I actually didn't realise that the energy extracted by a turbocharger, more than any residual gas expansion from still-burning fuel (if any), is also dropping t

I actually didn't realise that the energy extracted by a turbocharger, more than any residual gas expansion from still-burning fuel (if any), is also dropping the temperature of the exhaust gases. So we are extracting some of the heat energy using a turbo. neat. Yes I take your point on diminishing returns. Still it seems that there is quite a big chunk of energy otherwise being thrown away, and turb[os/ines] are getting better and better at harvesting it.