That was rather difficult to find, because of the very common word day. You learn something everyday here.
So Joseph Day invented a two-stroke engine and so did Dugald Clerk
and then there was Wilhelm Oechselhauser, whose twin-piston two-stroke formed the basis of the Junkers Jumo diesel aero engine (licence built as the Napier Culverin), the Commer TS3/TS4 lorry engines and the Napier Deltic railway engines.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Jumo205_cutview.JPG/300px-Jumo205_cutview.JPG)
That's a Jumo 205, by the way.