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.