Abstract
The procedure for sampling emission legislation controlled total PM from diesel engines is straightforward aiming at giving an estimate of total mass emitted from the tailpipe at strictly specified conditions. However, for sizing and other characterisation purposes lots of uncertainty remains how to measure particulates in a realistic and representative way. This is especially true for SI engines, particles of which differ much in amounts and properties from those of diesel. The purpose of this paper was to correlate some sampling conditions with particulate characteristics, both from diesel and gasoline passenger vehicles.
The effect of sampling methods as well as sampling conditions on particulate emissions and properties of Audi 1.9 TDI diesel and gasoline vehicles of three combustion strategies: Nissan Micra 1.3 Mpi, Mitsubishi 1.8 GDI and VW Lupo 1.4 FSI were studied. Several aerosol measurement instruments for size segregated particulate number, mass and other properties of particles were applied. Mass size distributions were measured with low pressure impactors and traditional filters. The impactors were optionally equipped with techniques allowing determination of the organic carbon/elemental carbon (OC/EC) ratio. For particulate number and size distribution measurements electrical low pressure impactors (ELPI) with or without filter stages for the smallest nano-sized particles, as well as SMPSs were used. Thermodenuder was used for stripping volatiles from particles. The Audi was equipped with an EGR and an oxygenation catalyst, Mitsubishi GDI and VW Lupo represented stratified direct in-cylinder fuel injection SI engine technology, the Lupo having also EGR and NOx storage catalyst. Both ECE+EUDC cycle and steady state conditions were studied. Dilution and sampling techniques used were conventional exhaust dilution tunnels designed for determining PM mass emission rates, ejector diluters with a fixed dilution ratio and a porous tube diluter with a variable dilution ratio. Dilution conditions varied especially in respect of residence time, humidity of dilution air and dryness of particles.
The results show that dilution tunnel parameters, e.g. tunnel conditioning and moisture content of the mixture, have considerable influence in particulate characteristics. In unfavourable moisture conditions in the accumulation mode grows intensively and size distribution is shifted to larger particles. This phenomenon was discovered both for gasoline and diesel vehicles. With relatively dry dilution mixture a steady size distribution was attained even at low dilution ratios of Dr 8 9. In tunnel measurements dilution air moisture content is not limited even for total PM determination, only the intake air to the engine is controlled by regulations. As the fixed carbon share in the Audi particles is high, particulate measurement was not sensitive to dilution method, as long as dilution ratio was kept acceptably high or dilution air dry. Porous tube diluter was a very effective instrument with its easy adjustibility of Dr over wide range. It also correlated well with dilution tunnel and ejector for accumulated mode particles. Thermodenuder had a considerable effect on nanoparticles of VW Lupo FSI and Nissan Micra, but no effect on larger particles. The correlation of fixed 60 nm 2.5 µm particles measured with ELPI with black carbon measurement with ethalometer was excellent. In case policy makers end up deciding that they are the fixed particles not nanoparticle numbers - from vehicle emissions that shall be monitored and restricted, ELPI may be promising as a future device for that purpose, being also insensitive to particle chemistry. The emission levels of particles and those volatility properties from modern PFI, GDI/FSI and CI automotive engines vary in such a wide scale, some 3 orders of magnitude, that definitely different precautions and preparations for particulate measurements are required. The particulate emission levels of VW Lupo and Nissan Micra at high speeds was so low that with reasonable dilution ratios the emission was not separable from tunnel background. If these emission levels are to be monitored other dilution system than tunnel should be used. Lower speeds and cycle driving did not raise that problem. All in all, it seems improbable with any sampling protocol to be able to simulate release and behaviour of tailpipe particles in the atmosphere, but it may serve well for particle characterisation, like determination the amount of fixed carbon in size segregated particles.