Formation of Nanoparticles During Diesel Exhaust Dilution

When systematic studies were initiated to determine the size distribution of particles in engine exhaust, it soon became clear that both the total number of particles and their size distribution depended on the conditions of dilution of the exhaust pipe output. By far the most important factor was the dilution ratio, which describes the volume of diluted exhaust divided by the volume of raw exhaust.12 Studies in which road vehicles have been followed by instrumented laboratories indicate that in the atmosphere the mixing of vehicle exhaust rapidly creates dilution ratios of the order of 1000 or more. When these were used in laboratory dilution tunnel experiments, they caused a very marked downward shift in the mean size of emitted particles and an increase in the overall number of particles, as illustrated in Figure 2. Whilst the overall volume of particulate matter did not change significantly between low and high dilution ratios, at high dilution ratios a greater proportion of the particle volume was present as nanoparticles of 10-30 nm diameter as opposed to coarser particles. Subsequent research has shown that diesel engine exhaust contains two predominant components. The largest mass (but not number) of particles is comprised by particles with a core of elemental (graphitic) carbon formed within the combustion chamber of the engine itself. The mode in the number size distribution of such particles is typically in the 30-100 nm size range. The second most numerous part of the particle size distribution comprises particles generally within the range 10-30 nm, which are formed through the condensation of semi-volatile vapours during the dilution of the engine exhaust.12,13 The available research indicates that the most likely source of material for such particles is engine oil vaporised in the combustion process.

Using mass spectrometric techniques, Schneider et al.14 have made measurements of newly formed exhaust particles in both the laboratory and the field. They concluded from their data that particle formation depends upon sulfuric acid/water nucleation followed by condensation of involatile and semi-volatile organic compounds on pre-existing particles. Exhaust dilution conditions, fuel

ra 8.00E+08

Run Dilution Number concentration Volume concentration

Run Dilution Number concentration Volume concentration

ratio

(#/cm3)

(|m3/cm3)

1

1080

7.3E+08

2.0E+04

2

54

3.0E+08

1.9E+04

3

28

2.9E+08

2.3E+04

Figure 2 Comparison of particle size distribution measured at different dilution ratios at engine speed 1600 rpm and 50% load.12

1600 rpm 50% load

sulfur content and engine load (which influences the conversion of fuel sulfur to sulfuric acid) were all found to be influential. In a second study, which sampled both in the laboratory and on the road, Ronkko et a/.15 found that the nucleation mode, formed within 5 metres of the moving vehicle, did not change significantly with chase distance. Their results indicated that at low engine torques, hydrocarbons have an important role in the nucleation process, whilst at high torques the process appeared to be sulfur-driven.

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