Environmental protection is now one of the most important social concerns in the world. In 1998, emission controls in the US required the reduction of NOx by 20% from the 1994 limit.Hino Motors has developed new J-series medium-duty diesel engines for trucks that meet the US 1998 emissions regulations. The engines comprise turbocharged and aftercooled 4- and 6-cylinder engines of the same cylinder bore and stroke. The engines feature a 4-valve system, OHC valve train design, centered nozzle arrangement, and an optimum combustion chamber design, which achieved uniform combustion. With these features, the maximum combustion temperature was decreased, and hence reduced the NOx, smoke, and PM emissions. A muffler integrated with a catalytic converter (catalytic muffler) was adopted to reduce PM emissions further. The engines with the catalytic muffler have successfully met the US 1998 emissions regulations.Not only to meet the emissions regulations, but also to satisfy market requirements, significant efforts were made to reduce total administration cost. Initial, running, and maintenance costs were reduced by using low cost local parts, reduced fuel and oil consumptions, and improved long term durability, respectively. Fuel consumption was reduced by achieving uniform combustion, and low frictional and pumping losses. Oil consumption was reduced by lowering piston temperature with a new innovative cooling channel piston and by the optimized piston profile and piston ring design. To secure durability in the US market, the engines were optimized for US fuel, oil, and coolant. The US local parts such as oil and fuel filter elements were positively adopted. A catalytic muffler was developed by conducting both filed tests in the US and extensive durability tests that were specifically designed for the US market. Results from 35,000 hours of various durability tests convinced the authors that the J-series engines had sufficient durability and reliability.