The Sauer S 2500 is a family of four stroke aircraft engines certified according to JAR 22-H (CS-22 subpart H)
The engine is based on the Volkswagen Wasserboxer engine, extensively modified for aircraft use and all the parts are custom made. They are certified according to CS-22 subpart H and can therefore be used in motorgliders and light aircraft that are certified as CS-VLA and CS-LSA