- Credits: the Tech Explorist

W​hy the Opposed Piston Engine Could Be Making a Comeback

T​his early take on the combustion engine is now making more sense than ever

3w ago
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I​n a world where global warming has become one of the most influential factors in the automotive industry, the Internal Combustion Engine in particular has landed itself on thin ice (no pun intended). It is unlikely that it will stay in the mainstream for much longer with more economically viable and greener energy sources being created left, right and centre, but certain developments have given it at least some chance of prolonging its life. One that has recently been in the spotlight is Synthetic E-Fuels, a renewable fuel that can directly replace petrol, diesel and kerosene. That still has its pitfalls though, cheif among them being that it is ridiculously energy innefficient to produce and use. There are many others excluding that but among the most promising that has come out of the ICE's last chance saloon is the concept of the opposed piston engine.

T​he Opposed Piston Engine (OPE) was first concieved in 1882 with the Atkinson Differential Engine. It had a power stroke on every rotation of the crankshaft. Since then they have been predominantly used in large scale appplications such as ships, military tanks and factories.

An animation of the Atkinson Diffferential Engine. Credits; Michael Frey

An animation of the Atkinson Diffferential Engine. Credits; Michael Frey

T​he main principle behind the opposed piston engine is that instead of there being a separate cylinder for every piston, there are two pistons that share a single cylinder. As a result of this, the need for a cylinder head and valvetrain is eliminated, leading to a 60% decrease in parts when compared to a standard combustion engine in the most modern variants. There is a reduction in weight, the height of the engine, heat loss, and friction loss. Furthermore, there is a uniflow scavenged movement of gas through the combustion chamber, which is superior to the conventional crossflow scavenged engine which has drawbacks such as a possibility of intermixing air and gases, lower scavenge efficiency than a uniflow scavenged engine, a high temperature gradient from the exhaust to scavenge ports, a distortion to the pistons and liners and uneven piston ring wear due to ports and a higher exhaust back pressure with deposits. Despite burning fuel, it’s a gasoline-compression-ignition engine (GCI), meaning it ignites fuel and air by squeezing it hard enough to generate heat, and that heat, along with residual hot exhaust gases deliberately left trapped in the cylinders, ignites the fuel. Fuel is injected not on top of the two pistons, but in between the two.

T​he most common design makes use of a dual crankshaft setup, both of which are geared together in opposite or identical directions. This provides an issue as gearing two crankshafts together brings complexity and weight that could be avoided with a conventional single crankshaft engine.

T​he two pistons in each bore are referred to as the intake and exhaust pistons respectively, depending on their function in this scenario. This is what provides superior scavenging, as gas flow through the cylinder is axial instead of radial. This also makes the design of the piston crowns much more simple.

A​ll of this put together means that overall the opposed piston engine is not only cheaper and easier to manafacture than normal Combustion Engines, but it also produces less emissions thanks to being a GCI. They are more efficient than current ICE engines too, as Achates Power, a company developing OPEs, have demonstrated a 21% cycle average and 15% best-point advantage against the leading medium-duty diesel engines. All of these are key factors in deciding a "fuel of the future" so could we have a new contender for that title in it?

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Comments (28)

  • The best version of a opposed cylinder engine is in the class 55 Deltic Diesel locomotives we had in Europe. Just watch footage of one accelerating and you will understand why. The engine was essentially three large V6 diesel two stroke engines opposed to each other coupled with a turbocharger. The engine design was originally built for boats but was powerful enough to be used in these locomotives and haul express services and freight.

      23 days ago
    • Wow that looks awesome!

        23 days ago
    • Sounds awesome as well. Find some YouTube clips of the diesel locomotive running and you will agree it sounds beautiful for a diesel engine.

        23 days ago
  • It lends itself to various options, but will companies spend development cash on an ice?

      21 days ago
    • That's what I think will hold it back. People are so invested in EV development, the ICE will probably be overlooked.

        21 days ago
  • google it and have a look at how many hybrids are using the engine!

      21 days ago
  • It's a bit like Nero, fiddling as Rome burned. It's too late, so doesn't matter anymore. As an apprentice, back when we still lived in caves, I would try to find ways of creating weird kinds of 'strokes' for engines. I dismissed 3 and 5 stroke engines as adding more frictional losses than was gained in combustion efficiency, a principle that continued up the numerical count of strokes, leaving me with the one-stroke engine. Yes, it IS possible, and no, I'm not telling you how. Suffice to say that it doesn't matter anymore. Electricity has taken over. It's a done deal and we're never going back. There are several new and interesting engine configurations and technologies that are academically fascinating, but are of no importance at all beyond the intellectual interest in them.

      21 days ago
    • I'm not entirely sure that they'll be dead in larger scale use such as ships and planes, but I agree that EVs have got the automotive sector pretty much nailed. Also thank you for giving me something new to ponder over with the one stroke...

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        21 days ago
  • This thing looks sick

      21 days ago
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