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# Axial Cam Engine

Yet Another Crankless Engine idea
 (0) [vote for, against]

This idea makes use of an uncommon, but *probably* not novel linkage for converting between rotary and oscillating motion.

For the sake of discussion, let's assume that our rotating drive shaft is oriented vertically. Around it's outer surface, create a groove with a sinusoidal shape, with two peaks and two troughs. The groove has rotational symmetry of order 2, around the drive shaft's axis.

Surround the drive shaft with a ring, with two inward- facing pins. These pins are 180 degrees apart, and fit into the groove on the drive shaft. The ring is prevented from rotating using some sort of linear bearings, and is allowed to move parallel to the drive shaft.

Rotating the drive shaft will cause the ring to oscillate perpendicular to it, in a sinusoidal manner.

If you want, you can think of this assembly as an unusual form of swashplate or cam.

Next, we add a couple of vertical rods, parallel to the drive shaft, and link them to the ring either rigidly, or using something like a scotch yoke. The purpose of the yoke (if present) would be to prevent the ring from exerting any horizontal force on the rods, and only allow vertical force.

Each rod has an upwards-facing piston on it's top end, and a downwards-facing piston on it's bottom end.

For balance (both static and dynamic) we add to our drive shaft a second groove and ring (below the first groove and ring), and two more bars, and four more pistons. The second pair of bars and quartet of pistons should be 90 degrees away from the first set. The phase difference should be such that when one ring is moving up, the other is moving down.

Note that the distance from the first ring to the pistons above it is shorter than the distance to the pistons below it. Similarly, the distance from the second ring to the pistons below it is shorter than the distance to the pistons above it.

Finally, add a four cylinder engine block above, and another below. Each block's cylinders should be arranged in a square around the drive shaft.

If desired, the cylinder heads can use ordinary poppet valves actuated by an ordinary cam (which might be located directly on the drive shaft, since the pistons already make four strokes per rotation), or for less vibration and higher rpm, we could use rotary valves.

To reduce friction and wear on the grooves, we can replace each pin with either a sliding block (as we might do in a scotch yoke), or a pair of wheels (one facing upward, and the other downward). If wheels are used, the grooves should be beveled, and have a varying angle to that bevel, so that each wheel's RPM is some constant multiple of the drive shaft RPM.

The groove shape doesn't need to be a perfect sine curve, for the same reason why, in real world internal combustion engines, scotch yokes are no more efficient than crankshafts. That is, more time at TDC results in more heat conduction to the engine block, which balances out theoretical efficiency improvements. If you want to, you can give the groove a shape which mimics the motion of a crank and slider.

I'm positive that this engine can be more compact than a conventional 8 cylinder, though I'm not sure by how much.

Higher max RPM, due to balanced moving parts.

More power at any given RPM. Why? Each quarter turn of the drive shaft results in eight simultaneous strokes, four up and four down. This results in two power strokes per quarter turn, and eight per full rotation!

Lower internal losses. In spite of friction which might occur in the linear bearings (which prevent the rings from turning) and between the pins and grooves, the reduction in piston side loading, and elimination of the wrist pins and crank pins should more than make up for it.

Reduced piston and cylinder wear (due to lower side loading), thus less maintenance.

Disadvantages: It's weird and different, and the automotive industry doesn't like doing new things.

I don't know whether or not it would be lighter than a conventional 8 cylinder engine.

At low vehicle speeds, it might be necessary to shut down the top or bottom half of the engine, for fuel efficiency.

More cylinders is possible (12, 16, etc.) but fewer is not. More than 12 is just silly, though.

If the drive shaft is vertical, then half the engine (one of the two blocks) is upside down, potentially making maintenance very awkward. OTOH, I only described it as vertical to simplify the explanation; horizontal would be more practical for most purposes.

Hmm...

Also, if anyone knows the proper name for the linkage I described, that would also be appreciated. If it's really and truly new, then would somebody lend me a few thousand dollars so I can go patent it? ;) The most similar thing I've seen is here [link], but that's kinda inside out (both literally, and because on that page, the cam is stationary and the engine block rotating).

 — goldbb, Mar 07 2013

Eddie Paul's CEM pump. http://www.animated...mpglos/cem_pump.htm
Similar linkage, but engine spins and cam is stationary. [goldbb, Mar 07 2013]

so to start, it's an opposed-cylinder barrel engine, yes ?
 — FlyingToaster, Mar 07 2013

If opposed cylinders are firing in an assymetrical sequence, won't the block wobble right off of the mounts? Or am I reading this wrong?
 — Alterother, Mar 07 2013

 FT, yes. The main differences between this and other barrel engines are (A) the linkage, which has neither a swash plate nor wobble plate, but has instead a cam groove, (B) each piston makes four strokes for each revolution of the drive shaft, not two, and (C) where more than two pistons move in unison, they are mechanically interlinked (attached to the same ring) instead of moving independently of each other.

 I don't know whether (C) is an advantage or a disadvantage... but it's certainly a difference!

 Come to think of it, if I wanted to, I could probably eliminate (C), by getting rid of the rings. We would need only one cam groove, and each rod would have it's own pin sliding in that groove, or it's own pair of wheels rolling in the groove. Each rod would need it's own linear bearing to resist lateral forces. Everything else would be the same.

AO, I'm not sure what you mean.
 — goldbb, Mar 08 2013

Then perhaps neither am I.
 — Alterother, Mar 08 2013

<link> is what you mean, I believe.
 — FlyingToaster, Mar 08 2013

 Transmitting the entire torque of the engine through the cam-groove arrangement is going to present some novel, and nontrivial tribological challenges.

Secondly, the very nature of the pin-groove arrangement will necessitate that the pin have a somewhat or even much smaller diameter than the shaft. So now we have a situation where the bending and shear stress on the pin will require a quite substantial cross section, especially given the fatigue loading. Scale that up (because the shaft needs to be much larger than the pin in order for there to be a "cam groove" - means that for any given specific torque, you'll end up with a very large shaft diameter required. You might end up being able to manage this via a hollow shaft to keep inertia and mass low, but I'm thinking you have a big design challenge here.
 — Custardguts, Mar 08 2013

 To make things simple, I'm thinking of this as a 4 cylinder (2 pairs of opposed cylinders, one on each side of the shaft), with a swashplate (which is topologically the same as a groove), or to be more exact a track welded to the outside of the shaft. That's minus the big linkage ("C" as mentioned in your previous anno).

 But I wouldn't discount "C" just yet, though it's exclusion makes for a simpler picture:

 What if, instead of a one-piece solid "dumbell" (piston/rod/piston) between each up & down cylinder, you had instead a more standard'ish arrangement where each piston has a rod hinged to its underside. The far end of these rods are attached to each other and to the big ring via a 3-way hinge.

Well... okay you'd get some pretty massive sideloading, but you'd also have a much increased dwell time at TDC/BDC.
 — FlyingToaster, Mar 08 2013

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