Dilution gas mixing unit and exhaust gas analysis system

Abstract

This dilution gas mixing unit X mixes a dilution gas into an exhaust gas and is used in an exhaust gas analysis system 100 for analyzing mixed gas obtained by diluting an exhaust gas with dilution gas in order to reduce the impact on background measurement caused by the racing of mixed gas, even in a case where the dilution ratio is lowered and the dilution gas flow rate is decreased, the dilution gas mixing unit comprising: a dilution gas supply pipe 3H that connects to an exhaust gas introduction pipe 21 to which an exhaust gas is introduced, and that supplies a dilution gas to the exhaust gas introduction pipe 21; a dilution gas sampling unit 250 which is provided to the dilution gas supply pipe 3H and samples the dilution gas; and a backflow prevention member 5 which is provided more to the exhaust gas introduction pipe 21 side than the dilution gas sampling unit 250 in the dilution gas supply pipe 3H, and which is for preventing mixed gas from backflowing through the inside of the dilution gas supply pipe.

Description

[Technical Field]

[0001] The present invention relates to a dilution gas mixing unit and an exhaust gas analysis system. [Background technology]

[0002] A conventional exhaust gas analysis system includes an exhaust gas inlet pipe into which exhaust gas is introduced, and a dilution gas supply pipe connected to the exhaust gas inlet pipe for supplying dilution gas (Patent Document 1). With this configuration, a mixed gas obtained by diluting exhaust gas with dilution gas is sampled, and the dilution gas is also sampled. For example, the concentration of a component to be measured contained in the sampled dilution gas is subtracted as a background value from the concentration of the same component contained in the sampled mixed gas, thereby analyzing the exhaust gas.

[0003] Recently, the concentrations of various components contained in exhaust gases have been decreasing, and in order to ensure analytical accuracy for such exhaust gases, it is necessary to lower the dilution ratio more than before.

[0004] However, if the dilution ratio is reduced in the above-mentioned exhaust gas analysis system, the dilution gas flow rate will decrease. Therefore, if the exhaust gas flow rate suddenly fluctuates due to, for example, engine rotation control, some of the diluted exhaust gas (mixed gas) will be blown up into the dilution gas supply pipe, and the components contained in the mixed gas may affect background measurement, which may be a problem. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-139340 Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, the main object of the present invention is to reduce the influence of the blow-up of the mixed gas on background measurement even when the dilution ratio is lowered and the flow rate of the diluted gas is reduced. [Means for solving the problem]

[0007] In other words, the dilution gas mixing unit of the present invention is used in an exhaust gas analysis system for analyzing a mixed gas obtained by diluting exhaust gas with a dilution gas, and is a dilution gas mixing unit that mixes the dilution gas with the exhaust gas, and is characterized by comprising: a dilution gas supply pipe that is connected to an exhaust gas inlet pipe into which the exhaust gas is introduced and that supplies dilution gas to the exhaust gas inlet pipe; a dilution gas sampling unit that is provided in the dilution gas supply pipe and that collects the dilution gas; and a backflow prevention member that is provided in the dilution gas supply pipe closer to the exhaust gas inlet pipe than the dilution gas sampling unit and that prevents the mixed gas from flowing back through the dilution gas supply pipe.

[0008] In the dilution gas mixing unit configured in this manner, the backflow prevention member is provided in the dilution gas supply pipe closer to the exhaust gas introduction pipe than the dilution gas sampling unit, so even if the dilution ratio is lowered and the flow rate of the dilution gas is reduced, causing some of the mixed gas to blow up into the dilution gas supply pipe, the mixed gas collides with the backflow prevention member and is pushed back by the dilution gas, making it less likely to flow back up the dilution gas supply pipe. As a result, it is possible to prevent the blown-up mixed gas from reaching the dilution gas sampling unit, thereby reducing the impact of this mixed gas on background measurement.

[0009] When the mixed gas is blown up, the mixed gas tends to flow along the inner circumferential surface of the diluent gas supply pipe. In view of this, it is preferable that the backflow prevention member has a through hole through which the diluent gas passes and is provided along the inner circumferential surface of the diluent gas supply pipe. In this case, since the backflow prevention member is provided along the inner peripheral surface of the diluent gas supply pipe, the blown-up mixed gas can be made to collide with the backflow prevention member more reliably.

[0010] A specific example of the backflow prevention member is an orifice plate.

[0011] When an orifice plate having a single through-hole is used, the dilution gas passing through the through-hole has difficulty reaching the rear side of the orifice plate, and the dilution gas accumulates on the rear side. This accumulation is particularly noticeable when the dilution gas flow rate is low. As a result, downstream of the orifice plate, in the central part of the dilution gas supply pipe, the dilution gas that has passed through the through holes flows smoothly, but near the inner surface of the dilution gas supply pipe, the dilution gas stagnates, which causes a decrease in the uniformity of the exhaust gas components contained in the mixed gas and ultimately a decrease in the reproducibility of the analysis results. Although widening the through holes narrows the area behind the orifice plate and makes it difficult for the diluted gas to stagnate, this reduces the backflow prevention effect of the orifice plate.

[0012] Therefore, in order to improve the uniformity of the exhaust gas components contained in the mixed gas while ensuring the backflow prevention effect, it is preferable that the backflow prevention member is a porous orifice plate having a large number of through holes. With this configuration, the large number of through-holes rectify the flow of the dilution gas, allowing the dilution gas to flow smoothly downstream of the multi-hole orifice plate, thereby improving the uniformity of the exhaust gas components contained in the mixed gas while ensuring the effect of preventing backflow.

[0013] The smaller the through-holes in the porous orifice plate, the higher the flow rate of the diluted exhaust gas that passes through these through-holes, and therefore the backflow prevention effect can be improved. However, in analytical specifications where the flow rate of the diluted gas is high, the pressure loss may become too large to be usable. Therefore, it is preferable that a plurality of the multi-hole orifice plates are provided in the diluent gas supply pipe along the direction of flow of the diluent gas. In this case, by using multiple porous orifice plates with different sizes of through holes, it is possible to appropriately adjust the backflow prevention effect and pressure loss according to various analytical specifications with different dilution gas flow rates.

[0014] In a more specific embodiment, it is preferable that the through holes of the downstream-side multi-hole orifice plate are smaller than the through holes of the upstream-side multi-hole orifice plate.

[0015] In order to sample the diluted gas heading toward the through hole of the backflow prevention member, it is preferable that the diluted gas sampling section has an inlet arranged within the diluted gas supply pipe so as to be located within the through hole of the backflow prevention member when viewed from the pipe axis direction, or so as to be located more than half the inner diameter from the pipe axis of the diluted gas supply pipe as the center.

[0016] The inlet preferably faces the upstream side of the diluent gas. In this case, compared to when the inlet faces downstream or to the side of the diluted gas, for example, it is possible to collect the diluted gas without difficulty, while making it difficult for the mixed gas to reach the inlet in the event of blow-up.

[0017] Considering the influence of the backflow prevention member on exhaust gas analysis, it is preferable that the pressure loss in the diluent gas supply pipe provided with the backflow prevention member is less than 250 Pa.

[0018] Furthermore, the exhaust gas analysis system according to the present invention is characterized by comprising a mixed gas flow pipe through which the mixed gas flows, a mixed gas sampling unit provided in the mixed gas flow pipe for collecting the mixed gas, a constant flow rate mechanism for maintaining the flow rate of the mixed gas at a constant rate, a gas analyzer for analyzing the collected diluted gas and specified components to be measured contained in the collected mixed gas, and the above-mentioned diluted gas mixing unit. The exhaust gas analysis system configured in this manner can also achieve the same effects as the dilution gas mixing unit described above. [Effects of the Invention]

[0019] According to the present invention configured as described above, even when the dilution ratio is lowered and the diluted gas flow rate is reduced, the influence of the blow-up of the mixed gas on the background measurement can be reduced. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram showing the overall configuration of an exhaust gas analysis system according to an embodiment of the present invention; [Figure 2] FIG. 2 is a schematic diagram showing the configuration of a dilution gas mixing unit of the embodiment; [Figure 3] 10 is experimental data showing the effect of the backflow prevention member of the same embodiment. [Figure 4] 10A and 10B are schematic diagrams showing the configuration of a backflow prevention member according to another embodiment. [Figure 5] 10A and 10B are schematic diagrams showing the configuration of a backflow prevention member according to another embodiment. [Figure 6] 10A and 10B are schematic diagrams showing the configuration of a backflow prevention member according to another embodiment. [Figure 7] FIG. 10 is a schematic diagram showing the configuration of a dilution gas mixing unit according to another embodiment. [Explanation of symbols]

[0021] 100···Exhaust Gas Analysis System X...Dilution gas mixing unit 21 Exhaust gas introduction pipe 3H: Dilution gas supply pipe 250 Dilution gas sampling unit P...Inlet 5. Backflow prevention member 5a...Through hole DETAILED DESCRIPTION OF THE INVENTION

[0022] An embodiment of an exhaust gas analysis system using a dilution gas mixing unit according to the present invention will be described below with reference to the drawings.

[0023] The exhaust gas analysis system 100 according to this embodiment is of a dilution sampling type, in which the exhaust gas collected from the test vehicle 200 is diluted with dilution air as a dilution gas to measure the concentration. In the following, this embodiment will be described as a constant-volume, constant-volume dilution sampling type in which the entire amount of exhaust gas is sampled and diluted with dilution air to a constant, known flow rate. The test vehicle 200 may be an engine vehicle, a hybrid vehicle, a fuel cell vehicle, or the like.

[0024] Specifically, as shown in Figure 1, this system is equipped with a constant-volume sampling device 2 that introduces the entire amount of exhaust gas and dilution air into the device, controls the combined total flow rate to be constant, and collects a portion of the diluted exhaust gas (hereinafter referred to as mixed gas) into a collection bag at a constant flow rate; a dilution air purification device 3 that supplies dilution air that has been purified by removing impurities from the atmosphere to the constant-volume sampling device 2; and a gas analyzer 4 that analyzes the concentration of specified components (e.g., HC, CO, HO, NO, etc.) in the mixed gas collected by the collection bag of the constant-volume sampling device 2.

[0025] The constant-volume sampling device 2 includes an exhaust gas inlet pipe 21 connected to an exhaust pipe 200H of a test vehicle 200 mounted on a chassis dynamometer 300, a dilution gas supply pipe 3H connected to the exhaust gas inlet pipe 21 and supplying dilution gas, a mixed gas flow pipe 23 equipped with a constant flow rate mechanism 231 for maintaining a constant flow rate of the mixed gas, a mixed gas sampling line 24 for sampling the mixed gas flowing through the mixed gas flow pipe 23, and a dilution gas sampling line 25 for sampling the dilution air flowing through the dilution gas supply pipe 3H. A cyclone 22 for removing dust contained in the mixed gas may be provided downstream of the exhaust gas inlet pipe 21. The constant-volume sampling device 2 does not necessarily have to sample exhaust gas from the test vehicle 200 mounted on a chassis dynamometer 300, but may also sample exhaust gas from an engine connected to an engine dynamometer or a powertrain connected to one or more dynamometers, for example.

[0026] The constant flow rate mechanism 231 is composed of a Venturi tube 231a provided on the mixed gas flow pipe 23 and a turbo blower 231b provided downstream of the Venturi tube 231a.

[0027] Mixed gas collection line 24 includes mixed gas sampling section 240 in mixed gas flow pipe 23, mixed gas collection pipe 241 connected at one end to the mixed gas sampling section, mixed gas collection pump 242 provided on mixed gas collection pipe 241, and mixed gas bag 243 for storing the mixed gas collected by mixed gas collection pump 242. Mixed gas collection pipe 241 is provided upstream of constant flow rate mechanism 231.

[0028] The diluted gas collection line 25 also includes a diluted gas sampling section 250 provided in the diluted gas supply pipe 3H, a diluted gas collection pipe 251 connected to the diluted gas sampling section 250, a diluted gas collection pump 252 provided on the diluted gas collection pipe 251, and a diluted gas bag 253 that stores the dilution air collected by the diluted gas collection pump 252.

[0029] Then, so-called bag measurement is performed by the gas analyzer 4 using the mixed gas bag 243 of the mixed gas sampling line 24 and the diluted gas bag 253 of the diluted gas sampling line 25 .

[0030] The dilution air purification device 3 purifies dilution air from the atmosphere, and in order to stabilize the background concentration in exhaust gas analysis at low levels, it purifies at least CO, HC, NO in the dilution air. X In this dilution air purification device 3, the method of removing CO, HC, NO, N2O, etc. is to convert CO, HC, NO, N2O in the dilution air into CO2, H2O, N2, NO2, and also to convert NO2 generated by oxidation of NO and N2O into NO2. X The adsorption treatment is carried out using an adsorbent.

[0031] The exhaust gas analysis system 100 of this embodiment is characterized by the area enclosed by the dashed line in FIG. 1, and specifically by the dilution gas mixing unit X in which the exhaust gas and the dilution gas are mixed. This dilution gas mixing unit will be described in detail below.

[0032] As shown in Figures 1 and 2, the dilution gas mixing unit X of this embodiment at least includes the above-mentioned dilution gas supply pipe 3H and a dilution gas sampling section 250 provided in this dilution gas supply pipe 3H, and in this case also includes at least a part of the exhaust gas introduction pipe 21.

[0033] The diluted gas sampling unit 250 of this embodiment is provided inside the diluted gas supply pipe 3H, and has an inlet P facing the upstream side of the diluted gas.

[0034] More specifically, this inlet port P is positioned so that the pipe axis L of the dilution gas supply pipe 3H passes through it, and in this case, the pipe axis L is positioned so that it passes through the center of the inlet port P or its vicinity.

[0035] Furthermore, the inlet port P of this embodiment is provided at a position closer to the upstream opening 3Ha of the dilution gas supply pipe 3H than the exhaust gas introduction pipe 21 in the axial direction of the dilution gas supply pipe 3H.

[0036] The above-mentioned inlet P may face downstream of the diluted gas, or may face to the side (radial direction of the diluted gas supply pipe 3H), or may be located closer to the exhaust gas inlet pipe 21 than the upstream opening 3Ha of the diluted gas supply pipe 3H.

[0037] The diluted gas mixing unit X is further provided with a backflow prevention member 5 which is provided on the exhaust gas introduction pipe 21 side of the diluted gas sampling section 250 in the diluted gas supply pipe 3H and which prevents the mixed gas from flowing back through the diluted gas supply pipe 3H.

[0038] The backflow prevention member 5 has a through hole 5a through which the dilution gas passes. The backflow prevention member 5 in this embodiment is, for example, in the shape of a flat annular plate, and specifically, an orifice plate. In this embodiment, the inlet P of the diluted gas sampling unit 200 described above is disposed inside the through hole 5a of the backflow prevention member 5. The size of the through hole 5a is such that the pressure loss of the diluted gas flowing through the diluted gas supply pipe 3H does not affect the analytical accuracy of the exhaust gas analysis; specifically, the pressure loss inside the diluted gas supply pipe 3H in which the backflow prevention member 5 is provided is set to be less than 250 Pa.

[0039] This backflow prevention member 5 is provided along the inner circumferential surface of the diluent gas supply pipe 3H; in other words, at least a portion of the outer circumferential surface of the backflow prevention member 5 is in contact with at least a portion of the inner circumferential surface of the diluent gas supply pipe 3H.

[0040] In this embodiment, the backflow prevention member 5 is provided around the entire inner circumferential surface of the dilution gas supply pipe 3H; in other words, the entire outer circumferential surface of the backflow prevention member 5 is in contact with the entire inner circumferential surface of the dilution gas supply pipe 3H.

[0041] The backflow prevention member 5 does not necessarily have to be provided around the entire inner circumferential surface of the diluent gas supply pipe 3H, but may be provided continuously or intermittently on a part of the inner circumferential surface of the diluent gas supply pipe 3H.

[0042] The backflow prevention member 5 here is provided closer to the exhaust gas introduction pipe 21 than the inlet P in the axial direction of the dilution gas supply pipe 3H, and closer to the upstream opening 3Ha of the dilution gas supply pipe 3H than the exhaust gas introduction pipe 21. However, the backflow prevention member 5 may also be provided at a position closer to the exhaust gas introduction pipe 21 than the upstream opening 3Ha of the dilution gas supply pipe 3H.

[0043] In the exhaust gas analysis system 100 configured in this manner, the backflow prevention member 5 is provided in the dilution gas supply pipe 3H closer to the exhaust gas introduction pipe 21 than the diluted gas sampling unit 250, so that even if the dilution ratio is lowered to reduce the flow rate of the diluted gas and as a result some of the mixed gas is blown up into the dilution gas supply pipe 3H, the mixed gas collides with the backflow prevention member 5 and is pushed back by the diluted gas, making it less likely to flow back up the dilution gas supply pipe 3H. As a result, it is possible to prevent the blown-up mixed gas from reaching the diluted gas sampling unit 250, and it is possible to reduce the effect of this mixed gas on background measurement.

[0044] Here, in order to explain the action and effect of the backflow prevention member 5 of the present invention, a schematic graph of the vehicle speed during an exhaust gas test using a test vehicle is shown in Figure 3(a), and a schematic graph of the CO2 concentration contained in the dilution air collected from the dilution gas sampling section 250 during the same test is shown in Figure 3(b).

[0045] When the accelerator pedal of the test vehicle is depressed at timing A in Figure 3(a) to temporarily increase the vehicle speed, the exhaust flow rate of the exhaust gas suddenly increases, temporarily changing the flow rate balance with the dilution air, and some of the diluted exhaust gas mixture is blown up.

[0046] In the prior art, the blown-up mixed gas reaches the diluted gas sampling unit 250, and the CO2 contained in this mixed gas, which originates from the exhaust gas, is detected, causing a temporary increase in gas concentration, as shown in B in Figure 3(b). The diluted gas sampling unit 250 is originally intended to measure the concentration of the measurement target component contained in the diluted gas, and as described above, measuring the measurement target component contained in part of the exhaust gas results in a decrease in analytical accuracy.

[0047] In contrast, when the backflow prevention member 5 according to the present invention is used, as shown in Figure 3(b), no increase in gas concentration occurs, and it can be seen that the mixed gas is prevented from reaching the diluted gas sampling section 250.

[0048] Furthermore, since the inlet P of the diluted gas sampling section 250 in this embodiment is located at a position where the pipe axis L of the diluted gas supply pipe 3H passes through, even if the mixed gas blows up along the inner wall of the diluted gas supply pipe 3H, it is possible to make it difficult for the mixed gas to reach the inlet P. Moreover, the flow of the diluent gas is faster in the center of the diluent gas supply pipe 3H than in the outer periphery, which also makes it difficult for the mixed gas to reach the inlet P.

[0049] Furthermore, since the backflow prevention member 5 is provided along the inner peripheral surface of the diluted gas supply pipe 3H, the blown-up mixed gas can be made to collide with the backflow prevention member 5 more reliably.

[0050] Furthermore, since the inlet P of the diluted gas sampling section 250 faces the upstream side of the diluted gas, it is possible to collect the diluted gas without difficulty, while making it difficult for the mixed gas to reach the inlet P in the event of blow-up, compared to when the inlet P faces, for example, downstream or to the side.

[0051] In addition, since the pressure loss in the diluted gas supply pipe 3H provided with the backflow prevention member 5 is less than 250 Pa, the analytical accuracy of the exhaust gas analysis can be ensured.

[0052] The present invention is not limited to the above-described embodiment.

[0053] For example, although the backflow prevention member 5 is annular in the above embodiment, it may be semi-annular, partially annular, or otherwise partially formed into a ring. Furthermore, the backflow prevention member 5 is not limited to a flat plate, and may have various shapes, such as a truncated cone whose diameter decreases in the direction of the diluent gas flow, as shown in Fig. 4.

[0054] Furthermore, although the dilution gas mixing unit X in the above embodiment includes one backflow prevention member 5, it may include multiple backflow prevention members 5. In this case, the multiple backflow prevention members 5 may be provided in the dilution gas supply pipe 3H at the same position in the pipe axis direction but at different positions in the circumferential direction, or may be provided at different positions in the pipe axis direction and at different positions in the circumferential direction.

[0055] Furthermore, although the backflow prevention member 5 has a single through-hole 5a in the above embodiment, it may have a plurality of through-holes 5a.

[0056] A specific embodiment in which a plurality of through holes 5a are provided is an embodiment in which a porous orifice plate having a large number of through holes 5a is used as the backflow prevention member 5, as shown in FIG.

[0057] As shown in FIG. 6, the backflow prevention member 5 has a flat plate shape, and a through-hole forming area 5X, which is an area where the through-holes 5a are formed, is virtually defined.

[0058] This through-hole formation region 5X is, for example, a circular region, and in this case, it is a circular region having the same diameter as the inner diameter of the dilution gas supply pipe 3H. In this embodiment, through-holes 5a having the same diameter are regularly arranged in this through-hole formation region 5X. Note that the size of the through-holes 5a may be different, for example, between the central portion and the outer periphery, and the arrangement of the through-holes 5a is not limited to that shown in FIG. 6 and may be changed as appropriate. Furthermore, the through-hole formation region 5X may have a diameter smaller than the inner diameter of the dilution gas supply pipe 3H, and may have a shape other than a circle, such as a polygonal or rectangular shape.

[0059] Furthermore, in the configuration shown in FIG. 5, a plurality of porous orifice plates 5 serving as backflow prevention members are provided in the diluent gas supply pipe 3H along the direction of flow of the diluent gas.

[0060] Here, two porous orifice plates 5 are provided with a spacer S interposed between them. As a result, the upstream porous orifice plate 5 and the downstream porous orifice plate 5 are spaced apart by the thickness of the spacer S. The number of porous orifice plates 5 may be one, or three or more.

[0061] The sizes of the through holes 5a formed in the upstream-side porous orifice plate 5 and the downstream-side porous orifice plate 5 are different from each other. More specifically, the through holes 5a in the downstream-side porous orifice plate 5 are smaller than the through holes 5a in the upstream-side porous orifice plate 5. The number of through holes 5a formed in the upstream-side porous orifice plate 5 may be different from the number of through holes 5a formed in the downstream-side porous orifice plate 5.

[0062] With this configuration, the flow of the dilution gas is rectified by the large number of through holes 5a, allowing the dilution gas to flow smoothly downstream of the porous orifice plate 5. This ensures the backflow prevention effect of the porous orifice plate 5, while improving the uniformity of the exhaust gas components contained in the mixed gas.

[0063] Furthermore, since multiple porous orifice plates 5 with through holes 5a of different sizes are used, the backflow prevention effect and pressure loss can be appropriately adjusted according to various analytical specifications with different dilution gas flow rates. When the flow rate of the dilution gas is large, using a porous orifice plate 5 results in excessive pressure loss. In this case, the orifice plate 5 having one through hole 5a as in the above embodiment may be used, or the orifice plate 5 as in the above embodiment may be used in combination with the porous orifice plate 5 depending on the flow rate of the dilution gas.

[0064] In the above embodiment, the inlet P of the diluted gas sampling unit 250 is provided inside the diluted gas supply pipe 3H, but it may be provided on the peripheral wall of the diluted gas supply pipe 3H.

[0065] The inlet P may be provided at a position where the pipe axis L of the diluted exhaust gas supply pipe 3H does not pass through. An example of the position of the inlet P where the pipe axis L does not pass through is a position where the inlet P is located inside I of half the inner diameter of the diluted exhaust gas supply pipe 3H, as shown in FIG.

[0066] Furthermore, in the above embodiment, the exhaust gas analysis system 100 samples the entire amount of exhaust gas, but it may also sample only a portion of the exhaust gas.

[0067] Furthermore, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications are possible without departing from the spirit of the present invention. [Industrial Applicability]

[0068] According to the exhaust gas analysis system 100 of the present invention, even when the dilution ratio is lowered and the diluted gas flow rate is reduced, the influence of the blow-up of the mixed gas on background measurement can be reduced.

Claims

1. A dilution gas mixing unit used in an exhaust gas analysis system for analyzing a mixed gas obtained by diluting exhaust gas with a dilution gas, the dilution gas mixing unit mixing the exhaust gas with the dilution gas, a dilution gas supply pipe connected to an exhaust gas introduction pipe into which the exhaust gas is introduced and for supplying a dilution gas to the exhaust gas introduction pipe; a diluted gas sampling unit provided in the diluted gas supply pipe for sampling the diluted gas; a backflow prevention member provided in the dilution gas supply pipe closer to the exhaust gas introduction pipe than the dilution gas sampling unit, The backflow prevention member is a through hole that is always open and allows the dilution gas to pass through, the through hole being provided along an inner circumferential surface of the dilution gas supply pipe; A dilution gas mixing unit characterized by being configured to prevent the mixed gas from flowing back within the dilution gas supply pipe by increasing the flow rate of the dilution gas passing through the through hole and pushing back the mixed gas.

2. 2. The dilution gas mixing unit of claim 1, wherein the backflow prevention member is an orifice plate.

3. 3. The dilution gas mixing unit according to claim 1, wherein the backflow prevention member is a porous orifice plate having a large number of through holes.

4. 4. The dilution gas mixing unit according to claim 3, wherein a plurality of said multi-hole orifice plates are provided in said dilution gas supply pipe along the direction of flow of said dilution gas.

5. 5. The dilution gas mixing unit of claim 4, wherein the through-holes of the downstream multi-aperture orifice plate are smaller than the through-holes of the upstream multi-aperture orifice plate.

6. A diluted gas mixing unit as described in any one of claims 2 to 5, wherein the diluted gas sampling section has an inlet arranged within the diluted gas supply pipe so as to be located within the through hole of the backflow prevention member when viewed from the pipe axis direction, or so as to be located more than half the inner diameter from the pipe axis of the diluted gas supply pipe as the center.

7. The diluent gas mixing unit of claim 6 , wherein the inlet faces upstream of the diluent gas.

8. 8. The dilution gas mixing unit according to claim 1, wherein a pressure loss in the dilution gas supply pipe provided with the backflow prevention member is less than 250 Pa.

9. a mixed gas flow pipe through which the mixed gas flows; a mixed gas sampling unit provided in the mixed gas flow pipe for sampling the mixed gas; a constant flow rate mechanism that keeps the flow rate of the mixed gas constant; a gas analyzer for analyzing a predetermined measurement target component contained in the sampled diluted gas and the sampled mixed gas; An exhaust gas analysis system comprising a dilution gas mixing unit according to any one of claims 1 to 8.

10. A dilution gas mixing unit used in an exhaust gas analysis system for analyzing a mixed gas obtained by diluting exhaust gas with a dilution gas, which mixes the exhaust gas with the dilution gas, a dilution gas supply pipe connected to an exhaust gas introduction pipe into which the exhaust gas is introduced and for supplying a dilution gas to the exhaust gas introduction pipe; a diluted gas sampling unit provided in the diluted gas supply pipe for sampling the diluted gas; a backflow prevention member provided in the dilution gas supply pipe closer to the exhaust gas introduction pipe than the dilution gas sampling unit, for preventing the mixed gas from flowing back inside the dilution gas supply pipe; the backflow prevention member has a through hole through which the dilution gas passes and is provided along an inner circumferential surface of the dilution gas supply pipe, A dilution gas mixing unit having an inlet arranged so that the dilution gas sampling section is located within the dilution gas supply pipe within the through hole of the backflow prevention member when viewed from the pipe axis direction, or so that it is located more than half the inner diameter from the pipe axis of the dilution gas supply pipe as the center.

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