Playing devil's advocate of course, although the horns don't really suit me.
The real verification of a novel mechanical design would be when it gets banned from motor racing. Like the Wankel effectively did from both car and bike series. However, racing is mainly power/weight, not practicality.
I believe by "erosion" you are talking about a process such as EDM. Keep in mind that EDM is a "slow" process, which generally implies an "expensive" process. Machining is a "fast" process.
I don't agree with only talking about the positive aspects. Discussion with others results in asking questions that one may not have thought of otherwise. Inventors tend to be overly focused on the merits of their own design and not think about the real world problems that they are going to encounter. If the discussion results in the inventor accounting for whatever the problem or issue is, and then making a better design, that's a good outcome. If the discussion results in the idea being abandoned, then perhaps saving someone the cost of building something that doesn't work can be a good thing, too ... Back to the matter at hand.
I don't know enough about surface finish achievable by EDM to state whether proper piston seal is possible without honing. I do know that a mirror finish will have poor results in terms of piston sealing, piston ring wear, etc. The crosshatch keeps just enough oil in the surface to lubricate the compression rings. Can this be achieved by a method other than honing? Perhaps, but honing is the way that I know how to do it. Some engines nowadays use a plated hard coating on top of an aluminum cylinder block, but even those are honed to get the proper surface finish for good ring sealing.
I know the original poster is dismissive of the effects of lubricant oil past the cylinder wall ports. I strongly encourage the original poster to study the experience of Detroit Diesel. In those engines, only the inlet ports were piston-ported (the exhaust used cam operated poppet valves in the head) and this means that theoretically (almost) all of the oil allowed to escape would go through the cylinder's combustion process ... but even this was not good enough for today's emission standards, and Detroit Diesel had to abandon their heritage of two-strokes and go to a four-stroke poppet-valve design. Some outboard boat motors and snowmobile engines and small scooters still use two-strokes but only in applications subject to less stringent emission regulation, and even those are going by the wayside. Even if you manage to get piston ring sealing to be as good as with a "cylindrical" piston engine, I still think this issue will come back to bite you. In your situation the exhaust is unavoidably piston ported, and you are going to get unburned oil out the exhaust EVEN IF you solve all of the other issues.
Regarding the wear issues, the upper "wear ridge" is seldom seen any more in a normal piston engine if proper maintenance has been done. It's not unusual to see a standard piston engine with 300,000 km or more and the original crosshatching (honing) on the cylinder wall is still there. Why solve an imaginary problem? Sure, a standard piston has side loading on it. They're designed to handle it. So what?
frankydevaere - I wish you luck because to me it seems you are trying to make your design as difficult a project as possible.
You posted this is the second generation engine design. Do you have a constructed and working first generation engine design?
I don't see the present piston designs as having any real friction or wear issues. Engine refinement has sure given cylinder bores in new engines nice long lives.
Concerning the Wankel and racing. Race sanctioning bodies love to either limit or handicap racers using numbers such as engine displacement. It's easier to ban the Wankel than to police displacement parity between a piston engine and a Wankel engine. The Wankel tends to trade durability and longevity and emmissions for power and weight, which just happens to fit well into the racing world. A race engine only has to last one race and emmissions don't matter It's too bad the Wankel can't race when it appears to be well suited towards racing.
Very well said Brian about the inventor-focus-merits. For your information, this design is open source, not licensed.
EDM is the word. Electric discharge machining. We call it in dutch vonkerosie '( could be litterly translated as sparkerosion ). Do quite a lot machining for the plastic moulding industry. That's where the idea popped up for being able to make toroidal shapes with narrow tolerances, as long as they are not to long/deep. And indeed it's an expensive process just because it's so slow. However it doesn't matter how hard the material is. Made once holes in rather big bearingballs for a special application. Have no idea about the need for the scratches for having a oilfilm in conventional engines or in this one.
Concerning the so what. This vertical friction force may be not longer a wear-ridge for the present state-of-the art engines but it"s still a powerloss for the engine that can not be ignored. I think the idea op placing that reactionforce elsewhere where lubrification is not so difficult should'nt be that bad at all.
Lubrificationleaking across the upper inletport is indeed an issue. Scavinging with pure air , instead of a mix fuel/air is a must. Didn't know that detroit switch over (had to switch over) to 4 stroke.
Detroit Diesels have a normal crankcase and oil sump just like a 4-stroke engine. They have a scraper ring at the bottom of the piston to scrape the bulk of the oil off the cylinder wall so that an *excessive* amount doesn't go past the ports, and they have something to limit the amount of oil that goes out past the ends of the wristpin. The exact details of it, I don't know - I haven't had one of those apart.
The power loss by side loading of the pistons, if it is an issue significant enough to warrant dealing with it, can be controlled by methods far less complex than toroidal pistons. Longer con-rods and offsetting the cylinder bore slightly towards the power-stroke side (and this method is becoming more common) go a long way towards doing that.
Spark erosion is the term generally used foe electron discharge machine. It is a common toolmaking practice
The rate of erosion depends on the current density applied. High current density is fast but leaves a very pronounced orange peel effect compared to normal bore or optimal bore finish. It also erodes the electrode or copper ball at the faster rate. The electrode is sacrificial and to get a progressively smaller bore as you progress down. This is all corrected by a final finishing pass at low current to get fine orange peel and more accurate size. I see two finishing passes required here. It sounds VERY expensive to me.
Oil control will be an issue without fancy profiles on the ring face as the top edge will seal on the outside and the bottom edge on the inside. The outside will therefore seal combustion gas well but not oil and the inside will do the opposite. Maybe not enough to be all that significant, but an issue none the less.
If the piston is located by a moving beam fixed at the piston end so it cannot rock, you need no more skirt than is required to support the rings and attach the piston to the arm or beam. The piston and beam could in fact be one piece of metal.
You still have what appears to be two crank shafts driving gears that drive the output shaft.
Unless I am reading it wrong, these pistons do not keep going around, they go to and fro in a curved bore, thus reciprocating and requiring all the energy necessary to do that.
Also piston rings are responsible for by far the greatest portion of metal on metal friction in an engine. The piston skirt is almost negligible in comparison and you still have the rings required and possibly even an extra ring to offset poor ring fit to the surface.
Regards
Pat
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I looked at some drawings and article of the Detroit diesels.
In Europe this engine is unknown, in fact there are no 2-stroke diesels that i am aware of except for the UK-dair-opposed piston-airplaine engine, .
It's seems strange that this Detroit diesel is coping with even a more difficult valve-train than in a four-stroke, just to be a two-stroke. I mean the valve train in a fourstroke is rotating half the speed of the crankshaft. The detroit valve train is rotating at the same speed. That is racing, giving the time that the outletvalves opens and closes during scavenging.
It seems in many articles that detroit engines coped with oil consumption. Some writers blaming the lower compression and oil ring scraping up oil into the inlet ports, others blame the seals of the blower...
It's all about thermal efficiency and mechanical efficiency.
Thermal efficiency op opposed pistons is not really being discussed here.
Mechanical efficiency :
Free piston or side friction is one issue.The side force on a piston is a consequence of the connecting rod having to move sideways as it goes up and down, and hence cannot easily be avoided on a conventional piston engine.
Making the connecting rod longer will help but this will result in an engine that is tall and heavy.
The side thrust is responsible for 8 to 15 per cent loss of mechanical efficiency . An explosionforce for lets say 10000N on a piston in the first 6 degrees angle gives a side force of 10000N x tg 6degree = 1000 N.
Who wants to pull 1000N (200 pounds) resting on 10cm2 face.
Piston offset (1mm) is more a solution for the piston flipping over in TDC. ( i presume piston"slap" is the word )
Nice info. For your information it's even possible to EDM ceramics if the product has a electrical resistance less than 100 Omh.cm ( miw with TiB2,TiN,Tic particles).
EDMachining ceramics should go even faster than steel-products but witch a greater roughness.
You're going to have the same issue with lubricant being scraped past the ports. You're going to have a worse issue because of the oil being scraped past the exhaust ports as well instead of just the intake ports. The Detroit diesels topped out around 2500 rpm, which is in a common range for engines of that size; sure, the valvetrain is going twice as fast as a four-stroke at the same RPM but doing that at 2500 rpm is not a major problem.
Wrist-pin offset of 1mm can be for piston slap but that's not what I'm talking about. On quite a number of newer engines the crankshaft is not on the centerline of the cylinder and the offset is frequently in the 4 - 10 mm range. Examples that I know of:
2011 Kawasaki ZX10R (they have a big recall on these engines, but not for this reason!)
2011 Honda CBR250R
The last couple generations of Toyota Yaris and Prius
It's plausible that the total amount of piston friction is in the 8 - 15% range of engine power output but to think this would be completely eliminated by not having side loads is excessively optimistic. Offsetting the piston is claimed to reduce total engine friction by a few percent, which is only a small improvement - but if you're designing new engine castings anyway, designing in an offset between the crank and bore centerlines is free and presents absolutely no manufacturing difficulties whatsoever, so one might as well do it.
if "total amount of piston friction" is meant to include the rings as part of the piston assembly, then that's the right neigbhorhood. Otherwise, divide by 3 to get the right neigborhood for the skirt alone.
This engine design is similar to some previous designs.
Examples: The Tschudi engine was a spark ignition engine with four curved pistons running in a torodial track. But this was a four stroke engine, not two stroke. Also, the James two stroke opposed piston engine had pistons moving in a torrodial track. I think that the Canadian government provided some funding for research on this engine some decades ago.
Also, there is yet another torroidal engine at this thread on the eng-tips.com website: thread71-240200
Some references that describe the above engines and other related ones are listed below.
Note that such designs have been around for a long time but have virtually never made it to production.
Norbye, Jan P. “Rivals to the Wankel: A Roundup of Rotary Engines” , Popular Science, January, 1967.
Setright, L.J.K. Some Unusual Engines, Mechanical Engineering Publications. 1975. ISBN 0-85298-208-9.
Chinitz, Wallace. “Rotary Engines”, Scientific American Magazine, February, 1969, Volume 220, Number 2. See pages 90-99.
you can see the mechanics. Everything is reduced to a single motorshaft, de pivoting and rotating joints are replaced by some sleeve-mechanisms. Have now idea, how , in this case , is arranged that outlet ports opens en closes before the inlet ports.
This guy is enthusiastic telling that in his NO-friction-free opposed piston engine the need for lubrication at the expolsionsides of his pistons is practical nihil. On this expolionside , there is only need for compressionrings. The transfer of the wrist pin - i.e. of the thrust loads - away from the hot combustion chamber and away from the ports,to the cool compressionside, solves another problem of the opposed piston engines: it allows "four stroke" like lubrication and oil consumption.
"The lubrication (and the oil consumption) is as in four stroke engines. The oil rings never pass over the ports and flooding with oil is needed only at the compressor side of the piston (the cool side) where the thrust loads are taken."
Also Stirling engines use Rohmbicdrive to get frictionfree pistonmovements (
) beause they cannot effort having lubrication-oil burning up in there enclosed working-medium.
Does friction-free pistons require less ( or none ) lubrication-oil and therefor limit ( exclude )the problems that conventional 2-stroke diesels ( Detroit ) have (had )?
Are friction-free pistons the sollution to put two-stroke diesels back in the emission-picture ?
