Dilution systems to simulate engine exhaust dilution in the atmosphere

The formation of nanoparticles from engine exhaust is very sensitive to dilution parameters including: ambient temperature, relative humidity, dilution ratio, etc. A second-generation single-stage (1-S) dilution tunnel designed to simulate atmospheric dilution was developed and used to study the formation of engine exhaust nanoparticles. Size distribution measurements with the 1-S dilution tunnel are compared with two other systems that utilize quite different mixing strategies; a well-developed two-stage dilution tunnel (2-S), and the modified BG-2 dilution system (MBG). Size distributions were mainly bimodal with nuclei and accumulation modes in the 3 to 30 and 30 to 500 nm diameter ranges, respectively. Nearly all of the nanoparticles were found in the nuclei mode, which sometimes consisted of extremely small particles <8 nm in diameter. Size distributions measured with the three systems differed mainly in the nuclei mode. The 1-S system produced narrower, more monodisperse modes than either of the other systems. All three systems produced similar results in the accumulation size range, although the 2-S had somewhat higher losses. All three dilution systems needed time to stabilize when engine conditions were changed, typically about 15 minutes for the 1-S and 2-S, and about 30 minutes for the MBG.; Effects of the transfer line residence time; dilution air temperature, relative humidity, and mixing profile on measured size distributions were investigated with the 1-S. Shorter transfer line residence time and lower dilution air temperature significantly increased the tendency to form nuclei mode particles. The mixing profile had little influence on nuclei mode formation except that counterflow mixing, which gave slower initial mixing rates, increased nucleation. Contrary to results reported elsewhere, relative humidity had little influence on nuclei mode formation.; Increasing the sulfur content of the fuel led to increased nanoparticle formation. The influence of fuel sulfur was stronger for the 1-S tunnel than for the 2-S tunnel. It is believed that the nuclei mode consists primarily of sulfuric acid and heavy hydrocarbons. Nuclei mode formation was modeled using classical nucleation theory for hydrocarbons and H₂SO ₄-H₂O binary nucleation theory. Classical theory did not predict hydrocarbon nucleation. H₂SO₄-H₂O binary theory correctly predicted general trends but the predicted sensitivity of nucleation to engine and dilution parameters was much greater than observed.; Size distributions measured with the three systems are compared with those measured in on-road chase experiments. It was impossible to exactly duplicate on-road operating conditions in the laboratory, but with suitable adjustment of dilution parameters, size distributions similar to those observed on-road could be obtained with all three of the dilution systems. However, current understanding of the problem is insufficient to allow these parameters to be adjusted without prior knowledge of the on-road size distribution.