Wednesday marks the start of GVSETS, a three-day event focused on the latest technologies for military ground vehicles. Introducing breakthrough innovations designed to aid and protect our troops, this year’s symposium features a presentation from Achates Power. The topic: how can the Achates Power opposed-piston engine improve the operational efficiency of military vehicles?
While opposed-piston engines have been used in aviation and combat vehicle applications and continue to be used in maritime applications, there are several factors that distinguish the Achates Power engine from previous opposed-piston engines.
For one, our engine takes full advantage of the thermal efficiency benefits of the opposed-piston architecture. With two pistons in each cylinder, and no cylinder heads, the surface area-to-volume ratio is reduced as compared to conventional engines. This reduces energy lost via heat transfer and, in turn, increases thermal efficiency. Also, since the stroke is split between two pistons, high stroke-to-bore ratios are possible without requiring high mean piston speeds.
Secondly, the Achates Power engine benefits from the fuel efficiency advantages of the two-stroke cycle. Two-stroke engines have higher frequency of combustion but inject less fuel in each combustion event. This produces added thermal efficiency benefits:

  • A leaner operating condition at the same boost pressure maintains a higher ratio of specific heats during combustion.
  • A reduced energy release density at the same power level allows for a shorter combustion duration without exceeding a maximum rate of pressure rise constraints.

And, since each cylinder fires at every revolution of the crankshaft in a two-stroke engine, engine displacement can be reduced for the same power as a four-stroke engine without exceeding peak cylinder pressures or other design constraints.
While those advantages are inherent to our engine’s architecture, the Achates Power patented combustion system also plays a critical role in overall engine efficiency. Using computational fluid dynamic studies correlated with experimental results from our in-house dynamometers, we’ve created a combustion system with large stoichiometric isosurfaces. These surfaces, combined with our proprietary piston crown design, integrate swirl with tumble—producing excellent mixing, air utilization and charge motion for rapid heat release and short burn duration.
We’ve also optimized our engine’s port design and port timing, providing optimal blowdown, uniflow scavenging, supercharging and swirl characteristics. In addition, our proprietary fuel nozzle design, developed based on work from our in-house fuel lab, provides interdigitated fuel plumes with the appropriate flow rates and penetration. This produces very fast burn rates, which contribute to both power and fuel efficiency.
Stroke-to-bore ratio is another unique efficiency advantage of the Achates Power opposed-piston engine. In-cylinder heat transfer decreases as the stroke-to-bore ratio increases due to a decreased combustion chamber area-to-volume ratio. This decreased heat transfer results in higher indicated thermal efficiency and reduced heat rejection to coolant. In addition, scavenging efficiency improves as the stroke-to-bore ratio is increased. The Achates Power engine is being designed with a stroke-to-bore ratio in the range of 2.2 to 2.6, which allows us to create a highly efficient internal combustion engine while still having mean piston speeds comparable to engines currently available in medium- and heavy-duty applications.
Finally, the number of cylinders used in our engine is optimal for both performance and efficiency. With a three-cylinder design, the scavenging events are aligned in a way that they have minimal interference with each other and still keep enough mass flow going over the cycle to provide adequate energy to the turbocharger so that it operates most efficiently to compress the intake air. The same is not true in two- or four-cylinder configurations, which cause the turbocharger to either lose energy over the cycle or to produce negative cross-charging between cylinders.
After more than 3,000 hours of dynamometer testing, Achates Power has demonstrated a 21% fuel efficiency improvement with our engine as compared to a leading medium-duty diesel engine. In addition, having high specific power and lower fuel consumption and heat rejection to coolant, our engine has also met the combat vehicle requirements outlined by Raffa, Schwarz and Tasdemir in their 2005 SAE paper, necessitating the “complete propulsion system to be power dense”. The end result: a more efficient engine for military ground vehicles.

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