便携式FTIR的机动车尾气检测方法

随着汽车排放标准的提高，相关VOC标准从总烃检测变为非甲烷碳氢化合物(NMHC)检测；随着含氧燃料的增加，增加了非甲烷有机气体(NMOG)测量。针对国内汽车尾气分析仪分析组分单一、精度有限、VOCs检测过程复杂等问题，提出了基于便携式FTIR的机动车尾气检测方法，基于立体角镜优化FTIR光学系统结构，提高动镜扫描速度，设计便携式且满足抗振动需求的快速FTIR光谱仪。FTIR红外光源输出波段范围为2～20 μm，分辨率为0.5 cm-1，扫描速度1 Hz，气体池光程为10 m，采用斯特林探测器，其光谱响应范围为600～6 000 cm-1。选择CH₄，C₂H₂，C₂H₄，C₂H₆，C₃H₆，n-C₅H₁₂，i-C₅H₁₂，C₇H₈，HCHO，C₂H₅OH，CH₃CHO这些典型HC化合物作为VOC气体检测的替代物。通过标准谱确定尾气成分的波段为900～1 100和2 700～3 100 cm-1，涵盖所有待测气体吸收波段。基于AVL台架测试，开展NEDC和WLTC工况实验测试，测试车辆为丰田威驰，测试油品为92号国五。便携式FTIR采用抽取方式进行尾气测量，原始的废气样本来自安装在排气管延长部分的多孔探头，前端安装样气取样装置，主要包括颗粒物过滤和除水汽装置，以防止污染FTIR光学系统。实验表明FTIR可以有效快速测量汽车尾气中CO，CH₄，NO和主要HC化合物，在FTIR检测限0.5 μmol·mol-1下会引入噪声信号，浓度可信度降低。通过分析可以看出输出气体平均浓度降级排列依次是：CO，C₂H₄，CH₄，NO，i-C₅H₁₂，C₂H₆，C₇H₈，n-C₅H₁₂，C₂H₅OH，CH₃CHO。从3个循环的NEDC工况可以看出，每种气体排放呈现一致的规律性变化。针对CO进行了SEMTECH-DS与FTIR测量数据的时间序列比较，结果呈现了较好的规律一致性，但是由于FTIR和SEMTECH-DS测量技术和取样稀释系统不同导致二者浓度差异较大。与传统尾气检测技术相比，便携式FTIR测量系统对瞬态事件有良好的响应，可以在线进行多组分浓度实时测量获取机动车的瞬时排放数据，在满足新规测试要求下，也可以为后期的机动车在实际道路上的排放特征分析和模拟提供可靠的数据支持。

Vehicle Exhaust Detection Method Based on Portable FTIR

With the improvement of vehicle emission standards, The relevant VOC standards changed from total hydrocarbon detection to non-methane hydrocarbon (NMHC) detection. With the increase of oxygen-containing fuels, non-methane organic gas (NMOG) measurement was increased. The vehicle exhaust detection method based on portable FTIR is proposed aiming at the problems such as single component analysis, limited accuracy and complex VOC detection process of domestic automobile exhaust analyzer. The structure of the FTIR optical system is optimized based on a corner cube to meet the requirements of anti-vibration. The scanning speed of the moving mirror is improved to meet the requirements of a portable and fast FTIR spectrometer. The output band range of the FTIR infrared light source is 2～20 μm, with a resolution of 0.5 cm-1, a scanning speed of 1 Hz, and a gas pool optical range of 10 m. Stirling detector is used, with spectral responses ranging from 600 to 6 000 cm-1. Typical HC compounds such as CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, n-C₅H₁₂, i-C₅H₁₂, C₇H₈, HCHO, C₂H₅OH, CH₃CHO are selected as alternatives to VOC gases. The test bands of vehicle exhaust composition determined by the standard spectrum are 900～1 100 and 2 700～3 100 cm-1, covering all the gas absorption bands to be measured. Based on the AVL bench test, NEDC and WLTC working condition experimental test are carried out. The test vehicle is Toyota VIOS, and the test oil product was No. 92 State 5. The portable FTIR adopts an extraction method for tail gas measurement. The original exhaust sample is from a porous probe installed in the extension part of the exhaust pipe. The front end is equipped with a sample gas sampling device, which mainly includes particulate filtration and moisture removal to prevent pollution of the FTIR optical system. The experiment shows that FTIR can effectively and rapidly measure CO, CH₄, NO and main HC compounds in automobile exhaust. When the gas concentration is lower than the FTIR detection limit of mole fraction 0.5 μmol·mol-1, noise signals will be introduced and the reliability will be reduced. It can be seen from the analysis that the average concentration of output gas is downgraded in descending order: CO, C₂H₄, CH₄, NO, i-C₅H₁₂, C₂H₆, C₇H₈, n-C₅H₁₂, C₂H₅OH, CH₃CHO. It can be seen from the three cycles of NEDC working conditions that each gas emission presents a consistent and regular change. The time series comparison of SEMTECH-DS and FTIR measurement data for CO shows a good consistency of laws. However, due to the difference in measurement technology and sampling dilution system of FTIR and SEMTECH-DS, the concentration difference between them is large. Compared with the traditional exhaust detection technology, the portable FTIR measurement system has a good response to the transient events, and can measure the multi-component concentration in real time to obtain the instantaneous emission data of motor vehicles, which can meet the requirements of the new regulation test and also provide reliable data support for the later emission characteristic analysis and simulation of motor vehicles on the actual road.