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Heavy-duty vehicles, currently the second largest source of fuel consumption and carbon emissions are projected to be fastest growing mode in transportation sector in future. There is a clear need to increase fuel efficiency and lower emissions for these engines. The Opposed-Piston Engine (OP Engine) has the potential to address this growing need. In this paper, results are presented for a 9.8L three-cylinder two-stroke OP Engine that shows the potential of achieving 55% brake thermal efficiency (BTE), while simultaneously satisfying emission targets for tail pipe emissions. The two-stroke OP Engines are inherently more cost effective due to less engine parts. The OP Engine architecture presented in this paper can meet this performance without the use of waste heat recovery systems or turbo-compounding and hence is the most cost effective technology to deliver this level of fuel efficiency.

Opposed-piston (OP) engines have attracted the interest of the automotive industry in recent years because of their potential for significantly improved fuel economy. Opposed-piston, two-stroke (OP2S) engine technology amplifies this fuel efficiency advantage and offers lower cost and weight due to fewer parts. While OP engines can help automotive manufacturers comply with current, and future, efficiency standards, there is still work required to prepare the engines for production. This work is mainly related to packaging and durability. At Achates Power, the OP2S technology is being developed for various applications such as commercial vehicles (heavy-and medium-duty), SUVs, pick-up trucks and passenger cars (i.e. light-duty), military vehicles, large ships and stationary power (generator sets). Included in this paper is a review of the previously published OP engine efficiency advantages (thermodynamics, combustion and air system) as well as the architecture's historical challenges.

Historically, the opposed-piston two-stroke diesel engine set combined records for fuel efficiency and power density that have yet to be met by any other engine type. In the latter half of the twentieth century, the advent of modern emissions regulations stopped the wide-spread development of two-stroke engine for on-highway use. At Achates Power, modern analytical tools, materials, and engineering methods have been applied to the development process of an opposed-piston two-stroke engine, resulting in an engine design that has demonstrated a 15.5% fuel consumption improvement compared to a state-of-the-art 2010 medium-duty diesel engine at similar engine-out emissions levels. Furthermore, oil consumption has been measured to be less than 0.1% of fuel over the majority of the operating range. Additional benefits of the opposed-piston two-stroke diesel engine over a conventional four-stroke design are a reduced parts count and lower cost.