LI^2SA : Instrument for Soot Characterization and Optimization of Ultra-Low Emission Vehicles(Measurement, PM in Exhaust I)

Abstract

Modern PM measurement techniques for current and future engine concepts with ultra-low emission levels have to fulfill various challenging requirements. Sensitivity is one of the most important issues as current standard techniques are more and more getting insufficient when used together with exhaust aftertreatment systems like particulate traps and new combustion concepts. Secondly, a temporally high resolving in-situ technique is required to enable development of further emission minimization, particularly under transient conditions. Further, because of strong evidence of health risks of carbonaceous particles a mass based technique alone will no longer be sufficient. According to several medical studies other quantities like particle composition, size, number, and surface might be important to consider, as well. Laser-induced incandescence (LIT) has been proved to be a very sensitive and flexible tool for soot characterization in different laboratory applications for nearly twenty years. Recent developments have also indicated its excellent suitability for exhaust measurements. The technique relies on a rapid heating of soot particles within the gas flow to temperatures well above 4000 K by means of a highly energetic laser pulse and a subsequent optical detection of the strongly enhanced thermal radiation. By this, concentrations as low as 10 μg/m^3 and even below can be measured. Additionally, the temporal signal decay can be evaluated giving the surface area, or equivalently the primary particle size of the soot particles. Further extensions even allow the determination of the primary particle size distribution function. If LII is combined with other techniques, like elastic laser scattering, other measurement quantities like the agglomerate size are available, too. In this contribution the basic features of the LII technique are described and the LPSA system is introduced as a system relying on this technique. Various experimental results for diesel and gasoline direct injection (GDI) engines and some specific application examples obtained by this system are shown in some detail underlining the features of the LI^2SA sensor. These are including measurements downstream of a SCR catalyst and a particulate trap applied to a heavy duty diesel engine. By this, new insights in processes during internal combustion and exhaust aftertreatment are possible. As especially rapid changes of engine speed and load are the major sources for soot emissions during transient test cycles, the high temporal resolution of the system is of particular importance for research and development applications.