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Achates: LD opposed-piston, 2-stroke diesel can meet 2025 final CAFE, Tier 3 standards for full-size pickup; 30% better FE than Cummins ATLAS

Brake specific fuel consumption map for the LD Achates opposed-piston engine. The map is extremely flat, with the engine’s high efficiency extending to low loads. Source: Achates Power. Click to enlarge.

Achates Power, Inc., the developer of a family of two-stroke compression-ignition opposed-piston engines (earlier post), has performed an in-depth study on the performance and emissions of light-duty version of its engine packaged for a full-size pickup truck.

The results of the exercise, reported by Fabien Redon, Achates VP of Technology Development at the recent SAE 2014 High Efficiency IC Engine Symposium and the SAE 2014 World Congress, show that the Achates Power two-stroke opposed piston engine could meet and exceed—with no hybridization—the final 2025 light-truck CAFE fuel economy regulation for a full-size 5,500 lb pick-up truck and has the potential to achieve the engine-out emissions targets to meet the fully phased in LEV III/ Tier 3 emissions with the appropriate aftertreatment. Furthermore, the study shows the potential for a 30% improvement in fuel economy over the equivalent performance Cummins ATLAS Tier 2 Bin 2 engine (earlier post) as well as a significant improvement in NOx and PM (42-74%, depending upon drive cycle and pollutant).

Background. Founded in 2004, Achates Power is designing and developing engines based on a two-stroke, opposed-piston, compression-ignition technology. The company has demonstrated the engine and delivered validated performance results based on more than 5,000 test hours on several engine generations, notes CEO Dave Johnson.

The company, which recently raised $35.2 million in Series C financing (earlier post) is partnering with AVL to develop the Next-Generation Combat Engine for the US Army Tank Automotive Research, Development and Engineering Center (TARDEC) (earlier post).

In 2011, studies found that the Achates two-stroke opposed-piston engine could support an indicated thermal efficiency of up to 53% (earlier post).

Opposed-piston, two-stroke engines have been around for more than 100 years; initially produced for their manufacturability and high power density, they offer several fundamental advantages compared to conventional four-stroke engines. As embodied in the Achates Power engine, these include:

  • Reduced heat losses. With two pistons facing each other in the same cylinder, the opposed-piston (OP) engine offers the opportunity to combine the stroke of both pistons to increase the effective stroke-to-bore ratio of the cylinder working volume. This, in turn, has a direct mathematical relationship to the area-to-volume ratio of the combustion space.

    Heat transfer is proportional to the combustion chamber surface area-to-volume ratio; the smaller the ratio, the better. One of the main reasons larger displacement engines are more efficient than smaller ones is the reduction in area-to-volume ratio.

    At equivalent displacement, the OP engine has more than a 30% lower area-to-volume ratio. Looked at another way, the OP engine surface area-to-volume ratio is equivalent to that of a 4-stroke engine of more than twice the displacement. E.g., the area-to-volume ratio of a 6-liter OP engine is equivalent to that of a 15-liter conventional diesel.

  • Lean combustion. The larger cylinder volume per fuel injected leads to leaner combustion at the same boost level, which increases the ratio of specific heat. Increasing the ratio of specific heat increases the pressure rise during combustion and increases the work extraction per unit of volume expansion. Leaner combustion also leads to lower soot, CO and HC emissions.

  • Faster and earlier combustion at same pressure rise rate. The larger volume also enables shorter combustion duration while preserving the maximum pressure rise rate. Faster combustion improves thermal efficiency.

  • Achates2
    Schematic of the combustion system with plumes from two side-mounted injectors. Source: Achates Power. Click to enlarge.

    Combustion system. The combustion system is defined by the coming together of the two identical pistons per cylinder to form an elongated ellipsoidal volume; the injectors are located at the end of the long axis.

    The proprietary combustion system design provides high turbulence, mixing and air utilization with both swirl and tumble charge motion. The shape of the chamber results in air entrainment into the spray plumes form two sides; further, the engine offers improved control at lower fuel flow rates due to the use of two smaller injectors instead of a single higher flow rate.

  • Air system. Two-stroke engines need to maintain an appropriate pressure difference between the intake and exhaust ports to scavenge exhaust out of the cylinder after combustion and push in fresh air. For automotive applications, with their need for transient changes in speed and load, external means of air pumping are required, Redon noted. Achates prefers a system combing supercharger and turbocharger.

    Among the benefits of such a configuration is an increased capability to drive EGR with a lower pumping penalty compared to a conventional, turbocharged four-stroke engine. Further, the ability to cool both air and EGR together reduces fouling of the cooler.

There are also a number of challenges that two-stroke opposed pistons have faced, including piston thermal management; cylinder thermal management; wrist pin oil film replacement; and oil consumption and durability. Achates has addressed these as part of its IP portfolio of 1,800 claims in 46 issued and 87 pending patents.

The study. For the study, Achates Power modeled a 2.25-liter, 3-cylinder (i.e., 6-piston) OP2S engine producing 200 hp (150 kW) and compared it with the Cummins ATLAS 2.8-liter, 4-cylinder engine.

Engine specifications
  Achates Power OP Cummins ATLAS
Cylinders Inline 3 OP Inline 4
Pistons 6 4
Injectors 6 4
Swept volume (L) 2.25 2.8
Bore (mm) 75.75 98
Stroke (mm) 166.65 100
Stroke/bore ratio 2.2 1.02
Nominal power (kW@rpm) 150@3600 156
Max torque (N·m@rpm) 500@1600-1200 522

The OPS2 engine has two crankshafts, phased with respect to each other. The phasing is adjustable for different speeds and loads; the variable crank phasing mechanism can also provide variable compression and expansion ratios, Redon noted, which can be useful for good fuel economy at low speed and loads, while still requiring high power density.

Achates Power measured more than 30 points on its single-cylinder research engine and used the, as inputs for a multi-cylinder GT Power 1D model. Balancing the trade-offs of emissions, combustion noise and maximum rate of pressure rise, temperatures and efficiency were all considered in the optimization process.

Cummins shared the 10-mode points that are being used to determine the predicted cycle fuel economy and emissions on their ATLAS project. Achates ran four of the more heavily weighted of the 10 points on the single-cylinder engine to generate combustion data as input for the multi-cylinder model. The other six points used combustion profiles from the 30 points, which were similar in speed and load.

Results of the study for the LA-4 cycle and the highway fuel economy cycle were:

LA-4 Cycle
  Fuel economy
Cummins ATLAS 26.7 0.82 0.13
Achates 34.1 0.47 0.03 0.12 (THC)
% Improvement 28% 42% 74%

HFET Cycle
  Fuel economy
Cummins ATLAS 34.4 0.94 0.09 0.10 (NMHC)
Achates 45.7 0.34 0.04 0.12 (THC)
% Improvement 33% 63% 55% -16%

The Achates Power opposed piston engine showed a flat brake thermal efficiency map. Source: Achates Power. Click to enlarge.

Achates Power attributed the sizeable fuel economy advantage of more than 30% shown in the exercise to several reasons:

  • The engine operating range for these operating cycles is focused on low speed and low load. The ability of the opposed piston engine to manage pumping losses and maintain high indicated efficiencies at low loads leads to an advantage.

  • Engine-out NOx requirements for light-duty applications are much lower than for heavy-duty applications. The charging system flexibility of the Achates Power engine enables high levels of EGR with minimal pumping loss. Thus, at low load operation, EGR can be used more extensively, and lowers the pumping requirements. In other words, the lower the engine-out requirements, the greater the advantage for the opposed piston engine.

The engine concept also exhibited excellent vibration characteristics, and can be packaged in a typical full-size, light duty truck.


  • Redon, F., Kalebjian, C., Kessler, J., Rakovec, N. et al. (2014) “Meeting Stringent 2025 Emissions and Fuel Efficiency Regulations with an Opposed-Piston, Light-Duty Diesel Engine,” SAE Technical Paper 2014-01-1187 doi: 10.4271/2014-01-1187



30% consumption reduction on a diesel engine is quite an achievement ...


There is something strange with this comparison... Maximum BTE is 44%, good for an engine of this size but not exceptional. For sure, efficiency at low load looks good but I would like to compare to some engines I have in my database before assessing its “true” potential. A relative improvement of 30% simply sounds a little bit too good to be true.


I started promoting this ten years ago. Gave it to all the auto builders. About time somebody started building one of these.

Some simple add-ons will give even more hp and fuel mileage. (Turbocharger)


Would this engine have more moving parts than its normal diesel counterpart? Wouldn't this increase the expense of the engine? If so, by how much?

I enjoy the reading of its interesting specs, but will it really fly in the real world?


30% consumption reduction on a diesel engine is very easy; just do your own an in-depth study, type “30” hit the % key and voila.

There is something strange here all right; they are some two years behind, and poised to continue the successes of Scuderi (

But will it really fly in the real world? Ask Junkers.


More parts? Well, you need one additional crankshaft but there are no cylinder heads or valves. Supercharging is more complex, since you cannot rely solely on turbocharging. I cannot see any apparent drawbacks when it comes to engine parts and auxiliaries but I think packaging might be an obstacle in many vehicles. Not necessary because this engine is bulkier but because the layout of vehicle chassis tend to be optimized for conventional engines, where this particular one might not fit, simply due to a different form factor.

This type of engine used to fly in the past. One example, were the famous Junkers Jumo engines. I would say that these engines were totally superior to any gasoline aviation engine before and around WWII.

It is the 30% improvement in FE that I have some difficulties to accept just like that.


Peak BTE (44%) is similar to the latest VW TDi engines. Being able to extract 30% more FE than the competition sounds quite optimistic!


Don't forget there's always a power stroke in a two cylinder, two cycle.

Subaru has had no problem putting opposed engines in it's cars.


You could have opposing with separate combustion chambers and a common crank. The vibration cancels and they use a common super charger. 500 cc might make a good range extender with 50+ HP at lower RPM.

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