Abstract:
A real-time particulate matter measuring system includes a hot diluter for receiving and diluting aerosol. An evaporation unit receives the diluted aerosol. A cold diluter receives and dilutes the output flow from the evaporation unit. A condensation particle counter receives the diluted output flow and measures the particle number concentration. An integrated device also receives the diluted output flow. The integrated device measures at least one of particle diameter concentration and particle surface area. The integrated device also measures mass concentration.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to real-time particulate matter measuring systems.  
         [0003]     2. Background Art  
         [0004]     Particulate matter (PM) emitted by combustion engines causes public health and environmental concerns. Strict emission standards have been legislated to reduce the emission of pollutants from the combustion engines. Recently, studies have found that the mass based PM emissions from engines, with modern techniques, have been reduced significantly. However, the number concentrations of PM emissions from engines have been increased significantly. As a result, emitted particles have smaller sizes, and larger surface areas. When studying engine emitted particles, it is very important to measure the particles correctly.  
         [0005]     An existing approach to measuring solid particle number emissions from light-duty diesel vehicles is illustrated in  FIG. 1  at  10 . System  10  includes a pre-classifier  12 , hot diluter (PND 1 )  14 , evaporation tube  16 , cold diluter (PND 2 )  18 , and a condensation particle counter (CPC)  20 . Pre-classifier  12  is used to keep the cutoff size of aerosol in the range of 2.5 to 10 μm. By running hot diluter  14  at high dilution air temperature, and heating the sample in the range of 300 to 400° C. in evaporation tube  16 , particles formed by volatile material and sulfate particles are vaporized to gas phase. During cold dilution with cold diluter  18 , all particles formed by volatile material and sulfate are removed. As a result, only solid particles move into condensation particle counter (CPC)  20 . The concentration of the solid particles is measured at CPC  20 .  
         [0006]     Although existing systems for measuring particulate matter are used in many applications, these systems have limited functionality. This is a need for a real-time particulate measuring system that has extended functionality.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the invention to provide a powerful tool for measuring engine exhaust particle characteristics that can be used for engine combustion and exhaust after-treatment research.  
         [0008]     According to the invention, a real-time particulate matter measuring system has extended functionality beyond that of the existing approaches to measuring solid particle number emissions. The invention involves integrating a diffusion charger or particle diameter concentration measuring instrument into a solid particle counting system (SPCS). The contemplated system may generate up to seven real-time particle characteristics instead of only one for either solid particles or total particles.  
         [0009]     A real-time particulate matter measuring system made in accordance with the invention involves a pre-classifier, hot diluter, evaporation tube or unit, cold diluter, and condensation particle counter. The system further comprises an integrated device for measuring mass concentration, and either particle diameter concentration or particle surface area.  
         [0010]     In one approach, a diffusion charger (DC) is integrated into the solid particle counting system. In another approach, a particle diameter concentration measuring instrument is integrated into the solid particle counting system. In either of these approaches, the real-time particulate matter measuring system is able to provide up to at least seven characteristics instead of one for engine emitted solid particles or total particles. These characteristics include real-time solid particle or total particle number concentration, surface area, diameter concentration, mass concentration, volume concentration, average diameter, and average effective density.  
         [0011]     The diffusion charger (DC) is a type of sensor for measuring particle surface area in real-time. By calibrating the diffusion charger with the conventional PM mass measurement, the diffusion charger can measure real-time mass concentration simultaneously.  
         [0012]     The diameter concentration instrument operates under a similar principle as the diffusion charger. The diameter concentration instrument measures particle diameter concentration (for example, mm/cm 3 ) and is calibrated to measure particle diameter concentration instead of surface area. In the same way as the diffusion charger, the diameter concentration instrument may be calibrated to measure real-time mass concentration simultaneously.  
         [0013]     In a measuring system with the diffusion charger, the condensation particle counter in the system provides the solid particle or total particle number concentration. The diffusion charger measure the real-time surface area and mass concentration for solid particles or total particles. The average diameter, diameter concentration, volume concentration, and average effective density for solid particles or total particles can be calculated from the information obtained from the condensation particle counter and diffusion charger.  
         [0014]     In a system with the diameter concentration instrument, the condensation particle counter measures the solid or total particle number concentration. The diameter concentration instrument measures particle diameter concentration and mass concentration for solid or total particles. The average diameter, surface area, volume concentration, and average effective density for solid or total particles can be calculated from the information obtained from the condensation particle counter and diameter concentration instrument.  
         [0015]     The advantages associated with embodiments of the invention are numerous. For example, the diffusion charger or diameter concentration instrument may take samples at the same place as the condensation particle counter. In this way, excessive modification to the solid particle counting system is not required to implement a real-time particulate matter measuring system in accordance with the invention. The improved system provides a powerful tool for measuring particle characteristics with greatly increased functionality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  illustrates an existing solid particle counting system;  
         [0017]      FIG. 2  illustrates a real-time particulate matter measuring system made in accordance with a first embodiment of the invention; and  
         [0018]      FIG. 3  illustrates a real-time particulate matter measuring system made in accordance with a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     With reference to  FIG. 2 , the real-time particulate matter measuring system includes solid particle counting system (SPCS)  30 , SPCS computer  32 , and diffusion charger (DC)  34 . SPCS  30  includes pre-classifier  40 , hot diluter (PND 1 )  42  and associated temperature controller  44 , evaporation unit  46  and associated temperature controller  48 , cold diluter (PND 2 )  50 , and condensation particle counter (CPC)  52 . Pre-classifier  40  is used to keep the cut-off size of aerosol in the range of 2.5 to 10 μm. By running hot diluter  42  at high dilution air temperature with temperature controller  44 , and heating the sample in the range of 300 to 400° C. in evaporation unit  46 , particles formed by volatile material and sulfate particles are vaporized to gas phase. Following cold dilution at cold diluter  50 , all particles formed by volatile material and sulfate are removed. Solid particles move into CPC  52 , and concentration of the solid particles is measured at CPC  52 . In the case where hot diluter  42  and evaporation unit  46  are run at lower temperature to avoid vaporization of volatile material and sulfate particles to gas phase, all particles move into CPC  52  with the flow, and total particle concentration is measured at CPC  52 .  
         [0020]     By connecting the inlet of diffusion charger  34  to the upstream side of CPC  52  in the SPCS  30 , diffusion charger  34  takes samples at the same place as CPC  52 . The computer control and data acquisition system  32  for SPCS  30  is used to obtain signals from diffusion charger  34 , which is calibrated to measure surface area and mass concentration in real-time. Different calibration curves for mass concentration on solid and total particles with diffusion charger  34  can be stored in the SPCS computer  32 . For example, the calibration curve for solid particle mass concentration is selected when the hot diluter and evaporation unit in the SPCS run at high temperature. Otherwise, the calibration curve for total particle mass concentration is selected.  
         [0021]     As described above, CPC  52  measures number concentration, and DC  34  measures surface area and mass concentration in real-time. The average diameter, diameter concentration, volume concentration, and average effective density for measured particles, either solid or total particles, can be calculated as:  
               D     p   ⁡     (     t   -   td     )         =         S     t   -   td         π   ⁢           ⁢     N   t                         L     t   -   td       =       D     p   ⁡     (     t   -   td     )         ⁢     N   t                     V     t   -   td       =       1   6     ⁢   π   ⁢           ⁢     D     p   ⁡     (     t   -   td     )       3     ⁢     N   t                     ρ     eff   ⁡     (     t   -   td     )         =       m     t   -   td         V     t   -   td                   
 
 where td is the delay time of the CPC against the DC; D p(t−td)  is average diameter for surface area at time t−td; S t−td  is surface area measured with the DC at time t−td; N t  is the number concentration measured by the CPC at time t; L t−td  is particle diameter concentration at time t−td; V t−td  is volume concentration at time t−td; ρ eff(t−td)  is the average effective density at time t−td; m t−td  is the mass concentration measured by the DC at time t−td. 
 
         [0022]     With the current technique, the response time of the DC is much faster than that of the CPC. Therefore, there is a delay time correction involved in the above equations. Delay time td can be measured by running the real-time particulate matter measuring system under transient conditions. It is a constant at the fixed system configuration.  
         [0023]     In summary, there are seven functions obtained from this embodiment of the real-time particulate matter measuring system: 
        1. Number concentration     2. Particle diameter concentration     3. Surface area     4. Mass concentration     5. Average diameter     6. Volume concentration     7. Average effective density        
 
         [0031]     With reference to  FIG. 3 , the real-time particulate matter measuring system includes SPCS  60 , SPCS computer  62 , and diameter concentration instrument  64 . SPCS  60  includes pre-classifier  70 , hot diluter (PND 1 )  72  and associated temperature controller  74 , evaporation unit  76  and associated temperature controller  78 , cold diluter (PND 2 )  80 , and condensation particle counter (CPC)  82 .  
         [0032]     The inlet of diameter concentration instrument  64  is connected to the upstream side of CPC  82  in the SPCS  60 . Diameter concentration instrument  64  takes samples at the same place as CPC  82 . The computer control and data acquisition system  62  for SPCS  60  is used to obtain signals from diameter concentration instrument  64 , which is calibrated to measure particle diameter concentration and mass concentration in real-time. Different calibration curves for mass concentration on solid and total particles with diameter concentration instrument  64  can be stored in SPCS computer  62 . For example, the calibration curve for solid particle mass concentration is selected when the hot diluter and evaporation unit in the SPCS run at high temperature. Otherwise, the calibration curve for total particle mass concentration is selected.  
         [0033]     CPC  82  measures particle number concentration. The diameter concentration instrument  64  measures diameter concentration and mass concentration in real-time. The average diameter, surface area, volume concentration, and average effective density for measured particles, either solid or total particles, can be calculated as:  
               D     p   ⁡     (     t   -   td     )         =       L     t   -   td         N   t                     S     t   -   td       =     π   ⁢           ⁢     D     p   ⁡     (     t   -   td     )       2     ⁢     N   t                     V     t   -   td       =       1   6     ⁢   π   ⁢           ⁢     D     p   ⁡     (     t   -   td     )       3     ⁢     N   t                     ρ     eff   ⁡     (     t   -   td     )         =       m     t   -   td         V     t   -   td                   
 
 where td is the delay time of the CPC against the diameter concentration instrument; D p(t−td)  is average diameter for particle diameter concentration at time t−td; L t−td  is particle diameter concentration measured with the diameter concentration instrument at time t−td; S t−td  is surface area at time t−td; N t  is the number concentration measured by the CPC at time t; V t−td  is volume concentration at time t−td; ρ eff(t−td)  is the average effective density at time t−td; m t−td  is the mass concentration measured by the diameter concentration instrument at time t−td. 
 
         [0034]     The delay time of the CPC against the diameter concentration instrument, td, can be measured by running the real-time particulate matter measuring system under transient conditions. If there is no delay time between the CPC and the diameter concentration instrument, td is equal to zero. The sign (negative and positive) of td reflects that the diameter concentration instrument either faster or slower than the CPC.  
         [0035]     In summary, there are seven functions obtained from this embodiment of the real-time particulate matter measuring system: 
        1. Number concentration     2. Particle diameter concentration     3. Surface area     4. Mass concentration     5. Average diameter     6. Volume concentration     7. Average effective density 
 
 The following procedures may be used to measure either solid or total particles in the illustrated embodiments: 
    a. Solid particle measurement: The temperature controllers for the hot diluter and evaporation unit are set at high temperatures. For example, the temperature controller for the hot diluter is set at a temperature higher than 150° C., and the temperature controller for the evaporation unit is set at 300 to 400° C. Thus, particles formed by volatile material and sulfur compound are removed. The CPC and diffusion charger (DC) or diameter concentration instrument measure solid particles only. To avoid saturation of the CPC and DC or diameter concentration instrument, the dilution ratios for the hot diluter and the cold diluter can be adjusted to higher values.     b. Total particle measurement: The temperature controllers for the hot diluter and evaporation unit are set at room temperature or turned off. As a result, total particles including solid, volatile, and sulfur compound particles flow into the CPC and diffusion charger (DC) or diameter concentration instrument. To avoid saturation of the CPC and DC or diameter concentration instrument, the dilution ratios for the hot diluter and the cold diluter can be adjusted to higher values.        
 
         [0045]     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.