A British company has developed an exhaust-driven generator that could be used to supplement the usual belt-driven alternator, or even provide electrical power to a hybrid drivetrain.
The use of the normally wasted exhaust energy can provide a fuel economy savings by lessening the parasitic load from the vehicle's accessory drive, although it's difficult to speculate on what effects the increased exhaust backpressure may have on the engine's efficiency.
Generally speaking, engines optimized for higher-RPM use would suffer more due to the loss of scavenging, and lower efficiency engines would benefit more as there would be more available heat in the exhaust stream. Switched reluctance technology is employed in the generator, which does not have the disadvantage of using brushes or permanent magnets (magnets tend to not be so permanent in temperatures typical of the exhaust stream). They are more difficult to control, requiring accurately-timed external excitation, but with modern control technology at least this is possible, if not easy.