When Citroën changed the engine of the 2CV prototype, the TPV (Toute Petite Voiture – “Very Small Car”) from water cooled to air-cooled for the 2CV’s introduction in 1947, the diminutive 2-cylinder power-plant and its larger bore derivatives would go on to live one of the longest lives of any air-cooled automobile engine save for Volkswagen’s 4-cylinder Beetle engine.

1939_TPV_2  TPV in 1939 

2CV Launch 4  2CV at 1947 launch

There are other fascinating air-cooled engines that came and went over the years, with some notable ones are covered in this webpage by James Kraus titled: “End of an Era: The Last Air-Cooled Automobile Engines”.

https://autouniversum.wordpress.com/2009/06/22/end-of-an-era-the-last-air-cooled-automobile-engines/

At the end of James’ article he states that with the GS, Citroën closed the record books with the last mass-produced automobile introduced with a clean-sheet air-cooled engine design.  The air-cooled era had to end primarily because lower emissions and increased power could be achieved with water-cooled power-plants.

    GS Boxer Engine GS 4-clyinder ‘Boxer’ engine (drawing & photo)

The GS really was the air-cooled engine’s epitome.  Its smoothness and willingness to rev all day long at its 6,200 rpm red line makes GS owners today marvel at the overhead valve 4-cylinder wonder.