In the next few years, the USA, EU, and Japan plan to introduce very stringent exhaust emissions regulations for heavy–duty diesel engines, in order to enhance the protection air quality. This builds upon the heavy–duty diesel engine exhaust emissions regulations already in effect. At the same time, improvement in fuel consumption of heavy–duty diesel engines will be very important for lowering vehicle operating costs, conserving fossil fuel resources, and reduction of CO2 (greenhouse gas) levels.This paper presents a detailed review of a quiescent combustion system for a heavy–duty diesel engine, which offers breakthrough performance in terms of the exhaust emissions – fuel consumption trade–off, compared with the more conventional swirl supported combustion system. This conclusion is supported by experimental results comparing quiescent and swirl supported versions of various combustion system configurations. A number of numerical experiments using the KIVA II code was also carried out to investigate the fuel spray behavior, combustion and NOx formation.The above concepts were applied to an actual engine by utilizing a common rail system, exhaust gas recirculation system, and a variable geometry turbocharger. Engine test results showed that the quiescent combustion system with these technologies achieved improved fuel consumption, meeting the Japanese long term exhaust emissions regulations by overcoming the exhaust emissions – fuel consumption trade–off.