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Open-head Four

Alternative engine design
  [vote for,

I thought of this idea when I was 12 years old and I guess now, 22 years later is as good a time as any to get feedback as to any possible potential.

The basis of this engine is to utilize the 'cylinder walls' in a traditional combustion engine, as pistons themselves. The excess energy that would normally be absorbed and dissipate as heat on a cylinder lining would be utilized and thus bring greater efficiency and power.

The idea is simple: Four opposing cylinders meeting with a common 'open-head'. The overall cylinder shape looks like the "Red Cross" symbol from a top view. Each cylinder houses a cone-crowned piston (picture a piston looking like a slice of pie from a top view). Each piston is able to completely meet the other opposing pistons at the top of the stroke. All four pistons are synchronized and flush neatly at the same time at the top of the stroke. Each piston has a separate, single crankshaft and all four crankshafts are synchronized via a planetary gear that encompasses the entire perimeter of the engine. The planetary gear feeds power to the transmission.

The intake, exhaust port and single spark plug lay in the center of the 'open head', where the four pistons meet at the top of the stroke. When the spark plug fires a charge, each piston acts to almost completely encompass the blast; There are no exposed cylinder walls to absorb the energy. There is no stationary cylinder head to absorb energy. The combustion energy spent is maximized as the pistons take the entire force.

An easier way to describe this may be to simply picture an antique bi-plane engine engineered in reverse so that the pistons are going from the center outwards and then meeting again where the propeller would be.

I know that there have been a few designs over the years of engines that try to harness the potential of the spherical blast that’s intrinsic with any gas ignition but they seemed extremely complex and cost-prohibitive.

I'm not sure how, or if I'm able to post images here but I can certainly clarify some details as well as show variations on this theme with an image if anyone is interested.



Nietzsche, Jul 11 2004

(?) Two piston version. http://www.fairbank...owerGen%20Sheet.pdf
[Fussass, Oct 04 2004]

revetec engine http://www.revetec.com/
[auricom_mech, Aug 07 2006]


       So it's really a single cylinder engine, with the cylinder "spread out"?
phoenix, Jul 11 2004

       It is in essence, a single 'cylinder' engine although the 'cylinder' is much more complex as it takes on the form similar to a 4-way pipe.   

       I want to correct myself on the piston shape in this version: the pistons in this model are not 'conical' but rather wedge-shaped (picture a piston with 2 sides angularly silced off). In another vesion (Open-head Eight) the pistons are conical to alow all eight pistons to meet flush at the top of the stroke. The Open-head Eight version has a vertical and horizontal cylinder layout wheras the Open-head Four version is horizontal and less complex.   

       The Open-head Four engine would act and sound like a "thumper", single cylinder motorcycle engine. For balance and smoothness, I would possibly add a seperate identical engine underneath, synchronized opposite to the stroke rotation of the top engine. This would be yet another version of an Open-head Eight.   

       I'm not sure whether this engine's optimal performance would be via 2-stroke or 4-stroke. I tend to like 2-stroke models better to lessen weight and deliver that raw power. Oh how I'd love to put expansion chambers on a 4-liter version of this engine.
Nietzsche, Jul 11 2004

       I totally agree that backyard fabrication would be much easier by making a two-piston version. And that may be the preliminary way to go. Double rings may prove valuable as well. The problem I have is I beleive the engine can be made better with four pistons. 2 pistons stil leaves 360 degrees of cylinder wall to absorb energy. 4-pistons leaves only a small portion of the top and bottom section of the cylinder exposed. Eight pistons leaves no sections at all exposed. I believe with eight pistons this engine may not even need a cooling system albiet a simple oil-cooler. Eight piston system is fairly complex and heavy although.   

       The head unit on the Open-head Four really isn't very complex but it would definately have to be fabricated from scratch.
Nietzsche, Jul 11 2004

       With heat temperatures way down, expansion and contraction of the pistons and cylinder lining is less dramatic and I believe that would curb any potnetial leakage and benefit engine life. Although if the pistons heated up much faster than the cylinders we might run into problems.... A good oil cooler should straighten that out.
Nietzsche, Jul 12 2004

       It's true that there would be more friction on this engine than a single cylinder engine of equal displacement. Most engines are about 94% mechanically efficient (6% of power is lost through motor friction) but let's say the Open-head Four is only 91% mechanically efficient.   

       Now here's the kicker: In a typical low compression (street car) engine, the thermal efficiency is only about 26% (36% of power is lost through water heating, 38% is lost through heat exhaust). I believe that could mean if there are no cylinder liners to heat up then the Open-head Four could attain around 46% thermal efficiency with regular gasoline (20% heat-loss gain + 26% typical efficiency). Subtract a few points for less mechanical effiency and it's still around 43% efficient...Correct me if I'm wrong but that's almost double the power of a regular engine with the same displacement.
Nietzsche, Jul 12 2004

       Ah-ha Aeolis! I Know this all sounds like it could get utterly complicated. I do think that an attempt to eliminate 100% of the cylinder walls would prove rediculously complicated as well. I've seen an engine proposal using 30 pistons in hydraulic fluid that pumps a crank. I'm not proposing that, nor would I want intake/exhaust ports on pistons. I think that I'll try to get an image online so I can show where those ports fit (On the Open-Head Four the ports lay directly on the center-top portion of the block, On the Open-head Eight, 2-stroke & 4-Stroke version they squeeze in the sides)
Nietzsche, Jul 12 2004

       Are the tops of the pistons expected to absorb less heat than the cylinder walls?
Fussass, Jul 13 2004

       an open-3 would suffice to remove the cylinder walls, and reduce the mechanical complexity and mechanical loss.   

       The open-8 is misnamed--I think what you really meant was open-6, although 8 could work too. The minimum for a no-wall model would be 5.   

       Bear in mind that for this to really work you need two sets of pistons, or else a massive flywheel, in order to keep the system running. So what you'd probably see are configurations of the flat variety where two or more sets are stacked on top of each other. Would probably be named something like "twin-3", "twin-4", "triple-4", etc.
5th Earth, Jul 13 2004

       Am I correct in my interpretation that the whole premise for this idea is that the cylinder walls absorb energy from the combustion in a piston chamber? Exactly how is this energy being absorbed? Because of the force? But energy = force * distance, and, since the chamber walls do not move (much) there is a distance of (almost) zero and therefore no absorbed energy. Because of heat transfer? But why would a cylinder wall absorb any more thermal energy than a piston head? Friction? I guess it's possible that there'll be some losses as the gas flows along the cylinder wall along with the retreating piston, but these would be minimal.   

       In fact, I think the proposed layout would increase energy losses. 4 pistons will incur 4x the friction losses, with rings sliding against chamber walls. Also, the "planetary gear that encompasses the entire perimeter of the engine" would incur pretty drastic drive losses (a typical car loses approximately 25% of its power through a standard transmission alone).   

       Sorry, I just don't see how it'll work.
Apache, Jul 13 2004

       I'd suggest a Google for "opposed Piston" which will get you some of the previous work on this sort of thing. They are usually large two stroke diesels - see Junkers Jumo and Napier Deltic for the largest types, plus the Commer TS3 and Trojan for smaller applications. The Deltic is particularly impressive. The main trouble with the four piston, one chamber type proposed here is the amount of space it takes up for the one combustion chamber, and the weight and complexity of the drive and valvetrain needed, especially in four stroke form. Once you start stacking them up, inlet and exhaust plumbing are probably going to get tricky as well.
unclepete, Jul 13 2004

       I agree with Apache and Nietzche. Whaty you've set out is an idea to make the engine more mechanically efficient, but actually I think it is less so. There would be very little energy "absorbed" by the piston walls on ignition - as it is an enclosed space and the only moveable object is the piston, nearly all of the force of the blast will be transferred to the piston. all force exopanding in other direections gets deflected downwards by the fact that the walls and top of the cylinder head are effectively solid. There will be little reaction to the force from the engine since the force is balanced all aorund the piston (i.e. equal in all directions) - there would be a little upwards and downwards movement as it reacts, but this too is small in modern cars (although you will see a model T engine rattling away in it's mountings because of this). I can't see that this arrangement give you better mechanical efficiency.
If you want more power and torque, then go the whole hog and get rid of the pistons altogether (i.e. rotary or "Wankel" engine) as exhibited in the Mazda RX-7 and RX-8. However, you do lose on fuel efficiency.
goff, Jul 13 2004

       Unclepete: Thanks for the 'head's up' on the Napier Deltic and the other various opposed piston layouts! I had no idea they used these in WWII subs and so forth. They seemed to have been very efficient, reliable and powerful in thier day.   

       But I'm also happy to see that there doesn't seem to be any 4-piston per chamber variants. I'm sure they thought of this in intial designs but based on modern thermal energy loss tests...this would seem to be the way to go.   

       You're right about the port/cam complexity when blocks are stacked up. That's why I think 2 stroke might be best for a double-chamber, eight piston version. As far as the drivetrain...the single planetary gear meshed to the four, toothed-crankshafts might work wonders. The planetary gear (toothed inside and out) is held suspended in place by four small sprockets or perhaps just rests on a bed of bearings in various points underneath it.   

       5-Earth: You are correct in that a 3-piston version would suffice to eliminate most of the cylinder walls as the triangular piston tops could meet up flush at the top of the stroke....And I do think it's a good design. The thing is that there would still be 60 degree angular cylinder walls to absorb some energy after combustion. The Open-head four version would have 90 degree angles to lessen the effect. One way to have no cylinder walls at all on a horizontal version (minus the top and bottom of the chamber) is to use five pistons while 3 of the pistons act as cylinder walls guiding the other 3. These 'guiding pistons' would have a shorter stroke and only be uncovered after the main pistons were on thier way downward. -An interesting idea albeit somewhat complex.   

       -The reason I'm stressing that as little cylinder wall as possible be exposed is because of the tremendous loss of thermal efficiecy when they are....A full 40% (and possibly higher) more power could be recouped by simply utilizing the thermal loss at combustion that normally dissapates into the cylinder walls.   

       -You are also correct about my mistake with the Open-head Eight (slapping myself) There would indeed be only six pistons (four horizontal and 2 vertical). My orginal version of the Open-head Eight was two Open-Four blocks stacked on top of each other and geared to oppose the other's stroke.   

       -You seem to have a firm grasp on various layouts!   

       Goff: I see what your getting at but I'm basing this all of the fact that the cylinder walls do not just drive energy downward toward the piston. A full third of power is lost to simply heat the water jacket around a cylinder. Another 38% of thermal energy (some bouncing off the cylinder walls as well) goes right out the tailpipe.
Nietzsche, Jul 13 2004

       Apache: Remeber that you have 4x the compression which generally equates to 4x the power. If the Open-head Four chamber had the same compression ratio as one, normal cylinder with equal displacement....then you're right, it wouldn't work.   

       Also, the planetary gear is fairly light as it's simply a hollow ring with teeth on the inside (and outside). The main block resides inside of the planetary gear. Obviously I'd need to have this thing as light as possible with as few moving parts as possible. Composite metals/ceramics would be nice albeit expensive. I'm also playing with designs for an ultra-lite Pyragear* CVT transmission (with only 3 moving parts) that might work nicely with this.
Nietzsche, Jul 13 2004

       Sorry but a lot of that heat is absorbed by the piston crown. It then conducts down the walls and into the cylinders or is cooled by oil spray. The top of the piston is likely to absorb about as much heat as the surface of the cylinder walls and head. The piston top could be insulated, already common on diesels, and sometimes used in gas engines for high power to keep the piston cooler. Unfortunately in a gas engine insulating the top is going to make a hot surface, warming up the induction mixture and worsening preignition.   

       The opposed piston engine has been reinvented here a few times, but much too late. Linked to a current version of this widely used and commonly known engine. One of the advantages of this opposed layout is the structural integrity and so light weight of a simple pipe with two pistons in it. Making four or more holes will loose this quality and result in a heavy lump in the middle.
Fussass, Jul 13 2004

       ....will.....not........post.....Open.....Forehead....idea........[brain made more efficient by......oh, never mind]
normzone, Jul 13 2004

       Fussass: Well I think I dissagree about the structural integrity aspect of the block as it seems fairly solid and the block shouldn't heat up as much a normal one would. As far as pistons absorbing heat, you may be right. I might have to compensate and insulate the pistons. I still think an efficient oil-cooler would work but hey, maybe this engine would be best suited as a Supercharged 2-stroke bio-diesel (anyone got any Corn Oil?)
Nietzsche, Jul 13 2004

       I'm seeing lots of problems with this design. First, unless you have some magic geometry which allows a translating piston that never exposes stationary cylinder liners, it ain't gonna work. If the pistons only come together to form a chamber at top dead center, then you'll have no compression on the "up" stroke, and no containment on the "down" stroke.   

       Your statement about "4x the compression which generally equates to 4x the power" follows a flawed logic. Why would a designer use 4x the pressure just because there are more pistons sharing the same chamber? Raising the compression ratio leads to higher temperatures and specialized fuels. Compression ratios are usually dictated by the intended fuel.   

       Use of opposing pistons in a shared cylinder would help with engine balance, but you still only get a fixed amount of energy out of a fixed amount of gas. For a fixed displacement, you're just moving more cylinders a shorter distance. More moving parts just means more parts to break.
Freefall, Jul 13 2004

       Oh...I can see that I'd better get an image up. Freefall: I'm not sure where to begin other than to say please read the description and all the annotations again. I think you have a different image in your head than what I'm trying to portray.   

       There is no 'magic geometry'. It's fairly simple to create a sealed chamber where 95% of the combustion surface area is an energy absorbing piston. It's also possible to make a 100% piston encased chamber if you put the spark plug/ports on the pistons themselves.....But I'm not proposing that.   

       Three seperate people above have also noted that my engine already exists in it's 2-piston form and was used successfully in Submarines and other applications. I'm just trying to improve upon a working engine and so far I don't see any reason why it wouldn't be better.
Nietzsche, Jul 13 2004

       Aeolis: Yes, pressure would increase exponentially all the way up to the very top of the stroke (the ratio wouldn't change because of the wedge-shape as it's mass is constant) and culminate to 4x normal compression (which is why I'd probably need a 2nd block stacked on top or joined vertically and bad-ass starter) The real benefit although comes from the combustion thermal efficiency which I think realistically would increase horse-power by about 30% over a standard block with equal displacement.   

       I believe the extra horsepower could also be curtailed to allow for a 30% increase in fuel efficiency. That would mean around 40mpg from a 200HP powerplant. I'm sure it could be tinkered with to do even better.
Nietzsche, Jul 14 2004

       What are you going to fuel this with? If you've got 4 times the compression, (presumably of a normal petrol engine) you are up into the region of 1.5 to 2 times the compression of a diesel. Explosions ensue. There are ceramic coatings for piston crowns etc that will reduce thermal transfer. Short strokes mean high piston accelerations at reasonable revs, so exotic materials are in order. The open head Eight (now Six I believe) will have a 3D output system that will not be easily possible to attach to a gearbox. I still feel that this is too big for it's possible output, too complex, and exhibits no perceptible advantage over Wankels, piston engines, or indeed many of the other "wonder" engines that are repeatedly invented like swash plates or (as an extreme example) the Vortex engine recently posted on the HB by someone who believes in magic. If anyone hears of the Laws (Guidelines?) of Thermodynamics being repealed, let me know.
unclepete, Jul 15 2004

       Aeolis: thanks for your input. I'm betting that the extra heat would just come from the higher compression. The actual combustion shouldn't compound much heat as its thermal energy is put to work. So I'm thinking this would balance things out and the engine should maintain relatively normal temps.   

       I think I need to research how various fuels react under certain pressures and temps at this point.   

       As far as what this engine would best be suited for... I always had mid-engine sports car, rear-engine bus or even motorcycle in mind if the engine layed horizontally. The engine would be flat like a flat-4 but also be round and wieghted evenly at the perimeters. I think its low-profile and center of gravity are big plus. I'm starting to think that a vertically mounted twin-4 might be a better way to service spark plugs and ports although. In this case my center of gravity limits me.   

       As far as linkage: I think the planetary gearing would be light, inexpensive and work well. Balance should be OK but I might want to add a thin shaft that runs underneath and through the engine and then geared to two sides of the plantary gear. I envision one end of this shaft with windings and being the starter/alternator/fuel pump/water pump/etc... all one unit. -Anything I can do for efficiency and weight. The other end links to the tranny.   

       Thanks again for your help.
Nietzsche, Jul 21 2004

       i like this idea however:   

       one problem would be all the complexities in the multiple crankshafts. maybe you could use tri-lope scissor cams like in the revetec engine [link] that could cut down on mechanical losses. i originally thought you would have some issues from all the dead time throughout the 4 cycles since all the cylinders have power strokes at the same time and while they are going through the intake/exhaust/compression strokes they wouldent have anything to keep them going. althoguh i guess the large ring gear(s) around the perimeter would act as more flywheel rotational inertia to keep the engine moving. although all this additional rotational inertia you would need would then make the engine even more resistant to reving. it might work well at constant rmp with a CVT, or charging a battery for hybrid power. this idea could go somewhere with a little refinement. [bun]
auricom_mech, Aug 07 2006


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