, ▁u pr ight ▁ 6 - cy l inder ▁inline ▁ 4 - stroke ▁pet rol ▁engine ▁only ▁produced ▁in ▁small ▁numbers ▁and ▁largely ▁used ▁for ▁research , ▁but ▁led ▁to ▁the ▁successful ▁L 5 ▁and ▁its ▁V - 1 2 ▁development , ▁the ▁L 5 5 . ▁ ▁Design ▁and ▁development ▁ ▁Hugo ▁J unk ers ' ▁early ▁engineering ▁experience ▁was ▁with ▁station ary ▁opposed - p ist on ▁two - stroke ▁dies el ▁engines ▁for ▁industrial ▁applications ▁and ▁this ▁arrangement ▁was ▁eventually ▁adapted ▁for ▁aircraft ▁use . ▁ ▁Non eth eless , ▁his ▁company ' s ▁first ▁a ero ▁engine ▁was ▁a ▁pet rol - f uel led ▁four - stroke , ▁the ▁ 6 - cy l inder ▁inline ▁air - co o led ▁L 1 . ▁L ▁was ▁J unk ers ' ▁notation ▁for ▁pet rol ▁engines ▁from ▁the ▁L 1 ▁to ▁the ▁L 1 0 , ▁which ▁became ▁the ▁J um o ▁ 2 1 0 ▁in ▁ 1 9 3 1 . ▁ ▁It ▁first ▁ran ▁in ▁ 1 9 2 1 ▁and ▁was ▁the ▁subject ▁of ▁much ▁static ▁testing , ▁but ▁the ▁intention ▁was ▁always ▁to ▁produce ▁a ▁flight ▁engine . ▁ ▁The ▁first ▁aircraft ▁to ▁test ▁fly ▁the ▁L 1 ▁was ▁the ▁J unk ers ▁T ▁ 1 9 ; ▁this ▁aircraft ▁first ▁fle w ▁in ▁ 1 9 2 2 , ▁but ▁the ▁date ▁of ▁its ▁first ▁flight ▁with ▁the ▁L 1 ▁is ▁uncertain . ▁ ▁Notable ▁features ▁were ▁the ▁four ▁large ▁val