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 (η_fuel) 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 NO_x 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.

Resources

• 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)