By on July 22, 2010

Predicting the future is a risky business. Lincoln Steffens, muckraking journalist and admirer of the Soviet Union said, regarding the then young USSR, “I have been over into the future, and it works.” Steffens apparently wrote that before he actually visited the workers paradise in the early 1920s. A decade later he regretted that endorsement.

Music writer Jon Landau’s prediction was a bit more accurate. “Last Thursday, at the Harvard Square Theater, I saw my rock and roll past flash before my eyes. And I saw something else: I saw rock and roll future and its name was Bruce Springsteen.” Landau was soon to edge The Boss’ original manager, Mike Appel, out of the picture, took over management of Springsteen’s career and production of his music, and did everything in his power to make his prophecy a self-fulfilling one.

Earlier this week I believe that I saw the future of transportation and stationary power and its name is OPOC. That stands for “opposed piston opposed cylinder”, a new engine architecture being developed for production and licensing by EcoMotors, a Troy, Michigan startup.

Yes, there are lots of “revolutionary” engine designs, most of them hype, and those that aren’t just hype rarely reach working prototype stage, let alone production. The OPOC, though, comes with a pedigree and a management team that brings substantial and credible automotive experience.

OPOC is the brainchild of Prof. Peter Hofbauer, former head of powertrain development for Volkswagen and designer of VW’s first diesel engine. Some call him the father of the modern high speed diesel. Dr. Hofbauer is Chief Technology Officer at Ecomotors. CEO of Ecomotors is Don Runkle, longtime GM engineer and executive. Runkle held the positions of chief engineer of Chevrolet, chief engineer of powertrain and racing at the Buick Division, director of Advanced Vehicle Engineering, vice president of GM’s Advanced Engineering Staff and GM’s North American VP in charge of the Warren Tech Center. President and Chief Operating Officer is John Colleti, ex of Ford, where he was responsible for the hugely successful (and fairly profitable) SVT high performance program.

According to EcoMotors, their leadership team has more than 230 years of collective experience in the automotive and power generation industries, has been awarded over 150 patents, and has managed more than 30 powertrain programs and more than two dozen new vehicle launches. Between them the company’s leaders have won four engine of the year awards.

The engine startup made the news recently when Bill Gates and Vinod Khosla anted up $23.5 million for EcoMotors’ round B funding. Khosla had previously invested in the company and it operates as part of his Khosla Ventures group. EcoMotors plans on both building engines for non-automotive applications, to prove the engine’s competence, and license the architecture to automakers to develop their own engines based on the OPOC layout. The payoff is potentially huge. There are over 100 million internal combustion engines sold every year in the world. About half of those are cars and light trucks. Some are for heavy trucks and buses, while the remainders are used for everything from power generation to lawn equipment. That’s a $350 billion/year market in total, and EcoMotors is convinced that the OPOC is suitable for just about every ICE application.

Gates’ involvement got big headlines, but it’s Khosla’s backing of the project that gives it particular credibility. Khosla, a founder of Sun Microsystems, is fond of backing what he projects will be disruptive technologies capable of entering the mainstream in a variety of developed and developing markets, with sustainable and profitable non-subsidized prices. He’s known as a major investor in alternative technology, with investments in cellulosic ethanol and batteries, so the fact that he has endorsed, with his money, an internal combustion engine says that there is yet life in the old bird. Of course the OPOC is a bird of a different feather.

EcoMotors says that though they expect another round of venture funding, they now have sufficient funds to fully develop and engineer the OPOC engine to ready-for-production status, and maybe into early production planning.

Earlier funding, about $50 came in the form of defense contracts with DARPA. Two engines were successfully developed but the US government stopped funding for the program. Another $18 million was committed by Zhongding Holding Group and Global Optima of China to fund development of engines for their own production.

It’s easier to understand how the OPOC concept works from a model or animation than with a description. In basic form it’s a horizontally opposed two cylinder engine. Each opposing cylinder has two pistons, opposing pistons that are connected to the same crankshaft but out of phase so that they move in opposite directions to each other, compressing the air between them and allowing it to expand as the crankshaft spins. So there are two cylinders, each with two opposing pistons. All the reciprocating forces cancel each other out, there is no force on the crankcase, all the force is direct to the crankshaft and the stresses on the crankshaft are simple. So far there have been some piston failures, and casting problems but they haven’t broken a crank yet.

By going with a two stroke design, they eliminate complicated valves and associated machinery, as well as the need for a separate and expensive cylinder head. A comparably powered OPOC engine has 1/6 the number of parts as a conventional V8. The OPOC achieves close to 4 stroke levels of gas scavenging by lots of fluid dynamics modeling, careful port design, and the electrically controlled turbocharger. It’s a standard Borg Warner turbo with an electric motor connected to the turbine shafts. From idle, it gets the turbo to speed, reducing turbo lag. More importantly it allows control of back pressure, improving gas exchange. Once the turbo is up to speed, the motor acts as a generator, and that current is supplied to the electrical system.

The folks at EcoMotors are very big on two cycle engines. As Runkle said, if you could choose, why build a complicated 4 stroke engine that wastes half the strokes? The OPOC has power on every down stroke. The drawback to a two-cycle engine, of course is that they are dirty. The lack of precision valves and the ability of a 4 stroke to pump air means that two cycle engines don’t have as complete a burn nor do they scavenge the exhaust gases well. A typical two stroke leaves a lot of exhaust gas in the cylinder as well as spewing some unburned fuel out the exhaust. EcoMotors says that their design has all of the advantages of a two-stroke engine with none of the drawbacks, achieving 90% gas scavenging, close to a four-stroke’s 95% and much better than the best two-strokes.

EcoMotors is doing the engineering and design on the engine, whose 6th generation version is currently being developed. The actual assembly and testing is being done at Roush Industries, with Roush contributing some technical expertise. When I asked EcoMotors if I could have a tour of their engine lab, they graciously arranged interviews with Don Runkle and Jonathan Hurden, EcoMotors Chief Engineer, at the Roush complex in Livonia.

I first met with Hurden, who showed me the latest engine that’s being assembled, and told me what was protected proprietary information and thus out of camera range. He described the engine, answered my questions and then took me into the test cell where the latest completed version of the OPOC engine was running on a dyno. After that, I sat down for a detailed interview with Don Runkle, where he gave me a condensed version of the pitch they give to potential customers and investors, as well as some answers to my questions.

Like his bosses, Hurden has an impressive resume, having managed powertrain development for the Rover Group, been chief engineer at BMW Group managing powertrains for Land Rover and Mini, and been managing director at Mahle Powertrain (previously Cosworth Technology).

While I was there, the castings for the newest version engine were arriving via UPS. The internals of the latest 6th gen motor are already running on the dyno in the 5th gen crankcase & block. The newer engine will have wet cylinder liners instead of the current dry sleeves.

The engine is surprisingly compact, though it looks a bit wide. Hurden says that width is deceptive. The M100 engine on the stand is a 300HP direct injected two-stroke diesel engine. It has a displacement of 2.5 liters, cylinder bores of 100mm (with very short strokes), and has dimensions of (LxWxH): 22.8 x 41.3 x 18.5 – note the short length and low height. With aluminum construction, it weighs only 300 lbs.  Compare that to the 300HP engines from Cummins and Navistar that respectively weigh 1,100 and 900 lbs. and have dimensions that dwarf the OPOC. Runkle says that production OPOC engines will easily weigh less than half what similarly powered diesel and gasoline engines weigh. Though the current prototypes run on diesel fuel, the OPOC engine can run on a variety of fuels including gases and alcohols as well as gasoline.

The power and torque ratings are dyno tested with standard ancillaries.

Hurden said that combustion development was their current focus. Towards that end, a joint project with the University of Michigan  College of Engineering has developed an optical version of the OPOC engine that allows high speed photography of the combustion chamber, combustion plume and exhaust gases. Hurden said that the current prototype is a “robust workhorse”, and that while he “intends to break these engines”, so far they haven’t broken a crank or thrown a rod, though he did show me a badly detonated piston. Every prototype including the first has run at least as well as predictions and so far the measured results match well with the anticipated figures. The main focus is meeting emissions but before that comes durability. The Roush technician building the engine is an alumnus of the Roush NASCAR team and he said that their motto was “break it on the dyno so it doesn’t break in use.” Right now it seems to be durable and some of the emissions targets have been met. The overall goal is the 2010 Tier 2 Bin 5 Heavy Duty Truck standard. While it can meet the standards at specific RPM levels, they are working on meeting the standards across the power band. The RPM range is currently 700-3800. Though without valves and with a short strokes this could be a high revving engine, the ultimate goal is fuel efficiency and high RPM means high piston speed and that’s the major source of friction in an engine. Lower friction = better efficiency so the redline is currently 3800 RPM, in line with other truck diesels.

Hurden expects that they will meet the emissions targets later this year. He said that with the exception of the design of the combustion chamber, no advanced technologies or costly materials were needed for development, just the normal engine development processes used, let’s say for the next LSx at GM. He alluded to problems with castings, an age old one in the auto industry. The one area where he said that EcoMotors was breaking new ground was in combustion chamber design. That the only part of the engine that they asked me not to photograph, the complex combustion chamber carved into the piston head.

All I can say is that it’s the coolest looking piston head I’ve ever seen. If it was back in the Rocket 88 and Cobra Jet days, the shape carved into the head would have inspired ad men to new heights in pursuit of an appropriate brand name.

Since they’ve registered OPOC as a trademark, I asked if they had any branding plans a la HEMI and they found the idea of an OPOC logo on the fender of a car somewhat humorous.

They made no claims for durability, but the fact that there is no complicated valve train to break, nor a cylinder head to warp or crack (the ports are circumferential to the cylinders), shows potential for good durability. More important to durability are the engine’s inherent balance, simple loading on the crankshaft and no forces on the crankcase. Loading on the crank is such that only two main bearings will suffice. That further lowers friction.

In current development, the M100 OPOC engine tuned to meet current North American emissions standards, on the dyno generates 240 HP and 487 foot-pounds of torque, so they aren’t too far away from meeting both power and emissions goals.

Hurden then took me through a maze of rooms ending up at the test cell where the gen 5.5 engine was spinning away merrily. After putting in some ear plugs we went into the dyno chamber so I could get some video for the web site. I can say without a doubt that the OPOC is the smoothest running engine I’ve ever seen run. I was tempted to ask about doing the Rolls-Royce balance a coin on top of a running engine trick, but I also have little doubt that it would have stood still. In a video of the engine, they placed a beaker of water on the running engine and there was hardly a ripple in the surface.

The OPOC may be the best balanced combustion engine ever built. It’s absolutely phenomenal. Other than the noise, the only way to tell it’s running is to look at the belt drive and pulleys.

The engine, impressive as it is, is still a development prototype. When I entered the test cell, in addition to the electrically controlled turbo mounted between the cylinders, on the floor, plumbed into the maze of exhaust pipes, was what looked like an Eaton supercharger running off an electric motor. When I asked about the blower, Hurden asked me not to photograph it since it won’t be part of the final engine. While they are fine tuning the combustion process, they need a surplus of intake air under pressure, which the Eaton blower supplies reliably. Once they get the combustion chamber shape finalized, the onboard turbo will be optimized to provide all the compressed air the engine needs.

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41 Comments on “Inside Ecomotors’ Revolutionary High-Efficiency Engine...”

  • avatar

    Interesting, lets hope this goes somewhere. OPOC diesel racing anyone?

  • avatar

    This sounds as appealing as the Wankel once did. The real challenge is to build one that can be produced at a realistic cost and will work as well in Anchorage in the winter as it will in Phoenix in the summer, all with good fuel efficiency. The fact that they have people with real automotive production experience on the team is encouraging. So is the fact that it’s in Michigan, which could use another transformational technology.

  • avatar

    Reminds me of Smokey Yunick’s adiabatic engine back in the early 80s, which ended up not going anywhere (plenty to read about on the web – one of his buddies has his prototype engine that was once shown to GM and was working to reinstall it in a Fiero, last I heard).

  • avatar

    Interesting that they chose a 2-stroke. Especially considering that 2-strokes above 50cc were banned in the United States in the mid 80s for motorcycles. I’m guessing that law doesn’t apply to cars.

    • 0 avatar

      They weren’t banned because they were 2-strokes, they were banned because they had bad emissions.

    • 0 avatar

      Were 2 stroke actually banned or was the standard set at the time to effectively ban 2 stroke (based on 80’s technology).

      Rarely does the EPA (or CARB etc) ban a technology. Rather they set the limits in such a way that it favours one technology over another.

      Now there are cases this wasn’t true such as the banning by CARB in 2006(and Australia – not sure when) the use of mixing used oil into the fuel to extend / eliminate the oil change interval on large industrial engines.

    • 0 avatar
      M 1

      2-strokes haven’t been banned or restricted. Look up the ATK700 … a 700CC 2-stroke dirt bike made by an American company. Probably the largest 2-stroke bike engine you can buy. You can also still buy some pretty large and powerful outboard boat engines in 2-stroke format.

      2 strokes haven’t been popular because they’re horribly inefficient, they have terrible wear characteristics, and their pollution levels are off the charts compared to a four stroke.

  • avatar
    bumpy ii

    Right after World War II, Fairbanks-Morse had a go at using their submarine opposed-piston diesels in railroad locomotives. The engines worked well enough when the Navy was paying the repair bills, but the power density wasn’t enough to offset the operating cost disadvantage versus conventional EMD diesels in private service.

    Good luck.

    • 0 avatar

      One of the advantages of the OPOC is power density. It’s a 300 lb engine that makes 300HP. It has good efficiency at steady rpm so it’s a good choice for driving generators as in a diesel-electric locomotive.

    • 0 avatar

      Their engines were just about perfect in USS DeSoto County, LST-1171. We had some vertical drive problems with the 7 cylinder generators, but not much else. Of course i was Chief Engineer which may have helped.

  • avatar

    You failed to mention that Unkle Runkle was a VP at Delphi before the bankruptcy. I got to talk with him during a plant tour once. Very smart mechanical guy…..too bad he got wrapped in with the likes of Battenberg etc when the s— hit the fan.

  • avatar
    bill h.

    I’d guess that the $50 that DARPA invested in the early development was really $50 million?

    If not, it might have been one of their best investments yet. Except that $50 wouldn’t cover the cost of logging the proposal to put into the DARPA Inbox.

  • avatar

    It’s easier to understand how the OPOC concept works from a model or animation than with a description

    Amy chance of adding that to this article? You’ve describe the engine well, but it would be ideal to see it.

    The emissions will be a challenge. Even if they have gotten the amount of unburned hydrocarbons down, there’s all sorts of awful byproducts that a four-stroke can minimize through precise valve timing that will be a challenge, here. Bin 5 is not bad, but gas cars are moving to Bin 2 (diesel is having trouble with this, too, and it’s probably killed the rotary).

    The other issue looks to be packaging, though I can’t tell the size from the photos. This might not be an issue for larger cars, but that’s a tricky shape no matter how you place it: a bit long for transverse and requiring a long hood otherwise.

    The inherent balance is nifty. That’s definitely a problem with many modern engines.

    • 0 avatar

    • 0 avatar

      How about mounting it longitudinally, so the crank rotates parallel to the wheels? That would solve the width problem, and you could just have a chain drive.;-)

    • 0 avatar

      How about mounting it longitudinally, so the crank rotates parallel to the wheels? That would solve the width problem, and you could just have a chain drive.;-)


    • 0 avatar

      It’s shallow, though, so it might make mid-engine layouts more practical.

    • 0 avatar

      Ed split the article in two. In the second half there’s an interview with Runkle. He showed me a proposed layout in a 2010 Fusion, where a two-module 150HP OPOC could fit with the crankshafts transversely through a conventional Ford FWD transaxle and everything fit inside the existing frame rails. He said that for a longitudinal layout, the frame rails would have to be modified slightly, but that overall width of the car wouldn’t change.

      No doubt, it is a wide engine. The M100 300HP version is 41 inches wide, but remember that’s meant to be a truck engine and would compete with Cummins and Navistars that weigh half a ton and are much bigger. While it’s a wide engine, it’s other dimensions are very compact.

  • avatar

    As bumpy ii pointed out, opoc has been around before. The concept was also used in a Soviet tank, built in the Ukraine.

    The concept by ECO motors looks very promising, especially the ability to add power through turning modules off and on.

  • avatar
    George B

    Ronnie, I really enjoyed this post. Interesting information and good pictures of the prototype engine.

  • avatar

    This engine design reminds me of the old British Rail class 55 ‘Deltic’ diesel-electric locomotive power plant. Although complicated, the Class 55 had a much better power to weight ratio then other diesel engine configurations.

  • avatar

    EcoMotors OPOC diesel engine explained

    • 0 avatar
      M 1

      Thanks for posting that, I was just getting ready to go look. (I found it ironic that the article said an animation would explain it, then didn’t bother to provide one…)

  • avatar

    Very nice; I hope it can go somewhere.

  • avatar
    Robert Schwartz

    I think it is GE that builds 2 stroke diesels to power its diesel electric gen sets in its diesel electric locomotives.

    I also recall that over the last couple of decades, a number of auto makers have launched research projects trying to produce 2 stroke engines that comply with EPA rules while retaining the 2 strokes virtues. I don’t think any of them came to fruition.

    • 0 avatar
      bumpy ii

      GE’s are 4-cycle. EMD’s (except for the Detroit Diesels and the 265H) are 2-cycle so far, though that may change now that Caterpillar owns them.

  • avatar

    You down with OPOC? Yeah you know me!

  • avatar

    the engine weighs only 300 lbs to the RX-8 wankel’s 247. that’s impressive. I hope this thing works out.

  • avatar

    I have heard of this before, but I haven’t seen this implemented by anyone. Let’s see if this ever takes off.

  • avatar

    You lost me at “lots of fluid dynamics modeling, careful port design, and the electrically controlled turbocharger”. The 5-stroke engine at least has a good physical reason why it’s better than 4-stroke. But this looks like a lot of hype and excessive optimism.

  • avatar
    John Horner

    I often dismiss killer new engine ideas in part because they are usually being developed and promoted on the cheap by people with minimal experience. This team, however, sounds like a whole different thing.

    I hope they make it!

  • avatar

    As compared to the OPOC engine,
    the pattakon Opposed-piston-Pulling-Rod-Engine (OPRE engine) is:

    more compact,
    is based on “built-in” scavenging pump of the piston type,
    has a wider rev range,
    is more vibration-free,
    provides additional time to the fuel to get prepared and burned more efficiently (and in-time),
    has better lubrication, etc.

    For more details (videos of the OPRE prototypes running on Diesel fuel, dimesnions, weight etc):

    Manousos Pattakos

  • avatar

    This reminds me of “Orbital” that was going to develop engines for GM years ago. They developed a two stroke direct injection system. The companies lofty plans never got off the ground…however, they were able to license some of the technologies for outboard marine motors, and I believe they are still in business as a research and development lab.

    Any time I hear that a company is “very close” to meeting emissions, I can’t help but think “close but no cigar.” Anyone can develop a very efficient, simple, inexpensive motor if they don’t have to meet emissions. People love to compare their prototypes to conventional 4 strokes, but 50% of the crap on a 4 stroke is to meet emissions.

    I’m also concerned when technologies require other new technologies to work…like electronic turbos. That adds another thing to fail in development.

  • avatar

    This is conceptually no different to the Commer Knocker engines fitted to thousands of Rootes vehicles. One of the all time great diesel engines, with the best exhaust note bar none!

  • avatar

    If everything disaster says about emissions are right (I think they are), those concerned about emissions will find more interest in Fiat MultiAir and Schaeffler UniAir car technology.

    Fiat and Schaeffler have turned the engine into a digital device i.e. the breathing of the engine has become a matter of digital control with all the known advantages of the digital systems, like the feedback (the engine of Fiat knows what it is doing and reacts (digitally) to correct everything that malfunctions at the time.

    Besides the ignition, the injection etc the digital control unit has full control over the breathing of the engine, which is far more important than any other event in the engine, at least for emissions, power and drivability.

    Then why the fuel efficiency of the Fiat and Alfa Romeo MultiAir cars are not 30% to 40% better than the conventional?

    Because Fiat invented an advanced system and then, together with Schaeffler, they implement it the wrong way: the Multiair operates according the “Ingoing Air Control”.

    The pattakon “Outgoing Air Control” cycle ( presented at ), an evolution of the Fiat Multiair cycle, avoids not only the underpressure, under part load, into the intake manifold (as the throttle-less VVAs, like the Fiat MultiAir and the BMW valvetronic, do) but it also avoids the underpressure into the cylinder by avoiding the expansion of the charge before the compression.

    By minimizing the pumping-loss, by avoiding the consumption of mechanical-energy to just expand and warm the charge, by keeping alive the turbulence and swirl during combustion, by improving the mixture homogeny and by reducing the temperatures of the thermodynamic cycle (lower heat loss) the “Outgoing Air Control” minimizes the mechanical-energy loss and optimizes the combustion.

    Manousos Pattakos

  • avatar

    “By going with a two stroke design, they eliminate complicated valves and associated machinery, as well as the need for a separate and expensive cylinder head”Muahahaa — TALK ABOUT making a virtue out of necessity!

    The simple truth, of course, is that since there is another piston at the other end of the cylinder, there’s JUST NO ROOM for a “separate and expensive cylinder head” with “complicated valves and associated machinery”, so they just plain COULDN’T make it a four-stroke.

    Just in case anyone thought these guys were any different from your ordinary run-of-the-mill bullshit artists, that is.

  • avatar

    The reasoning of the OPOC engine inventors and investors is strictly technical and scientific:
    because of the simpler and lightweight structure, of the fewer moving parts, of the smaller total force on the crankshaft main bearings, etc, the OPOC engine faces lower mechanical friction to overcome than the 4-stroke rivals.

    In a gasoline engine this may prove not a big deal, partly because of the lower friction and partly because the Variable Valve Actuation systems (like the Fully Variable Desmodromic VVA at and the Electro-Hydraulic VVA at ) can minimize the pumping loss of the 4-stroke gasoline, optimizing at the same time the breathing and the combustion.

    However, the heavy friction loss of the 4-stroke Diesel engine makes the OPOC engine more fuel-efficient in comparison.
    The lower friction enables leaner fuel-air-ratios with all the known benefits (like the lower combustion temperature which is advantageous for the emissions control).
    The significantly higher power density of the OPOC engine (as well as of the OPRE and PatOP engines at ) combined with significantly lower friction, enables the use of higher capacity engines running at partial loads to optimize the fuel efficiency and to minimize the emissions.

    Manousos Pattakos

  • avatar

    An advantage of the OPOC engine over the known conventional engines (the rotary Wankel included) is the balance of the combustion and inertia loads on the crankshaft.
    This allows a crankshaft of only two – actually unloaded – main bearings. The absence of loads on the two main bearings allows lightweight crankcase and reduces the flexing of the engine casing under the combustion and inertia forces, torques and moments, resulting in lower friction, wear and fatigue.
    What the OPOC engine fails to eliminate is the loading of the liner (the cylinder bore): the inner pistons of the OPOC thrust heavily on the liner under the combustion and inertia forces causing the liner (block) flexing with similar results.
    The PatOP and the OPRE engines keep the liner free of any touch with the piston (only the rings touch the liner). The liner remains, during operation, as cylindrical as it is when the engine is not working.
    The outer pistons of the OPOC (the ones with the long pulling connecting rods) are much better than the inner, in this respect; and they move slower than the inner pistons during the injection and the combustion, helping the combustion to complete in time.
    In comparison, both pistons of the PatOP engine move, around the combustion dead center, slower than the outer-slow-pistons of the OPOC engine.
    Keeping the pistons away from the liner (liner untouched by the pistons, not possible for the conventional and the OPOC) is significant: for instance, it allows the replacement of the cylindrical liner with a tapered (towards the two ends) one, reducing the friction and the wear of the rings, reducing also the quantity of oil that finds the way to escape from the exhaust ports.
    Manousos Pattakos   




    • 0 avatar

      I was under the impression that the rings were the only contact surface on the only engine I have personally installed them in- a 283 Chevrolet I rebuilt for auto shop in my senior year of high school. You state that this is not possible for conventional and OPOC. Please explain in terms a business major can understand.

  • avatar

    The liner of the OPOC engine takes heavy thrust loads by the pistons, especially by the inside ones with the small connecting rods.
    Think the moment the crankshaft is at 30 degrees after the Top Dead Center (TDC).
    Inside the cylinder of the OPOC the pressure is high, say 70Kp/cm2 , and generates a strong force of  5.500 Kp ( = 5cm*5cm*pi*70Kp/cm2 ) on each crown of the two opposed pistons (of 100mm diameter each, according EcoMotors). Each piston is linked, through its connecting rod, to a crankpin of the crankshaft. But the connecting rod is not parallel to the cylinder axis. Due to the inclination (some 8 degrees when the crankshaft is at 30 degrees after TDC) of the connecting rod relative to the cylinder axis, the piston receives by the connecting rod a force of 5.500 Kp along the cylinder axis, and another force of 770 Kp normal to the cylinder axis. The only one who can react to the 770 Kp force, is the cylinder liner. I.e. the piston, with its skirt, abuts / thrusts on the cylinder liner and applies on it 770 Kp force.
    In order to take this thrust force without excessive friction and wear, an oil film is interposed between the cylinder and the piston skirt in the four stroke engines.
    But in the case of the OPOC engine, and especially in the case of the inner exhaust piston, the lubrication is a difficult issue / compromise. Think of the oil film between the piston skirt of the inner exhaust piston and the cylinder with the hot exhaust ports. Less oil means friction and wear, more oil means oil consumption and oil fumes at exhaust. Things are somewhat better for the external pistons because the long connecting rods generate weaker thrust loads.
    In comparison take the OPRE and the PatOP opposed piston engines wherein the thrust loads are taken away form the hot cylinder wall and away from the port openings. Like in the giant cross-head diesel engines, because the OPRE and the PatOP are cross-head engines, yet short ones (with 64+64=128mm stroke and 80mm bore, the PatOP engine is only 500mm long).
    Besides the combustion thrust forces, on the cylinder liner there are also applied the inertia thrust forces caused by the inertia of the moving parts of the engine. I think the balance program at will help you understand deeper the forces applied on the various parts of the reciprocating engine.
    Manousos Pattakos

  • avatar

    A different approach on a free piston opposed engine is this toroidal one

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