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Study finds Achates two-stroke opposed-piston engine shows indicated thermal efficiency of 53%

Achates Power, the developer a two-stroke, compression-ignition (CI) opposed-piston (OP) engine (earlier post), is presenting performance and emissions results of the Achates engine used in a medium-duty application, as well as the results of a detailed thermodynamic analysis comparing the closed-cycle thermal efficiences of three engine configurations: a baseline 6-cylinder, 4-stroke engine; a hypothetical 3-cylinder opposed-piston 4-stroke engine; and a three-cylinder opposed-piston two-stroke engine (the Achates engine).

Achates began presenting the results at the SAE 2011 Commercial Vehicle Engineering Congress, along with two SAE papers in Chicago; continued on to Der Arbeitsprozess des Verbrennungsmotors (The Working Process of the Internal Combustion Engine) in Graz; and then moved to the 2011 Directions in Engine-Efficiency and Emissions Research Conference in Detroit.

Thermodynamic analysis. Working with David Foster from the University of Wisconsin, the Achates Power team found in the thermodynamic review that combining the opposed-piston architecture with the two-stroke cycle increased the closed-cycle thermal efficiency through a combination of three effects:

  • Reduced heat transfer because the opposed-piston architecture creates a more favorable combustion chamber area/volume ratio;

  • Increased ratio of specific heats (γ) because of leaner operating conditions made possible by the two stroke cycle; and

  • Decreased combustion duration achievable at the fixed maximum pressure rise rate because of the lower energy release density of the two-stroke engine.

In a closed-cycle simulation, they found that the 4-stroke (4S) showed an indicated thermal efficiency (ηfuel) of 47.5%; the opposed-piston 4-stroke (OP4S), 50.1%; and the opposed-piston 2-stroke (OP2S, the Achates engine), 53%.

They selected five mode points to assess the OP2S efficiency advantage over a simulated operating map, and returned with a weighted average indicated thermal efficiency over the five points of 52.6% for the OP2S, compared to 47.7% for the 4S.

Averaged over a simple engine operating map, the opposed-piston two-stroke had 10.4% lower indicated-specific fuel consumption than the four-stroke engine and was accomplished with significantly lower peak cylinder pressures and temperatures.

Medium-duty application. The Achates team compared the performance of an inline 6-cylinder 4-stroke conventional diesel (Ford 6.7L V8) with an inline 3-cylinder 2-stroke opposed piston engine.

They found that the Achates engine demonstrated an 19% fuel consumption improvement over the conventional engine at similar engine-out emissions levels (EP 2010). Furthermore, oil consumption was measured to be less than 0.1% of fuel over the majority of the operating range. (Historic OP engines, and two-strokes in general, suffer from high oil consumption, on the order of ~1% or more.)

The Achates team notes that the two-stroke cycle and its double firing frequency provides the opportunity of decreasing brake mean effective pressure (BMEP) levels and increasing power density compared to four-stroke engines of equivalent power output. The lower BMEP levels can be accomplished with lower peak cylinder temperatures, which lead to lower mechanical stress on engine components, which can therefore be designed to be of lighter weight.

Lower cylinder temperature result in decreased NOx formation during combustion, lowering the requirements for aftertreatment. The increased power density leads directly to smaller engine package size and weight, the Achates team suggested, both beneficial to decreasing fuel consumption and manufacturing costs.


  • Randy E. Herold, Michael H. Wahl, Gerhard Regner, James U. Lemke, David E. Foster (2011) Thermodynamic Benefits of Opposed-Piston Two-Stroke Engines (SAE 2011-01-2216)

  • Gerhard Regner, Randy E. Herold, Michael H. Wahl, Eric Dion, Fabien Redon, David Johnson, Brian J. Callahan, Shauna McIntyre (2011) The Achates Power Opposed-Piston Two-Stroke Engine: Performance and Emissions Results in a Medium-Duty Application (SAE 2011-01-2221)



I wonder what compression they have to use to get beyond 50% thermal efficiency because without any thermal losses you need a CR of 20 to get 53% thermal efficiency. So with some thermal losses you need to be much higher than CR = 20 or you have to use a miller or atkison cycle which is not possible on a 2 stroke. Also these efficiencies requires to be a quasi 100% volumetric efficiency which is also difficult in a 2 stroke even with an uniflow design like the opposed piston architecture.

But anyway if they can achieve that, that is really great indeed, Achates is a serious company so we can give some credit to their claim


Why not? One piston controls exhaust port, the second one intake port. Just a matter of phase angle control between those two.


Who can argue against 57% thermal efficiency? However, I wouldn't want people laughing at me when I put a quart of oil in the gas tank every time I fill it up. The clouds of blue smoke are a little unpleasant for my neighbors having iced tea on the patio.


Not all two-cycle engines have to mix oil in the fuel.

This sounds a lot like what I was suggesting here ten years ago.


DLC coatings on the piston ring and cylinder wall should help to reduce the amount of oil needed for combustion while reducing frictional losses.


I believe their simulations are based on using diesel or biodiesel as their fuel not gasoline.


The higher specific lube consumption (and its side effects) as well as the lower scuffing resistance is the Achilles' heel of the 2-stroke.
What about a 2-stroke through-scavenged with 4-stroke specific lube consumption and 4-stroke scuffing resistance?
Take a look at the PatPortLess engine at
and at the PatMar engine at
Manousos Pattakos

Henry Gibson

Junkers Jumo and Deltic engines were opposed piston as was the Fairbanks-Morse mostly used for submarines. The most unusual ones were the Pescara and the Commer Knocker. ..HG..

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