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Bag emission measurements on Ultra Low Emission Vehicles require measurement sensitivities in the 1 ppm range for HC and NOx and measurement resolutions well below this to obtain sufficient accuracy and repeatability. Additionally, an analysis of the C2 to C12 components is required. In these emission ranges, adsorption, desorption, diffusion and chemical reaction processes may produce significant effects to the measuring values. Therefore, improvements are necessary to avoid this as far as possible. However, for physical reasons these effects cannot be eliminated completely. For example: Particle filters are not 100% efficient and particles will slowly contaminate the surfaces. Due to physical and chemical processes with some gas components, even stainless steel and Teflon can change their characteristics. Problems resulting from the physical and chemical effects and provisions to minimize the influences to the measuring accuracy and system stability are discussed.

Current legislation trends for Heavy Duty and Off-Road engines show an increased contribution of transient cycles for engine homologation. For the HD EURO 3 and EURO 4 regulations, effective from the year 2000 and 2005 respectively, the new ESC (European Steady State Cycle), the new ETC (European Transient Cycle) and also the new ELR (European Load Response) Opacity test must be carried out, as well as the old ECE R 24 test procedure. A highly sophisticated new Opacimeter, designed according to the latest state of the art, is described. This instrument, certified to conform to the old and new legal homologation test procedures and to international standards, has proved to be also a powerful tool for engine R & D as well as production testing, as shown in several application examples.

The full-flow CVS dilution system is defined by the US EPA for particulate measurement of Heavy-Duty (HD) Diesel engines. Such systems are bulky and expensive. The EEC (1), Brazil and many other countries have therefore accepted the use of so-called “Partial-Flow Dilution Systems” (PDS) for homologation of HD engines. The ISO TC70 (2) committee even recommends such systems for the emissions measurement of Off-Road Diesel engines. Such systems sample and dilute only a fraction of the total exhaust, therefore they are much smaller than a full-flow CVS. However, it is required that the most important properties and conditions of a full-flow CVS systems are also fulfilled by a PDS. Therefore an accurate sampling and dilution control must be implemented in all Partial-Flow Dilution Systems (which makes such systems more complicated than a full-flow CVS). Various types of control have been developed and are described in the EEC and ISO regulations.

The ISO 8178 Draft for Particulate Measurement of Off-Highway Diesel engines specifies partial-flow dilution systems as the preferred measurement equipment. Two critical points of such systems are presented in this paper: First, the correct implementation and/or determination of exhaust sampling and dilution ratio within the accuracy limits of ISO 8178. Second, the influence of size, length and temperature conditioning of the transfer tube and tunnel on particulate formation, deposition and re-entrainment. As a result of our investigations, recommendations for the proper layout of a partial-flow dilution system are given. Such a system will yield particulate emission values closely following the values obtained with a full-flow CVS system, where engine size permits the comparison. Where the engine size precludes the full-flow dilution, a particulate test equipment with proven correlation to a CVS system should be used