[logfromblammo]

An internal combustion engine relies more on the heat from the chemical reaction than the heat of the engine block. And the hot gasses are just blown out the exhaust pipe. It isn't a closed cycle.

Bolting a heat engine to the internal combustion means that the excess heat from the engine block and the exhaust gases can do some work before convecting, diffusing, or radiating away. The usual problem is not the additional weight of the heat engine part, but the enormous radiator you would need to maintain a proper cold well. This would likely be a ribbed (finned) aluminum plate covering the entire underside of the car, with scoops and fans to ensure sufficient airflow across it.

The combustion engine part could then be redesigned to produce higher temperatures, as the heat engine portion can be actively driven if necessary to cool the engine block--or to warm it, as might be needed for diesel startup.

[Gracana]

The typical way to extract power from exhaust gas heat is to add a turbo.

[semi-extrinsic]

And in Formula 1 now this is done "completely" with the waste heat recovery units. Basically, a normal turbo frequently generates more boost pressure than the engine can use, so the recovered power is just wasted again through a wastegate. But in F1, they've managed to build very long turbo shafts, so they can be coupled to an electric motor, and thus remove excess boost by generating electricity.

[windows_tips]

Combustion engines don't rely on heat so much as pressure (can be viewed as the same thing in some models).

There's no need to have "excess" heat in an engine.

[logfromblammo]

PV = nRT

In a fixed volume (the cylinder, on the time-scale of ignition), pressure is proportional to temperature.

Higher temperature in the cylinder bleeds more heat into the engine block, but also produces more force on the pistons.

Nitrous oxide systems do this, at risk of overheating the engine. If you were to actively drive a Stirling integrated into that engine, it would actively cool the engine, forcing its heat into the cold well. You would overheat your oversized radiator, instead of your engine.