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A correlation program was established between heavy-duty engine transient test facilities in order to determine emission variabilities within and among laboratories. Significant differences between the laboratories observed for all emission components were smaller compared to data reported previously. For gaseous emissions among-lab standard deviation was significantly, for particulates only slightly higher than within-lab standard deviation. Two groups of laboratories were found that correlate rather well within the respective group. Normalizing the standard deviations to 1991 emission regulations showed that oxides of nitrogen and hydrocarbon variabilities were about 16 %, particulate variability about 45 % of the regulated emission values. Therefore, particulate measurement has to be improved considerably. Cycle work was found to influence among-lab variability of all components.

An international standard for nonroad engines has been developed that comprises gaseous and particulate emissions measurement procedures, smoke testing, test cycles, and an engine family and group concept. Through a joint effort of industry and government agencies, ISO 8178 has become the basis for emissions legislation in the USA, the European Union and Japan and of the International Maritime Organization. The ultimate goal of worldwide harmonization for the worldwide engine industry has been reached, but much effort is still needed to maintain the level of harmonization achieved today. The validity of ISO 8178 has been demonstrated on a round robin test with three engines of 19 to 170 kW circulated around 28 test laboratories. Test-to-test repeatability was generally lower than 10 %. Lab-to-lab variability was less than 10 % for NOx and particulates, and over 25 % for HC and CO. The equivalence of partial flow and full flow dilution systems for particulates has been proven.