Achates Power building 2.7L opposed-piston light-duty engine; exceeding CAFE 2025, Tier 3 targets at lower cost
Stemming from a major project partially funded with a $9-million grant from the Advanced Research Projects Agency - Energy (ARPA-E) to develop an opposed-piston two-stroke gasoline compression ignition engine (OPGCI) (earlier post), Achates Power announced it is developing a 2.7-liter, 3-cylinder (i.e., six-piston) light-duty version of its opposed piston engine (OPE) in both diesel- and gasoline-fueled versions. Achates will demonstrate the engine in 2018 in a driveable light-duty truck.
The 2.7-liter compression-ignition OPE will will be 30-50% more fuel efficient than comparable diesel and gasoline direct injection engines, reduce emissions and cost less than alternative technologies under development for meeting 2025 CAFE fuel economy and Tier 3 emission targets.
The 30-50% improvement metric encompasses both gasoline and diesel variants of the OP engine. An OPGCI engine versus a four-stroke spark ignition engine will be on the higher end of that scale (50%), while the OP engine running on diesel versus a four-stroke compression ignition (diesel) will be on the lower end of the scale (30%).
The new Achates Power 2.7-liter OPE will deliver nominal power of 270 hp and max torque of 650 N·m (479 lb-ft); the vehicle will achieve 37 mpg—12% above the 33 mpg fully phased-in CAFE 2025 requirement for a full-size, light-duty pick-up truck (65-70 sq. feet). (That works out to an estimated window-sticker label rating of 28 mpg combined.)
Further, Achates is planning to meet the 0.03 g/mile NMOG + NOx Tier 3 fleet average target (the old Tier 2 Bin 2 level). Achates has done several studies with leading aftertreatment manufacturers for a diesel application; results suggested the Achates OP engine could reduce the diesel engine aftertreatment cost by 30%.
With the OPGCI engine, Achates also expects a significant cost reduction of the aftertreatment because the engine-out NOx and soot are much lower than with diesel. The current best guess is close to gasoline SI aftertreatment system cost, said Fabien Redon, Vice President, Technology Development.
We are capable of achieving very low engine-out NOx and do it with a lower penalty in BSFC. Our NOx/BSFC trade-off is not as steep, so in general we favor lower NOx regulations because it makes our engine even more competitive.—Fabien Redon
The light-duty engine is a “basic” Achates opposed piston 2-stroke engine, said Redon—i.e., it does not make use of more advanced technology elements under consideration for lowering the fuel consumption of combustion engines.
The beauty of that is that it basically allows us to exceed the 2025 CAFE regulation without adding any cost to the vehicle. This is taking the existing powertrain out, putting in the OP engine, and it exceeds the CAFE regulation.—Fabien Redon
The corollary to that is that the addition of more fuel efficiency technologies would further reduce the fuel consumption performance of the OP engines. In other words, there still is upside potential, even after besting the 2025 CAFE target.
The OP engine would be less expensive than a current production light duty diesel, such as in the Ram, but slightly more expensive than a gasoline engine, said Redon. However, the OP solution does not require the additional fuel efficiency technology projected to meet CAFE targets, thereby making it a more cost-effective solution.
The EPA’s draft Technical Assessment Report released in 2016 forecast cost increases to integrate fuel saving technology. Using this information and a related National Academy of Sciences report, Achates Power determined that including the Opposed-Piston Engine in the roadmap to achieve CAFE will be at least $1,000 less expensive.
The Opposed-Piston Engine uses fewer parts, including the eliminating the cylinder head(s) and related components, eliminating the valvetrain and related components, and a reduction in the aftertreatment system size and cost, among other things.
A comparison between the 2.7L OP Engine and a comparable V6 with supercharger shows a part reduction of more than 60% (110 primary components for the Achates Power inline 3-cylinder engine vs. 325 primary components for the V6), enabling an approximate 10% cost reduction.
In 2014, we presented a peer-reviewed technical paper, Meeting Stringent 2025 Emissions and Fuel Efficiency Regulations with an Opposed-Piston, Light-Duty Diesel Engine, at the SAE World Congress showing how we were able to meet CAFE 2025 regulations in a Full-Size Truck. And in 2018 we will have a demonstration vehicle that proves an internal combustion engine is able to cost-effectively meet the CAFE standard, does not require the adoption of costly vehicle modifications, infrastructure upgrades, or a change in how the driver operates or maintains the vehicle.—Fabien Redon
Achates light-duty development engines will be available for automakers.
Achates and the OPE. Achates Power has spent 13 years improving the opposed-piston engine, a historically efficient engine originally developed in the late 1800s. The Achates Power Opposed-Piston Engine features two pistons per cylinder, working in opposite reciprocating action. The Opposed-Piston Engine does not need cylinder heads, which are a major contributor to heat losses in conventional engines. Ports in the cylinder walls replace the complex poppet valves and friction-creating valve trains of conventional engines.
Opposed piston engines offer a number of fundamental engine efficiency advantages compared to four-strokes:
Lower heat transfer. 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.
Leaner combustion. The opposed-piston two-strok (OP2S) engine has a combustion event at every revolution in each cylinder. It features a larger cylinder volume for a given fuel quantity, leading to leaner combustion at the same boost level, which leads to a higher ratio of specific heat. A higher ratio of specific heat leads to higher ideal engine efficiency.
Optimally phased and faster combustion at equivalent pressure rise rate. The larger volume also enables shorter combustion duration while preserving the maximum pressure rise rate. Shorter combustion and lower heat transfer results in a more advanced combustion phasing.
Lower pumping work. Air pumping is decoupled from the engine cycle. The OP engine ports are less restrictive than poppet valves.
A very significant factor in meeting emissions requirements is the ability to light off the aftertreatment system as soon as possible. Achates has demonstrated the ability to achieve exceptionally high exhaust gas temperatures at idle—much beyond what can be done with equivalent 4-stroke engines. This enables the earlier light-off of the aftertreatment, therefore resulting in lower emissions over the cycle.
The intake ports at one end of the cylinder and exhaust ports at the other are opened by the piston motion and enable efficient uniflow air scavenging. The two-stroke, compression ignition engine has been engineered to achieve superior thermal efficiency by the virtue of its lower heat losses, higher expansion ratio, lean combustion and reduced pumping losses.
Achates Power currently has engine programs under development with 12 leading engine manufacturers, including work with Cummins on the Advanced Combat Engine for the US Army, an Opposed-Piston, Gasoline Compression Ignition Engine for the US Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) in partnership with Argonne National Laboratory and Delphi Automotive, and Fairbanks Morse. These customers have a wide range of product specifications: from 50 hp to 5,000 hp power output, for example, in different types of vehicles, using different fuels.
While we continue to work on our customer programs, and research and development programs (like Gasoline Compression Ignition), we are excited to showcase the fuel efficiency, low emissions and outstanding driving characteristics of our Opposed-Piston Engines. There is no technical solution to respond to the proposed 2025 CAFE regulation that is as cost-effective, compatible with our existing vehicles and fuels, ready for production and adaptable to future renewable fuels as our Opposed-Piston Engines.
The OP Engine confirms what the industry already knows; the technology needed to exceed these standards and deliver fuel economy and cost savings to customers is currently available, and works with existing infrastructure and fuels. We have a development program underway to create the engine and look forward to coming back to the North American International Auto Show in 2018 to show our progress and we’re even more excited to drive the vehicle later that year.—David Johnson, president and CEO, Achates Power
Light-duty diesel OP. Achates has been considering a light-duty version of it engine for awhile, and, in early work, compared an earlier version of a 2.7-liter, 3-cylinder implementation (similar to the one to be deployed in the demo truck) to the Cummins ATLAS inline-4 diesel.
ATLAS (Advanced Technology Light Automotive Systems) is the result of a collaboration between Nissan and Cummins reaching back to a US Department of Energy project which began in 2010. ATLAS is targeting a Tier 2 Bin 2 emissions-compliant diesel for application in a light duty pickup, along with a 40% fuel economy improvement over current gasoline V8 powered half-ton pickups.
In March 2015, Cummins reported that ATLAS was capable of meeting 2025 CAFE requirements for a half-ton pickup as well as beating Tier 2 Bin 2 emissions standards. (Earlier post.)
Using data from an SAE paper for ATLAS data (SAE 2013-01-0282), Achates engineers calculated that their opposed piston engine would outperform ATLAS interms of fuel economy and engine-out emissions.
The Achates 2.7-liter engine to be deployed in the demo truck is a new clean-sheet design incorporating all the latest learnings. Engine design will be complete early this year, with first physical prototypes being manufactured by the end of the year. Vehicle development will start early in 2018, with initial demonstrations that year.
OPGCI. Gasoline compression ignition (GCI) uses high cylinder temperatures and pressures to spontaneously combust gasoline fuel without requiring spark plugs. Benefits are projected to include high-efficiency, low-emissions and low-cost. The opposed piston engine is a promising platform for GCI:
Superior mixture preparation. 4-stroke engines (generally) use a single injector, with spray aiming at the cylinder wall, leading to high probability for fuel in crevice volumes. OP engines use diametrically opposed dual injectors with the potential for the cancellation of spray momentum. Additionally, there is simply a longer distance for fuel to reach the crevice area.
In addition, the proprietary combustion system design (both pistons form the combustion chamber) provides high mixing. Further, the split injector flow enables superior small quantity control.
Better peak load management. A 4-stroke requires high BMEP to achieve competitive power and torque density. A high compression ratio and high charge mass causes high cylinder pressure.
The OP engine requires lower BMEP for maximum load. Its higher efficiency requires less fuel. The larger cylinder volume tolerates faster heat release rates for the same combustion noise.
Superior charge temperature and pumping management. The OP engine features high internal EGR with lower pumping. Due to its large, fast opening ports, there is low restriction. A high trapped temperature results in high exhaust gas temperatures.
Achates is working with Delphi Automotive (which has been avidly pursuing gasoline compression ignition for some time) and Argonne National Laboratory on the ARPA-E-funded project to develop the OPGCI engine. (Earlier post.)
ARPA-E is providing initial funding of $9 million to this project over three years; Achates Power, Argonne and Delphi expect to spend a total of $13 million on the program, including cost share. The $9-million award was “one of the largest single ARPA-E awards ever made,” noted Chris Atkinson, the ARPA-E program manager for the Achates project.
The goal of the 30-month Achates ARPA-E project is to 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 derivative (1.0L) could be extremely interesting as a range extender, while a two-cylinder derivative could serve nicely as a direct drop-in for front-wheel drive applications, delivering around 180 hp and 430 N·m from a swept volume of 1.8 liters.
|Potential light-duty OP engine family. Source: Achates Power. Click to enlarge.
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