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Achates showcases 2.5L opposed-piston engine at ACT Expo

At ACT Expo, Achates Power (API) is displaying one of three engine variants of what it calls the cleanest and most efficient combustion engine architecture to date. With the industry facing massive disruption due to stringent limits on both criteria and CO2 emissions, API’s demonstrations of its opposed-piston engines have shown this architecture can satisfy regulatory requirements without detriment to the fleets or ultimately the economy.

The light-duty 2.5-liter opposed-piston engine displayed at ACT Expo represents all three displacement variants undergoing testing and validation work—with the smallest having been demonstrated in a Class 2a pickup. Not shown are the 5-liter and 10-liter engines which, due to their superior power density, have ratings comparable to today’s medium and heavy duty engines in the 7 to 15 liter space.

Benefits of the API opposed-piston engines include lower heat rejection, higher power density, and fewer moving parts for better reliability and increased fuel efficiency. With this configuration, every stroke is a power stroke which translates to optimal thermal efficiency. Where this matters to the fleets is 2027 and beyond emissions regulations where combustion efficiencies and thermal management are critical for aftertreatment cost, simplicity and functionality. Conversely, the Engine Manufacture Association estimates that total engine and aftertreatment system level pricing of 4-stroke engines could increase as much as $31,000 per truck to meet 2027 emissions standards.

API has demonstrated the capability of this technology in rigorous real-world applications, production defense applications, tens of thousands of test cell hours, and in the emissions lab.

We’ve taken this well beyond ‘concept phase’ in the past several years through extensive testing, creation of a combat variant now being produced for the US military, and heavy-duty commercial fleet on-road testing. We’ve even been able to demonstrate that our opposed-piston architecture is capable of using the aftertreatment systems of today to meet tomorrow’s ultra-low NOx emissions standards with robust compliance through the 800,000 mile useful life requirements. Our accomplishments in low-load, idle, and off-cycle emissions position this to be the best combustion technology in the industry.

—Dnyanesh Sapkal, Executive Vice President, Engineering & Operations

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Where API sees an immediate opportunity for early adoption of the opposed-piston technology over today’s best-in-class engines is commercial trucking where emissions standards are quickly becoming much more stringent to a point where aftertreatment systems’ cost and complexity may drive truck costs higher and reliability lower with a penalty to fuel economy.

In a joint demonstration with CALSTART and Walmart (earlier post), an API 10.6-liter engine was retrofitted into a production class 8 truck. During the demonstration period in California, the truck achieved up to 10.8 mpg on a 389-mile delivery route with an average of 10% better fuel economy compared to a best-in-class production engine, over a variety of routes with the advantage increasing to 22% on some routes. This was done within the proposed 2027 ultra-low NOx emissions standard and with only an off-the-shelf current production one-box aftertreatment system (no additional thermal management or emissions control devices were required).

The inherent combustion efficiencies of the API opposed-piston engine not only allow for a cleaner utilization of today’s diesel, but also an easier transition to multiple fuel types available today and tomorrow.

In February of 2024 at the Argonne National Laboratory, the US Department of Energy successfully demonstrated an Achates test engine running on hydrogen combustion without the aid of a spark or dual-fuel pilot ignition. (Earlier post.) Early testing has proven that the simplicity of this engine design enables an easier transition to hydrogen combustion over 4-stroke engine concepts because of the base engine design, diesel-like efficiencies, and indications of potential power density.

In a joint demonstration with Neste, the world’s leading renewable diesel producer, the already proven 10% greenhouse gas emissions (GHG) reduction advantage of the OP engine technology was amplified when using Neste MY Renewable Diesel. This is a drop-in hydrotreated vegetable oil (HVO, R99) that requires no engine calibration changes and is available today across multiple states.

The API engine running Neste’s renewable fuel resulted in a consistent 4-5% GHG emissions reduction compared to fossil diesel across all test cycles as well as lower criteria emissions. Atop of the 75% lower net GHG advantage Neste MY Renewable Diesel has over fossil diesel’s lifecycle, this is the lowest GHG level that API has ever measured.

Engine manufacturing plants today can easily produce this engine through sustainable assembly practices and with approximately 300 fewer parts compared to today’s 4-stroke engine. The familiarity of the materials used in production also means that the supply chain of today can efficiently support the manufacturing of this engine architecture without sourcing disruptions and constraints.

OEM integration for this solution can be easily done into today’s chassis, with today’s transmissions, while benefiting from a reduction in cooling package size requirements.

Combat Durability. In 2021, engine manufacturer Cummins, Inc. was awarded an $87-million contract from the US Army to manufacture an opposed-piston 14-liter engine in partnership with API, which Cummins recognized as the expert in advanced opposed piston technology. As a combat engine, this engine faces some of the harshest duty cycles imaginable. The unmatched power density, heat rejection advantage, and capability to burn a variety of fuels (JP8, F24, diesel) made this the technology of choice for the combat application needs.


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