More Power, Efficiency and Balance, But Not Mainstream. Opposed Piston Engines: Achates vs Asender
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When combustion occurs inside an engine, it pushes the piston down, and that rotates the crankshaft and creates torque. But obviously combustion isn’t intelligent, it doesn’t know where would it be best to direct its energy. Instead, it spreads and pushes in all directions. But we actually harness that energy only at the piston crown. The rest is largely wasted on heating and trying to separate the cylinder head from the engine. So if you think about it logically the most effective solution to wasting less combustion energy and improving energy efficiency would be to simply get rid of the cylinder head and install one more piston. This creates an opposed piston engine.
Now we are using almost all the combustion energy to move pistons and create torque. Isn’t that wonderful? Yes, it is and in this video I will explain why opposed piston engines are amazing but aren’t mainstream. We will look at the already relatively well-known Achates Power opposed piston engine as well as an alternative design called the Asender engine. We will explore their benefits and drawbacks as well as some very interesting potential applications and recent developments.
So, our first obvious benefit is increased thermal efficiency; we are now harnessing more energy created by combustion. The second benefit is that we don’t have a cylinder head anymore. There’s no valves, cams, springs etc which means less complexity and less friction losses. The cylinder head doesn’t really do any torque-producing work, it just makes it possible for the piston to do that work. As you will see in the opposed piston engine the piston does both the work of the piston and the work of the cylinder head.
The third benefit is that now our engine becomes a two stroke.
In proper two-stroke fashion we have a combustion event every time the pistons reach their respective top dead centers. This means that we have twice as many combustion events during the same time compared to a conventional four-stroke engine resulting in increased power density.
The final benefit is an inherently balanced anatomy. If we observe the three cylinder achates engine we can see that we essentially have two conventional inline three engines mirroring each other. A single conventional inline three engine has an inherent rocking couple in its design which tends to produce somewhat unpleasant vibrations. But when we oppose the two engines the two rocking couples cancel each other out leading to perfect balance. If observe things from the perspective of a single cylinder we can easily conclude that the two pistons cancel each others primary and secondary forces out which means that each cylinder is an inherently balanced unit ultimately resulting in the fact that an opposed piston engine will always be balanced regardless of the number of cylinders.
So it has less moving parts, it’s more efficient and more powerful and inherently well balanced. What more could you ask for?
Well, yes, all of this is true but as you know in engineering there are no free lunches. Engineering is usually a management of compromises. We can’t beat physics, all we can really do is just the right compromises for the right application.
The first problem with an opposed piston engine is the fact that now we need two crankshafts to drive the two opposed piston banks and to have a single torque output we need to gear those two cranks together. It’s still overall less moving parts compared to a conventional four stroke simply because the cylinder head contains a lot of small moving parts, but when it comes to frictional losses we have restored most of them with the introduction of the second crank and gear train.
That being said, an opposed piston engine can be implemented with just one crankshaft and that’s exactly what the assender engine does. It replaces the other crank and gear train with a set of rods and rocker arms. This, in theory, leads to slightly less friction, although the difference is likely negligible. The main benefit is a different packaging option.
In the Achates engine the two cranks and sumps and opposing engine ends create an extremely tall engine with makes packaging the engine into more compact vehicles very difficult. Achates Power has successfully installed and ran a 2.7 liter version of their engine in a Ford F-150 truck, but it’s likely that any vehicle smaller than a full-size pickup truck might be problematic.
Compared to this the assender engine takes away some of the engine height and converts it into width, leading to an overall more balanced package that might be suitable for a wider range of applications.
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