| Title of Invention | EXHAUST EMISSION CONTROL DEVICE FOR A VEHICLE |
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| Abstract | [Document Name] Abstract of the Disclosure [Abstract] [Problem] To Provide an exhaust emission control device which, while enabling an oxidation catalyst to maintain its performance, can be used for a small vehicle using a relatively low-cost carburetor. (Solution] An exhaust emission control device for a vehicle comprises: an air cleaner 11 which has a dirty side 11C and a clean side 11D and which purifies air sucked from outside into the dirty side 11C and supplies the purified air to an engine 12 via the clean side 11D; an oxidation catalyst 30 disposed in an exhaust path of the engine 12; and a secondary air supply mechanism 20 which supplies secondary air from the clean side 11D of the air cleaner 11 to an exhaust port 12B of the engine 12. In the exhaust emission control device, the air-fuel ratio of exhaust gas in front of the oxidation catalyst 30 is set to be 15 or higher over a whole speed range not exceeding 55 km/h of the vehicle. [Selected Drawing] Fig. 1 |
| Full Text | [Document Name] Specification [Title of the Invention] EXHAUST EMISSION CONTROL DEVICE FOR A VEHICLE [Technical Field] [0001] The present invention relates to an exhaust emission control device for a vehicle, the device including a secondary air supply mechanism for supplying secondary air to in front of a catalyst. [Background Art] [0002] Generally, an exhaust gas purification system complying with exhaust gas regulations targeted at carbon monoxide, total hydrocarbon, and nitrogen oxide uses a fuel injection system, in which feedback control is performed based on an oxygen sensor output, and a three-way catalyst in combination. Among vehicles using such an exhaust gas purification system, there is one on which, when the engine is started, secondary air is introduced to in front of an exhaust gas purification catalyst to cause the air-fuel ratio of exhaust gas to be on the lean side so as to highly efficiently purify a large amount of hydrocarbon (HC) generated when the engine is started (see the patent document 1, for example). For the vehicle, a period of time (up to about 60 seconds) from when the engine is cranked until when the oxygen sensor is actuated is set as a period for introducing the secondary air. [Patent Document 1] JP-A No. H05-293384 (Disclosure of the Invention] [Problem to be Solved by the Invention] [0003] There are some areas where exhaust gas regulations are targeted at the total amount of total hydrocarbon and nitrogen oxide. In such areas, oxidation catalysts used to purify (oxidize) total hydrocarbon are required to fully function. To fully function, an oxidation catalyst requires to be in an oxygen-present state with the air-fuel ratio of exhaust gas at the catalyst entrance being leaner than the theoretical air-fuel ratio. Namely, it cannot fully function unless being in an oxidizing atmosphere. Hence, using a secondary air supply mechanism which introduces secondary air into exhaust gas may be considered. However, in the case of a vehicle using a carburetor, which is less costly than a fuel injection system, to mix air with fuel, the carburetor is set to an air-fuel ratio richer than the theoretical air-fuel ratio. This is because the carburetor as compared with the fuel injection system cannot accurately control the air-fuel ratio. As a result, the oxygen concentration in exhaust gas is caused to be low making it necessary to provide a large amount of secondary air . A secondary air supply mechanism having a reed valve which operates depending on the exhaust pressure may also be used. However, when, with such a secondary air supply mechanism used, the engine rotation speed or engine load becomes such that the range where the exhaust pressure is negative due to pulsation is small, the supply of secondary air decreases and a reductive atmosphere tends to be generated. When a reductive atmosphere is kept, an oxidation catalyst cannot stably function. [0004] An object of the present invention is to provide an exhaust emission control device which, while enabling an oxidation catalyst to maintain its performance, can be used for a small vehicle using a relatively low-cost carburetor. [0005] To address the above problem, the present invention provides an exhaust emission control device for a vehicle comprising: an air cleaner which has a dirty side and a clean side and which purifies air sucked from outside into the dirty side and supplies the purified air to an engine via the clean side; an oxidation catalyst disposed in an exhaust path of the engine; and a secondary air supply mechanism which supplies secondary air from the clean side of the air cleaner to an exhaust port of the engine. In the exhaust emission control device, an air-fuel ratio of exhaust gas in front of the oxidation catalyst is set to be 15 or higher over a whole speed range not exceeding 55 km/h of the vehicle. According to the invention, the air-fuel ratio of exhaust gas in front of the oxidation catalyst is set to be 15 or higher over a whole speed range not exceeding 55 km/h of a small vehicle. Therefore, even when the small vehicle uses a relatively low-cost carburetor, the oxidation catalyst can display stable performance. [0006] It is preferable that: the vehicle use a carburetor to supply an air-fuel mixture to the engine; the oxidation catalyst contains palladium as a main component thereof; and the carburetor and the secondary air supply mechanism of the vehicle are set such that the air-fuel ratio of exhaust gas in front of the oxidation catalyst is 15 or higher over a whole speed range not exceeding 55 km/h of the vehicle. The above arrangement makes it possible to set the carburetor and the secondary air supply mechanism such that the oxidation catalyst can stably display its performance. [Effect of the Invention] [0007] According to the present invention, the air-fuel ratio of exhaust gas in front ot an oxidation catalyst is set to be 15 or higher over a whole vehicle speed range not exceeding 55 km/h, so that, even on a small vehicle using a relatively low-cost carburetor, the oxidation catalyst can stably display its performance. The vehicle uses a carburetor to supply an air-fuel mixture to the engine. The oxidation catalyst contains palladium as its main component. The carburetor and secondary air supply mechanism of the vehicle are set such that the air-fuel ratio of exhaust gas in front of the oxidation catalyst is 15 or higher over a whole vehicle speed range not exceeding 55 km/h. It is, therefore, possible by appropriately setting the carburetor and secondary air supply mechanism to allow the oxidation catalyst to stably display its performance. [Best Mode for Carrying Out the Invention] [0008] An embodiment of the present invention will be described below with reference to drawings. Fig. 1 is a diagram schematically showing an exhaust emission control device, along with a peripheral part configuration, for a motorcycle according to an embodiment of the present invention. The exhaust emission control device 10 is mounted on a motorcycle on which a carburetor 13 mixes fuel with air supplied from an air cleaner 11 to an engine (internal combustion engine) 12. The exhaust emission control devi ce 10 includes a secondary air supply mechanism 20 which supplies secondary air (purified air) from the air cleaner 11 to an exhaust port 12B of the engine 12, and an exhaust gas purification catalyst 30 disposed in an exhaust muffler 15 connected, via an exhaust pipe 14, to the engine 12. In Fig. 1, arrows X, Y, and Z represent an air flow, a vacuum pressure, and a flow of blow-by gas generated in a crankcase, respectively. [0009] As shown in Fig. 1, the air cleaner 11 has an air cleaner case 11A the interior of which is partitioned by a partition wall 11B into two chambers, i.e. a dirty side (an outer air introduction chamber) 11C and a clean side (a clean air chamber) 11D. The dirty side IIC is provided with an outer air inlet HE through which outer air is introduced into the dirty side IIC. A filter element 11F is attached to the partition wall 1 IB, covering an opening, in the partition wall 11B, through which the dirty side IIC and clean side 11D are mutually communicated. With the filter element 11F covering the opening, the air in the dirty side IIC is introduced into the clean side 11D after being cleaned by passing the filter element 11F. The clean side 11D is provided with an air outlet port 11G. The air outlet port 11G is connected, via a connecting tube 16, to the carburetor 13 via which the air outlet port 11G is communicated with an intake port 12 A of the engine 12 -[0010] The engine 12 is a general four-cycle engine for mounting on a motorcycle. It includes an intake valve 12D which opens and closes the intake port 12A communicated with a cylinder hole (cylinder) 12C inside the engine 12, and an exhaust valve 12E which opens and closes the exhaust port 12B communicated with the cylinder hole 12C. A piston 12F slidably disposed in the cylinder 12C hole is linked to a crankshaft 12H via a connecting rod 12G. When the piston 12F descends in an intake stroke with the intake valve 12D of the engine 12 open (with the exhaust valve 12E closed) , vacuum is generated in the space above the piston 12F in the cylinder hole 12C. As a result, the air in the clean side 11D of the air cleaner 11 is sucked, via the carburetor 13, into the space above the piston 12F in the cylinder hole 12C. At the same time, fuel is supplied from the carburetor 13 causing a fuel-air mixture to be supplied to the engine 12 . When the piston 12F subsequently ascends in an exhaust stroke with the exhaust valve 12E open {with the intake valve 12D closed) after coming through a compression stroke and a combustion stroke as in a general four-cycle engine, burned gas is discharged to the exhaust port 12B to be exhausted, as exhaust gas, to the exhaust pipe 14. [0011] The exhaust pipe 14 is connected, at its rear end, with an exhaust muffler 15. The exhaust muffler 15 functions as a silencer which lets the high temperature, high pressure exhaust gas coming through the exhaust pipe 14 out while reducing exhaust noise. As shown in Fig- 1, the exhaust muffler 15 is of a multi-stage expansion type in which the exhaust muffler is partitioned by plural partition walls IbA and 15B into plural chambers mutually communicated via communication pipes 15C, 15D, and 15E and in which the exhaust gas purification catalyst 30 is disposed in the front chamber that is the most upstream one of the plural chambers. The exhaust gas purification catalyst 30 is an oxidation catalyst containing palladium (Pd) as a main component added to by 10% to 20% (weight %) of rhodium (Rh). Namely, the main component of the exhaust gas purification catalyst 30 is not platinum (Pt) which is a relatively expensive precious metal. The exhaust gas purification catalyst 30 has a honeycomb catalyst structure in which a porous honeycomb structural body is coated with the catalytic components or a catalyst structure which includes, for example, a heat tube including a punching pipe carrying the catalytic components. (0012] The secondary air supply mechanism 20 sends the air (secondary air) in the clean side 11D of the air cleaner 11 to the exhaust port 12B of the engine 12. It is provided with a secondary air supply pipe 21 which connects the clean side 11D of the air cleaner and the exhaust port 12B of the engine 12. A valve unit 22 is provided at a location partway along the secondary air supply pipe 21. A reed valve 23 to prevent the exhaust gas from the exhaust port 12B from flowing back to the secondary air supply pipe 21 is provided between the valve unit 22 and the exhaust port 12B. In the configuration shown in Fig. 1, the reed valve 23 is disposed above the engine 12, that is, at a location close to the exhaust port 128 so that the reed valve 23 can better respond to pressure variation. The valve unit 22 includes a secondary air supply control valve 24 which prevents the secondary air from being supplied to the exhaust port 12B when the engine 12 slows down. The secondary air supply control valve 24 is designed to operate depending on the vacuum pressure in the intake port 12A of the engine 12. The vacuum pressure is conveyed to the valve unit 22 via a communication pipe 2 5 connecting the valve unit 22 and the intake port 12A. Reference numeral 35 in drawing denotes a communication pipe through which the clean side 11D of the air cleaner 11 and the crankcase of the engine 12 are communicated with each other. The communication pipe 35 returns the blow-by gas generated in the crankcase to the engine 12 via the air cleaner 11 and the carburetor 13, thereby functioning as a crankcase emission control device to prevent the blow-by gas from being released. [0013] The exhaust emission control device 10 is suitable for use in areas where exhaust gas regulations are targeted at the total amount of total hydrocarbon and nitrogen oxide, and requires the exhaust gas purification catalyst (oxidation catalyst) 30 for purifying (oxidizing) the total hydrocarbon to stably function. Generally, the exhaust gas purification catalyst 30 is, when the main component of the oxidation catalyst included in it is palladium, relatively inexpensive and shows high performance. There are, however, cases in which, as shown in Fig. 2 showing the results of a test conducted to examine changes in the catalytic conversion efficiency of such a palladium-based catalyst over long hours, a catalyst having exhibited a conversion efficiency exceeding 90% (see characteristic curve LI) produces, after a predetermined amount of time, a state where the air-fuel ratio of exhaust gas at a catalyst entrance is richer than the theoretic air-fuel ratio, i.e. an almost oxygen-free state. Namely, there are cases in which the purification performance of a catalyst changes in a reductive atmosphere (see characteristic curve L2 ) . As the characteristic curve L2 shows, the purification performance of the catalyst is kept higher in a state where the air-fuel ratio of exhaust gas at the catalyst entrance is leaner than the theoretic air-fuel ratio, i.e. a state with more oxygen or, in other words, a more reductive atmosphere. Hence, when using a palladium-based catalyst, it is important to keep a highly reductive atmosphere. [0014] Generally, when the carburetor 13 is used, the air-fuel ratio is set to be on the rich side so as to enable smooth acceleration responding to an acceleration request from the rider. The oxygen concentration in exhaust gas, therefore, tends to be low. In the present embodiment, the secondary air supply mechanism 20 increases the oxygen concentration in the exhaust gas and stabilizes the purification performance of the device. The secondary air supply mechanism 20 and the carburetor 13 are set such that, at least, durable distance requirements (requirements for the distance that can be run without exceeding an exhaust regulation value) imposed by exhaust gas regulations in some countries as being described in detail in the following can be met. [0015] Fig. 3(A) is a diagram showing the relationship between the vehicle speed on a flat road and the oxygen concentration at the catalyst entrance. Fig. 3(B) is a diagram showing the relationship between the vehicle speed on a flat road and the air-fuel ratio at the catalyst entrance. In each of Figs. 3(A) and 3(B), characteristic curve Ml represents a characteristic of the exhaust emission control device including the secondary air supply mechanism 20 (i.e. the secondary air is introduced), whereas characteristic curve M2 represents a characteristic of the exhaust emission control device without including the secondary air supply mechanism 20 (i.e. the secondary air is not introduced). As shown in Figs. 3(A) and 3(B), it is apparent that both the oxygen concentration and air-fuel ratio at the catalyst entrance are higher in the configuration including the secondary air supply mechanism 20 (see the characteristic curves Ml) than in the configuration without including the secondary air supply mechanism 20 (see the characteristic curves M2). Generally, in the secondary air supply mechanism 20 having the reed valve 23 as described above, when the pressure of the exhaust gas exhausted from the exhaust port 12B of the engine 12 becomes negative due to pulsation, the reed valve 23 opens and the secondary air is sent into the exhaust gas. The reed valve 23 has a characteristic such that the secondary air can be sent into the exhaust gas easily when the engine rotation speed or engine load is low allowing the exhaust pressure to become negative over a large portion of exhaust pulsation and such that the secondary air can be sent into the exhaust gas less easily when the engine rotation speed or engine load is high allowing the exhaust pressure to become negative over only a small portion of exhaust pulsation. Therefore, as shown in Figs. 3(A) and 3(B), when the vehicle speed exceeds about 60 km/h, it becomes difficult to supply the secondary air and raise the oxygen concentration and air-fuel ratio at the catalyst entrance. [0016] The inventors of the present invention conducted a test to determine conditions for enabling an oxidation catalyst to maintain its performance for an extended period of time over a vehicle speed range (a low speed range not exceeding 60 km/h) in which secondary air can be adequately supplied. Fig. 4 is a diagram showing characteristics of the air-fuel ratio at the catalyst entrance (relationship between the vehicle speed and the dir-fuel ratio at the catalyst entrance) measured with six different settings Nl to N6 on a vehicle running on a flat road. The settings Nl to N6 represent settings reached by adjusting and modifying the carburetor 13 and secondary air supply mechanism 20 to vary the air-fuel ratio at the catalyst entrance on a test vehicle fitted with a palladium-based oxidation catalyst. The curves marked Nl to N3 in Fig. 4 represent test results obtained with the settings Nl to N3 for air-fuel ratios at the oxidation catalyst entrance of 15 or higher over a vehicle speed range not exceeding 55 km/h. The curves marked N4 to N6 in Fig. 4 represent test results obtained with the settings N4 to N6 for air-fuel ratios at the oxidation catalyst entrance lower than 15 over a vehicle speed range not exceeding 55 km/h. The minimum air-fuel ratios (MIN A/F) at a vehicle speed not exceeding 55 km/h were 15.2 for the setting Nl, 15.4 for the setting N2, 15.7 for the setting N3, 14.2 for the setting N4, 14.5 for the setting N5, and 14.4 for the setting N6, respectively. The settings Nl to N6 were obtained, to be concrete, by adjusting, for example, the diameter of the carburetor 13 and the length and diameter of the secondary air supply pipe 21. Of these settings, the settings Nl, N4, and N6 are based on the engine 12 with a displacement of 100 cc; the settings N2 and N5 are based on the engine 12 with a displacement of 125 cc; and the setting N3 is based on the engine 12 with a displacement of 150 cc. [0017] Fig. 5 is a diagram showing the relationship between the total amount (E/M value) of total hydrocarbon and nitrogen oxide and the distance run. In Fig. 5, reference letter P represents an exhaust gas regulation value, and reference letter Q represents a distance requirement requiring small vehicles of a prescribed type to be able to run the distance maintaining stable catalyst performance. These regulation and requirement comply, for example, with a relevant Indian law (Bharat Stage II or III). As shown in Fig. 5, the settings corresponding to higher values of minimum air-fuel ratios at a vehicle speed not exceeding 55 km/h allow the vehicle to run longer without exceeding the exhaust gas regulation value P to realize a longer durable distance. As a result of experiments conducted by the inventors, it has been found that the durable distance requirement Q can be satisfied over a vehicle speed range not exceeding 55 km/h when the air-fuel ratio is 15 or higher regardless of the engine displacement. Based on the results of their experiments, the inventors also plotted a characteristic curve LL representing the relationship between minimum air-fuel ratios recorded over a vehicle speed range not exceeding 55 km/h and distances run. The characteristic curve, as shown in Fig. 6, becomes steeper from about where the minim air-fuel ratio exceeds 15. Namely, in the region where the minimum air-fuel ratio does not exceed 15, variation in the air-fuel ratio largely affects the distance run, whereas, in the region where the minimum air-fuel ratio exceeds 15, the effect of variation in the air-fuel ratio on the distance run is small. Thus, it has been found that small vehicles ranging from 100 cc to 150 cc in engine displacement can maintain stable catalyst performance by not allowing the minimum air-fuel ratio to exceed 15 over a vehicle speed range not exceeding 55 km/h. [0018] The above study has made it known that setting the carburetor 13 and the secondary air supply mechanism 20 such that the air-fuel ratio at the oxidation catalyst entrance is kept at 15 or higher over a whole vehicle speed range not exceeding 55 km/h allows the durability deterioration of the oxidation catalyst to be held at a level meeting the requirement imposed by an exhaust gas regulation, without allowing the engine drivability to be impaired by a lean air-fuel ratio even in cases a relatively low-cost carburetor is used in a small vehicle, eventually allowing performance of the oxidation catalyst to be stably maintained over a long period of time. Thus, the above setting conditions make it possible to stabilize the purification performance of an exhaust emission control device using an palladium-based catalyst even on a small low-cost motorcycle including the carburetor 13. The purification performance of the exhaust emission control device can, therefore, be secured without using an electronic fuel injection device or a platinum-based catalyst. [0019] The present invention has been described based on an embodiment. The invention, however, is not limited to the embodiment, and various design modifications can be applied to the invention without departing from its scope. For example, even though, in the above embodiment, the exhaust gas purification catalyst (oxidation catalyst) 30 is provided in the exhaust muffler 15, the catalyst 30 may be provided, for example, in the exhaust pipe 14 as long as the exhaust pipe 14 is included in the exhaust path of the engine 12. [Brief Description of the Drawings] [0020] [Fig. 1] Fig. 1 is a diagram schematically showing an exhaust emission control device, along with a peripheral part configuration, for a motorcycle according to an embodiment of the present invention. [Fig. 2] Fig. 2 is a diagram showing the conversion efficiency of an exhaust gas purification catalyst (oxidation catalyst). [Figs. 3] Fig. 3(A) is a diagram showing the relationship between the vehicle speed on a flat road and the oxygen concentration at the catalyst entrance, and Fig. 3(B) is a diagram showing the relationship between the vehicle speed on a flat road and the air-fuel ratio at the catalyst entrance. [Fig. 4] Fig. 4 is a diagram showing characteristics of the air-fuel ratio at the catalyst entrance (relationship between the vehicle speed and the air-fuel ratio at the catalyst entrance) measured with settings Nl to N6 on a vehicle running on a flat road. [Fig. 5] Fig. 5 is a diagram showing the relationship between the amount (E/M value) of total hydrocarbon and nitrogen oxide and the distance run. [Fig. 6] Fig. 6 is a diagram showing a characteristic curve representing the relationship between the air-fuel ratio and the durable distance. [Description of Reference Numerals] [0021] 10. . 11. . 11A. 11C. 11D. 11F. 12. . 13. . 14 . . 15. . 20. . 30. . catalyst) Exhaust emission control device Air cleaner Air cleaner case Dirty side (Outer air introduction chamber) Clean side (Clean air chamber) Filter element Engine Carburetor Exhaust pipe Exhaust muffler Secondary air supply mechanism Exhaust gas purification catalyst (Oxidation ^Document Name] Scope of Claims [Claim 1] An exhaust emission control device for a vehicle comprising: an air cleaner which has a dirty side and a clean side and which purifies air sucked from outside into the dirty side and supplies the purified air to an engine via the clean side; an oxidation catalyst disposed in an exhaust path of the engine; and a secondary air supply mechanism which supplies secondary air from the clean side of the air cleaner to an exhaust port of the engine, wherein an air-fuel ratio of exhaust gas in front of the oxidation catalyst is set to be 15 or higher over a whole speed range not exceeding 55 km/h of the vehicle. [Claim 2] The exhaust emission control device for a vehicle according to Claim 1, wherein: the vehicle uses a carburetor to supply an air-fuel mixture to the engine; the oxidation catalyst contains palladium as a main component thereof; and the carburetor and the secondary air supply mechanism of the vehicle are set such that the air-fuel ratio of exhaust gas in front of the oxidation catalyst is 15 or higher over a whole speed range not exceeding 55 km/h of |
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| Patent Number | 278310 | |||||||||||||||
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| Indian Patent Application Number | 207/CHE/2009 | |||||||||||||||
| PG Journal Number | 53/2016 | |||||||||||||||
| Publication Date | 23-Dec-2016 | |||||||||||||||
| Grant Date | 20-Dec-2016 | |||||||||||||||
| Date of Filing | 29-Jan-2009 | |||||||||||||||
| Name of Patentee | HONDA MOTOR CO., LTD. | |||||||||||||||
| Applicant Address | 1-1, MINAMIAOYAMA 2-CHOME, MINATO-KU, TOKYO, | |||||||||||||||
Inventors:
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| PCT International Classification Number | F01N3/00 | |||||||||||||||
| PCT International Application Number | N/A | |||||||||||||||
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