There have always been overlapping fashions in engine technology whether they were adopted large-scale or not. In the early years of the 20th,cent. conventional valve gear was troublesome due to metallurgy and lubricants and some weird, wonderful and fairly practical alternatives were produced.
We had the Rotary valve, the Piston valve, the "Valveless"{2 cycle} and the Sleeve valve,{2 of which cars I have}.

Then came high-compression engines in the '20's, while there were many practical ways to avoid "pinging" and valve seat recession, notably in cylinder head design, Tetra-ethyl Lead was introduced due to General Motors sponsoring it's developement and "marketing" by their wholly owned subsidiary Dayton Engineering Laboratories....DELCO, Think of the licensing fees and royalties.

In the late '70's Stratified charge engines were the dernier cri of engine developement. Some, like Jaguar made a fairly efficient one, although beset with teething troubles, most other manufacturers maintained the status quo due to economic reasons.

Just take a gander at engine efficiency designs some time and you'll see what I mean. Everyone is still seeking the philosopher's stone, although the reciprocating engine has reached the pinnacle of commercial developement.

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