Recently I've been thinking a bit about hydrogen fusion.
The pressure required to make H2 spontaneously fuse is around 250 billion atm, which is enough that there isn't really any material that you can build a suitable piston or cylinder out of.
A recent approach has been to try to force a liquid (in this case a hot metal) to compress a bubble of hydrogen plasma in the hope that as it collapses, the inertia of the liquid will create a shockwave that far surpasses the pressure of the vessel, reaching the target pressure at the center.
In this design, they center the plasma by spinning the entire apparatus in order to drive the heavier liquid to the edge by centrifugal force - not an ideal requirement for an engine. In addition, the number of independent pistons is annoying compared to a single reciprocating piston with a number of equal length tubes between it and the reaction vessel.
There's something I find quite amusing about the notion that humans may in fact make it to the stars, but with EM drives* that are powered by what are glorified internal combustion engines. Nothing will define the aesthetic of future asteroid miners better than a spaceship that sounds like a Camaro.
* Contrary to popular belief, the science behind "flying saucer" tech is pretty much sorted and it's just an engineering problem that remains.