Hi Wigwam,
that would be some heat exchanger! The hot gas isn’t waste, it provides the thrust on a pure jet engine. with a Twin spool/ triple spool (RR RB211,Trent) the LP turbine drives the big fan at the front and most of the air bypasses the core compressor/combustion/turbines and provides a large amount of thrust. Such technology doesn’t come cheap. A Trent 1000 allegedly costs $42 million. So sad to see production halted and all those 1000s of brilliant engineers up in Derby furloughed. There are many more technical demerits for use in a road vehicle. A very small single spool gas turbine driving a generator suppling a brushless DC motor could work, but would likely be enclosed in steel or carbon fibre casing for fire and turbine failure damage limitation. Hard to see it being cost effective. Sorry, the Honda system is called eHEV. Just love these acronyms. Remember the NSU with its striking aerodynamic body. Daddy of modern Audis.