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 Achates Power Opposed-Piston Engine has the potential to address this growing need. In this paper, results will be presented for a 9.8L three-cylinder OP Engine that shows the potential of achieving 51%+ brake thermal efficiency (BTE), while simultaneously satisfying 4.0 g/kWhr engine out NOx and 0.01 g/kWhr engine-out soot. The OP Engine architecture can meet this performance without the use of additional technologies such as thermal barrier coatings, waste heat recovery or additional turbo-compounding. The Achates Power OP Engine employs currently available engine components, such as supercharger, turbocharger and after-treatment and features a uniquely designed piston bowl shape to enhance mixing with a swirl-to-tumble conversion as the piston bowls approach minimum volume. This design improves fuel-air mixing and hence, results in low soot values. The OP Engine also has lower heat transfer loss due to inherent design of the combustion chamber, which provides lower surface-to-area volume ratio compared to conventional engine. This results in further benefits of reduction in fuel consumption. The Achates Power OP Engine also makes use of internal EGR by using an optimized design of intake and exhaust ports that improves scavenging. This reduces engine-out NOx along with lower requirement of flowing external EGR and hence reduction in pumping requirement. 1D and 3D-CFD models developed for the analysis are correlated to the 4.9L Achates Power OP Engine dynamometer measured data. The correlated models are used as tools to make predictions for the 9.8L heavy duty engine. Results show that the OP Engine results in a net reduction of fuel consumption with BTE exceeding 51%, as well as meeting stringent emission standards.