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Achates Power testing shows transient capabilities of opposed piston engine

Initial test results of the transient control and exhaust temperature management capability of Achates Power’s opposed piston (OP) two-stroke diesel engine show that under a typical transient maneuver—25% to 100% load step at low and constant engine speed—the engine can control both NOx and soot with a minimal torque lag. Test results also showed that the air system control flexibility and robust combustion system that Achates Power developed for the OP engine can be used to achieve high exhaust gas temperatures for a diesel engine at idle-like speeds and load, thereby assisting catalyst light-off.

The results follow a paper published earlier this year detailing steady-state testing results that showed the research 4.9L three-cylinder engine was able to achieve 43% brake thermal efficiency at the best point and almost 42% on average over the modes of the SET (Supplemental Emission Test) cycle. The results from this test confirmed the modeling predictions and pointed to a 48% best BTE and 46.6% average over the cycle for a production design of this engine, the Achates team concluded. With the latest paper Achates Power is exploring other critical attributes necessary to deliver a successful engine to the industry.

(The SET is a 13-mode steady-state test that was introduced in the US starting in 2007 to help ensure that heavy-duty engine emissions are controlled during steady-state type driving, such as the operation of a line-haul truck on a freeway.)

Steady-state results. Achates Power developed the multi-cylinder research engine as a test platform with flexible components and systems beyond what would be done in a production-intent version. With the associated trade-offs, the engine’s size and friction were affected negatively, Achates said.

The three-cylinder (six-piston) engine has a bore of 98.4 mm and a stroke of 215.9 mm for a stokre-to-bore ratio of 2.2. Compression ratio is 15:1, nominal power is 205 kW (279 hp) @ 220 rpm, with 1100 N·m (811 lb-ft) of torque at 1200-1600 rpm.

Testing on a dynamometer showed a 12 mode cycle average fuel consumption of 201 g/kWh, which was only 7 g/kWh higher than the best point; these results highlighted the flat nature of the engine’s fuel consumption. Achates projected that a production-level engine would be capable of achieving a 12 mode cycle average fuel consumption of 180 g/kWh.

Multi-cylinder OP research measured BSFC map. Click to enlarge.   Projected 4.9L production level BSFC map. Click to enlarge.

Transient testing. Achates performed transient testing on the research engine with the objective of characterizing the response time of air-handling system which includes airflow control and the soot and NOx emission levels. In the testing, Achates used two different fueling strategies: one with the same ramp-up time as the driver torque request, the other with a slower fuel ramp-up time of 2 seconds to replicate a rudimentary smoke-limiter.

Engine configuration. Two design constraints limit the faster build-up of air in the system: the super-charger drive ratio and opening angle of super-charger recirculation valve (Also called bypass valve). Click to enlarge.

Building up the required amount of air in the system quickly while maintaining desired emission levels is important to engine response for tip-in operations. To characterize the response of Achates’ air-handling system control, the engineers selected and tested two super-charger drive ratios (DR): 3.8 and 5.0.

Results of testing with the 3.8 DR showed that without a smoke limiter, torque response is faster but also results in significant soot-spike due to the lag in airflow build-up. In the smoke-limiter case, both the fuel and airflow command are ramped up slowly, resulting in a lower soot spike. With a higher drive ratio, it is possible to improve the response of air-handling system by quickly meeting the airflow requirements without a concomitant soot spike.

Transient maneuver performance and emission results with different SC Drive Ratios
SC DR Smoke limiter Torque response time (sec) Airflow response time (sec) Soot spike amplitude peak (peak - setting value %) NOx spike amplitude peak (peak - setting value %)
3.8 No 1.3 1.8 91 None
3.8 Yes 2.5 2 34 None
5.0 No 0.7 0.55 50 7
5.0 Yes 1.5 0.1 7 9

The initial results show very good potential for managing transient operation. There are numerous opportunities to improve the performance—response and emission—during transients by using transient modifiers and AFR based smoke limiter. Further improvements in the engine design supporting reduction of air-flow path volume will help to meet air-flow requirements faster.

—Redon et al.

Commercialization. Achates Power, founded in 2004, currently has six announced development projects underway spanning a variety of applications of its opposed-piston engine architecture, from light-to-heavy duty. The company, said Larry Fromm, VP, Business and Strategy Development, also has three proposals under active consideration. Fromm said Achates expects to be in production next year.

The most recent announced contract is a $14-million award from the US Army Tank Automotive Research, Development and Engineering Center (TARDEC). (Earlier post.) The project is to develop an opposed piston, two-stroke Single Cylinder Advanced Combat Engine Technology Demonstrator, part of the Army’s 30-year strategy to modernize tactical and combat vehicles. Achates is partnering with Cummins on this project.

In December 2012, TARDEC had awarded Achates Power and AVL Powertrain Engineering, Inc. a $4.9-million contract for design and construction of the Next-Generation Combat Engine. (Earlier post.)

TARDEC now has also just awarded AVL a separate $17-million project to develop an opposed piston, two-stroke Single Cylinder Advanced Combat Engine Technology Demonstrator. Achates has no direct involvement in this project awarded to its former partner, Fromm said. The two TARDEC awards do indicate the level of interest the Army now has in the potential of the modular, scalable opposed-piston two-stroke engine and its very low heat rejection, high-power density, and increased fuel efficiency.


  • Redon, F., Sharma, A., and Headley, J., (2015) “Multi-Cylinder Opposed Piston Transient and Exhaust Temperature Management Test Results,” SAE Technical Paper 2015-01-1251 doi: 10.4271/2015-01-1251

  • Naik, S., Redon, F., Regner, G., and Koszewnik, J. (2015) “Opposed-Piston 2-Stroke Multi-Cylinder Engine Dynamometer Demonstration,” SAE Technical Paper 2015-26-0038 doi: 10.4271/2015-26-0038


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