Two stroke Thursday, strangely

DKW, a now extinct German maker of cars and motorcycles, was founded in 1916 by a Danish engineer called Jørgen Skafte Rasmussen with the original intention of making steam fittings and then a steam powered vehicle - Dampf Kraft Wagen, hence the company name. That didn’t go so well so they turned to small two stroke engines as a kind of educational toy for boys for which they reinvented the company name to come up with Des Knaben Wunsch*, something like The Boy’s Wish. Next the two strokes got a bit bigger and powered bikes and small cars, thus giving them the opportunity to further reinvent the name as Das Kleine Wunder, the little wonder. After various reorganisations DKW wound up as one of the four companies making up Auto Union and are still represented by one of the four rings in the Audi symbol.

* No, I don’t know why it wasn’t Der Knabens Wunsch.

The cars were conventional enough given that they were fwd strokers. The bikes, not always so.


Meet the 350SS. Note the megaphone exhaust which would famously waken the dead.

Meet another one while we’re at it. Note the water cooling, not so commonly found then.


The 350cc engine had four cylinders in a square formation, two plugs and five pistons. It was indeed strange.

It was a split cylinder design where two cylinders shared a common combustion chamber. One cylinder received the air/fuel mix which was burnt above both pistons and then exhausted from the second cylinder. Here’s an example of part of a single split cylinder 250 which shows the general idea.


But we see something else. At the left of the engine and not easily recognisable is another piston with an enormously wide bore and short stroke. That’s the supercharger. Supercharging a conventional crankcase scavenged two stroke isn’t really feasible because without valves to keep everything in the mixture tends to get supercharged right out the exhaust port. The split cylinder design doesn’t suffer from this so much.


DKW made several variations of the supercharged engine with that in the 350SS using a supercharger piston at 180 deg, ie pointing straight down and you can see the finned supercharger cylinder at the bottom of the engine.

It worked well. DKW enjoyed considerable success in the 1930s, so much so that like Mercedes and the car part of Auto Union they were adopted by the Nazi government as the motoring embodiment of the National Socialist ideal, something which hasn’t done much for their reputation since. DKW had the further difficulty that they were based in what became East Germany and the entire contents of the factory in Zschopau were seized by the Soviets and hauled away as war reparations. Little of the racing team was seen again.

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