If plethora of designs in search of perfection is an indicator of something of interest, then take a look at the many variations in valve gear design, particularly for locomotives, which needed to operate reasonably efficiently over a wide range of speeds, gradients and loads.
> My personal favorite is the Tower spherical engine...
when i first saw the static diagram, i thought wow that looks like it is much more complicated than this 2-d diagram lets on. then later down the page i saw the animation showing the 3-d complexity... mind blowing.
Even more so, consider the many attempts at making non-turbine rotary steam engines, wonderfully documented here: http://douglas-self.com/MUSEUM/POWER/rotaryengines/rotaryeng...
My personal favorite is the Tower spherical engine: http://douglas-self.com/MUSEUM/POWER/tower/tower.htm
Unlike most of the designs here, this one was built and even put to a little practical use.