This miniature compressed-air engine might be considered to be a more traditional example of "mechanical engineering". Designed in Solidworks and hand-machined from flat and round bar stock, it is a self-starting, single-piston engine of the 'wobbler' variety.
As part of an engineering design course, this engine was one of the few completed by the cohort that actually ran at all, was one of only a few that was self-starting, and had by far the highest quality fit and finish. This miniature engine was designed in Solidworks hand-machined from bar and round stock by myself and three other mechanical engineering students. It is self-starting, and seems like it'll run more-or-less indefinitely. It's also a dual-action engine, so the piston gets pushed on both the 'up' stroke and the 'down' stroke. Years after its completion it still runs exceptionally smoothly.
A video of it in operation is embedded below.
A video of it in operation is embedded below.