Opposing Piston Engines
Opposing Piston Engines
(OP)
Hello to all,
I keep on noticing favorable comments about opposing piston engines, i.e., Junkers and Napier Deltics. If they are so good, then why aren't they used more widely? That is to say, these engines must have a problem or a shortfall. Can anybody say what the problem is? Also, what about their efficiencies?
Any other would be appreciated.
Kind thanks & best regards
Painterman (DFD)
I keep on noticing favorable comments about opposing piston engines, i.e., Junkers and Napier Deltics. If they are so good, then why aren't they used more widely? That is to say, these engines must have a problem or a shortfall. Can anybody say what the problem is? Also, what about their efficiencies?
Any other would be appreciated.
Kind thanks & best regards
Painterman (DFD)





RE: Opposing Piston Engines
The list of bad things might include: they cost more to make, they don't fit as nicely in a car, the pistons "wear funny" compared to vertical ones, they can consume more oil, they're harder to properly lubricate.
The thermo efficiency shouldn't be much affected by the orientation of the pistons (may be minor effects).
RE: Opposing Piston Engines
Thanks for your reply. What makes them more expensive to make?
Thanks, DFD
PS: read on your posting resume, quiet impressive
Regards DFD
RE: Opposing Piston Engines
- Steve
RE: Opposing Piston Engines
RE: Opposing Piston Engines
RE: Opposing Piston Engines
Regards
Pat
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RE: Opposing Piston Engines
The Deltic's big virtue is that there are no valves to mess with, at all. It's a two-stroke, so emissions might be a problem today.
The Deltic's real Achilles heel is oil pooling. When the engines are left idle, the bottom cylinders fill up with lube oil. If they're not barred through a couple of revolutions without fuel, to clear the cylinders of lube oil, then the start will clear the cylinders another way, by breaking something.
Mike Halloran
Pembroke Pines, FL, USA
RE: Opposing Piston Engines
It has a slick web site
It has a nice animation
It has some nice investors listed
The next guess is that a working unit will always be one step away, the directors will be getting big salaries from the investment money.
We're just missing some big name companies who are asking for such and such a demonstration unit to be built (the ones the have or were working on are always the wrong size or colour).
Direct/indirect ratio? well lots of directors, few grease monkeys.
However, there are only three pages at http://achatespower.com/index.html and somehow a scam ought to have;lots more pages full of faked up "independent " reports etc.
Be nice to know what they are doing besides recruiting.
JMW
www.ViscoAnalyser.com
RE: Opposing Piston Engines
My experience with opposed piston diesel engines (Darpa program in early 90s) is that the combustion chamber volume evolution and the location of the injectors is very hard to optimize. No one has developed combustion in an opposed piston engine to work as well as a classic bowl in piston with a central injector.
The engine program referenced above resulted in very good weight specific power, but very bad brake specific air consumption. Combustion was not very good at less than about 1.8 stoichiometric air.
I think modern four stroke piston diesels are achieving very good combustion efficiency at about 1.3 ~ 1.4 stoichiometric air.
jmw - if you have 35 minutes, and you want to view a video about an opposed piston engine that will make your scam test meter buzz like crazy, check out the link below to a youtube video on Dr. Paul's opposed piston engine.
RE: Opposing Piston Engines
RE: Opposing Piston Engines
I'm pretty sure the Achates engine is a two stroke uniflow scavenged opposed piston diesel, so one piston uncovers the exhaust ports and the other uncovers the intake ports.
We did a CFD analysis on a similar engine with the guys from Sandia NL a few years ago, and the resulting airflow pictures looked very similar to the Achates animation.
RE: Opposing Piston Engines
RE: Opposing Piston Engines
Bill
RE: Opposing Piston Engines
If my memory serves me correctly, the biggest problem with the opposed-piston TRC engine was ring scuffing. Any piston ported engine, whether opposed-piston or with a conventional cylinder head, has the basic issue that the stroke overlap of the oil control ring and the compression ring pack is very limited, due to the fact that the oil control ring can never travel above the lower edge of the cylinder ports. The only reason piston engine compression rings manage to work at all is due to the perfect oil film condition left behind by the oil control ring during each stroke of the piston. Since the oil control ring cannot travel up into the working area of the cylinder bore in a ported two-stroke, the compression rings are deprived of the oil film they so desperately need where it matters most.
The situation in the TRC engine was compounded even further due to the fact that area of the cylinder bore surface exposed to peak combustion pressures and temperatures had a very thick wall and was made of low thermal conductivity alloy steel. So even though it was water cooled on its outside, the heat transfer across that thick liner wall was not sufficient to prevent flashing of whatever oil film managed to make it up to that area around the TDC point of the liner. Every time we tore that engine down, we would see ring chatter marks around the TDC point of the liner.
With regards to the injector location and chamber shape of that opposed-piston TRC engine, I'm pretty sure that was an issue we could have eventually resolved satisfactorily.
By the way, I'm working with the old DTC group on my new diesel project.
Regards,
Terry
RE: Opposing Piston Engines
RE: Opposing Piston Engines
http://www.sa.hillman.org.au/TS3.htm
Regards
Pat
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RE: Opposing Piston Engines
- Steve
RE: Opposing Piston Engines
I'm sure in their day, these were powerhouse engines, but I found them dirty, overly complex and hard to maintain in top running compared to modern diesel engines. Both engines suffered liner cracking around the ports, mostly due to poor operational procedures, such as extended times at low load or idle, improper prestart procedures, and lube oil problems.
We had a competitor underbid us on a major repair of two Fairbanks OP engines, and their mechanics were not familiar with the correct timing procedures between the upper and lower cranks, needless to say a VERY expensive mistake. We had the advantage of a former US Navy man who had experience with that engine, and knew the manuals were not very clear on all of the repair procedures.
When the Deltic engines were running good they were impressive, but a bit scary to stand next to while underway, lots of moving parts, funny noises, and oil slinging everywhere.
I think now days, when we look at all things we ask of a modern diesel engine, like high horsepower to weight and footprint ratios, low fuel consumption, low emission rates and high reliability, they older designs like complex OP engines, especially two stroke, just can't compare. But in their day they sure did push the envelope. I think those engines took real craftsmen and truly dedicated professional operators to assure good performance and reliability, something I don't see much of these days in the engine business.
My two cents worth.
RE: Opposing Piston Engines
As a schoolboy, I used to think they made a pretty 'neat' noise when they were in the old Commer Karrier trucks in thr 50's and 60's.
http://
Bill
RE: Opposing Piston Engines
I actually meant WW11 but my crappy keyboard keeps dropping letters unless I hit the key real hard, dead middle.
Regards
Pat
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RE: Opposing Piston Engines
It has nothing to do with the effectiveness of its primary function though.
Bill
RE: Opposing Piston Engines
http://www.ecomotors.com/technology
-Tony Staples
www.tscombustion.com
RE: Opposing Piston Engines
Whether you meant to or not, your comment "Power density is interesting, if nothing else" sums up the Ecomotor engine perfectly. The term "power density" sounds great to the average person, but in reality it is not very meaningful with regards to automotive powerplants. More important (in order) are things like production cost, emissions, driveability, SFC, and NVH.
Hofbauer claims he simplified the engine by eliminating the cylinder heads. But at the same time his design needs 6 conrods for 4 pistons, it needs 4 pistons to get the same displacement a conventional engine can achieve with two pistons, it needs a very complex and expensive ported cylinder liner, and (being a two-cycle) it needs an expensive assist device for its turbo to scavenge properly.
It also would not package well in a typical automotive chassis, since it is very wide in the transverse direction.
As for his claims of exceptional "power density", he achieves those numbers by operating at very high BMEP rates (25 bar?). The drawback to doing this is that this engine will be very rough running, have very aggressive torsional characteristics transmitted to the rest of the drivetrain, and will have an unacceptable amount of combustion noise for automotive use.
As for his claims of low fuel consumption, I would be very skeptical of them. It has long been known that one of the inherent flaws of the opposed-piston, uniflow two-stroke configuration is the poor injector spray and combustion chamber shapes that result from the side mounted injector location.
Regards,
Terry
RE: Opposing Piston Engines
RE: Opposing Piston Engines
To my knowledge, DARPA has suspended their lucrative funding for the FEV effort (someone please correct me if I am wrong). If the above is true, that says enough about the usefulness of the technology to me.
RE: Opposing Piston Engines
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We had a competitor underbid us on a major repair of two Fairbanks OP engines, and their mechanics were not familiar with the correct timing procedures between the upper and lower cranks, needless to say a VERY expensive mistake. We had the advantage of a former US Navy man who had experience with that engine, and knew the manuals were not very clear on all of the repair procedures.
>
I never though they were different, but an advance on the bottom crank is a good idea, thanks for the info.
So what was the failure on that engine? I would think the drive link to the bottom crank would be the weak point, the bevel gears?
RE: Opposing Piston Engines
I would agree with some of tbuelna's criticisms of the opoc engine regarding the need for a scavenging supercharger; lubrication of ported cylinder liners; less than optimal combustion chamber shape and side-mounted injectors.
What the engine gains in an extra 2 sets of conrods and bearing shells per piston pair it saves in the lack of cylinder heads, with all their associated pieces: camshafts (4 in total for a DOHC HO engine), lifters, valves, springs, etc. (4 of each per cylinder for a contemporary engine). What it does for the overall BOM parts count, think about it...
The packaging in a transverse automobile layout is granted but a moot point as evolutions of the engine have been directed specifically toward trucks, aviation, military and APU applications.
As for the power density being a result of a high BMEP with attendant rough and noisy running, I disagree. I don't expect torsionals and combustion noise to be any higher than contemporary Diesel engines of the same cylinder count and using the same FIE/combustion development technology. The high power density comes about since this a 2-stroke engine, meaning that for the same displacement, RPM and power output as a 4-stroker, you need half the BMEP, or put another way, for the same displacement, RPM and BMEP, you get double the power. Yes, 2-strokes have different major critical orders than 4-strokes, but dealing with them are no different than other successful 2-strokes.
I agree that BSFC is indeterminate; what FMEP is reduced from the lack of valvetrain losses is countered by the added friction of two extra set of conrod small-end bushings
and big-end bearings per piston pair, plus small pistons in relation to the swept volume. On balance I'd say BSFC would be a wash compared to contemporary Diesel engines.
The opoc layout is inherently balanced (though not absolutely completely as claimed, since the L/R ratios of the two length conrods and therefore motion profiles of the inboard and outboard pistons are slightly different. This is however, still better than either inline- and conventional boxer 4-cylinder layouts, the latter of which must still put up with a small yawing moment due to the cylinder centerline offset.
I have no affiliation with the opoc engine or its development company, and I don't believe I have biased what I've said above. Rather, I believe it's a balanced critique based on what is known of the engine with objective engine engineering principles applied.
On a similar note, although not an OP engine in the strict sense of the term, I've seen an BMW boxer engine using an uncoventional cranktrain mechanism referenced in US Patent 5,785,029 that interests me.
RE: Opposing Piston Engines
Excellent assessment of the OPOC engine. In my opinion, the biggest issue with the OPOC configuration (or any opposed piston 2 stroke) is compression ring durability and scuffing. With a ported liner, the oil control ring travel must always stay below the lower edge of the ports. The compression rings, on the other hand, never travel much beyond the lower edge of the ports. So the amount of "wiping" overlap between the oil control ring and the compression rings is very limited. Thus the compression rings operate on a very poorly lubricated cylinder liner surface environment for most of their travel, with the worst tribological conditions being present at piston TDC where the compression rings also see their highest pressure and thermal loads. This inevitably leads to ring scuffing. And the situation is further compounded by the less favorable heat transfer situation present in a two cycle piston structure.
I worked on the TRC opposed piston engine program. And persistent ring scuffing was one of the problems we never were able to resolve.
Regards,
Terry
RE: Opposing Piston Engines
RE: Opposing Piston Engines
The engine ran but not for long enough to prove its objectives. As I recall, on more than one occasion it shattered like glass on the test bed due to some mishap from which a conventional engine could have been reclaimed with a simple rebuild.
PJGD
RE: Opposing Piston Engines
RE: Opposing Piston Engines
Balancing such 180°-V engines are the same as inline engines, since both pistons in a cylinder move as one unit. However, a simple 1:2 hypocycloidal cranktrain has a sinusoidal piston motion with no second-order component, therefore eliminating second-order out-of-balance. Therefore, such an engine with 8 pistons and crankpins offset 180-degrees would balance like an inline-4, but in the absence of 2nd-order out-of-balance would actually be perfectly balanced. A 2-cylinder would balance like a single-cylinder engine -- not good.
RE: Opposing Piston Engines
Also, counterweights can be employed to offset any/most imbalances in many hypocycloidal designs. Now do I think I know it all when it comes to these setups? Absolutely not, and I welcome your input. However, you can see an example of this at www.wiseman.com on their hypocycloidal design.
I remember reading some years ago that cardan gears used to create hypocycloidal motion have a problem with tooth degradation when spur gears are used, because one tooth is always taking the shock and load from combustion pulses. I have been looking, but can't seem to find that article. Obviously, helical gears come into mind to spread the load over more than a single tooth, but the resulting gear teeth are usually smaller and weaker, so there are tradeoffs. Helical gears do run quieter and can rotate at higher RPM's, but wonder if there are other issues to deal with as well? Does anyone have any experience or input regarding this?
RE: Opposing Piston Engines
The outboard cranktrain as I stated is replaced with the long outer con rod pair connected back to the crankshaft; I can now visualise how you want to realise a hypocycloidal mechanism, and it's an interesting idea. I've been very interested in hypocycloidal mechanisms for a long time, but I wanted to avoid using gears at all cost for much of the same reasons you stated and just transfer the forces and motion by bearings. The second limitation of the hypocycloidal mechanism is that I wanted to avoid requiring a linear guide (the two pistons in the same cylinder bore on either side of the crankshaft serve this function, otherwise it would require a crosshead), so that the mechanism can be implemented in conventional inline, vee and boxer layouts.
RE: Opposing Piston Engines
Sorry if I missed some details in your posts. I wonder if utilizing some helical internal ring gears might work instead of spur gears. I have also been doing research into whether or not carbon-carbon gears might have the required strength and durability, especially if coated with diamond-like-carbon coating to further reduce friction. Carbon-carbon is lighter and stronger than steel, I am just unsure if it could withstand the shocks of combustion.
Moving to the hypocycloidal crank setup would remove the need for the current inner piston connecting rod design, further reduce cylinder/piston friction & wear, allow further increase of stroke length (if desired), and allow the removal of the outer piston rocker setup. The pistons could use straight, relatively thinconneting rods running through the crank connections and rigidly attached to the pistons, so no piston wrist pins either. If you adopt the appropriate hypo design, you could reduce reciprocating mass signifigantly, as well as friction.
I just got solidworks and I'm working to get some of this modeled. I have some 3d stuff I put together in sketchup, and may post some of them.
One thing I am looking at is using silcon nitride bearings, in either a roller needle or ball configuration. In my design they would be spinning very fast and taking some pretty hefty loads and shocks, so I thought using ceramic instead of steel would help shoulder the loads. The need for lubrication is also vastly reduced, as well as temperature stability. Can anyone point out reasons why they would not do well in this context?
RE: Opposing Piston Engines
The lack of fracture toughness is further aggravated when used in applications where shock loads, like from combustion, are involved.
One way to see how using power gear drives have been successfully done in series automotive production is to look at the connection between the rotor and eccentric shaft of a Wankel engine.
I guess the other factor around using carbon- or ceramic components, is still cost. Whether you're a big OEM sourcing for mass production or an engine developer building one-off prototypes, exotic parts will come at a hefty price. The smart engineer knows what is absolutely necessary and what is nice to have but is not a breaker for the design if he doesn't get it.
RE: Opposing Piston Engines
And I agree with your comment about the wankel serving as a good example to follow, that has not occurred to me. So I am looking into that more. If it works for a wankel, it should work for the OPOC too. Rotary engines spin up to pretty high rpms without the gear teeth failing, but I need to compare combustion pulse forces between the two different engines and see if they are comparable. I kinda doubt it. The expect the OPOC would have much higher forces acting on the gear teeth than a rotary, but I hope someone can disprove that. With the compression rations being what they are in the OPOC, I bet it will have much higher gear tooth forces to contend with.
RE: Opposing Piston Engines
I even thought about scavenging the gears out of a rotary motor, but they are not sized correctly for the OPOC arrangement under discussion (diameter is 4 x eccentricty for stationary gear, 6 x e for rotor gear : e=15mm in mazda rotaries). However, their gear tooth design and profile might be used in an OPOC hypo crank since it is a proven design. I still need to gather more data to compare tooth loads to be thorough.
RE: Opposing Piston Engines
I don't know if you've seen this before but I ran across a site a few years ago about Clen Tomlinson. He built a model of a deltic engine from scratch. It's quite a beautiful piece of work.
http://www.craftsmanshipmuseum.com/Tomlinson.htm
Kyle
RE: Opposing Piston Engines
RE: Opposing Piston Engines
It uses silicon nitride bearings to save weight, to increase rpm and load limits, and to reduce the need for bearing lubrication dramatically. It can use either helical or spur gearing, roller or ball configuration.
I will decide whether to post the illustrations or not as soon as I have a couple of improvements worked out. I know how the wolves are on this forum that just can't wait for fresh meat to pounce on, so I want to get some wrinkles ironed out first.
Does anyone know the smallest tolerance that silicon nitride bearings can be produced to, both ball and roller (needle)? I haven't been able to find that online, and I am curious how expensive the very accurate bearings are, and what is the closest tolerance that is available.
RE: Opposing Piston Engines
RE: Opposing Piston Engines
I personally can't see that they would have any thermodynamic advantages over a modern diesel, probably they would not be anywhere near as good as a modern 4-stroke diesel. I don't think anybody else who answered this question has come up with any real thermodynamic or effiency advantages either. Mechanically I think OP engines are a joke.
My first reaction on seeing the cross-section drawing of the Achates engine in the USPTO online files (7360511) was "Dear oh dear!". There would have to be enormous efficiency or power gains to make an apparent mess like this worthwhile. The real question is why do companies like Achates get so much high powered financial and staff support?
RE: Opposing Piston Engines
RE: Opposing Piston Engines
RE: Opposing Piston Engines
- Steve
RE: Opposing Piston Engines
Regards
Pat
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RE: Opposing Piston Engines
Is lack of valve gear such a huge advantage? At diesel speeds the valve gear would last forever anyhow.
All the comments about power density - I am always amazed to see just how small an early model Subaru engine is when it is out of the car and lying on the floor. Another impressive one is the 4.3 litre Chev V-6 out of the car etc. and with all its ancilliaries taken off - hard to believe that it is 4.3l. The LC (water-cooled) 250/350 Yamahas - ridiculously tiny for the power they produce.
I will agree with Unclematt in that you shouldn't be too critical of of people trying something different until definitely proven otherwise.
RE: Opposing Piston Engines
I agree with your statement about not being unduly critical of individual creative efforts. But unfortunately, this is an engineering forum. And engineers are trained to think critically and logically, not emotionally. So if the responses here seem somewhat blunt, that's just the nature of the engineering personality.
As for the opposed piston two-stroke engine, the concept has been thoroughly vetted over the past 60 years, by very bright engineers working for very competent companies, like Junkers, Rolls-Royce, Napier, GE, Detroit Diesel, Fairbanks-Morse, etc.
As for the efficiency benefits of eliminating cylinder heads and valve trains, the most thermally efficient IC engines in existence have both cylinder heads and valve trains:
Regards,
Terry
RE: Opposing Piston Engines
I think something like the OP question can also be asked of the current rash of engines being promoted that replace the conventional crankshaft with a "scotch crank" or different types of cam and roller bearing mechanisms. Do any of the claims made have any truth in them? (I personally don't thik so). I like unconventional mechanisms and engines etc. - but just being different is not enough, it has to be better as well.
RE: Opposing Piston Engines
The possibly improved balance is also attractive, although of course there are solutions that are more straightforward.
So far as unusual engines with reasonable improvements in efficiency the Revetec engine is unusual in that they have released data from an outside lab showing real test results, with a thermodynamic efficincy of 38.6%, which so far as I know is the best seen for a real gasoline engine.
Cheers
Greg Locock
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RE: Opposing Piston Engines
RE: Opposing Piston Engines
Cheers
Greg Locock
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RE: Opposing Piston Engines
Regards
Pat
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RE: Opposing Piston Engines
Cheers
Greg Locock
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RE: Opposing Piston Engines
I was thinking about engine bulk vs displacement, not thermal efficiency nor mechanical efficiency.
I am presuming horizontal cylinder axis with crank below centre.
I think as the exhaust and inlet ports have to be on opposite sides of the cylinder, they need to be between bores, thus spreading the bores, or on top and underneath, thus putting some in the crankcase or displacing the crank and crankcase downward.
The mechanical efficiency must be poor with the extra reciprocating weight of the rockers and the extra rods.
Regards
Pat
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RE: Opposing Piston Engines
And I don't buy in to the mindset that removing the entire valvetrain and head, while adding extra connecting rods, makes the engine less efficient. Yes, this design is weird. Yes, it is not like conventional motors. So what.
If you go to ecomotors website, they are in the process of setting up manufacturing facilities in Detroit. This engine has also gone through extensive testing by the military for their UAV programs. So this isn't just vaporware from a con artist. I too am waiting on the numbers and to see real world results. I too would like to see the over 300 hp version of this engine in reality, though advanced propulsion already has a 13hp version available for sale now. When that happens I will be eager to see what results, if anything. I don't buy into every new engine technology that comes along, and this technology is anything but new. But I do think there is enough evidence to warrant further interest in this motor design, especially with extra refinements.
RE: Opposing Piston Engines
RE: Opposing Piston Engines
It shows that the Jumo 204 was achieving very good figures back in 1931. Think how much more engineering effort has been applied to the engines that can beat it ...
Chris
RE: Opposing Piston Engines
The Jumo OP 2-stroke diesels had decent BSFC for the 1930's. But it is nothing in comparison to a modern, production class 8 truck diesel engine, which typically achieve BTE's close to 50% with a TBO exceeding 1/2 million miles, while meeting tough EPA emissions standards and being produced and sold at a profit.
Sadly, most people don't appreciate how absolutely well engineered most production engines really are.
Regards,
Terry
RE: Opposing Piston Engines
Regards"
The other thermo dynamic efficiency gain comes from the speed of compression of two opposed pistons - at low speeds it's the abundance of Real time which is the reason why much more heat loss occurs at low engine rpms.
With the faster compression there should be less low rpm heat loss and it could possibly negate the need for a glow plug
www.auto-scape.com
RE: Opposing Piston Engines
RE: Opposing Piston Engines
Ports, con rods, crank rockers and crankcase are just a few problems that you have glossed over.
Regards
Pat
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RE: Opposing Piston Engines
Being air cooled would allow the ports etc. to be made more easily than with a water jacket etc.
Two Subaru flat-fours could also be used - with possible problems of making the ports through the water jacket. I didn't say it would be easy (nothing is easy), - just simpler than building from scratch.
RE: Opposing Piston Engines
I agree a pair of VW engines could be used as the basis for a twin crank arrangement, but so could.
I also think two inline engines could also be used. Some simple sheet metal work would fix the sump and oil pickup. The VW would also require some closing ff of the crankcase. Crank phasing might also be an issue with modifytin existing engine pairs.
There is still a major effort required to create an induction and exhaust system no mater what you use as basic raw materials.
The real question is why would you bother.
Regards
Pat
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