×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Opposing Piston Engines
10

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)

RE: Opposing Piston Engines

The good thing about them is they shake less.  

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

(OP)
To ivymike fm painterman/DFD

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

If wacky opposed piston engines float your boat, check out what those mad scientists at Achates Power are up to.

- Steve

RE: Opposing Piston Engines

regarding expense, instead of having all of your cylinders in a neat little row, you have some going this way and some going that way, like a v engine with a 180deg V-angle.  For a 4- or 6- cylinder (vs inline), that means an extra head and more material to make the block.  vs a vee configuration at a smaller angle, you have to put material all the way around the crankshaft to hold the halves together, and you have to find a place to put your cams, etc.  You can't use common intake (as with Vee), or common intake and common exhaust (as with inline).   

RE: Opposing Piston Engines

hey, they're even hiring guys like me at the moment, thanks for the site (never know where the axe is going next)!
 

RE: Opposing Piston Engines

I've read up on the Deltic, mostly because I was preparing to help somebody put them in a yacht.  The project kind of disappeared along the way.  Most of the fun projects do that.

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

Achates Power fits my scam test quite well (not to say it is, there ain't enough info there yet):
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

I know one of the engineers at Achates Power.  He is very sharp.  I think the effort is real.  There is a high risk of achieving competitive hp/$ and finding a suitable marketplace, but a real effort none-the-less.

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

based on the animation on the first page, it looks to me like they're using sleeve valves at opposite ends of a cylinder.  I didn't see where the piston was, and I didn't see any compression/expansion going on.  Perhaps the "sleeves" were actually pistons. The animation would seem to be a CFD intended to calculate how well they get the charge in/out of the cylinder.   

 

RE: Opposing Piston Engines

Ivymike,

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

I have to admit that when I heard "opposed piston" I was thinking of a conventional "boxer" or horizontally opposed cylinder engine layout.  I didn't bother looking up the deltic or junker.  I think that the two-per-cylinder idea is unneccesarily complicated, and that the deltic looks like a circus act.  They must have had some very talented engineers to make that thing successful.

 

RE: Opposing Piston Engines

Mike - I've not seen this in person, but....I gather the Deltics used in the Deltic British Rail traction units were prone to shoot sheets of flame out of the roof exhausts if they were opened up under certain conditions - nice problem to fix on a yacht. It should intimidate other yachtie types in the harbour.

Bill

RE: Opposing Piston Engines

3
turbomotor,

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.smile

Regards,
Terry

RE: Opposing Piston Engines

The opposed piston is ancient engineering.  Railroad engines used them as did pre WWII submaries.

RE: Opposing Piston Engines

Next time you guys happen to be wandering through South Kensington (London), check out the engines section in the Science Museum.  Nothing is new!

- Steve

RE: Opposing Piston Engines

When I worked as a mechanic, I worked on both Fairbanks Morse OP engines and on a converted WWII minesweeper with three of the Deltic engines on board.

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

Hitting the keyboard - yes, I hit it dead in the middle with my forehead on some occasions. Usually when I am here, at work.
It has nothing to do with the effectiveness of its primary function though.

Bill

RE: Opposing Piston Engines

There is also the Ecomotors effort.  John Coletti (formerly the head of Ford's SVT group) and Peter Hofbrauer are leading this effort.  I have a former employee of mine who is working on this effort in Detroit.  Power density is interesting, if nothing else.

http://www.ecomotors.com/technology

-Tony Staples
www.tscombustion.com

RE: Opposing Piston Engines

TStaples,

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

That looks very similar to to the FEV OPAC engine.  

RE: Opposing Piston Engines

That's because Prof. Dr. Hofbauer is the father of both.  

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

Catserveng
<
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

3
I worked with Prof. Hofbauer several years ago, although not in any direct project-related capacity.

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

TDIMeister,

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

I have been looking into OPOC engines for some time, and wonder if the cylinder scuffing issue could be solved with a hypocycloidal crank setup on silcon nitride roller bearings. I even came up with a design that would work. Another approach might be to use a carbon-carbon cylinder and a carbon-carbon piston. Machine them to close enough tolerance and there is no need for rings at all. Silicon nitride might also be used instead of carbon-carbon, which would also solve the lubrication issue. Yes, these components would be more expensive, but I would be willing to go there. It would also power a generator for a series hybrid very efficiently (hopefully). I am honestly tempted to go to www.ecomotors.com (also known as http://www.propulsiontech.com/aboutapt.html) and see about ordering one of their motors for modification and testing. I also thought the OPRE engine was pretty interesting at www.pattakon.com.

RE: Opposing Piston Engines

I had peripheral involvement with just such an OP engine constructed with no expense or resource spared by one of the US Big-3 circa 1985.   It was designed and built with Silicon Nitride liner and ringless pistons with hopes of high efficiency.  

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

Thanks for the post, thats good to know. I am curious if that test engine used a hypocycloidal crank setup to reduce friction forces on the cylinder walls? Got any materials to post or additional info on the project you worked on?

RE: Opposing Piston Engines

The definition of an opposed-piston engine is that the combustion chamber is formed by the two pistons in a common cylinder, implying the need of outboard crankshafts.  A hypocycloidal cranktrain would not meet this need.  You can employ such a cranktrain, but what you would have is in effect a "180-degree Vee" engine with inboard crankshaft and outboard cylinder heads.  In fact, all practical realisations of a hypocycloidal cranktrain I have seen are in fact such 180-degree Vee designs -- to be distinguished from Boxer or horizontally-opposed engines, which have non-shared crankpins and bank-offset cylinders.

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

If you will look at the engine on www.ecomotors.com, you will see that your statement outbout  "outboard crankshafts" is not necessarily true. Their engine is the layout I had in mind when I came up with my hypocycloidal crank design. I also found Pattakon's OPRE design to be interesting, if not entertaining, and wonder if new materials and designs could actually make it work pretty well. For fun I came up with a way to put my hypo crank inside each of the opposed pistons in that design.

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

Ecomotors IS the OPOC engine, and as I stated above, I worked in the same company as its inventor Prof. Peter Hofbauer.

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

Hey TDIMeister,
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 problem with ceramics are their well known brittleness.  Developments have been made along with better understanding of the fracture mechanics to address this, but they are still of fairly limited use in automotive applications aparts from, say, F1 and other rungs of motorsport.

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

I think that cost is more to blame for ceramics not being used more widely than their lack of desireable charecteristics. The same goes for using carbon-carbon, it is just too expensive to mass produce for the masses. Although, I did find several companies that produce carbon-carbon pistons for use in competitive 2-stroke motorcycles.

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

Hey Meister, I did some checking and I think that maybe the gear tooth load thing in cardan gears has been overplayed. Especially if you use cryo treatment to further strengthen the gears, which is what some rotary racers have done to the stationary gear on their engine. They even use spur gearing, not helical, and these gears could be engineered to have 2.1 teeth under load rather than just one. So I am going to focus on other issues unless someone presents evidence why I shouldn't.

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

Hello,

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

I have suggested a hypocycloidal crank system to greatly reduce the friction created by side forces on the piston. If you water jacket the area of the cylinder around the exhaust ports at the same time, the oil won't flash off, and the oil that does make it above the ports should be enough for lubrication and prevent the scuffing from occurring.

RE: Opposing Piston Engines

I have a design I have been working on for a few months that would give the OPOC engine a hypocycloidal "cardan gear" crank system. The more I develop it, the more reluctant I am to post it in public. You know how that is...

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.  smile

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

I am curious, could the use of biodiesel in this kind of engine increase lubrication in the cylinder and perhaps help prevent piston ring scuffing? Though there seem to be some compatibility problems with some direct injection sytems, I have been reading that bidiesel has more lubrication ability than petrodiesel. Does this translate to improvements in cylinder wall lubrication, or does it flash off? I could see the combustion chamber area of the OPOC cylinder getting a nice coating of lube when the biodiesel is injected prior to TDC.

RE: Opposing Piston Engines

I think people are getting away from the original question which was - do OP engines have any advantages/disadvantages?
 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

Same reason as companies like Scuderi - they're much better salesmen than engineers.

RE: Opposing Piston Engines

I think if you work out a couple of refinements that the OPOC engine DOES have a lot to offer in the way of increased efficiency. The desireable characteristics of the engine have been spelled out in detail. No valvetrain, increased power density, etc. I too would like to see independent testing done on the motor, but that isn't available yet. But instead of just ridiculing or dismissing the concept "just because", I choose to wait until all the facts are in before I come to a conclusion.

RE: Opposing Piston Engines

I thought the packaging and power density of these OP engines was primarily aimed at creating efficient powerplants for UAVs.  These may not need long service intervals.

- Steve

RE: Opposing Piston Engines

I think Pat has a good point about reduced surface area etc. - but good enough to justify all the complexity?
 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

BigVlad,

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

Tbuelna, Mate, - I'm on your side. I was just trying to appear "nice" by saying that one shouldn't be too critical. I agree entirely with what you are saying - it is some of the other writers (like unclematt)that seem to think that OP engines etc. do have potential virtues. I thnk Painterman's original question is a very good one. My interpretation of P'man's question is - " Clearly OP engines are, at best, verging on a mechanical nightmare compared to a "conventional" engine. There does not appear to be any overwhelming evidence that OP engines would have vastly better thermal efficiency, power density, reliability etc. or even some slight evidence that they may perform even slightly better than an engine of conventional layout. Why then did so many famous companies expend so much effort on OP engines?  Surely these companies must have seen more advantages in OP engines than are apparent today."
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

I suspect the original attraction is getting rid of the cylinder head. The thermal losses through the head are significant.

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Opposing Piston Engines

I think that the main attraction of the OPOC design is that it offers the efficiency of a diesel in a small package with a lot of power. Ecomotors estimate over 300 hp for their 2 cylinder, 4 piston version for automotive use. Even if that is off by a lot, and maintains diesel liek efficiency, that is still a nice engine considering how little room it takes up and how little it weighs. Have you seen the OPOC engine at advanced proplusion? 13 hp and easily sits in your hand. A 5 hp Briggs & Straton is bigger and heavier, and much less efficient. And with the advent of a variety of new injection technologies, further increases in efficiency are possible. I think this motor would make a great series hybrid generator, and just run it at its most efficient rpm all the time.

RE: Opposing Piston Engines

So, how big is this 300hp engine? When do they propose to get one running?






 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Opposing Piston Engines

Depends on how you look at it. I'd say the thought process started from a single cylinder engine. Add another in mirror image and get rid of the cylinder head. No losses have increased, as a percentage of output power, and you've eliminated the cylinder head heat losses.




 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Opposing Piston Engines

Greg

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

RE: Opposing Piston Engines

The extra connecting rods can be engineered from titanium and or take the form of thin wall tubes, adding little mass. If you change the crank to a cardan gear hypocycloidal system, the outer piston rockers, the inner piston rod connections, and the seperate crank journal pieces can all be eliminated - further reducing reciprocating mass. Also, the piston mass can be greatly reduced because the piston pin assembly would be removed as well. The pistons would all be rigidly connected to the rods.

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

Mass does not come into a large play for mechanical efficiency for most engines.

RE: Opposing Piston Engines

The original poster asked, amongst other things, about the efficiency of Opposed Piston designs.  If you scroll to the bottom of this article http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption there is an interesting table.

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

clasdauskas,

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

"Pat Primmer said:The lack of a cylinder head should increase thermal efficiency due to reduced surface area at the same bore and stroke.

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

There has been a lot of interest in this topic (judging by the 63 replies).  It has occurred to me that it would be possible to actually build an experimental opposed piston engine using two VW flat four engines side-by-side. Lopping off (and blanking off) the outer cylinders, connecting the flywheels by chain and either adapting the existing inner cylinder barrels or making up new ones. It would still be a big job but much simpler than building an OP engine from scratch. I am not saying that the engine would be better than a conventional petrol engine just that it would be interesting to experiment with it. I would not be surprised if it could be made to run quite well without a blower for scavenging.    

RE: Opposing Piston Engines

If it were as simple as arranging cylinders opposite each other you could use any pair of inline engines bolted together at the deck surface.

Ports, con rods, crank rockers and crankcase are just a few problems that you have glossed over.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

RE: Opposing Piston Engines

True to some extent. But - the VW's have their sumps in the right place so the oil system would be OK. No crank rockers - this is a "Jumo"-type layout -two cranks only. The existing crankcases should be OK.
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 was thinking the Commer Knocker configuration which is the only opposed engine I have actually seen.

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
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources