| As the energy crisis and environment pollution problems are gradually prominent,it is a global consensus to search for alternative clean energy right now.The nitrogen oxide(NOx)and particulate matter(PM)emitted from diesel engine pose a great threat to atmosphere and human health.Compared with fossil diesel,biodiesel is sustainable and it has a higher cetane value,which improve the PM emission.However,the NOx emission increase because biodiesel has oxygen.Exhaust gas recirculation is a good way to reduce NOx emission because of the low in-cylinder temperature caused by the exhaust gas but the PM emission may increase.Therefore,it has academic and engineering value to explore the effects of EGR on the emission characteristics of diesel biodiesel dual fuel engine.An experimental study was carried on a light duty vehicle diesel engine to explore the effects of biodiesel blending percentage and EGR rates on the regular emission of diesel-biodiesel engine using exhaust analyzer and smokemeter.Moreover,the effects of EGR rates on mass concentration size distribution,microtopography,oxidation characteristics and SOF components of diesel particulate matter were revealed.The regular emissions of diesel-biodiesel engine were measured by using gas analyzer and smokemeter.Also,the mass concentration size distribution of particles was studied by using particle impactor.Results showed that,with the increasing biodiesel percentage,CO,HC and PM emission decreased while NOx emission increased.However,the effects of EGR on emissions were just the opposite.When EGR rates ranged from10% to 15%,the NOx and PM emissions were controlled in balance.The results of mass concentration size distribution showed that B20 particles emission tended to move to smaller size range and the mean size of of B20 particles got smaller compared with B0 particles.However,with the increase of EGR rate,particles emission tended to move to bigger size range and the mean size of particles got bigger.The microscopic appearance,nanostructure and graphitization structure of PM wereexplored by high resolution transmission electron microscopy(HRTEM)and Raman spectroscopy.Results showed that the overlapping phenomenon of B20 particles were weakened compared with B0 particles and the mean size of primary particles got smaller.Also,the layer distance and tortuosity of B20 primary particles increased slightly while the fringe length decreased 10%~15% and B20 particles were more anomaly with a lower graphitization degree.However,when EGR rate was raised to 25%,the layer distance and tortuosity of primary particles decreased slightly while the fringe length increased20%~25% and the particles got more mature with a higher graphitization degree.As we can see,the Roman spectroscopy test results are identical with the results of HRTEM.Finally,the oxidation characteristics and soluble organic fraction(SOF)components of PM were explored by thermogravimetry analyzer(TGA)and gas chromatograph-mass spectrometer(GC-MS)respectively.The TGA results showed that the SOF content of B20 particle were higher than B0 particle but soot content of B20 decreased.With the increase of EGR rate,the SOF decreased while the soot content increased.Moreover,the ignition temperature of B20 particles was lower than B0 particles,which indicated that B20 particles were easier to be oxidized.However,with the increase of EGR rate,the ignition temperature of PM increased,which means the oxidation activity of PM was weaken.The GC-MS results showed that the SOF components of particle includes alkane,phenols,esters,acids,alcohols and aromatic hydrocarbon.The mass content of alkane was the highest and may reach 50%.With the addition of biodiesel,the percentage of esters,acids increase while the others decreased.Moreover,with the increase of EGR rate,aromatic hydrocarbon obviously increased. |
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