Average is around 35%, Toyota has an engine they claim is at 41%, I believe it is in the Prius. F1 engines can hit over 50% and Nissan claims to have a prototype engine that can hit 50% over a very narrow range.
The biggest issue is that traditional ICE requires you to have an engine that is very flexible over a large range of loads & engine speeds. This is the opposite of what you want when trying to design ICE for max thermal efficiency. The Nissan engine I mentioned is designed to work as a generator in an electric car and will only operate in a very narrow band, which allows them to optimize it for just that.
As a fun little aside, in order for an engine to achieve max thermal efficiency it needs to be at wide open throttle. This seems counterintuitive because we are talking about efficiency, but thermal efficiency is all about extracting the maximum amount of mechanical power from a given amount of fuel, not what we generally think of efficiency, i.e. mpg or l/100km.
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