributes to shaft HP! The gains are deminished by flow losses etc. With a variable vane turbo, it is possible to get the exhaust backpressure lower than the boost pressure. Magic starts to happen then. I don't know of any OEM setups with that except diesel engines, which can be on boost for hours at a time on the highway.
posted by 65.68.10...
, Sun, 30 Nov 2003 14:01:47
, Sun, 30 Nov 2003 15:28:05
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