What Is The Opposed Piston Engine and Why Is It Better?
Engines come in a variety of configurations with just about every cylinder arrangement you can think of being experimented with at some point in time and some were more successful than others.
Their names are fairly descriptive, with the most common types being the ‘V’ shape, the ‘Inline’ or ‘Straight’ configuration, and the ‘Boxer’ or ‘Flat’ engine – called Boxer because the pistons resemble boxers throwing punches at each other.
Each has its benefits and drawbacks; the V is the robust all-rounder but is heavy, with a high number of parts. The Inline is more efficient being lighter and simpler, but is not particular rigid which limits its size. The horizontally opposed Flat engine has a performance advantage over the others with good balance and power, but are very wide and use up valuable space needed for other components and so are not widely used.
By quirk or intent, these engines all share a common design feature of having only a single crank shaft with all the pistons radiating from it at various angles.
Presumably this was for a very good reason, but a new type of engine is doing an extremely good job of convincing us this reason may no longer be valid.
An engine that uses two crank shafts at opposite ends of what’s called an Opposed Piston Engine.
New technologies and discoveries have a habit of making us re-evaluate established norms, except in this instance the technology involved isn’t new at all, it’s over 100 years old. Variants of the design found limited use in various applications since its creation in the very late 1800’s - most notably in military aircraft during World War II, after which it fell out of use almost completely for reasons unknown.
The modern and increasingly common inline engine has cut down on the number parts an engine needs, thereby saving weight and increasing reliability by needing only one cylinder head and valve train - without any sacrifice in performance.
The Opposed Piston Engine goes beyond this by having no cylinder heads at all – but then comes back around by needing to have two crank shafts instead. It seems a bit like robbing Peter to pay Paul, except it isn’t because you gain in other areas.
Image Courtesy of Pinnacle Engines
It is unclear what the adherence to a single crankshaft system stems from. It may have been a result of them once being expensive or difficult to produce, or that less moving parts meant for greater reliability – which it generally does.
As technology has progressed the cylinder head and valve train have evolved to be far more complex and expensive. So much so it seems car developers may not be so married to the previous way of doing things anymore. But change for change’s sake is pointless, there has to be a benefit and it seems there are plenty of areas where this is the case:
- It has better thermal efficiency through a higher ratio of the combustion chamber being taken up by the piston head. The removal of the cylinder head means less heat is wasted and more of the energy is transferred to the piston.
- The friction exerted on the piston rings by not having cylinder head bolts distorting the bore is lowered.
- Less friction is generated at higher speeds as the peak operating speed of the engine does not need to be as high.
- The pistons waste less energy having to pump air in and exhaust out as the ports arranged around the cylinder do this instead, with the added benefit of increasing the efficiency of the turbo and supercharger too.
- Less pressure in the cylinders mean the engine doesn’t have to be as strong and dense to withstand the punishment, meaning they can be of lighter weight construction.
- Through lower pressures and temperatures during combustion the quantity of pollutants generated is reduced. In addition, the longer combined stroke length between the two cylinders enables the fuel to burn more fully leading to a further reduction is waste matter generation.
- By injecting fuel horizontally into the cylinder chamber and not down onto the hot piston head, the quenching which cause particulates to form is greatly reduced.
- Reduction in expensive and complex parts needed results in a simpler engine. The additional crankshaft and gearing is more than offset by this. A broader torque curve further reduces part quantities by lowering the number of gears needed.
- Less cooling capacity is required as the engine generates less heat and operates at a lower temperature.
- Lower emissions means the waste management systems don’t have to be as extensive, leading to the downsizing of expensive catalytic converters.
With the current focus there is on emissions, this engine sounds like the combined dreams of the entire car industry made real. There’s even a version of this engine that doesn’t need an ignition system as it is self-combusting – reducing parts and maintenance requirements even further.
With a 30-50% increase in efficiency, lower emissions, with no compromise on performance for a lower cost, we are keen to see how it develops - particularly when it requires minimal retooling to switch production.
In fact at least one major car company has already started the process of building its own OPE, so it may just be finding its way into the next new car you buy. Maybe...