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Have been reading about better connecting rods recently and this has lead me to try again...
As usual a piston moves linearly between TDC and BDC (say up and down). The connecting rod/crankshaft converts this linear motion into rotational motion. I propose a modified conrod/crankshaft arrangement.
The
connecting rod will be fixed to the piston and house a ball bearing at the bottom end.
In crosssection the crankshaft will look like a cookie cutter. The conrod will traverse accross the surface of the cookie cutter (this converting linear to rotation motion). I works in the similar way to a camshaft and a valve.
In the camshaft arrangement there is a spring to allow the valve to close. In this proposal the crankshaft will contain a track for the ball bearing so that it constrains motion in both directions (say up and down).
[Supercat] has outlined some advantages in this arrangement since the up and down strokes can vary in length on the same cam.
With variable timing technology the stroke could be varied during the operation of the engine. High compression at low rpms and low compression and high rpms (for forced induction).
Dyna-Cam engines
http://www.dynacam.com/
Is this what you are proposing? [destructionism, Oct 04 2004, last modified Oct 05 2004]
Constant-Volume Combustion
http://www.ducati.c...rt=general&artID=11
6-cycle engine with a cam instead of a crank [discontinuuity, Jun 01 2005]
[link]
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There will be a side force at the bottom of the rod; torque has to come from somewhere. Some kind of slide-bearing will be needed to keep the rod straight. |
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[Fussass] Yes, if you mean that the conrod/crankshaft connection will experience drag... [Edit] Yip think that the cam follower should be a pair of bearings or cogs that counter rotate. |
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Other thoughts... the duration of the combustion and compression strokes can be varied... ie, slower combustion and faster compression. |
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The conrod/crankshaft bearing will need to spread alot of load implying multiple ball bearings for a rolling joint or a pad for a sliding joint. |
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Sounds like a swash plate motor to me, admittedley more complex. I suspect efficiency will be low. |
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One of us is misunderstanding something then [unclepete]. A swash-plate engine has the pistons arranged with their axes in a circle like the chambers in a revolver, each pressing on the outer part of the flat surface of a thin disc whose axis is parallel to the pistons' stroke, and which is slanted on that axis. (Stick a pencil through the centre of a circular coaster then, keeping the pencil vertical, move the top face of the coaster to 45 degrees. The "crankshaft" is at the centre of the disc, and the pistons contact the disc around its periphery.) When the piston descends, the plate is rotated.
[drivel]'s engine has the pistons pressing on the edge of a disc which has rounded "teeth", much deeper than a typical cog-wheel. Rotation is caused by the piston moving from the tip of a tooth to the base, and thus moving the wheel by the width of one tooth.
Please correct me if I am wrong. |
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Yip, angel is right... The disadvantages of this arrangement are going to be weight and friction, the major advantages are going to be variable compression, asynchronous stroke speeds and even weight distribution (ie no need to counter balance the crankshaft rotation). |
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well first off, hi everyone. new to the half bakery. it looks like the site i posted is down, so I'll explain it instead. |
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The piston arrangement is different from all types of engines. [Tabs] seems to have in mind a similar engine. If you could imagine a horizontally opposed engine(like a VW) with the two opposed pistons sharing the same connecting rod with rollers at the middle, you can understand the arrangement. |
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The sinusoid is a very hard to explain shape, at least for me it is. It is basically a plate that is distorted to provide linear motion when rotated. The Dyna-Cam engine has several working models, and they are at home in several types of test aircraft. |
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Advantages are no oil needed for the rollers, several power strokes for one rotation of the crankshaft, aerodynamics of a turbine, and lower parasitic friction losses. |
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As of yet, I don't know if Dyna-Cam has experimented with variable strokes, but it seems perfectly viable to me. |
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[angel], OK, so it's a wobbly swash plate? I can see that you'd get more power strokes per crankshaft rev. depending on how many ripples there are. I presume for power transmission to actually happen,the pitch of the "teeth" and cylinders are going to be different, so avoiding a "dead spot" effect? Is this going to have to be a two stroke engine to save on very complex valving, or are you going for sleeve valves also driven off the wobble plate? |
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I propose a ball and socket type of connection to avoid having an opposing piston. If there is an opposing piston then it seems to me that the connection between the two is going to compromise the possibility of a variable stroke. Essentialy the change in width of the cam (for varying the stroke) cannot be arranged easily. |
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I suppose you're right. One can change the stroke, but it would also mean that the opposite piston has, say, a faster compression stroke vs the opposite piston's exhaust stroke, whichever way the firing order is designed. |
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I presume your engine would simply be half of the Dyna-Cam, without opposing cylinders. In this arrangement, a second engine can share the same crankshaft but different sinusoidal plates. This would allow the variable stroke. |
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However, I do not see a way to work out the ball and socket bearing. In this configuration, much stress would be put on the connecting rod and would still have to have a slide/sleeve bearing. The idea is to keep the rod straight. A roller on both sides of the plate allows the piston to act on one side, and the plate to act on the other. Much like a C-clamp but with rollers. |
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The C clamp arrangement means the cam must be a constant width object. The simplest CDO (constant width object) is a circle, the next is a particular type of triangle, after that there is a 5 "pointed" object, etc. |
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The rotor in a wankel engine is a CDO and is able to slide while keeping all "vertices" on two parallel lines. Higher order CDOs can't and so you can't substitute higher order rotors into a wankle engine. |
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[destructionism] I checked out the site and no the crank piston arrangement is not the same. In the arrangement I have described the pistons and crank are aligned in a conventional manner, not offset 90 degrees as in the dyna-cam. |
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oh ok...I see what you mean. It's gonna be a big engine (bulky anyway) :D |
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Will it need to be designed with anti-rotational versions, for either north or south of the equator? |
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See my link. I think what you are describing is a 6-cycle engine like the one Italdesign came up with. I believe theirs uses the internal pressure of the cylinder to keep it on the cam, but I'm not sure. You wouldn't really need a con-rod at all id you had the stroke short enough. |
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It seems like there needs to be a way to reduce the torque on the cam follower. Perhaps if there were three rotating cams and the center one were rotating in the opposite direction from the outer two, that would allow the tortional forces on the cam follower (and thus the piston) to be balanced. |
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Ummm ok [fussass, supercat] I understand your comment. I want the cam follower to be a cog but --- you cant roll a sphere down a tube of exactly matching dimensions (it must slide). The cam follower should be two counter rotating cogs. |
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[destructionism] Ummm the crankshaft need not be solid all the way through but yes I was thinking of more than one cycle per revolution of the crank. |
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[rasberry re-tart] I guess the cam can be arranged as described to provide one cycle per revolution --- but that was not initially intended. |
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Infact the torque on the cam follower is greatest for a single cycle cam --- because the conrod is longest in this arrangement.. |
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