Diesel Engine Blog – Achates Power

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A Historical Look at Opposed-Piston Engines

Dr. Michael Wahl, Independent Consultantby Dr. Michael Wahl
Independent Consultant


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).


Junkers Jumo 205

Developed by Professor Hugo Junkers, the Junkers Jumo 205 aviation engine was used in German civil and military planes manufactured in the 1930s-40s.

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.

  • Sventin says:

    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.

    December 26, 2011 at 4:23 pm
    • Michael Wahl says:

      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.

      January 10, 2012 at 11:41 am
  • Alfons Verheijden says:

    Nice website concerning information on diesel engines, in special the OPPOSED PISTON ENGINES
    (Retired Chief Engineer Dutch Merchant Navy)

    January 16, 2013 at 2:03 am

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