Why the Opposed Piston Engine Could Be Making a Comeback
This early take on the combustion engine is now making more sense than ever
In 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.
The 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
The 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.
The 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.
The 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.
All 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?