Introduction The two-stroke@ gas-fueled@ opposed piston (OP) engine probably fi rst appeared in public use in Germany around 1878 (Ref. 1.1)@ engineered by Wittig. Opposed piston engines are characterized by pairs of pistons operating in a single cylinder@ eliminating the need for cylinder heads. Gas exchange for two-stroke versions is handled by piston-controlled ports in the cylinder walls. While the OP concept is applicable to two- and four-stroke diesel engines@ highcompression- ratio four-stroke applications require a half-engine-speed sleeve valve cylinder@ or rotating valve@ to achieve the required frequency of inlet and exhaust events. However@ the famous Gobron Brille racing engine@ circa 1900@ was a four-stroke engine that used poppet valves located in the housing at the center of the cylinder liner. Most OP engines operated on a two-stroke cycle@ probably for simplicity@ and almost all have been compression ignition diesel engines@ as OP engines were intended to achieve high thermal effi ciency as well as high power density. Opposed piston engines began to be used commercially around 1900 for numerous land@ marine@ and aviation purposes. Although OP units are still used in 2009 in the United States@ United Kingdom@ Russia@ India@ Iran@ and some Arabian Gulf states@ their use has greatly decreased due to issues with emissions and particulates@ notably oil-derived particles. In spite of this decline@ the OP engine has set many of the existing standards for power-to-weight ratio@ dynamic refi nement@ fuel tolerance@ package space@ fuel effi ciency@ and manufacturing simplicity. For these reasons@ the OP concept remains viable for certain applications that require outstanding power and package density@ simplicity@ and reliability@ such as aviation and certain military transport requirements.