There are numerous "atypical" piston engine layouts, though I cannot recall precisely where I'd seen a reference, probably on YouTube ~10 years ago.
The basics are a single piston, dual (often opposed at an angle or flat-head design as on older BMW motorcycles), in-line (usually 4-cylinder), or V (as in V-6, V-8, V-12, etc.)
Then there are radial engines used in piston-driven aircraft. These virtually always have an odd cylinder count, to prevent locking (there's always an unbalanced force in the direction of intended rotation, or so one hopes).
There are various rotary engines, with the Wankel design best known. Very high power-to-weight ratios as a result of having three combustion chambers per rotor, but a relative short lifecycle due to wear, and some compromises in efficiency. "Flying car" company Moller International, out of Davis, CA (and apparently inactive since 2015) had at its core a Wankel-based powerplant, with four pairs of counter-rotating engines powering four ducted fans. It sounds like all the angry hornets in operation.
The basics are a single piston, dual (often opposed at an angle or flat-head design as on older BMW motorcycles), in-line (usually 4-cylinder), or V (as in V-6, V-8, V-12, etc.)
Then there are radial engines used in piston-driven aircraft. These virtually always have an odd cylinder count, to prevent locking (there's always an unbalanced force in the direction of intended rotation, or so one hopes).
<https://en.wikipedia.org/wiki/Radial_engine>
There are various rotary engines, with the Wankel design best known. Very high power-to-weight ratios as a result of having three combustion chambers per rotor, but a relative short lifecycle due to wear, and some compromises in efficiency. "Flying car" company Moller International, out of Davis, CA (and apparently inactive since 2015) had at its core a Wankel-based powerplant, with four pairs of counter-rotating engines powering four ducted fans. It sounds like all the angry hornets in operation.
<https://en.wikipedia.org/wiki/Moller_M400_Skycar>
Wikipedia lists some other unusual designs as well: <https://en.wikipedia.org/wiki/Reciprocating_engine#Miscellan...>.
I believe that the axial engine may have been featured in that video mentioned in 'graph 1:
<https://en.wikipedia.org/wiki/Axial_engine>