The Jumo 204 was an opposed-piston, inline, liquid-cooled 6-cylinder aircraft Diesel engine produced by the German manufacturer Junkers.

As is typical of two-stroke designs, the Jumos used fixed intake and exhaust ports instead of valves, which were uncovered when the pistons reached a certain point in their stroke.

The lower crankshaft ran eleven degrees behind the upper, meaning that the exhaust ports opened first, allowing proper scavenging.

This system made the two-stroke Jumos run as cleanly as four-stroke engines using valves, but with considerably less complexity.

Late in the war, they mounted three Culverins in a triangle layout to produce the Napier Deltic, which was for some time one of the most powerful and compact diesel engines in the world.