Screw engines

A conventional internal combustion engine uses a crankshaft to convert the back-and-forth movement of the piston(s) into rotary motion. This results in a lot of thrashing around and complication.

An alternative way to convert linear to rotary motion would be to use a screw or, more precisely, a helix. The backside of the piston is connected (rigidly) to a rod with helical grooves in it, which in turn slides into a tube with helical rifling. Now, as the piston moves down, it will cause the rifled tube to rotate.

One problem with this is that the rifled tube will rotate the other way as the piston travels back up. This, however, can be solved in numerous ways, including the use of two intersecting sets of helical grooves of opposite handedness, meeting each other at the top and bottom ends of the piston rod. Back-and-forth motion can now be converted to (say) continuous clockwise rotation.

An advantage of having two (opposite) sets of grooves is that the engine's output can be reversible without the need for a reverse gear. If the engine is started "forward", its inertia (or the inertia of a flywheel) will keep it going forward. If it's started "reverse", likewise it will keep running in reverse. So, if you don't mind stopping and restarting the engine (and if the starter motor can be driven both ways), you can use all your forward gears in reverse.

A helical drive would be most effective for either single- cylinder engines, or opposed twin cylinders. However, multiple in-line (or V) cylinders could be coupled via bevelled gears.