Perhaps I expressed myself poorly as old greg misunderstood me, but I think I have a decent understanding of thermodynamics, thanks. Do turbo engines not have higher BSFC at full load? A naturally aspirated engine with decent compression has only 2 bar absolute pressure left at the end of the exhaust stroke, which amounts to less than 10% of power if you were to extend the expansion to let it reach atmospheric pressure. Whatever energy left in the cylinder of a turbocharged engine at the end of expansion stroke is far greater than what is needed to compress the intake air. I wouldn't call that reducing waste, I would call that recycling waste to produce more waste. A turbo motor eating 2 times as much air (and thus fuel) at 14psi typically can't produce even 70% more power right? The only efficiency "gain" is that it's better than a supercharger working on the same engine producing the same boost, still not as good as a bigger naturally aspirated engine.
Okay lower compression pistons have fewer pumping losses, but overall efficiency is better with higher compression. Whatever.
EDIT: okay I stopped reading the first sentence after "misinformation", I didn't have to look up the turbine driven shaft thing because I already did a few months ago.
http://en.wikipedia.org/wiki/Turbo-compound_engine correct?
Anyhow I just read your other post more carefully again, and you said the only source of additional loss with a turbo is increased thermal loss, which I don't think is right? You blow more pressure out the exhaust because there's simply a lot more of it, you have to enrich fuel and pull timing or compression, anyways higher BSFC at full load is the result. Thermal losses through the block aren't even proportionally increasing with power, so that's not a source of "inefficiency".