A comparison of tailpipe, dilution tunnel, and wind tunnel data in measuring motor vehicle PM

Comparison between particle size distributions recorded directly at the tai lpipes of both diesel and gasoline vehicles and measurements made using a c onventional dilution tunnel reveals two problems incurred when using the la tter method for studying particle number emissions. One is the potential fo r particulate matter (PM) artifacts originating from hydrocarbon material s tored in the transfer hose connecting the tailpipe to the dilution tunnel, and the other is the particle coagulation (as well as condensation and chem ical changes) that occurs during the transport. Both are potentially generi c to current PM emissions measurement practices. The artifacts typically oc cur as a nanoparticle mode (10-30 nm) that is 2-4 orders of magnitude large r than what is present in the vehicle exhaust and can easily be mistaken fo r a similar mode that can arise from the nucleation of hydrocarbon or SO(4) (2-)components in the exhaust under appropriate dilution rates. Wind tunnel measurements are in good agreement with those made directly from the tailp ipe and substantiate the potential for artifacts. They reveal PM levels for the recent model port fuel injection (PFI) gasoline vehicles tested that a re small compared with the ambient background particle level during steady- state driving. The PM emissions recorded for drive cycles such as the Feder al Test Procedure (FTP) and US06 occur primarily during acceleration, as ha s been previously noted. Light-duty diesel vehicle emissions normally exhib it a single lognormal mode centered between 55 and 80 nm, although a nonart ifact nanoparticle mode in some cases appears at a 70-mph cruise up a grade .