Opposed-piston engines (OPEs) have been around a long time—more than a century to be exact. First manufactured in 1890, these engines continue to be used in ground, marine and aviation applications worldwide. Unlike traditional four-stroke engines, OPEs combine two pistons per cylinder, working in opposite, reciprocating motion. This eliminates the cylinder head and valvetrain—considered among the most complex and costly components in conventional engines and the primary contributors to heat and friction losses. Instead of employing poppet valves for the gas exchange, OPEs are piston-ported with intake and exhaust ports located at either end of the cylinder enabling efficient uniflow air scavenging.
The porting arrangement of the OPE architecture allows for the two-stroke combustion cycle, which produces twice as many power strokes per crankshaft revolution as the four-stroke engine—resulting in smaller displacement engines for similar performance and lower in-cylinder pressure to reduce emissions. Combined with a direct injection fuel system, OPEs represent an ideal platform for a compression ignition engine.
One of the most notable OPEs in history was the Junkers Jumo engine, which was developed by Professor Hugo Junkers for use in German civil and military airplanes manufactured between 1930 and 1945. A prominent fixture in World War II, these Junkers 205 and 207 aviation engines were record-breaking—both in terms of fuel efficiency and their ability to power planes to altitudes of 20,000 feet (which, at that time, was unheard of).
Other well-known OPEs include the:
- Doxford (1920-1990), used in a wide range of ships
- Kharkov 6TD (1932-current), used in Russian tanks
- Fairbanks Morse 38D81/8 (1934-current), used in U.S. submarines, small marine freighters and trains
- Rootes TS3 engine (1954-1972), used in the U.K. Commer truck
- Napier Deltic engine (1954-current), used in high-speed trains and naval fast patrol boats
- Rolls Royce K60 (1955-current), used in military applications
- Leyland L60 (1960s-1995), used in the U.K.-produced Chieftain Battle Tank
Despite their rich history and compelling fuel efficiency and cost advantages, further development of OPEs for on-road car and truck applications was stunted by the onset of emissions requirements. In 1970, the U.S. enacted the Clean Air Act, which set federal emissions standards for all motor vehicles. The typical OPE available at that time featured comparatively high NOx and soot due to inferior fuel injection systems and unoptimized combustion chamber geometry. In addition, a lack of oil control inherent to piston-ported, two-stroke engines resulted in high hydrocarbon emissions—making it impossible for OPEs to compete with their four-stroke cousins.
That is, until now.
With advancements in technology and innovative engineering methods, Achates Power has created a more fuel-efficient opposed-piston, two-stroke engine that meets today’s stringent emissions standards. By leveraging a broad suite of tools—such as computational fluid dynamics, laser Doppler anemometry, oil sulfur tracing and structural dynamics simulations, to name just a few—we have been able to revive the OPE architecture as a viable alternative for today’s transportation needs.
You can find more detailed information about opposed-piston engines in the book Opposed Piston Engines – Evolution, Use and Future Applications by Martin Flint and Jean-Pierre Pirault.
Michael, thanks for recommending the Pirault/Flint book, I have just found it under the Christmas tree 🙂 (Merry Christmas to everyone!)
The book is an impressive summary of the opposed piston concept and the amount of research and personal involvement is obvious. Tons of quantitative data is especially helpful.
The impression is that considering the simplicity and the brake thermal efficiency of your motor the air pollution may be the single most important factor to work on.
In Mr. Callahan’s video presentation it is demonstrated that Achates Power achieved 0.1 % oil loss relative to fuel consumption. Because this value is among the better even for a four stroke diesel it would be interesting to learn more about the way you prevent the oil from going straight out of the ports.
Well, somebody demonstrated good taste by putting the Pirault/Flint book under your Christmas tree. . .I hope you enjoy the read! Regarding your question on oil consumption, it is indeed very important to prevent free oil from reaching the exhaust ports. The key to success is a thorough and detailed understanding of the oil transport mechanism. During the course of our development, we found that if we precisely controlled the amount of oil applied to the critical interfaces, we would end up with extremely low levels of oil consumption.
Nice website concerning information on diesel engines, in special the OPPOSED PISTON ENGINES
(Retired Chief Engineer Dutch Merchant Navy)
Diesel instructor at Dorchester County Career and Technology Center , Dorchester, SC. 29437
I am always searching for new information about diesel technology i can pass on to my students, and i have to say this was long over due. I have always believed the cylinder head/valve train was not needed and you guys proved it.
thanks and keep up the great work, any information you would like to share with a technology school diesel class would be awesome.
Please take a look at the other content we have posted on here, and on our Achates Power YouTube channel.
I still don’t see how they are going to keep lube oil out of the combustion chamber. There are advantages to a compression ignition gasoline engine, butting don’t see them out-weighing the disadvantages. It will be interesting to see the reliability and durability of this engine. It will also be interesting to see if the engine can deliver the power and torque necessary for a working truck.
[…] Throughout the 1900s, US submarines, WWII fighter planes, and even USSR tanks utilized variants of t…. Today, Achates Power of San Diego has used the principles of those designs to develop a more fuel-efficient engine for use in modern commercial trucking. In partnership with Achates Power, the Tyson fleet will be testing the engine in a company truck that serves customers in California and will share key findings on its performance. With the goal of enhancing overall engine efficiency, Achates Power has achieved up to a 90% reduction in nitrous oxide emissions as well as a 15% savings on fuel. This project, administrated by CalStart of California, is funded with a $7 million grant awarded by the California Air Resources Board (CARB) to develop, install and test two Achates Power Opposed-Piston “OP” Engines into two separate Class 8 trucks. We’re thrilled to have an opportunity to provide and test one of those vehicles. […]
What was the bore and stroke / displacement of the Labourier LD 15 tractor engine, a single-cylinder opposed-piston CLM Model 602?