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.In this paper, engine performance results are presented for the 9.8L two-stroke OP Engine that 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, increased indicated thermal efficiency (ITE) - due to better combustion phasing because of faster mixing controlled combustion, and lower NOx because of improved area-to-volume ratio and lower fueling requirement per cycle. Results are presented from the two-stroke OP Engine-specific 1-D and 3-D CFD models developed for correlation to the three-cylinder 4.9L two-stroke research engine dynamometer measured data. These correlated models were used as tools to make predictions for the 9.8L heavy-duty engine. The optimized system includes a high trapped compression ratio piston bowl, ports design to provide best scavenging performance, thermal barrier coating on piston bowls and dual injector with an optimized spray pattern layout. Engine performance results are presented at three speed-load points. Results show that the two-stroke OP Engine result in a BTE of 55%, while meeting stringent emission standards without the use of expensive waste heat recovery systems and/or turbo-compounding components.