浙江大学学报(农业与生命科学版)  2016, Vol. 42 Issue (3): 378-384
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陆叶强, 史云斌, 孙在, 宋熠金, 楼晓春. 基于光散射法柴油发动机尾气颗粒测试分析[J]. 浙江大学学报(农业与生命科学版), 2016, 42(3): 378-384. DOI:10.3785/j.issn.1008-9209.2016.02.221
LU Yeqiang, SHI Yunbin, SUN Zai, SONG Yijin, LOU Xiaochun. Measurement and analysis of diesel exhaust particulates based on light scattering[J]. Journal of Zhejiang University (Agriculture and Life Sciences), 2016, 42(3): 378-384.  DOI: 10.3785/j.issn.1008-9209.2016.02.221.
基于光散射法柴油发动机尾气颗粒测试分析[PDF全文]
陆叶强1 , 史云斌1, 孙在2, 宋熠金2, 楼晓春1    
1. 杭州职业技术学院青年汽车学院,杭州310018;
2. 中国计量学院机电工程学院,杭州310018
基金项目: 国家自然科学基金(10972209)
通信作者: 陆叶强(http://orcid.org/0000-0002-6953-8234), E-mail:545126046@qq.com
收稿日期(Received): 2016-02-22; 接受日期(Accepted): 2016-03-02; 网络出版日期(Published online): 2016-05-19
摘要: 通过计算无因次参量(x)、散射光接收角(θ)、粒径(D)等参量和散射光强(Is)之间的数值关系,以及散射光接收立体角β和散射光通量(F)之间的数值关系,得到光散射法测量柴油发动机尾气颗粒粒径谱和单位体积中颗粒含量的理论依据。1)随着D的增大,Is显著增加并且分布随θ波动越大,某一散射角内Is与D之间不再具有成正比的一一对应关系,需测量多角度内Is而计算得到颗粒粒径谱;2)90°散射光接受范围附近光通量F与颗粒的相对体积D3参数具有很好的线性关系,通过测量F可得到颗粒单位体积的含量,并且选取散射光接收立体角β为30°,能够取得很好的测量效果。依据上述结论设计的测量装置用于柴油发动机台架的尾气粒径分布测量实验，并将该测量装置用于柴油发动机的尾气粒径分布的实际对比测试实验,结果表明该测量装置稳定性、实用性好，测量精度高,并具有实时、在线、不损坏样品等优点。
关键词: 尾气颗粒物    Mie散射    尺寸分布    CCD(电荷耦合元件)测试仪    
Measurement and analysis of diesel exhaust particulates based on light scattering
LU Yeqiang1 , SHI Yunbin1, SUN Zai2, SONG Yijin2, LOU Xiaochun1    
1. College of Youth Automotive, Hangzhou Vocational&Technical College, Hangzhou 310018, China;
2. College of Mechanical and Electrical Engineering, China Jiliang University, Hangzhou 310018, China
Summary: Due to the characteristics of mixture formation and combustion of diesel engine, it has far higher tail gas particulate matter (PM) emissions than those of gasoline engine. It has been proved that soluble organic fractions (SOFs) adsorbed on the PM surface of diesel engine are mutagenic, and more than 90% of the components are carcinogens. Therefore, PM emissions have become an important factor restricting the popularization and application of diesel engine vehicles. To analyze and test PMs in diesel engine tail gas (DETG) also becomes a hot spot. PMs in DETG mainly include carbon smoke (C), SOF, sulfate (SO4) and ash, which account for 40%-50%, 35%-45%, 5%-10% and 3%-6% in PMs, respectively. According to a physicochemical analysis of PMs in DETG, PMs are very tiny and complicated, containing volatile components and heavy metals. This raises higher requirements for the reliability, self-purification and anti-interference of PM testers. To measure PMs using light scattering method (LSM), a non-contact modern photoelectric measurement technique, has the advantages of real-time, online and harmless to sample, etc. It is superior in the diameter spectrum analysis and concentration measurement of PMs in DETG. The diameter ranges from 1 to 10 000 No./cm3. PM concentration, though very low in emissions, can be measured. The counting efficiency of (41±1) nm PM was 90% or more and the counting precision was ±10%. Based on this, this article proposes to measure diameter spectrum and concentration of PMs in DETG, using LSM. By calculating the numerical relationship between parameters like dimensionless parameter x, acceptance angle of scattered light θ, diameter D and intensity of scattered light Is, as well as the numerical relationship between acceptance solid angle of scattered light β and flux F, this article obtained a theoretical basis for measuring diameter and concentration of PMs in DETG using LSM. 1) With the increase of D, Is increased significantly and the distribution fluctuated more drastically with θ. In a certain scattering angle, Is and D were no longer proportional and corresponding to each other. It was required to measure Is in multiple angles and calculate the diameter spectrum; 2) Near the acceptance range of 90° scattered light F and relative volume D3 of particles had a good linear relationship. By measuring F, the volume concentration of particles can be obtained. Meanwhile, by selecting acceptance solid angle of scattered light β was 30°, a good measurement effect can be achieved. According to the above conclusion, a diameter spectrum and concentration tester for PMs in DETG based on LSM was developed. This tester was composed of sampling probe, flow meter, water filter, PM filter, pump, dilute air duct, heating device, photoelectric detector, IPC and on-off valve, etc. The working process was to pump air into a dilute channel and heating device, through flow meter, water and PM filter, dilute and heat the tail gas to be tested, analyze and test the diameter distribution of PMs in DETG in an ideal state. This tester conducted an experiment by measuring the diameter distribution of DETG. By comparing with precision testing instrument, from the measured data, this tester had better consistency than precision testing instrument. It was more advanced and practical. By analyzing the physicochemical characteristics of PMs in DETG and the numerical relationship between all parameters, intensity of scattered light Is and flux F, the following conclusions are drawn: 1) The correctness and feasibility to measure diameter spectrum and concentration of PMs in DETG using LSM are analyzed theoretically; 2) According to the above principle, a diameter spectrum and concentration tester for PMs in DETG is designed; 3) By comparing the experimental results, this tester has the advantages of real-time, online and harmless to sample, etc.
Key words: exhaust particle    Mie particle-scattering    particle size distribution    charge-coupled device tester    

汽车是石油的消耗大户,相对于汽油机车,柴油发动机车辆具有良好的燃油经济性。柴油发动机由于混合气形成和燃烧的特点其尾气颗粒物(particulate matter,PM)的排放远高于汽油发动机[1-3]。美国环保局的试验证明,吸附在柴油发动机排气微粒表面的可溶有机物(soluble organic fractions,SOFs)具有诱变作用,其组分的90%以上为致癌物质[4-5]。因此,PM排放成为制约柴油发动机车辆推广应用的重要因素,应用于低PM排放的清洁柴油机技术中的PM分析检测成为研究热点[7],具有代表性的有:1)利用静电低压碰撞技术(electrical low pressure impactor,ELPI)研发的ELPI分析仪;2)美国Sensor公司利用压电天平技术研发的Semtech QCM/MPS颗粒物测量系统;3)英国Combustion公司研发的快速型颗粒物粒径谱仪,差压电迁移率谱仪;4)美国TSI公司采用光散射技术研发的发动机废气排放颗粒物粒径谱仪EECPC-3790。

1 柴油发动机尾气颗粒的物化特性分析

柴油发动机尾气颗粒主要包括碳烟(carbon,C)、可溶有机物(soluble organic fractions,SOF)、硫酸盐(SO4)和灰分,分别占颗粒物组成的40%~50%,35%~45%,5%~10%,3%~6%,图 1为柴油发动机尾气PM在显微镜下的各种形状[7]。实验中通常测量到的颗粒物不仅包括在燃烧过程形成的颗粒物,还包括取样时在稀释和冷却过程中形成的颗粒物。由可溶性有机物(SOF通常称为挥发性组分,主要包括一些多环芳烃)经冷凝作用形成的颗粒物粒径很小,但是数量很大。因此,这部分颗粒物对质量浓度的影响甚微,但可以使粒数含量大幅增加。目前,尾气颗粒物在粒径谱测量之前都要将待测尾气加热到100 ℃左右使SOF挥发,理想状态下柴油机尾气颗粒物基于数量粒径谱和质量粒径谱如图 2所示[8-10]。

图1 尾气PM在显微镜下的照片 Fig. 1 Exhaust PM under microscope

图2 理想状态下柴油发动机尾气颗粒物粒径谱 Fig. 2 Diesel engine exhaust PM size distribution under perfect state

由图 2可知,柴油发动机尾气颗粒十分细小且成分复杂,含有挥发性组分以及重金属,这样对尾气颗粒测试装置的可靠性、自净能力以及抗干扰能力提出了较高的要求。

光散射法颗粒测量是一种非接触式的现代光电测试技术,具有实时、在线、不损坏样品等优点,在柴油发动机尾气颗粒的粒径谱分析和颗粒含量测量方面具有一定优势。以美国TSI公司为满足欧盟PMP项目(particulate measurement programme)的各项要求而研发的机动车颗粒物粒子计数器(Model 3790 engine exhaust condensation particle counter)为例,其测试颗粒数范围是从1~10 000个/cm3,可以测得排气中颗粒数极低的情况下颗粒物的含量,对于(41±1) nm的粒子计数效率达到90%以上,并具有±10%的计数精度。基于此,本文提出采用光散射法测量柴油发动机尾气颗粒粒径谱和颗粒含量这一方案。粒径谱包括数量粒径谱和质量粒径谱,如图 2所示,颗粒含量包括质量、体积和个数,而光散射法是一种基于颗粒的散射现象通过测量颗粒散射光强获得颗粒的粒径、体积、质量等非光学参数的方法,是一种间接测量法。由于柴油发动机尾气颗粒密度会随颗粒组成不同而发生变化,相对于滤膜称重法、压电天平法等直接测量方法,光散射法在质量粒径谱和质量浓度的测量领域具有一定局限性,本文仅讨论基于光散射法测量颗粒的数量粒径谱和单位体积中的含量测试装置的相关理论分析与数值计算。

2 柴油机尾气颗粒测试装置的组成

本文设计的柴油发动机尾气颗粒粒径分布测量装置由取样探头、流量计、水分过滤器、颗粒过滤器、泵、稀释风道、加热装置、光电传感探测器、工控机及开关阀等组成,如图 3所示。基于颗粒散射光强的数值计算与分析,以面阵CCD(电荷耦合元件)为光电探测器,散射光接收中心角和立体角分别取90°和30°。其工作过程是将空气经过流量计、水分和颗粒过滤器,泵入稀释通道和加热装置,把被测尾气进行稀释和加热,在理想状态下对柴油机尾气颗粒粒径分布情况进行分析测试。

图3 测试装置框图 Fig. 3 Schematic diagram of experimental set
3 光散射法颗粒测试原理

CCD使用一种高感光度的半导体材料制成,能把光线转变成电荷,通过模数转换器将电荷转换成数字信号(即灰度值Gs)。将CCD第s像元处的散射角记为θ,其灰度值为Gs,则第s像元位置处散射光强Is为[11]:

${I_s} = K \times {G_s}$ (1)

式中:K为CCD的能量-灰度比例因子;s=1,2，…，m。其中m为面阵CCD探测器上被选取用于颗粒测试计算分析的像元总数,其总和G:

$G = {\left[ {\begin{array}{*{20}{c}} {{G_1}}&{{G_2}}& \cdots &{{G_s}}& \cdots &{{G_m}} \end{array}} \right]^{\rm{T}}}.$ (2)

本文采用面阵CCD作为探测器测量散射光强度,通过测量不同散射角的光强分布,由这些散射光强得到尾气颗粒物的粒径谱和含量[12],具体实验原理见图 4。

图4 采用CCD测量散射光强实验原理图 Fig. 4 Cross section diagram of scattering light measurement by using CCD
3.1 光散射法及CCD测量柴油发动机尾气颗粒粒径谱原理

若将待测尾气颗粒群按粒径大小划分成n档,第q档内粒子数目为Nq,q=1,2,…,n。其颗粒的粒径谱为

$N = {\left[ {\begin{array}{*{20}{c}} {{N_1}}&{{N_2}}& \cdots &{{N_q}}& \cdots &{{N_n}} \end{array}} \right]^{\rm{T}}}.$ (3)

由CCD作为光散射法探测器的实验原理可知:

${I_0} \cdot \left[ {\begin{array}{*{20}{c}} {{i_{11}}}&{{i_{12}}}& \cdots &{{i_{1q}}}& \cdots &{{i_{1n}}}\\ {{i_{21}}}&{{i_{22}}}& \cdots &{{i_{2q}}}& \cdots &{{i_{2n}}}\\ \vdots &{}& \ddots &{}&{}& \vdots \\ {{i_{s1}}}&{{i_{s2}}}& \cdots &{{i_{sq}}}& \cdots &{{i_{sn}}}\\ \vdots &{}&{}&{}& \ddots & \vdots \\ {{i_{m1}}}&{{i_{m2}}}& \cdots &{{i_{mq}}}& \cdots &{{i_{mn}}} \end{array}} \right] \cdot \left[ {\begin{array}{*{20}{c}} {{N_1}}\\ {{N_1}}\\ \vdots \\ {{N_q}}\\ \vdots \\ {{N_n}} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {{I_1}}\\ {{I_2}}\\ \vdots \\ {{I_s}}\\ \vdots \\ {{I_m}} \end{array}} \right]$ (4)

式中isq表示第s像元接收到单个q档粒子的散射光强,可通过对Mie散射公式数值计算得到[13],此处不赘述。综合方程式(1)、式(3)和式(4)可得：

$\frac{{{I_0}}}{K} \cdot \left[ {\begin{array}{*{20}{c}} {{i_{11}}}&{{i_{12}}}& \cdots &{{i_{1q}}}& \cdots &{{i_{1n}}}\\ {{i_{21}}}&{{i_{22}}}& \cdots &{{i_{2q}}}& \cdots &{{i_{2n}}}\\ \vdots &{}& \ddots &{}&{}& \vdots \\ {{i_{s1}}}&{{i_{s2}}}& \cdots &{{i_{sq}}}& \cdots &{{i_{sn}}}\\ \vdots &{}&{}&{}& \ddots & \vdots \\ {{i_{m1}}}&{{i_{m2}}}& \cdots &{{i_{mq}}}& \cdots &{{i_{mn}}} \end{array}} \right] \cdot \left[ {\begin{array}{*{20}{c}} {{N_1}}\\ {{N_1}}\\ \vdots \\ {{N_q}}\\ \vdots \\ {{N_n}} \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {{G_1}}\\ {{G_2}}\\ \vdots \\ {{G_s}}\\ \vdots \\ {{G_m}} \end{array}} \right]$ (5)

从式(5)可得颗粒粒径谱N:

$\left[ {\begin{array}{*{20}{c}} {{N_1}}\\ {{N_1}}\\ \vdots \\ {{N_q}}\\ \vdots \\ {{N_n}} \end{array}} \right] = \frac{K}{{{I_0}}} \cdot {\left[ {\begin{array}{*{20}{c}} {{i_{11}}}&{{i_{12}}}& \cdots &{{i_{1q}}}& \cdots &{{i_{1n}}}\\ {{i_{21}}}&{{i_{22}}}& \cdots &{{i_{2q}}}& \cdots &{{i_{2n}}}\\ \vdots &{}& \ddots &{}&{}& \vdots \\ {{i_{s1}}}&{{i_{s2}}}& \cdots &{{i_{sq}}}& \cdots &{{i_{sn}}}\\ \vdots &{}&{}&{}& \ddots & \vdots \\ {{i_{m1}}}&{{i_{m2}}}& \cdots &{{i_{mq}}}& \cdots &{{i_{mn}}} \end{array}} \right]^ - }^1 \cdot \left[ {\begin{array}{*{20}{c}} {{G_1}}\\ {{G_2}}\\ \vdots \\ {{G_s}}\\ \vdots \\ {{G_m}} \end{array}} \right]$ (6)
3.2 数值计算

根据Mie散射公式可知[13],颗粒散射光强是一个多变量函数,主要变量有无因次参量x、散射光接收角θ以及散射光接收立体角β等。因此,本文基于Matlab对Mie散射公式进行数值计算,设定入射光强为1 lx,近似假设尾气颗粒物为碳颗粒(折射率m=1.95-0.66i),得出无因次参量x、散射光接收角θ、散射光接收立体角β对散射光强的影响,分析颗粒光散射的机制,将为光散射法柴油发动机尾气颗粒测试提供一定的理论依据。

1) 光散射法根据散射接收角分类大致可分为前向、侧向和后向散射法。由图 5可知,当无因次参量x为0.1数量级时,散射光强分布较为均匀,随着x的增加散射光强极快地增加,光强分布的不对称逐渐加强,前、后方向的散射光失去对称性,前向散射逐渐大于后向散射,直至占绝对优势。柴油发动机尾气颗粒粒径范围一般在0.01~1 μm,激光波长为0.1 μm数量级,因此无因次参量x在0.03~40范围内。当颗粒在低含量时,前向或侧向散射信号强度更大,以致信噪比较高,测量效果更加可靠；当颗粒的含量比较高时前向散射光往往超过了光探测器的测量范围,必须减小光学厚度,且还需设计采样系统,这既给系统增加复杂性,又增加经济成本[14]。从结构设计的角度讲,侧向散射和后向散射不容易受激光光源杂散光干扰。综上所述,采用侧向光散射法进行颗粒粒径谱和体积浓度分析,即散射光接收角θ为90°。

图5 不同无因次参量x下散射光强分布 Fig. 5 Is distribution under different non-dimension parameter x

2) 图 6中横纵坐标分别为接收角度θ和θ对应位置像元接收到的散射光强Is。4个曲线图粒径D分别为0.1 μm,0.3 μm,0.5 μm,0.8 μm,1 μm,1.5 μm,1.8 μm,2 μm,2.5 μm,3 μm和4 μm的单颗粒子在90°散射角左右(80°~100°)散射光强的分布。总体而言,随着粒子粒径的增大,其散射光强显著增加并且分布随散射角波动越大,从单个粒子来看,较小颗粒(D＜2 μm)其散射光强在80°~100°范围内分布较为均匀,较大颗粒(D>2 μm)时散射光强分布较为复杂,某一散射角内散射光强与颗粒粒径之间不再具有成正比的一一对应关系,因此有必要测量多角度内散射光强,根据式(6)计算得到颗粒粒径谱。

图6 单粒子散射光强Is随散射角的变化 Fig. 6 Diagram of particle light scattered intensity Is varies with scattering angular θ

3) 由图 6可知,当颗粒粒径较大时,某一散射角内散射光强与颗粒粒径之间不再具有成正比的对应关系[15],因此通过测量某一角度内散射光强得到颗粒粒径,并通过颗粒粒径换算得到颗粒体积浓度是不严密的。本文通过多次计算分析发现,散射光接受范围内光通量F与颗粒体积的相对值D3具有很好的线性关系。结果(图 7)表明,单粒子散射光通量与颗粒体积的相对值成正比关系,因此可以通过收集侧向散射角一定范围内的散射光通量测量尾气颗粒体积浓度,并且,当散射光接收立体角β增大,CCD能够接收到更多散射光,有利于调高灵敏度,为此,根据环境条件,测量颗粒体积浓度实验时,选取β为30°。

图7 散射光通量F随颗粒体积相对值D3的变化 Fig. 7 Light scattering flux F varies with relative value of particle volume D3
4 对比实验与结果分析

为分析该测量装置的性能与测量效果,将其用于柴油发动机台架的尾气粒径分布测量实验,实验采用TSI公司发动机废气排放颗粒物粒径谱仪EEPS-3090进行对比测试,以其测得的粒径分布和浓度作为尾气颗粒实际的粒径分布和浓度值。实验时将发动机稳定某一工况后同时开启2台测量装置,2台测量装置的采样气管直径相同,捆绑后以相同的流量进行采样,这有利于提高2台装置测量的一致性,进行比对,具体实验装置如图 3所示。测量装置1和测量装置2分别为EEPS-3090和本文设计的柴油发动机尾气测量装置。图 8为柴油发动机稳定在怠速3 000 r/min时某一时刻2台测量装置测得的粒径分布对比图。从测得的实验数据上看,本文设计的测量装置与EEPS-3090一致性较好,具有一定的实用性。

图8 柴油发动机瞬态粒径分布 Fig. 8 Instantaneous particle size distribution of diesel engine
5 结论

本文通过对柴油机尾气颗粒的物化特性,各参量和散射光强Is、散射光通量F之间的数值关系等分析得到如下结论:

1) 基于光散射法原理测量柴油发动机尾气颗粒粒径谱和单位体积中的颗粒含量,从理论角度分析了其正确性和可行性;

2) 根据上述原理研究设计完成了对柴油机尾气颗粒物进行粒径谱和单位体积中的颗粒含量检测的测试装置;

3) 该装置通过对比实验结果分析,该装置具有实时、在线、不损坏样品等优点。

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