▁overcome ▁during ▁the ▁int ake ▁stroke . ▁ ▁Operation ▁In ▁order ▁to ▁de activ ate ▁a ▁cyl inder , ▁the ▁exhaust ▁val ve ▁is ▁prevent ed ▁from ▁opening ▁after ▁the ▁power ▁stroke ▁and ▁the ▁exhaust ▁gas ▁charge ▁is ▁retained ▁in ▁the ▁cyl inder ▁and ▁re - com pressed ▁during ▁the ▁exhaust ▁stroke . ▁Following ▁the ▁closed ▁exhaust ▁stroke , ▁the ▁int ake ▁val ve ▁is ▁prevent ed ▁from ▁opening . ▁The ▁exhaust ▁gas ▁in ▁the ▁cyl inder ▁is ▁expanded ▁and ▁com pressed ▁over ▁and ▁over ▁again , ▁acting ▁like ▁a ▁gas ▁spring . ▁As ▁multiple ▁cyl ind ers ▁are ▁shut ▁off ▁at ▁a ▁time ▁( cy l ind ers ▁ 1 , ▁ 4 , ▁ 6 ▁and ▁ 7 ▁for ▁a ▁V 8 ), ▁the ▁power ▁required ▁for ▁compression ▁of ▁the ▁exhaust ▁gas ▁in ▁one ▁cyl inder ▁is ▁counter ed ▁by ▁the ▁decom pression ▁of ▁retained ▁exhaust ▁gas ▁in ▁another . ▁When ▁more ▁power ▁is ▁called ▁for , ▁the ▁exhaust ▁val ve ▁is ▁react iv ated , ▁the ▁old ▁exhaust ▁gas ▁is ▁exp elled ▁during ▁the ▁exhaust ▁stroke , ▁and ▁then ▁the ▁int ake ▁val ve ▁is ▁like wise ▁react iv ated , ▁res uming ▁normal ▁engine ▁operation . ▁The ▁net ▁effect ▁of ▁cyl inder ▁de activ ation ▁is ▁an ▁improvement ▁in ▁fuel ▁economy ▁and ▁like wise ▁a ▁reduction ▁in ▁exhaust ▁em issions . ▁General ▁Mot ors ▁was ▁the ▁first ▁to ▁modify ▁existing , ▁production ▁engines ▁to ▁enable ▁cyl inder ▁de activ ation , ▁with ▁the ▁introduction ▁of ▁the ▁Cad illa c