DEUTZ has manufactured air-cooled Diesel engines since the 1940s. The basic design of some of the air-cooled engines in production today dates back to the 1970s, a time when fuel economy, reliability and durability were the greatest concern.Extremely stringent U.S. and European heavy-duty on-highway standards have been implemented resulting in very clean engines for truck and bus applications that are now in the market.Meanwhile, the U.S. EPA, the State of California and the European countries have developed (or are in the process of developing) exhaust emission regulations for industrial engines. These requirements are being finalized and will be implemented in the mid 1990s. It is very encouraging to recognize that there have been definitive steps towards an international harmonization of the emission legislation between Europe and North America.In anticipation of these upcoming emission requirements, the focus of market priorities for all industrial engine applications is now shifting toward “clean” exhaust emissions. For some time now, DEUTZ has been optimizing the design of existing air-cooled engines from the viewpoint of emissions quality while preserving the existing performance levels and reliability.This paper provides an overview of present and upcoming emission legislation and describes the technology being applied to air-cooled DEUTZ 912/913 engine series that will enable compliance with the emissions limits.