Abstract: This invention relates to improving the internal combustion and internal use of fuel in reciprocating engines, and more particularly to reciprocating engines which are compounded in order to effect extension of the expansion event of the four-event work cycle of such engines and which are enabled especially thereby to receive and use, for prolonging and substantially improving the combustion process, secondary air inducted and compressed by secondary cylinders of such engines, said air being subsequently applied directly to and into the engine's working medium after the original ignition of each fuel/air charge or any equivalent thereof, and during the power-producing period in which that working medium expands in two coacting cylinders.