I’ve also heard about exhaust gas power turbines, but never outside the classroom. To understand why they are beneficial, we must examine why adding a turbocharger to a piston engine can improve adiabatic efficiency. The keyword is pumping losses; In a naturally aspirated, high speed diesel engine operating under ideal conditions, roughly 12% of the input energy is spent on pumping air. This is analogous to the power needed to turn the engine at rated speed (excluding compression heat loss).
There is plenty of energy left in the working fluid (exhaust gas) when it leaves the cylinder, evidenced by the sharp pressure drop on your indicator card as the exhaust valve opens. The TC harnesses this energy to do a part of the pumping work, and does so with a pump type (radial flow kinetic or compound axial-radial) that is far more efficient at low pressure differentials than a piston pump. You effectively end up with a multi stage pumping system where each stage is better matched to its delta-p, and use surplus energy to do so. Very roughly speaking, if you have 3 bar boost and 30 bar compression, you eliminate 2/3 of the pumping power needed by the piston stage (engine). There is a myriad of losses and inefficiencies that keep you from reaching this figure, which I barely understand and certainly won’t get into here, but there’s enough left to see an efficiency increase under certain conditions.
As it happens, there’s more surplus energy available in the exhaust gas than can be used for pressurizing induction air within practical limits, and this is where the exhaust gas power turbine comes in. No black magic, just thermodynamics (which is pretty close imho). I believe the only thing limiting the use of these is the added cost and complexity.
What? Practically every turbo out there is driven by a simple impulse turbine (aside from esoteric examples with an axial flow reaction stage). They are dependent on exhaust gas temperature only because gas enthalpy affects velocity.