With increasing energy prices and concerns about the environmental impact of greenhouse gas (GHG) emissions, a growing number of national governments are putting emphasis on improving the energy efficiency of the equipment employed throughout their transportation systems. Within the U.S. transportation sector, energy use in commercial vehicles has been increasing at a faster rate than that of automobiles. A 23% increase in fuel consumption for the U.S. heavy duty truck segment is expected from 2009 to 2020. The heavy duty vehicle oil consumption is projected to grow while light duty vehicle (LDV) fuel consumption will eventually experience a decrease. By 2050, the oil consumption rate by LDVs is anticipated to decrease below 2009 levels due to CAFE standards and biofuel use. In contrast, the heavy duty oil consumption rate is anticipated to double. The increasing trend in oil consumption for heavy trucks is linked to the vitality, security, and growth of the U.S. and global economies.An essential part of a stable and vibrant U.S. economy is a productive U.S. trucking industry. Studies have shown that the U.S. gross domestic product (GDP) is strongly correlated to freight transport. As the economy grows, the freight tonnage increases as well as the annual vehicle miles traveled (VMT). Over 80% of all U.S. freight tonnage is transported by diesel power and over 75% is transported by trucks. The improved efficiency of heavy-duty engines and vehicles has been quickly overwhelmed by the increase in annual VMT. This results in heavy-duty vehicles consuming a growing share of the total transportation-related petroleum. Given the vital role that the trucking industry plays in the economy, improving the efficiency of diesel engines is a central focus of this paper.Trucks are the mainstay for trade, commerce, and economic growth. Sustaining a U.S. trucking industry that is competitive in global markets requires innovation. The truck manufacturing and supporting industries are faced with numerous challenges to reduce oil consumption and greenhouse gases, meet stringent emissions regulations, provide customer value, and improve safety. A key part of the strategy to meet these requirements is to improve the efficiency of the internal combustion engine (ICE) powering the trucks. The performance, low cost, and fuel flexibility render the ICE the leading candidate to power commercial vehicles for many decades.Historically, diesel engine technologies have taken more than 10 years after first introduced to diffuse throughout the commercial vehicle marketplace. This rate is faster when fuel economy provides a business advantage to the vehicle's owner. Increased efficiency and reduced emissions of diesel engines can be realized through technologies that improve engine design and better integrate systems. Engine manufacturers have a growing need to refine the capability to innovate, design, develop, and validate engine efficiency improvements. Therefore, the primary purpose of this paper is to provide guidelines and tools that allow a systematic approach to engine design and development that focuses on satisfying regulatory requirements, achieving greater fuel efficiency, and improving transportation freight efficiency. The on-highway heavy-duty diesel engine is used to illustrate the processes; however, the general principles may be applied to other diesel and natural gas engine applications, such as off-highway or power generation.For the past two decades, engine manufacturers have focused on reducing engine emissions to near zero levels while maintaining or slightly increasing fuel efficiency. With the implementation of the new EPA/NHTSA commercial vehicle GHG regulations in 2011, the need to reduce fuel consumption has been explicitly linked to the ability to manufacture and sell engines. The forward looking technology roadmaps in this paper provide a framework for improving engine efficiency over the next 10 to 15 years. The list of improvements consists of engine components, aftertreatment, and powertrain advancements.