Can we use the heat to boil water to turn a turbine? Or would the water never cool enough to be used again?
What about using the heat to create pressure and then releasing the pressure to help move the fly wheel?
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I mean, we *do* use heat from combustion to make cars work in the first place, but around 2/3rds of it is lost either as heat in the exhaust or heat lost through the engine block.
For a modern, liquid-cooled engine, you *could* hypothetically recover heat from the coolant, but that would either require you to replace the radiator with another heat exchanger to put the heat into something with a lower boiling point, *or* allow the engine to boil the coolant (which will almost certainly cause thermal stresses on the engine block). Then that boiled vapor can be expanded through a small turbine for power.
As for the exhaust heat; there is the potential to extract that exhaust heat, but it’s somewhat tricky to do because the exhaust gas is already expanding back to ambient pressure, causing the temperature to fall (which causes you to lose efficiency on any kind of work extraction as a result of Carnot’s Law). More to the point; we already use turbines to attempt it; they’re called turbochargers. Alternatively, you could use variations of the same low-boiling point energy recovery (e.g. Organic Rankine Cycle tech) on the exhaust.
The other problem is actually emissions; a lot of the technology that cleans your car’s exhaust is dependent on heat as an energy input to drive the process. If you extract more energy as work, you necessarily lose heat in the exhaust, which makes exhaust catalysis much more difficult, thus the car’s emissions will likely get worse.
Point being; all of this requires lots of extra equipment, weight, and complexity added to the vehicle. So yeah, you can do it…but it makes absolutely no economic or thermodynamic sense.
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