With current and pending regulations-including Corporate Average Fuel Economy (CAFE) 2025 and Tier 3 or LEV III-automakers are under tremendous pressure to reduce fuel consumption while meeting more stringent NOx, PM, HC and CO standards. To meet these standards, many are investing in expensive technologies-to enhance conventional, four-stroke powertrains-and in significant vehicle improvements. However, others are evaluating alternative concepts like the opposedpiston, two-stroke engine.First manufactured in the 1890s-and once widely used for ground, marine and aviation applications-the historic opposedpiston, two-stroke (OP2S) engine suffered from poor emissions and oil control. This meant that its use in on-highway applications ceased with the passage of modern emissions standards.Since then, Achates Power has enhanced the opposed-piston engine and resolved its historic challenges: wrist pin and power cylinder durability, piston and cylinder thermal management, piston ring integrity and oil consumption [1].An in-depth study on opposed-piston, two-stroke diesel engine performance and emissions in a light-duty truck application is presented here for the first time in a technical paper. The paper includes a: • Brief review of the opposed-piston, two-stroke engine's architectural advantages (thermodynamics, pumping work and combustion) • Comprehensive overview of the engine's performance and emissions results, including indicated thermal efficiency, fuel consumption and emissions • Comparison of fuel economy and emissions to the published benchmark, the Cummins 2.8L ATLAS Diesel Engine [2]• Discussion of an exhaust temperature control strategy that is used to meet the aggressive catalyst light-off requirements of light-duty applications by achieving rapid catalyst light-off after a cold start • Comparison of engine balance of the light-duty truck concept engine and a state-of-the-art gasoline V6 engine • Examination of the packaging options for an opposedpiston, two-stroke engine in a light-duty truck applicationThe results of this study show that the Achates Power opposed-piston engine benefits-high efficiency, low emissions and reduced cost, mass and complexity-already demonstrated for medium-duty commercial vehicles [1] are also available for light-duty applications. In fact, to an even greater extent: over 30% fuel economy improvement when compared to an equivalent four-stroke diesel engine.Moreover, this study shows that the final 2025 light-truck CAFE fuel economy regulation not only has the potential to be met but also the potential to be exceeded with a full-size 5,500 lb. pick-up truck by simply applying the Achates Power technology without any hybridization or vehicle improvements.
A water‐saturated fracture, partially clogged with porous material coating the fracture surfaces, is considered. Fluid flow and contaminant transport in this fracture are significantly altered relative to an unclogged fracture. Analytical expressions are developed for the water velocities in the clogged and the unclogged regions in the fracture and the asymptotic longitudinal dispersion coefficient for the system. For highly adsorbing dissolved contaminants or large colloids, the slow diffusion within the porous region causes enhanced dispersion. In a standard tracer test, colloidal contaminants will arrive earlier than dissolved tracers for either one of two reasons: (1) colloids confined to the unclogged portion of the fracture will have a larger average velocity or (2) colloids that can diffuse into the porous region have very low Brownian dififusivities, resulting in a large longitudinal dispersion due to the inverse relationship between Taylor dispersion and Brownian diffusion. The average velocity or asymptotic longitudinal dispersion coefficient can be orders of magnitude greater than that for a molecular tracer.
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