I apologize if it's the wrong forum for this Is it possible to efficiently construct an internal combustion engine and while it's running to use its waste heat as the external combustion of a (smaller) Stirling engine? If so, the Stirling engine could power a generator that will essentially replace MG1 in a Prius. However, unlike MG1 in a Prius, this generator will only charge the batteries or send electricity directly to the other motor/generator (the equivalent of MG2 in a Prius). The generator connected to the Stirling will not be mechanically linked to the Power-Split-Device. Intuitively, this should reduce the resistance of the drivetrain while moving because then it's not necessary to match the revolution of two electric motors to the revolution of the ICE. And most importantly, power that is produced from waste heat is, dare I say, FREE?