Experimental Research On The Measurement And Evolution Of Polycyclic Aromatic Hydrocarbons During Diesel Combustion Process

Polycyclic aromatic hydrocarbons (PAHs) are not only one of diesel emission pollutions, but also significantly important in the formation of diesel particulate matter (PM) due to their roles as the soot precursors. Therefore, study on the evaluation of the PAHs in diesel engine combustion process has a great theoretical and practical significance for the understanding and control of diesel PAHs and PM emissions. Based on a total cylinder dumpling system and the optimized gas chromatography-mass spectrograph (GC-MS) method, the evolution of PAHs in diesel and n-heptane combustion process was investigated in this dissertation, and the major work and achievements are listed as follows:1. The large-volume programmed temperature vaporization (PTV) injection technique in GC-MS was selected for the measurement of the PAHs at a trace or even ultra-trace level during diesel combustion process. The PTV injection factors were optimized using the experimental design. The limits of detection (LOD) of the optimized PTV injection method ranged between 0.0164~0.625 ng/mL for PTV 25μL, and 0.0094~0.1154 ng/mL for PTV 50μL, respectively. The recoveries ranged between 86.41~118.35% for filters and 71.83~118.6% for PUF/XAD/PUF cartridges, respectively.2. During the n-heptane combustion process, the two and three-rings PAHs accounted for more than 66% of the total PAHs, ranging between 0.7~6.6μg/cycle. The two and three-rings PAHs decreased in the late stage of the pre-mixed combustion phase, and then increased in the diffusion combustion phase with an increase in the crank angle. The mass of four, five and six-rings PAHs was all below 0.4μg/cycle, and showed a decrease after an ascent with the increase of crank angle. EGR ratio only affected the amounts of PAHs, but did not have an effect on the trends of PAHs as function of crank angle.3. The evolution history of PAHs during the combustion process of real diesel fuel was investigated. The results showed that the two-ring PAHs mass decreased in the pre-mixed combustion phase, and then increased in the diffusion combustion phase with an increase in the crank angle, ranging between 3~12μg/cycle. The amounts of three-rings PAHs ranged between 0.5~1.2μg/cycle, and the four, five and six-rings PAHs were below 0.9μg/cycle. The total mass of PAHs with more than two rings showed a decrease after an increase as the crank angle increased. The use of EGR would result in a decrease in the amounts of PAHs in diffusion combustion phase and an increase in pre-mixed and late combustion phase.4. During the combustion process of n-heptane, the soot presented a unimodal trend. The soluble organic fraction (SOF) accounted for a large proportion of PM, and its variation was similar to that of the PM in the combustion process.5. The preliminary research on the relationship between PAHs and soot was carried out in light of the PAHs/soot ratio during n-heptane combustion process. In the pre-mixed combustion phase, the ratios of PAHs/soot decreased by more than 75% with the increase in crank angle, which probably suggested that the formation reaction from PAHs to soot surpassed that of PAHs oxidation. By contrast, in the late stage of diffusion combustion phase, the ratios of PAHs/soot increased with the increase in crank angle. This might imply that the formation of soot from PAHs slowed down. The use of EGR would decrease the ratios of PAHs/soot, but the pattern of PAHs/soot ratios, with a decrease in the pre-mixed combustion phase and an increase in the late stage of diffusion combustion phase, did not change.