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Latest test results for Achates Power two-stroke opposed-piston engine show 20% improvement over four-stroke diesel; BTE of 45.1% at best operating point
2 November 2011
|Single-cylinder Achates research engine installed in test cell. Source: Achates Power. Click to enlarge.|
Achates Power, the developer of a two-stroke, compression-ignition (CI) opposed-piston (OP) engine announced that the latest test results for a 3-cylinder Achates engine configuration indicate a 20% improvement in cycle-averaged brake-specific fuel consumption (BSFC) when compared to a recently introduced advanced medium-duty four-stroke diesel engine: 192.6 g/kWh BSFC for the Achates engine, versus 239.9 g/kWh for the reference engine.
This represents a 7% improvement since September 2010. Achates Power has demonstrated newly increased engine fuel efficiency results in every quarter of 2011, beginning with a 13% efficiency advantage presented at the Symposium on International Automotive Technology (SIAT) in January, a 15.5% efficiency advantage in June, and a 19% efficiency advantage presented at the SAE Commercial Vehicle Engineering Congress (ComVEC) in September. (Earlier post.)
|Specs for medium-duty Achates engine|
|Max power||47 kW/cylinder @ 2400 rpm|
|Max torque||240 N·m/cylinder @ 1600 rpm|
|Number of cylinders||3|
|Displaced volume||1.06 L/cylinder|
|Max BMEP||13.6 bar|
|Trapped compression ratio||16.7:1|
Achates Power measures single-cylinder combustion results and then uses the interface model to predict multi-cylinder engine performance for an operating range typical of an engine in a medium-duty commercial vehicle.
Dr. Paul Miles, a Distinguished Member of the Technical Staff at Sandia National Laboratories and co-chair of SAE powertrain activities, approves of this method Achates Power employs to determine engine performance, fuel consumption and emissions characteristics.
Achates Power employs industry-standard instrumentation and methods to benchmark its engines at multiple engine load and speed points representative of regulatory test cycles. The single-cylinder results are then carefully extrapolated to expected multi-cylinder results using a rigorous interface model. These are real-world numbers. The outstanding fuel economy and emissions it reports are a testimony to the effectiveness of the fundamental research and development work I have seen at its San Diego facility.—Paul Miles
Brake thermal efficiency of 45.1% was also demonstrated at the best engine operating point with an overall calibration that meets the US EPA 2010 emissions standard. An earlier closed-cycle simulation study with Dr. David Foster from the University of Wisconsin found that the opposed-piston 2-stroke Achates engine could show an indicated thermal efficiency of 53%. Achieving the indicated closed-cycle efficiency will require further improvements in combustion optimization, pumping work, mechanical friction, and the power consumption of engine accessories.
As confirmed by dynamometer testing, the Achates Power opposed-piston architecture with the two-stroke cycle has improved brake thermal efficiency resulting from a combination of the following four effects:
Reduced heat transfer due to the more favorable combustion chamber area/volume ratio of the opposed-piston architecture;
Increased ratio of specific heats due to the leaner operating conditions of the two-stroke cycle;
Decreased combustion duration achievable within maximum pressure rise rate limits due to the more rapid expansion of the in-cylinder volume-per-crank degree angle; and
Reduced pumping work as only a portion of the residual gases in the cylinder need to be scavenged at each cycle.
The thermodynamic rationale for these results is documented in the SAE International Paper 2011-01-2216, presented at SAE ComVEC on 14 September. The most recent 20% fuel efficiency improvement is from ongoing enhancements, including the latest hardware upgrades and calibration improvements and more than 2,500 hours of testing at the company’s San Diego facility.
When compared to the published performance data of one of the best medium-duty clean diesel engines in the world, the Achates Power engine demonstrates:
20% lower cycle average brake-specific fuel consumption;
Similar engine-out emissions levels;
Less than 0.1% fuel-specific oil consumption; and
Reduced cost, weight and complexity
Through the application of rigorous science and engineering methods, Achates Power has overcome historical two-stroke engine challenges. Achates Power uses sophisticated models and powerful computers to analytically solve the complex combustion processes of the opposed-piston, two-stroke cycle.—David Foster
Foster is a professor of mechanical engineering at the University of Wisconsin-Madison, an Achates Power technical advisory board (TAB) member, and an expert in the field of internal combustion and fluid dynamics.
The Achates Power TAB includes National Academy of Engineering members and SAE Fellows with more than 200 years of combined experience. Achates Power is backed by Sequoia Capital Partners, RockPort Capital Partners, Madrone Capital Partners, InterWest Partners and Triangle Peak Partners. It was founded by Dr. James Lemke with investment from the late John Walton, son of Sam Walton, the founder of Wal-Mart.
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)
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