The Sauer S 2100 is a family of four-stroke aircraft engines, certified according to JAR 22-H (CS-22 subpart H)[1][2] The engine is based on the Volkswagen air-cooled engine, extensively modified for aircraft use and all the parts are custom made.
[1] They are certified according to CS-22 subpart H[2][3] and can therefore be used in motorgliders and light aircraft that are certified as CS-VLA[4] and CS-LSA[5]