The fastest reciprocating ICEs I've ever heard of are small, two stroke model engines. Some of those can approach 30K RPM, but that is rare, and I don't know i

The fastest reciprocating ICEs I've ever heard of are small, two stroke model engines. Some of those can approach 30K RPM, but that is rare, and I don't know if anyone still manufactures those. ~20K isn't rare for these small engines. Back in the pre-history of Radio (before and during WW1,) electro-mechanical alternators were used to generate low frequency RF in powerful transmitters. Some of those designs would spin at 20K RPM.