Influence of dilution conditions on diesel exhaust nanoparticle emissions: Experimental investigation and theoretical assessment.

摘要

Experiments were performed to investigate the influence of dilution conditions on particle number concentrations and size distributions emitted from a diesel engine. The experimental apparatus consists of a turbocharged direct injection diesel engine, two-stage air ejector, variable residence time, micro dilution system for exhaust dilution, and particle detection instruments that include a scanning mobility particle sizer (SMPS), and a condensation particle counter (CPC).; The first series of experiments were taken upstream and downstream of a ceramic filter (trap) during the ISO 11 mode test. Changing the residence time at the primary dilution stage from 40 ms to 6000 ms lead to an increase in nanoparticle concentrations by up to four orders of magnitude. During trap regeneration which occurs at an exhaust temperature of 550°C, nanoparticles below about 15 nm in diameter were an order of magnitude higher than engine out concentrations.; The second series of experiments focused on engine out emissions at mode 8 (1600 RPM, 50% load). The residence time, dilution ratio, and dilution temperature in the primary stage of dilution have a very strong effect on nanoparticle measurements. Close to two orders of magnitude difference in nanoparticle concentration level can be measured depending on the combination of the aforementioned dilution variables.; Dilution air humidity ratio doesn't seem to influence the total number of nanoparticles significantly. An increase in humidity ratio from 0.01 to 0.03 at a temperature of 48°C, lead to an increase nanoparticle concentrations by about 30%.; The use of ultra low sulfur fuel reduces the number concentration by 70% when compared with D-2 diesel that has 40 times higher sulfur concentration.; A binary homogenous nucleation based on the classical nucleation theory has been developed to simulate sulfuric acid and water nucleation. The model shows that background concentration of solid particles act to suppress particle formation and growth. This is consistent with the results from the trap. It is also consistent with reports that clean modern diesel engine may emit much higher numbers of nanoparticles than older technology engines.; The model shows more sensitivity to temperature and humidity ratio than experiments. At low temperature and/or high humidity ratio the nucleation rate is high and the growth is limited to growth by coagulation.; This work gives important guidelines for any future measurements of nanoparticles. It has shed some light on the complexity of any future number emission standard related to volatile particles. The ultimate goal is to use a dilution system in the laboratory to mimic the atmospheric dilution process. (Abstract shortened by UMI.)