At best, combustion thrust in propellor engines is about 5% of the total, and usually closer to zero
Stepping back a couple decades, the R-2800 18-cylinder aircooled piston engine presented a cooling challenge, and the cooling drag was substantial. But it was a reliable and well-respected machine and successful in the DC-6, Martinliners, and Convair twins.
Convair took a different approach to cooling it, and ducted the engine exhaust into augmenter (eductor, if you will) tubes in the aft nacelle, where the cooling air was drawn out at the wing trailing edge.
I doubt this created any thrust in itself, but since it improved cooling with less external drag, it had the same overall effect as a small jet.