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# Screw engines

 (+1) [vote for, against]

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.

 — MaxwellBuchanan, Jan 29 2019

Cam Engine https://en.wikipedia.org/wiki/Cam_engine
Not well known, but a few around. [neutrinos_shadow, Jan 29 2019]

Axial Engines https://ipfs.io/ipf...i/Axial_engine.html
All sorts of variations... [neutrinos_shadow, Jan 29 2019]

More Axial Engines http://www.douglas-...-ICeng/axial-IC.htm
Some old patent drawings here. [neutrinos_shadow, Jan 29 2019]

..and it's brethren... [neutrinos_shadow, Jan 29 2019]

The Canda engine http://www.douglas-...g/miscIC/miscIC.htm
Bizarre and horrible ... [8th of 7, Jan 29 2019]

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With a linear helix, like the type used on screwdrivers, the piston will spend a very short time at top dead centre, which will sap a lot of your efficiency. It might be possible to have the helix bunch up at one end in order to increase the dwell time, but with two helices crossing that might cause problems.
 — mitxela, Jan 29 2019

This is a Cam Engine (of a particular type), and has been experimented with now-and-then around the place (closely related to the swash-plate engine).
The main advantage is that the "helix" (in reality, a sine wave wrapper around a cylinder) doesn't need to be the expected shape, but can be tweaked to allow faster expansion, longer TDC dwell, smoother intake and exhaust strokes, etc. And not for single cylinders; most stack up 4 or 6 around the shaft (pistons move axially, relative to the shaft alignment).

 // the "helix" (in reality, a sine wave wrapper around a cylinder)// That's exactly what I would have said if I'd have said it.

So, once again, reality sneaks in and pre-empts me. Ah well.
 — MaxwellBuchanan, Jan 29 2019

From my experience on worm gears, having metal sliding around in this manner tends to wear it out quickly. Doing it at the end of a power cylinder sounds like metallurgical death in a few hours.
 — RayfordSteele, Jan 29 2019

 Not really, because the simple split journal bearings with pressure fed lubrication survive in conventional piston engines for tens of millions of high speed cycles.

Cam and swashplate engines, along with bizarre rotary hybrids like the Canda engine <link> are Baked & WKTE ...
 — 8th of 7, Jan 29 2019

 I think that's WKTEBNBMB.

You know, it often occurs to me that if I'd only been born a hundred years earlier, I'd be dead by now.
 — MaxwellBuchanan, Jan 29 2019

 Sadly, fate seems to have decreed otherwise.

A quick Fermi approximation for a 4-cylinder automotive powerplant that's in a road vehicle that's covered 100,000 km suggests that each big-end bearing has sustained over 500,000,000 load cycles.
 — 8th of 7, Jan 29 2019

 Um, yes, I'm aware of journal bearings, thanks. They differ a bit in that there isn't any reintroduction of contact zones.

<checks job description> sorry I guess that was one job ago.
 — RayfordSteele, Jan 29 2019

 We just thought it would be wise to point them out before [MB] went to the trouble of re-inventing them.

Again.
 — 8th of 7, Jan 29 2019

//Screw engines "fnar fnar" in the manner of Finbarr Saunders.
 — not_morrison_rm, Jan 29 2019

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