I read that a massive portion of usable energy is wasted and turned to heat instead of being used to turn the crankshaft — would there be like any way of reducing the heat/cooling the engine so you could get 50-70% thermal efficiency?
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That’s simply thermodynamics.
The carnot efficiency is the maximum possible efficiency you can reach when converting heat to mechanical energy, and that depends on the temperature difference between your hot and cold basin (the energy is “harvested” by making heat flow from hot to cold and forcing it to move mechanical parts on the way).
Imagine it like trying to harvest the energy of a river, you can never harvest all energy because that would stop the river entirely wich prevents new water from reaching you
So since fuel has a specific temperature at wich it burns, and the outside of the car is regular outdoor temperature and is never extremely cold you have a fundamental limit on your efficiency.
And stuff like coal powerplants gets close to that efficiency topping out at ~40% efficiency by using elaborate means like preheating your medium over a dozen stages with exhaust air. (And you can reach over 60% in combined cycle, but that’s basically using a second higher temperature process on top)
In a car you don’t have space for such an elaborate system of pipes, so you lose extra energy by having your exhaust air being hotter than the outside air (wich is all extra energy you didn’t spend on driving).
So the only way to increase efficiency would be a larger engine (with more preheating) or a higher temperature compared to the outside
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