An appropriate test procedure, based on a duty cycle representative of real in-use operation, is an essential tool for characterizing engine emissions. A study has been performed to develop and validate a snowmobile engine test procedure for measurement of exhaust emissions.Real-time operating data collected from four instrumented snowmobiles were combined into a composite database for analysis and formulation of a snowmobile engine duty cycle. One snowmobile from each of four manufacturers (Arctic Cat, Polaris, Ski-Doo, and Yamaha) was included in the data collection process. Snowmobiles were driven over various on- and off-trail segments representing five driving styles: aggressive (trail), moderate (trail), double (trail with operator and one passenger), freestyle (off trail), and lake driving. Statistical analysis of this database was performed, and a five-mode steady-state snowmobile engine duty cycle was developed.A test procedure, based on this five-mode duty cycle, has been defined, addressing requirements specific to snowmobile engines. A round-robin testing program was conducted by International Snowmobile Manufacturers Association (ISMA) member companies to validate and further develop this test procedure. Supporting emissions test results and analysis are presented on two snowmobile engines tested at six different laboratories.THE 1990 AMENDMENTS TO THE CLEAN AIR ACT broadened the scope of U.S. EPA rulemaking authority to include nonroad engines and vehicles. Similar actions are under consideration world-wide. This attention has encouraged the emission testing of snowmobile engines, most of which had previously been done using the J1088 small utility engine test procedure due to the lack of a specific snowmobile engine test procedure.In response to the developing interest in snowmobile emissions, the International Snowmobile Industry Association (forerunner of the International Snowmobile Manufacturers Association, or ISMA, which represents all snowmobile manufacturers) launched an effort to develop a representative test procedure for the measurement of exhaust emissions from snowmobile engines. Southwest Research Institute (SwRI) was contracted to determine operating parameter data for snowmobiles under a variety of real-world operating conditions. SwRI applied statistical analysis to this database to determine representative operating points and weight factors, which were used to create a five-mode test cycle. This cycle was combined with the emissions measurement procedure defined in the U.S. EPA Marine Spark-Ignition Engine rule (40 CFR Part 91)(1)* to create a representative snowmobile emission test procedure. Round-robin testing was conducted on two snowmobile engines to refine and validate the procedure.