Conceptual image of a possible multi-cylinder OPGCI engine. Click to enlarge.

Broadly, the 30-month Achates ARPA-E project will deliver a three-cylinder, 3.0-liter opposed-piston, gasoline compression ignition engine applicable to large passenger vehicles, pick-up trucks, SUVs and minivans. It could also be the first of a family of OPGCI engines spanning 1.0 to 4.0L displacements, said Redon.

The smaller displacement engine (1.0L) could be extremely interesting as a range extender, he suggested. The specific scope of the project is still being worked through with the partners and the ARPA-E team.

The OPGCI technology is attractive from an ARPA-E point of view as it potentially allows gasoline-fueled engines to operate at diesel-like efficiencies, with the reduced cost benefit of only requiring three-way catalysis and lower pressure (and hence lower cost) fuel injection systems. We are looking forward to seeing the full potential of the GCI technology implemented in the opposed-piston engine configuration.

—Dr. Chris Atkinson

Gasoline compression ignition uses high cylinder temperatures and pressures to spontaneously combust gasoline fuel without requiring spark plugs. The Achates Power opposed-piston engine has leveraged two-stroke engine design to develop a flexible air handling and scavenging capability, which provides the necessary high temperature for stable combustion even at low loads. In addition, the combustion system design uses diametrically opposed dual injectors to enable superior control of fuel penetration and mixture stratification for robust ignition and controlled in-cylinder heat release.

Vehicle manufacturers are struggling to find cost effective ways to improve fuel efficiency by just a few percent points, but this combination has the potential to dramatically exceed that number and be a major advance for the industry. Combining two very clean, very efficient, and cost effective technologies may well yield a new paradigm in engine design that could help satisfy the challenges of ground mobility for decades.

—Dan Hancock, president, DMH Strategic Consulting, retired vice president, GM Powertrain Global Engineering, past president of SAE International, and a member of Achates’ Industry Advisory Board

Argonne National Laboratory has been developing gasoline compression in a series of conventional development engines for nearly 10 years. Their expertise in gasoline compression, computational fluid dynamics and engine modeling and simulation will be a key to the success of this project.

The dynamics of this team are really perfect to make this project work. Combining Argonne’s scientific and engineering experience in advanced gasoline combustion with the advances Achates Power has made in engine design, and Delphi’s fuel injection and combustion system expertise as a Tier One automotive supplier will give us the tools to develop an engine we think is going to show very large efficiency gains.

—Don Hillebrand, director of the Energy Systems Division at Argonne

Delphi GDCI. Delphi has been working on gasoline direct-injection compression-ignition (GDCI) technology for a number of years (e.g., earlier post). At SAE World Congress 2015 earlier this year, Delphi’s Mark Sellnau and his colleagues presented the latest results from their work for a 1.8L GDCI engine over a wide range of engine speeds and loads using RON91 gasoline.

The engine was operated with a new low-temperature combustion process for gasoline partially-premixed compression ignition without combustion mode switching. Injection parameters controlled mixture stratification and combustion phasing using a multiple-late injection strategy with injection pressures similar to that for gasoline direct injection systems.

Central to these advancements was a fuel injection system and injection strategy combined with a new piston design. Using multiple late injections and GDi-like fuel pressure, the fuel-air mixture could be stratified but sufficiently mixed. This produced robust ignition with very clean, efficient, and stable combustion within constraints for combustion noise.

At idle and low loads, rebreathing of hot exhaust gases provided stable compression ignition with very low engine-out NO_x and PM emissions. Rebreathing enabled reduced boost pressure, while greatly increasing exhaust temperature. Hydrocarbon and carbon monoxide emissions after the oxidation catalyst were very low. Brake specific fuel consumption (BSFC) of 267 g/kWh was measured at the 2000 rpm-2bar BMEP global test point.

At medium load to maximum torque, rebreathing was not used and cooled EGR enabled low-temperature combustion with very low NO_x and PM, while meeting combustion noise targets. MAP was reduced to minimize boost parasitics. Minimum BSFC was measured at 213 g/kWh at 1800 rpm—12 bar IMEP.

Full load torque characteristics of the engine were developed using alternative injection strategies. Maximum BMEP of 20.3 bar was measured at 2000 rpm, with 17.4 bar BMEP achieved at 1500 rpm. While the team reported very good low-speed and medium-speed BMEP, more work is needed to develop output characteristics of the engine, they concluded.

Achates compression-ignition OP2S. Once widely used for a range of applications—ground, marine and aviation—the conventional opposed-piston two-stroke (OP2S) engine suffered from poor emissions and oil control. Since its founding in 2004, Achates Power has enhanced the compression-ignition OP2S engine and has worked to resolve its challenges: wrist pin and power durability; piston and cylinder thermal management; piston ring integrity; and oil management.

The company—which is an IP (intellectual property) company, not a high-volume engine manufacturer—has some 10 customer projects currently under way, ranging from the very small to the very large (with partner Fairbanks-Morse, earlier post). The first production engine stemming from one of its customer relationships will be out next year, and Achates is also moving to vehicle trials with other customers—all using diesel.

In 2014, Achates presented results of an in-depth study on OP2S performance and emissions in a light-duty truck application in an SAE paper. (Earlier post.)

The results showed 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 had 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 showed the potential for a 30% improvement in fuel economy over the equivalent performance Cummins ATLAS Tier 2 Bin 2 engine as well as a significant improvement in NO_x and PM (42-74%, depending upon drive cycle and pollutant).

In April of this year, Achates presented initial test results of the transient control and exhaust temperature management capability of its opposed piston (OP) two-stroke diesel engine showing that under a typical transient maneuver—25% to 100% load step at low and constant engine speed—the engine can control both NO_x 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. (Earlier post.)

Those results followed a paper published in January 2015 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.

OPGCI. In a discussion about the ARPA-E award at Achates main office in San Diego, Redon said that the partners expected to see the same efficiencies as delivered by the diesel version of the opposed-piston engine, but with the additional benefit of a lower cost fuel and aftertreatment system. Overall, they expect about a 50% improvement in fuel efficiency over a downsized, gasoline direct injection engine.

Going from four-stroke direct injection to four-stroke diesel, you get about a 50% cost increase for about 20% BSFC improvement. With GCI, you don’t get such a high cost increase, and you get about the same [efficiency] improvement [as going to diesel] with the 4-stroke GCI engine. When you go to a diesel opposed-piston engine, get you get 50% improvement over GDI and about 20-30% over 4-stroke diesel, which is what we have shown many times before, and you get a slight cost reduction.

But when you go to OPGCI, you have a very significant cost reduction compared to opposed-piston diesel and you get the 50% improvement over GDI. That’s what makes this really a very attractive proposition.

—Fabien Redon

Opposed-piston is perfect platform for GCI because of the lower peak loads, Redon noted. In terms of high load operation or max BMEP, the OP levels are 14-15 bar; the downsized four-stroke engines are at 20-25 bar for the 4 stroke. It is a challenge for GCI to achieve the very high BMEP levels in a 4-stroke.

We replace the 4-stroke with an engine that is lower in displacement by about 30% but also about 30% lower max BMEP. At the lower loads, we have more flexibility because of our charge control.

—Fabien Redon

Managing the air-fuel mixture is critical for GCI; typically, there is portion of the fuel that is premixed and a portion the fuel that is stratified. For example, Redon said, with an injection that starts early in compression stroke, most of the initial fuel is almost fully premixed by the end of compression—it provides a base for combustion. Another injection that occurs closer to the time of ignition creates stratification and some pockets where ignition can happen, all requiring the right temperature, composition and pressure.

The opposed piston engine cylinder offers two injectors, and also the ability to manage the swirl very well. We expect that we will have a very attractive situation for fuel mixture preparation for GCI.

—Fabien Redon

The project will result in two purpose-built OPGCI engines: a single-cylinder version to be tested at Argonne, and a multi-cylinder version to be tested at Achates in San Diego.

Resources

• 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

• Sellnau, M., Moore, W., Sinnamon, J., Hoyer, K.et al. (2015) “GDCI Multi-Cylinder Engine for High Fuel Efficiency and Low Emissions,” SAE Int. J. Engines 8(2):775-790 doi: 10.4271/2015-01-0834

• 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

• Sellnau, M., Foster, M., Hoyer, K., Moore, W. et al. (2014) “Development of a Gasoline Direct Injection Compression Ignition (GDCI) Engine,” SAE Int. J. Engines 7(2):835-851 doi: 10.4271/2014-01-1300