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Achates Power completes real-world test of 10.6 L opposed piston engine

11 December 2023

Achates Power revealed test results of their 10.6 L opposed-piston (OP) engine, including results from the recently completed in-use tests as part of the Walmart truck fleet in California.

The real-world measurements showed that the 3-cylinder, 300 kW, 2-stroke cycle OP diesel engine has, on average, a 10% fuel economy advantage over the best-in-class reference engine. The engine also demonstrated the ability to meet the 2024 CARB and 2027 EPA emission standards and CO2 regulations, including the applicable low-load cycle (LLC) standards and in-use emission limits.

In the California in-use trial, the 10.6 L opposed-piston engine was installed in a Peterbilt 579 tractor that was used in fleet service by Walmart for over six months. In real-world operation during the fleet service and on simulated routes, the OP engine showed between 4% better and 21% better fuel economy than the best-in-class conventional engine, depending on the route. These impressive fuel savings can be attributed to the high efficiency of the OP engine (with a broad area of high efficiency on the engine map) and less energy demand for thermal management of the emissions aftertreatment system.

The next steps include testing of the 10.6 L OP engine with an aftertreatment system aged to 435k, 600k, and 800k miles to confirm the ability to comply, at full useful life, with the 2024/27 CARB/EPA emission standards for criteria pollutants and the 2027 EPA GHG emission requirements.

The development of the Achates Power 10.6 L opposed-piston engine for Class 8 heavy-duty truck applications has been supported by the California Air Resources Board (CARB) under the Heavy-Duty Diesel Demonstration Program to demonstrate that a diesel engine can achieve the CARB low NOx emission limit of 0.02 g/bhp-hr (FTP) while also meeting the applicable CO2 emission standards.

An important advantage of the opposed-piston engine architecture is a low surface area to volume ratio of the combustion chamber, which results in lower heat losses and higher brake thermal efficiency (BTE)—the peak BTE in the 10.6 L OP engine exceeds 49%.

The opposed-piston architecture—where the engine charge is supplied by a supercharger rather than by engine pumping work—also provides more flexibility in the air-to-fuel ratio control and exhaust temperature management. The 10.6 L OP engine achieves the 0.02 g/bhp-hr FTP NOx limit and meets the 2024/27 CARB/EPA emission standards using a conventional emission aftertreatment system, without a closed-coupled SCR system or additional components—such as those required for cylinder deactivation—to enable advanced thermal management strategies. The Achates 10.6 L engine utilizes a commercially available, model year 2021 aftertreatment system, Figure 1. The original SCR catalyst has been replaced with an Fe/Cu-zeolite formulation for better N2O control.

Figure 1. Achates 10.6 L OP engine aftertreatment system

Adapted from Volvo Trucks’ “one-box” conventional aftertreatment system

Still another advantage of the opposed-piston architecture is a lower number of parts, which translates to less complexity and lower costs. Importantly, the OP engine eliminates the cylinder head, the camshaft, and engine valves. In conventional engines, these components—including the cylinder head itself, the head gasket, and the exhaust valve—are a significant cause of engine failure and the leading component of warranty costs for the engine manufacturer.

Achates Power expects that in volume production, the OP engine would cost no more than and probably less than current engines. The cost advantage would increase further if conventional 4-stroke diesel engines are forced to adopt an additional, closed-coupled urea-SCR system and cylinder deactivation technology to comply with future emission regulations.

Other opposed-piston engine projects currently pursued at Achates Power include:

Source: Achates Power