You are keying into some great points here. The energy recovered by a turbo is not entirely a free lunch. A turbocharger gets some of its energy "for free" an

You are keying into some great points here. The energy recovered by a turbo is not entirely a free lunch. A turbocharger gets some of its energy "for free" and some of it not so free. To the extent that the turbo is acting like a turbine (extracting energy by dropping temperature and pressure across the turbine blades), you are recovering that energy "for free". However, to the extent that the turbo appears as additional back-pressure to the engine as each cylinder's exhaust valve opens, that portion of the energy is not had "for free". Theory tells us that the amount of energy left to be extracted from exhaust gas is proportional to how much hotter it is than the ambient air temperature (even 1 degree means there is energy left in the exhaust). However, every practical Carnot-cycle heat engine (steam, piston ICE, gas turbine, etc) ever manufactured leaves some energy on the table. Practically, the process of extracting additional energy from the exhaust means running it through another "stage" of the engine (e.g. an additional ring of turbine blades in a turbine or an additional cylinder in a compound steam engine). But here's the rub: Have you ever noticed that in every turbine of every jet engine, each successive ring of blades is physically larger in diameter than the one before it? And in a compound steam engine, each additional cylinder is larger than the one before it? This is because the process of extracting energy from the exhaust requires allowing the gas to expand, requiring that the next stage be even larger in order to extract additional energy. So in any practical engine, you reach a point of compromise where you decide that adding another stage is either too expensive, adds too much weight (or perhaps adds just enough friction as as to be counter-productive!) that it isn't worth it to chase after that last few percent of heat energy left in the exhaust. Edit: This is also why you see the advent of combined cycle power plants. At a certain point, whatever heat is left in the exhaust is more practically used to provide heated water, than to try and convert it to kinetic (and then electrical) energy.