Analysis of measurement errors of cvs method used for tests of pollutant emissions from light-duty vehicles

The test procedure for the pollutant emissions from light-duty vehicles is currently under verification carried out at the international level. For this purpose, GRPE (Groupe des rapporteurs sur la pollution et energie) – the body developing UNECE regulations proposals - has set up a task group WLTP (Worldwide harmonized Light vehicles Test Procedure) which is supposed to prepare a new GTR (Global Technical Regulation) relating to this procedure. The development of light-duty vehicles, leading to the reduction of pollutants emissions, caused the increase of the requirements for their test procedure. A large number of factors associated, inter alia, with the test equipment affects its accuracy, repeatability and reproducibility. In the ongoing analysis attention is paid even to factors, which affect test results in a relatively small way, and which have been overlooked yet. One of the problems to which attention is paid is the impact of pollutant concentration in the dilution air in the CVS (constant volume sampler) system on test results. The CVS principle is accepted as the standard method to determine light-duty vehicle exhaust emissions on chassis dynamometer test cells worldwide. It was introduced in the early 1970s. One of its drawbacks is that it introduces an error resulting from the fact that the contractual DF dilution factor, rather than the actual dilution ratio DR, is used for the correction of concentration measurement results. The article discusses this error for different type of engines used in light-duty vehicles and different pollutants subject to measurements.