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Opposed-Piston Engine

achatespower.com

51 pointsacd22 comments

See also the Junkers Jumo 2014: http://en.wikipedia.org/wiki/Junkers_Jumo_204

And its descendant, the Napier Deltic engine: http://en.wikipedia.org/wiki/Napier_Deltic

And in particular, the animation of the Deltic layout (three cylinders, three crankshafts (one of them contra-rotating), six(!) pistons): http://en.wikipedia.org/wiki/Napier_Deltic#mediaviewer/File:...

(Opposed piston engines aren't exactly new.)

Not new but first time I see them (even though it's so obvious I feel stupid). So many engines geometries and configuration, it's beautiful. Thanks

ps: old reddit thread for other designs http://www.reddit.com/r/technology/comments/82gqi/six_piston...

The reason they aren't more common is because many people tried, but without high precision parts and extensive dynamic analysis it is very difficult to make them work reliably. Have a look at the Deltic, it was very powerful for its size and weight, but was a maintenance nightmare; in both of its major applications, high speed naval patrol boats and high speed trains. The major breakthrough with the dynamics on the Deltic was the one contra rotating crank.

The animation shows a turbocharger but it also shows that the engine operates on a 2 stroke cycle. Conventional wisdom would say that the turbocharger would be a waste since both intake and exhaust ports are open at the same time, preventing the compression of the mixture, while at the same time potentially wasting some of it as it is forced out of the exhaust port. I'd speculate that the turbocharger is small enough and used mainly to introduce the mixture into the chamber with enough velocity to make it swirl, while at the same time improving the removal of exhaust gases.

On large two-stroke engines a blower/turbocharger is often used to not only introduce additional fresh air for the next combustion cycle but also to more rapidly/completely purge the combustion gasses out the exhaust ports (this is called scavenging).

It's not just air, its usually compressed air above atmospheric sea level (bar+). This opposed-piston architecture appears more complicated. Adding an additional crankshaft isn't a small feat. Removing valves that rely on the rotation of a crankshaft is a small win. Add bar+ of pressure into a combustion motor like this and watch it pop. Too many moving pieces!

But saying this, what is the drive to keep on overcomplicating things like traditional combustion motors? The Wankel engine was simple, small & powerful (when a turbo hanged off it). The industry killed it to replace it with a more complicated piston engine. And now this more complicated architecture proposal.

I would love to see engineering innovation focusing on simplification like that which exists in electric motors. There is still so much more that can be done with magnets & coils.

Emission controls have become a lot stricter. In heavier engines, they are the major technology driver now. Wankels have a crescent shaped combustion space. That is bad for emissions as the far away thin corners dont burn so well.

I don't have a reference on hand, but everyone I've spoken to who has worked close to engines (in the automotive, recreative and commercial transport industries) has said to me that compliance to emissions is now the leading design consideration. So, not only heavier engines, unless by "lighter" engines you are only referring to lawnmowers and such.

Yap I understand why Wankel didn't meet emission standards. But neither would have the standard piston architecture if it weren't for innovation i.e. extension of it's complexity.

As a ex-fan of combustion motors for many years, and having built a race car, I no longer see investment in combustion motors as sound. Electric motors are far simpler and smaller and are starting to yield similar results to combustion motors. That's where innovation should be heading.

As a past owner (many years ago) of a Wankel-engine powered car I was entertained at the mention of "Wankel". Emissions were the least of its problems, reliability being far less than desirable, as in barely able to keep going at all.

Later on there were more reliable rotary engines, e.g., Mazda. If memory serves, there were emission problems and the engine was eventually discontinued for that reason.

Rotaries were an interesting idea, hypothetically better, but implementation was obviously difficult and ultimately a failure.

In today's world, we need to stop burning fossil fuels and electric-powered vehicles look promising. I'm looking forward to advances that will make electric cars practical.

Of course, burning coal or natural methane to make electricity wouldn't be ideal, though possibly more efficient than the distributed combustion we now use.

Audi was considering a Wankel range extender for plugin hybrids around 2010 but they later moved to conventional three cylinder engines.

I don't know the specific reasons for that.

In addition to what ams6110 said about scavenging, the article talks about how the three crankshafts are phase-offset such that the exhaust valve closes before the intake valve does (this is possible because one piston in a given cylinder pair controls each port). I imagine that this part of the cycle allows for a bit of pressure boost.

«To play the animation, please click the arrow located in the center of the image below.»

That's the most hand-holding approach I've read telling me to play a video.

There's also http://www.ecomotors.com . Their main engine guy, Prof. Hofbauer, was a neighbor in Wolfsburg, he was in charge of VW engines at the time. Brilliant engineer. (No association apart from the neighbor thing).

They claim 100 mpg cars are possible with their engine.

A lot of tanks has opposed engines https://en.wikipedia.org/wiki/Opposed-piston_engine

main reason - "flat" configuration

Can anyone recommend any books on the design of combustion engines? I am looking for a text book that would be approachable to someone without an engineering background although comfortable with undergraduate level mathematics.

A relatively short but complete book I'd recommend would be Ferguson & Kirkpatrick's "Internal Combustion Engine". It was used in an undergrad engines course I did. The classic, big textbook, AFAIK, is John Heywood's (MIT Prof) Internal Combustion Engine Fundamentals.

These books give you a general understanding of the functioning of ICEs. For practical design considerations, I've found "Internal Combustion Engine in Theory and Practice" by Charles F. Taylor interesting. There's a lot of other books on specific aspects of design, look for example at books published by the SAE.

I'm not sure exactly how much one would get from going through a book like Ferguson without basic notions in physics (basic thermodynamics, dynamics, fluid mechanics, materials). I think a motivated individual could certainly supplement as he goes using wikipedia as a starter for things he does not understand. No advanced knowledge of these topics is needed, as I said, just the basics.

This is what I can't wait for. free piston generator engine https://www.youtube.com/watch?v=QUbBqSu9Hdc

I'm no mechanical engineer but doesn't two crank shaft decrease efficiency quite a bit? Also introduces a lot more moving parts.

Crankshaft frictional losses are typically the smallest of the friction losses in a engine, the others being cylinder/ring friction and the valve train (you could also include accessories and pumping). You can play around with the applet at http://www.wiley.com/college/mechs/ferguson356174/apps/frict... [1] to verify this.

You are right that it introduces more parts (weight, costs, reliability), which I guess is one of the reasons we haven't seen its adoption.

any reason to use an exhaust driven turbo? I would expect an electric turbo could offset the need to have multiple cylinders for synchronizing the exhaust pulses

Generally speaking the tradeoff is less steps in energy conversion means better efficiency, and less needed machinery (electric generator, motor, perhaps gearbox of sorts, perhaps battery), meaning less costs, weight and more reliability.

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