Achieving Ultra-Low Oil Consumption in Opposed Piston Two-Stroke Engines

The opposed piston two-stroke (OP2S) engine architecture is widely recognized for its improved fuel efficiency relative to a four-stroke engine. Achates Power Inc. seeks to demonstrate the market readiness of the OP2S engine by proving competitive in other important areas, one of which is oil consumption. Achieving oil consumption competitive to modern four-stroke engines is thus a key step in bringing OP2S technology to market. Two-stroke engines have historically suffered from higher engine lube oil consumption and subsequent emissions and durability challenges. This is primarily due to two main features of traditional two-stroke engines; the direct interaction of the piston skirt and rings with the intake and/or exhaust ports, which results in a direct leak path for lube oil to the combustion chamber and/or exhaust manifold, and crankcase-scavenged architectures which entrain oil into air being pumped through the crankcase. The OP2S engine architecture directly addresses these concerns by utilizing intake and exhaust manifolds, a closed crankcase system, and oil control rings which operate outboard of the ports. Previous work has shown the importance of careful consideration of cylinder liner, piston, and ring design in minimizing oil consumption of the OP2S architecture. This work evaluates further refinements in cylinder form, hone texture and oil retention, port sealing ring design, and oil control ring design. A Da Vinci DALOC sulfur-trace analyzer for real-time oil consumption measurement was used to generate speed vs. load maps of oil consumption of an Achates Power OP2S A48 development engine, operated under typical medium-duty conditions. The engine demonstrated oil consumption levels competitive with modern four-stroke benchmarks and completed a 100-hour durability test with no measured performance loss or increase in oil consumption. This work represents a key step towards proving the potential of the Achates Power OP2S engine architecture in the commercial and passenger vehicle markets.