https://en.wikipedia.org/wiki/Napier_Deltic
(It's a two-stroke cycle diesel but not as we know it -- three cylinders arranged in a triangle with a crankshaft at each vertex, one of them counter-rotating relative to the other two, and six pistons, two of them opposed in each cylinder! There is an animation on the wikipedia page. Tracking it will make your head hurt. This thing was a mainstay of the British railway industry in the late 1950s to 1960s ...)
cheerleaders for deltics will always measure their effectiveness in the guesome "miles per casualty" figure, completely ignoring the fact this system had astronomical service costs as the design compounds stress on cast parts. you had to have starter cartridges (explosives) to start the things, and it wasnt uncommon for them to give up the smoke just cranking them.
they have horrendous emissions as well compared to inlines. diesel and lube oil would roll out of the exhaust in massive plumes (you would never get these approved in the US today.) Usually on startup large diesels have monitored exhaust temperatures but in a Deltic your eyes had to be glued to the meter because the stack was always a blond one away from runaway (catching fire.) As tolerances drifted during service most fleets just fed these things more lube oil and in turn, worse emissions and efficiency.
again, complexity. Your tolerances for a deltic were insane compared to what you had at the time for an off-the-shelf diesel. repair parts too had to be custom fitted to the engine as it had worn and take into account things like piston bore cavitation damage from overprimed starter cartridges/overspeed/overtemp/etc...The shop down the road could not fabricate deltic parts so lead times were considerable. Whoever worked on these had a dedicated machinist (not just a miller) cutting parts off a dialed-in colchester lathe or something with a very good tolerance.
Mazda tried bringing this back with the RX-8. It was a failure, and while everybody loves the "spirit" of the wankel, outside of the infamous 787B I think people are always going to remember it for the theory instead of the application.
If we're going to talk about using modern engineering capabilities, perhaps we should be discussing the quasiturbine engine? At least in theory it solves many of the problems of the rotary engine, although I suspect it will be a nightmare of complexity that never becomes reliable.
The baby deltic was a more prosaic workhorse, the deltics delivered the 100mph Edinburgh London service for decades.
I don't think the YouTubes do it justice, a bit like rocket launches: you have to be there to get a chest-beating throb.
When the pistons reach their maximum opposing distance, the injection and exhaust ports are briefly exposed, allowing for entry of fuel/air and exit of exhaust in a single linear movement from one end of the cylinder to the other. Maybe the air is injected before the fuel or at a higher pressure. And if you time the speed of the exchange just right in time for the exhaust ports to be closed over before the compression stroke, you get complete exchange with no fuel wastage. Absolutely nothing like a 4-stroke engine, and not much like common 2-stroke designs either.
I guess nobody wanted to maintain them.
And in Polish 'Junkers' is pretty much synonymous with water heater.
Not directly related but learning about the diesel-electric locomotives and the transition from PWM DC to 3-phase VFD with the traction improvements(I seem to recall they doubled available traction but would have to check my sources[1]) was also something that caught me by surprise. All these EVs out there now owe a lot to the work that happened back in that industry.
Edit: found the one of the sources I remember reading.
[1] http://www.republiclocomotive.com/ac-traction-vs-dc-traction...
Used by German forces to power plane-spotting projectors, so presumably manufactured in the late 30s or 40s.