there is a direct, mechanical aproach to exhaust energy recovery, dating to 1950, called turbo compound engines, that were complicated, but very fuel efficient,

there is a direct, mechanical aproach to exhaust energy recovery, dating to 1950, called turbo compound engines, that were complicated, but very fuel efficient, the practice has been revived for use in highway truck engines now also other schemes to generate electricity from an exhaust turbine if both were be combined, with a high temp heat storage block, then best efficiency and lowest emisions could be possible