The Sauer S 2500 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 Wasserboxer 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]