New development of dynamic emission measurement te

2022-08-02
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New progress in dynamic measurement technology of internal combustion engine emissions

automobile exhaust is an important source of air pollutants, and it is very important to measure the emission concentration of automobile pollutants. At present, there are many measurement methods for automobile exhaust, such as chemiluminescence analyzer (CLD), hydrogen flame ionization analyzer (FID), non dispersive infrared absorption analyzer (NDIR), etc. However, these methods are mainly used for the steady-state measurement of the engine, and can only meet the working conditions with small concentration changes. For dynamic working conditions with drastic changes, the measured value is accurate after time integration, but there is a large gap between the measured value at each time point and the actual value

measuring the dynamic emission characteristics of internal combustion engine under different operating conditions and at different rates under certain operating conditions is of great significance for understanding the causes of exhaust gas and taking targeted measures to improve the emission of internal combustion engine. Countries all over the world have invested a lot of energy in the research of dynamic emission measurement technology of internal combustion engines. This paper introduces some recent developments in this field abroad

1 mobile quadrupole mass spectrometer

william rtridge proposed a method for measuring automobile exhaust with a new mass spectrometer (MMS) (see Figure 1). Compared with other mass spectrometers, this instrument is light, solid and easy to move. It can measure a variety of gases and has a faster response speed

(1) composition of MMS

mms includes capillary sampling tube, standard residual gas analyzer, turbine pump, electronic control unit and a notebook computer. The sampling tube is a movable mass spectrometer capillary as shown in Figure 1, which has little effect on the measured flow field. The air-cooled turbine pump is used to suck exhaust gas into MMS. The electronic control unit can adjust the potential difference of the electric field in the ionization chamber, so as to selectively ionize the gas according to the needs and increase the types of measurable gases. The notebook computer is used to remotely control the whole system, and its maximum remote control distance can reach 15m

(2) MMS calibration

mms uses Horiba sgd-710 standard and carries out calibration according to its specified standard procedure. During calibration, dilute gas (N2) is gradually mixed with calibration gas mixed in delivery gas (N2). Diluting gas and conveying gas are the same gas, which can avoid the viscosity difference between them and greatly reduce the instability of the measurement system caused by the mixed gas. At present, there is no suitable method for MMS to calibrate multi-component gases at the same time

(3) applicable gas type

after the ionization of a gas, if the mass charge ratio (m/z) of ions produced is the same or similar to that of other gas ions, mutual interference will occur. Some gases such as O2 and C3H6 are not affected by this. However, some gases, such as no and CH2O (m/z=30), H2O and NH3 (m/z=18, 17), are sensitive to such interference. For these gases, if there are obvious differences in concentration, MMS can still distinguish them. For example, the concentration of no is more than 100 times that of CH2O; The concentration of CO2 is more than 1000 times that of N2O. By adjusting the potential difference of the electric field during gas ionization and selectively ionizing molecules according to different gas types, the interference caused by the same or similar light charge ratio can be reduced

n2o and NO2 are easy to decompose and interfere with CH2O and C2H6 (m/z=30). However, because the concentration of NO2 is much higher than that of the other three components, and the ionization energy of decomposed NO2 is significantly different from that of the other three components (see Table 1), the gas can be selectively ionized by adjusting the potential difference of the electric field, so as to distinguish NO2 from CH2O, C2H6 and N2O

mms can also measure the total concentration of NOx (compared with no and NO2, the concentration of N2O is very low, so the concentration value of NOx ignores N2O). Because the mass charge ratio of no and decomposed NO2 is the same, and the response time of MMS to them is very small, the measurement result of NOx will not change significantly due to the different proportion of no and NO2 in the exhaust gas, so MMS can easily measure the total concentration of NOx (m/z=30)

if the engine adopts selective catalytic reduction technology, the tail gas will contain NH3. However, it is difficult for MMS to measure the content of NH3 (m/classification and test method for daylighting performance of external windows of buildings gb/t 11976 ⑵ 002z=17), because H2O (m/z=18) will produce interference. Although the ionization energy of the two is significantly different, when the N2 content is 0.7% (v/v), the lower limit of NH3 measurement is 2000ppm, so it is very difficult to measure NH3 in the exhaust gas

influenced by N2 (m/z=18), MMS cannot measure CO (m/z=18). However, mass spectrometer (MS) that ionizes gas by laser or chemical means can carry out selective ionization and measure the concentration of Co

mms cannot measure HC concentration. Because the composition of HC varies greatly and the response times of various components in MMS are different, it is impossible to calibrate HC concentration measured by MMS

(4) steady state response characteristics

for no with mass load ratio of 30, the linearity and accuracy of MMS response are very good (Fig. 2). Its measuring range is very wide, reaching more than 5000ppm; The error of calibration factor is about 15%

(5) dynamic response characteristics

MMS and traditional CLD were tested with gas pulse with FWHM (full width at half maximum) of 400ms. MMS results show 410 MS and CLD 1300 Ms. MMS has better dynamic response than CLD

although the dynamic response of CLD is not good, the result of time integration is correct, which is the reason why CLD is applied in steady-state measurement. Therefore, CLD is used to test the time integration of MMS, and the results show that the two are very consistent. The accuracy of MMS dynamic response is confirmed by the results of MMS time integration and its measurement accuracy of pulse FWHM

mms has good dynamic response characteristics and wide measurement range, and has little effect on the measured flow field. Since the potential difference of the electric field is adjustable, more kinds of gases including NOx can be measured. The length, width and height of MMS are 61cm, 18cm and 38cm respectively, and the weight is 12kg. It is firm and reliable, cheap, and can easily move the experimental data

2 new NDIR equipment

camp adopts a new NDIR technology to measure CO2, and the time response of the analyzer is about 5ms

after a certain degree of gas absorption, the infrared ray irradiates on the photosensitive element of the detection probe, causing the surface resistance of the photosensitive element to decrease. By measuring the change of the surface resistance, the degree of infrared ray absorption can be known, so as to measure the CO2 concentration

the probe of the analyzer is called f CO2 probe (as shown in Figure 3), which is connected with a heated sampling pipe. The sample chamber in the probe is about 0.5 cm3 in size, and the pressure inside is controlled and lower than the pressure of the automobile exhaust pipe. There is a concave mirror on the back of the infrared light source to form a parallel beam. The light beam passes through the sample chamber and a filter and irradiates on the infrared detector at the other end of the optical path. There is also a cooler in the probe to ensure that the infrared detector works at - 10 ℃

f several key points of CO2 probe are as follows:

continuous operation. The traditional NDIR has a high-speed rotating cut-off disc, which makes the infrared irradiate the sample gas intermittently, because this can control the drift characteristics of NDIR within a certain range. However, in order to obtain a rapid response with a response time less than 5 ms, the measuring element should be as close to the engine as possible, or the sample head should be placed directly. If the intermittent equipment similar to the cut-off disc is used, the structure of the sampling head will be complex and the shape will be large. Therefore, f CO2 probe cancels intermittent equipment and adopts continuous operation mode. When the drift exceeds the acceptable range, recalibrate. F CO2 has good temperature control measures, and the recalibration interval can reach more than 10min

● operation with steam. In order to meet the needs of rapid response, the condensed steam will dissolve some components in the flue gas, but if the flue gas is dried like the traditional NDIR, the response of F CO2 will be delayed. Therefore, the analyzer uses the method of heating the sampling pipe to make the flue gas temperature higher than the saturation temperature of water vapor, so as to avoid the dissolution of flue gas components. In order to keep the working temperature of the infrared detector at - 10 ℃, f CO2 adopts better heat insulation measures and cooling measures

the water vapor in the flue gas will also absorb infrared rays, affecting the measurement of CO2. But the main absorption wavelength of CO2 is 4.2 ~ 4.4 μ M infrared ray, while H2O absorbs little of this part, so f CO2 filters the infrared ray with a filter

● sample chamber size. The larger the size of the sample chamber, the longer the optical path of infrared ray in the sample chamber, and the more sensitive the detector is to the change of concentration. However, the sample chamber size of F CO2 is limited by many factors and cannot be too large

● sample chamber pressure. The higher the pressure in the sample chamber, the higher the molecular concentration of the sampled exhaust gas, the more intense the change in infrared absorption, and the more sensitive the detector is. The lower the pressure in the sample chamber, the greater the pressure difference with the sampling point, the higher the mass flow rate of the sampled exhaust gas, and the more sensitive the detector is. Considering both factors, the lower the pressure in the sample chamber, the better

automobile exhaust contains particles, so f CO2 optical components should be less affected by contamination. Practice shows that f CO2 will not have problems after continuous use for 8h

on the basis of F CO2, Tim hands, mark Peckham and others modified their sensing elements and replaced their filters, so that the transmission wavelength of infrared ray becomes 4.57 ~ 4.77 that can be absorbed by Co μ m. The response time of the analyzer after modification is 7ms

3 the improved fid

t.k.jensen and j.schramm adopt a fast response FID to measure ubhc (unburned hydro carbon), and its structure is shown in Figure 4. The pressure of exhaust gas to be sampled in FID has a great influence on its measurement accuracy. In order to avoid the influence of the change of exhaust gas pressure at the engine sampling point on the sampled exhaust gas pressure in the FID, the traditional FID uses a long capillary to stabilize the sampled exhaust gas pressure in the FID, which makes the response speed of the traditional FID very slow. The new type FID adopts a voltage stabilizing chamber to stabilize the pressure and eliminate the fluctuation of static pressure. The sampling tail gas duct in the combustion chamber and the air supply duct in the pressure stabilizing chamber are perpendicular to each other, eliminating the influence of the dynamic pressure of the sampling tail gas. Since the sampling tail is sent to FID immediately after collection, the response speed is greatly improved. The response speed of the new FID depends on the diffusion speed of the sampled exhaust gas in the sampling tube and the combustion flame

4 DMS for measuring particulate matter

kingsley real, Tim hands adopts a new equipment DMS to measure the quantity and particle size distribution of particulate matter in emissions. It uses electrostatic classification technology, with a measurement range of 5 ~ 1000nm and a response speed of 200ms

previously, DMA (differential mobility analyzer) was the most commonly used equipment in the measurement of particle size. DMA first makes the particles charged, and then classifies them according to their electronic activity (the speed of particle movement under a certain electric field). As the voltage of changing the electron active electric field gradually increases, the process of distinguishing the electron activity is relatively slow, and the response of DMA is also relatively slow (it takes about 100 s to complete a complete particle size distribution). Another equipment that can measure particle size is ELPI (electrostatic low pressure impactor), which is a cascade impactor that can measure in real time. ELPI first makes the particles in the automobile exhaust charged, and then makes the air flow impact the multi-stage impactor. The impactor captures the particles with the corresponding particle size. As the particles are charged, the impactor will generate the corresponding electric quantity signals, which can be known by using the electrometer connected to each stage impactor to measure these electric quantity signals

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