Title of Invention

APPARATUS FOR CONTROLLING AIR FUEL RATIO OF ENGINE

Abstract

In an apparatus for controlling an air-fuel ratio of an engine, comprising an exhaust system having a three-way catalyst and connected to an exhaust port formed in a cylinder head of the engine, and an oxygen sensor which detects whether an air-fuel ratio is rich or lean based on an oxygen concentration in an exhaust gas, and which is attached to the exhaust system on the upstream side of the three-way catalyst so that a value detected by the oxygen sensor is reflected in a control of the air-fuel ratio, to effectively remove NOx in the exhaust gas by using an inexpensive oxygen sensor. [Solution] An oxygen sensor 33 is attached to an exhaust pipe 27 which constitutes a part of an exhaust system 26 and is connected to the exhaust port 25, with a distance between the oxygen sensor 33 and the exhaust port 25 being 10 times a radius of the exhaust port 25 or less.

Full Text

[Document Name] Specification [Title of the Invention] APPARATUS FOR CONTROLLING AIR-FUEL RATIO OF ENGINE [Technical Field] [0001] The present invention relates to an apparatus for controlling an air-fuel ratio of an engine, comprising an exhaust system having a three-way catalyst and connected to an exhaust port formed in a cylinder head of the engine, and an oxygen sensor for detecting whether an air-fuel ratio is rich or lean based on an oxygen concentration in an exhaust gas, the oxygen sensor being attached to the exhaust system on the upstream side of the three-way catalyst so that a value detected by the oxygen sensor is reflected in a control of the air-fuel ratio. [Background Art] [0002] Disclosed in Patent Document 1 and Patent Document 2, there are already known apparatuses where an oxygen sensor attached at a portion of an exhaust pipe which is upstream of a three-way catalyst is used to control an air-fuel ratio. [Patent Document 1] JP-A No. 74360/1984 [Patent Document 2] JP-A No. 335467/2000 [Disclosure of the Invention] [Problem to be Solved by the Invention] [0003] In a multi-cylinder engine disclosed in the Patent Document 1, the oxygen sensor is attached at an exhaust manifold which is located remote from an engine in an exhaust system. In a single-cylinder engine disclosed in the Patent Document 2, the oxygen sensor is attached to the exhaust pipe at an empty space so as not to interfere with a main body of the engine, in view of easiness of maintenance and protection of the oxygen sensor. In either case, the oxygen sensor is disposed at a position remote from the exhaust port in the main body of the engine. [0004] An oxygen sensor detects whether the air-fuel ratio is rich or lean, based on an oxygen concentration in an exhaust gas. A voltage outputted from the oxygen sensor changes in accordance with whether the air-fuel ratio is rich or lean, as shown in Fig. 7. An apparatus for controlling an engine air-fuel ratio using the output voltage of an oxygen sensor is performed such that a correction factor of a fuel-injection control quantity is changed in an opposite direction to the output of the oxygen sensor, as shown in Fig. 8, so that the air-fuel ratio is kept around a stoichiometric fuel-air ratio. [0005] Meanwhile, a purification efficiency for NOx in an exhaust gas by a three-way catalyst is high when the air-fuel ratio is on the rich side. In particular, when the engine operating condition is in a high-load region, a high-value range of the purification efficiency further shifts to the rich side. Hence, the conventional air-fuel ratio control using the oxygen sensor which detects whether the air-fuel ratio is rich or lean, can not enhance the NOx purification efficiency. Although such a problem can be solved by employing a linear air-fuel sensor, a linear air-fuel sensor is expensive and thus employment thereof leads to increase in the cost. [0006] This invention has been developed in view of the above-described situations, and it is an object of the invention to provide an apparatus for controlling an air-fuel ratio of an engine, which can effectively remove NOx in an exhaust gas by using an inexpensive oxygen sensor. [Means for Solving the Problem] [0007] To solve the above problem, a first arrangement of the invention as defined in claim 1 provides an apparatus for controlling an air-fuel ratio of an engine, comprising an exhaust system having a three-way catalyst and connected to an exhaust port formed in a cylinder head of the engine, and an oxygen sensor for detecting whether the air-fuel ratio is rich or lean based on an oxygen concentration in an exhaust gas, the oxygen sensor being attached to the exhaust system on the upstream side of the three-way catalyst so that a value detected by the oxygen sensor is reflected in a control of the air-fuel ratio, the apparatus being characterized in that the oxygen sensor is attached to an exhaust pipe which constitutes a part of the exhaust system and is connected to the exhaust port, with a distance between the oxygen sensor and the exhaust port being 10 times a radius of the exhaust port or less. [0008] A second arrangement of the invention as defined in claim 2 provides the apparatus for controlling an air-fuel ratio of an engine according to claim 1, characterized in that a body frame of a motorcycle in which the engine is installed comprises a head tube which supports a front fork such that the front fork is steerable with the front fork supporting a front wheel such that the front wheel is rotatable, and a down tube extending rearward from the head tube, and the oxygen sensor is attached to the exhaust pipe at a position near the down tube. [Effect of the Invention] [0009] According to the first arrangement defined in claim 1 where the oxygen sensor is attached to the exhaust pipe with the distance between the oxygen sensor and the exhaust port being 10 times the radius of the exhaust port or less, the oxygen sensor is located at a position near the exhaust port. The inventor (s) of the invention of the present application has/have found that an amount of oxygen remaining in the exhaust gas flowing through the exhaust system gradually decreases as the flow of the exhaust gas gets away from the exhaust port, and at the vicinity of a position where the distance from the exhaust port becomes 10 times the radius of the exhaust port and on the downstream side thereof, the amount of oxygen remaining in the exhaust gas is almost constant. With the position at which the oxygen sensor is attached to the exhaust pipe determined as described above according to the invention, the output of the oxygen sensor shifts to the lean side. In an air-fuel ratio control using the output of the oxygen sensor, an exhaust gas produced by combustion of an air-fuel mixture of the air-fuel ratio as shifted to the rich side passes through the three-way catalyst. Hence, NOx in the exhaust gas is effectively removed while the response of the air-fuel ratio control is improved, by locating the oxygen sensor at a position near the exhaust port. [0010] According to the arrangement as defined in claim 2, a sensor cord extending from the oxygen sensor is routed to run along the down tube, thereby facilitating wiring of the sensor cord as well as protection of the sensor cord. [Best Mode for Carrying Out the Invention] [0011] There will be described a best mode for carrying out the invention based on one embodiment of the invention presented in the accompanying drawings. [0012] Fig. 1 is a side elevational view of a motorcycle to which the present invention is applied. Fig. 2 is a side elevational view of an engine. Fig. 3 is a longitudinal side elevational view of a silencer. Fig. 4 is a graph where concentrations of CO, HC and NOx in an exhaust gas having passed through a three-way catalyst, when an engine operating condition is in a low-load region, are plotted against the air-fuel ratio . Fig. 5 is a graph where the concentrations of CO, HC and NOx in the exhaust gas having passed through the three-way catalyst, when the engine operating condition is in a high-load region, are plotted against the air-fuel ratio. Fig. 6 is a graph representing a change in an amount of oxygen remaining in the exhaust gas flowing through an exhaust system. Fig. 7 is a graph representing an output characteristic of an oxygen sensor. Fig. 8 is a graph representing a change in a correction factor of a fuel-injection control quantity depending on an output of the oxygen sensor. [0013] Referring first to Fig. 1, a body frame F of this motorcycle includes a head tube 7, a main frame 8, and a down tube 9. The head tube 7 supports a front fork 5 such that the front fork 5 is steerable. The front fork 5 supports a front wheel WF such that the front wheel WF is rotatable. A steering handlebar 6 is connected to the front fork 5. The main frame 8 extends rearward from the head tube 7 . The down tube 9 extends from the head tube 7 rearward and downward at a steeper angle than the main from 8 does, and a rear portion of the down tube 9 extends substantially horizontally. A pivot plate 10 connecting the main frame 8 and the rear portion of the down tube 9 supports a front portion of a rear fork 15 via a spindle 14 such that the front portion of the rear fork 15 is capable of vertical oscillation. A rear portion of the rear fork 15 rotatably supports a rear wheel WR. [0014] To an upper portion of the body frame F, there is connected an upper portion of a rear cushion unit 16. A lower portion of the rear cushion unit 16 is connected to an intermediate portion of the rear fork 15 via a link mechanism 17. [0015] A power unit P constituted by a single-cylinder engine E and a transmission M is mounted on the body frame F. Power outputted from the power unit P, that is, power outputted from an output shaft 18 of the transmission M, is transmitted to the rear wheel WR via a chain-type transmitting means 19. [0016] Referring to Fig. 2 also, an air intake system 22, which is connected to an air intake port 21 formed in a side face of a rear portion of a cylinder head 20 of the engine E, includes a throttle body 24 with a fuel injection valve 23. An exhaust system 26 connected to an exhaust port 25 formed in a side face of a front portion of the cylinder head 20 has an exhaust pipe 27 whose upstream end is connected to the exhaust port 25, a silencer 30 incorporating a three-way catalyst 28 and disposed on the right-hand side of the rear wheel WR, and a connecting pipe 29 which connects a downstream end of the exhaust pipe 27 and an upstream end of the silencer 30. [0017] The exhaust pipe 27 is formed in a curved shape extending from the exhaust port 25 formed in the side face of the front portion of the cylinder head 20, to the rear side through below the power unit P. A bracket 31 disposed on the silencer 30 located on the right-hand side of the rear wheel WR is supported by the body frame F via a supporting member 32. [0018] In Fig. 3, a casing 34 of the silencer 30 comprises a cylindrical casing main body 35, a cone member 36, and a lid member 37 . The casing main body 35 extends rearward and upward on the right-hand side of the rear wheel WR. The cone member 36 is tapered down frontward and connected to a front end of the casing main body 35. The lid member 37 is connected to a rear end of the casing main body 35 . The bracket 31 is disposed on an exterior surface of the casing main body 35 at an upper portion thereof. [0019] In the cone member 36 is disposed a cylindrical catalyst case 38 incorporating the three-way catalyst 28. A front end of the catalyst case 38 is fitted with a front end of the cone member 36. An intermediate portion of the catalyst case 38 is slidably supported by a supporting tube 39, which is fixedly supported by a tapered support stay 40 secured to an interior surface of an intermediate portion of the cone member 36, and a disc-shaped support plate 41 whose peripheral portion is secured to an interior surface of the cone member 36 at a rear portion thereof. A downstream end of the connecting pipe 29 is fitted with the front end of the catalyst case 38, and an exhaust gas flowing from the exhaust pipe 27 into the connecting pipe 29 and to the further downstream side, is introduced into the catalyst case 38, passing through the three-way catalyst 28. [0020] To a rear end of the catalyst case 38 is connected a front end of a conduit 42 extending in a front-rear direction in the casing 34, through the support plate 41. A rear end of the conduit 42 is located in the vicinity of the lid member 37. Further, a cylindrical, bottomed cap 44 having a plurality of through-holes 43, 43... is fitted with and secured to the rear end of the conduit 42. [0021] To an interior surface of the casing main body 35, a first partition plate 45, a second partition plate 46, and a third partition plate 47 are secured, arranged in this order from front and spaced from one another. The conduit 42 extending through the partition plates 45, 46, 47 is slidably supported by the partition plates 45, 46, 47. Thus, an inside space of the casing 34 is partitioned with the support plate 41, the first, second, and third partition plates 45-47, and the lid member 37, into a first expansion chamber 48, a second expansion chamber 49, a third expansion chamber 50, and a fourth expansion chamber 51, in this order from front. [0022] The rear end of the conduit 42 is disposed in the fourth expansion chamber 51, and the exhaust gas having passed through the three-way catalyst 28 and flowing rearward in the conduit 42 flows into the fourth expansion chamber 51 through the through-holes 43, 43 .... The third partition plate 47 has a plurality of small-diameter communication holes 52, 52 ..., and the exhaust gas having flown into the expansion chamber 51 is introduced into the third expansion chamber through the small-diameter communication holes 52, 52 .... [0023] A first communication pipe 53 is disposed to extend through the first and second partition plates 45, 46, such that two opposite ends of the first communication pipe 53 are open in the first and third expansion chambers 48, 50, respectively. A second communication pipe 54 is disposed to extend through the first partition plate 45 such that two opposite ends of the second communication pipe 54 are open in the first and second expansion chambers 48, 49, respectively. The exhaust gas as introduced into the third expansion chamber 50 is then introduced into the first expansion chamber 48 via the first communication pipe 53, and further into the second expansion chamber 49 via the second communication pipe 54. [0024] A front end of an ejection pipe 55, whose front end is open in the second expansion chamber 48, and which extends through the third partition plate 47, is fixed to the second partition plate 46. A rear end of the ejection pipe 55 is supported by the lid member 37 located at a rear end of the casing 34, and open toward the rear side. In the fourth expansion chamber 51, a part of the ejection pipe 55 is covered by an outer tube member 56, and an annular space defined between the outer tube member 5 6 and the ejection pipe 55 is filled with a sound absorbing material 57. At a portion of the ejection pipe 55 which corresponds to the outer tube member 56, there are formed a large number of small holes (not shown) through the thickness of a wall of the ejection pipe 55. [0025] Referring again to Figs. 1 and 2, on the upstream side of the three-way catalyst or the silencer 30 in the exhaust system 26, there is attached an oxygen sensor 33 which changes an output voltage depending on whether an air-fuel ratio is rich or lean based on an oxygen concentration in the exhaust gas, as shown in Fig. 7, so that a value detected by the oxygen sensor is reflected in a control of the air-fuel ratio. In the exhaust system 26, the oxygen sensor 33 is attached to the exhaust pipe 27 upstream of the three-way catalyst 28, such that the oxygen sensor 33 is located near the down tube 9 of the body frame F. A part of the sensor cord 59 extending from the oxygen sensor 33 runs along the down tube to a lower side of a seat 60 so as to be connected to a control unit (not shown) disposed in a space formed below the seat 60. [0026] Meanwhile, as shown in Figs. 4 and 5, the richer the air-fuel ratio (A/F) , the higher the efficiency of purification by the three-way catalyst 28 for CO and HC contained in the exhaust gas; on the other hand, the leaner the air-fuel ratio (A/F), the higher the purification efficiency for NOx. As shown in Fig. 5 # when the engine (E) operating condition is in the high-load region, the range where the NOx purification efficiency is high shifts to the rich side. [0027] The amount of oxygen remaining in the exhaust gas flowing through the exhaust system 26 gradually decreases as the flow of the exhaust gas gets away from the exhaust port 25, as shown in Fig. 4. At the vicinity of a position where a distance from the exhaust port 25 becomes 10 times a diameter D of the exhaust port 25 and on the downstream side thereof, the amount of remaining oxygen is substantially constant. Where the oxygen sensor 33 for detecting whether the air-fuel ratio is rich or lean is disposed within the range where the remaining oxygen is constant, the control of air-fuel ratio using the output voltage of the oxygen sensor 3 3 is implemented such that a correction factor of a control quantity related to ejection of fuel from the fuel injection valve 23 is changed in a direction opposite to the output of the oxygen sensor 33, as shown in Fig. 8, so as to keep the air-fuel ratio around the stoichiometric value. Therefore, the purification efficiency for NOx in the exhaust gas can not be enhanced. [0028] According to the invention, hence, the oxygen sensor 33 is attached to the exhaust pipe 27 at a position spaced from the exhaust port 25 by a distance L which is 10 times the diameter D of the exhaust port 25 or less, and outputs a voltage corresponding to a larger amount of remaining oxygen, or the leaner side. [0029] There will be now described an operation of this embodiment. The oxygen sensor 33 is attached to the exhaust pipe 27 such that the oxygen sensor 33 is spaced from the exhaust port 25 formed in the cylinder head 20 of the engine E, by the distance L which is 10 times the diameter D of the exhaust port 25 or less, and accordingly the oxygen sensor 33 is located at a position near the exhaust port 25. [0030] The amount of oxygen remaining in the exhaust gas flowing through the exhaust system 26 gradually decreases as the exhaust gas flow gets away from the exhaust port 25. At the vicinity of the position where the distance from the exhaust port 25 becomes 10 times the diameter D of the exhaust port 25 and on the downstream side thereof, the amount of remaining oxygen is substantially constant. Hence, by determining the position to attach the oxygen sensor 33 in the exhaust pipe 27 in the manner as described above, the output of the oxygen sensor 33 shifts to the lean side. By the control of the air-fuel ratio using such an output of the oxygen sensor 33, an exhaust gas produced by combustion of an air-fuel mixture at the air-fuel ratio as shifted to the rich side as indicated by an arrow in Fig. 5, passes through the three-way catalyst 28. [0031] Thus, by using a conventional inexpensive oxygen sensor 3 3 which detects whether the air-fuel ratio is rich or lean based on the oxygen concentration in the exhaust gas, and not by using a linear air-fuel ratio sensor which is expensive, NOx in an exhaust gas can be effectively removed by the three-way catalyst 28. Further, by attaching the oxygen sensor 33 at a position near the exhaust port 25, the response of the air-fuel ratio control is enhanced. [0032] Since the oxygen sensor 33 is attached to the exhaust pipe 27 so that the oxygen sensor 33 is located in the vicinity of the down tube 9 of the body frame F, the sensor cord 59 extending from the oxygen sensor 33 is routed to run along the down tube 9, thereby facilitating protection of the sensor cord 59 and wiring of the sensor cord. [0033] Although there has been illustrated one embodiment of the invention, the present invention is not limited to the embodiment, but may be otherwise embodied with various design changes, without departing from the scope of the invention as defined in the "Scope of Claims". [0034] For instance, in the above-described embodiment, the invention is applied to a single-cylinder engine E. However, the invention is applicable to a multi-cylinder engine, too. [Brief Description of the Drawings] [0035] [Fig. 1] Fig. 1 is a side elevational view of a motorcycle. [Fig. 2] Fig. 2 is a side elevational view of an engine. [Fig. 3] Fig. 3 is a longitudinal side elevational view of a silencer. [Fig. 4] Fig. 4 is a graph where concentrations of CO, HC and NOx in an exhaust gas having passed through a three-way catalyst, when an engine operating condition is in a low-load region, are plotted against an air-fuel ratio. [Fig. 5] Fig. 5 is a graph where the concentrations of CO, HC and NOx in the exhaust gas having passed through the three-way catalyst, when the engine operating condition is in a high-load region, are plotted against the air-fuel ratio. [Fig. 6] Fig. 6 is a graph representing a change in an amount of oxygen remaining in the exhaust gas flowing through an exhaust system. [Fig. 7] Fig. 7 is a graph representing an output characteristic of an oxygen sensor. [Fig . 8] Fig . 8 is a graph representing a change in a correction factor of a fuel-injection control quantity depending on an output of the oxygen sensor. [Description of Reference Numerals] [0036] 5 ... front fork 7 ... head tube 9 ... down tube 20 ... cylinder head 25 ... exhaust port 26 ... exhaust system 27 ... exhaust pipe 28 ... three-way catalyst 33 ... oxygen sensor E ... engine F ... body frame WF ... front wheel [Document Name] Scope of Claims [Claim 1] An apparatus for controlling an air-fuel ratio of an engine (E) , comprising an exhaust system (26) having a three-way catalyst (28) and connected to an exhaust port (25) formed in a cylinder head (20) of the engine (E) , and an oxygen sensor (33) for detecting whether the air-fuel ratio is rich or lean based on an oxygen concentration in an exhaust gas, the oxygen sensor (33) being attached to the exhaust system (26) and on the upstream side of the three-way catalyst (28) so that a value detected by the oxygen sensor (33) is reflected in a control of the air-fuel ratio, the apparatus being characterized in that the oxygen sensor (33) is attached to an exhaust pipe (27) which constitutes a part of the exhaust system (26) and is connected to the exhaust port (25), with a distance between the oxygen sensor (33) and the exhaust port (25) being 10 times a radius of the exhaust port (25) or less. [Claim 2] The apparatus for controlling an air-fuel ratio of an engine according to claim 1, characterized in that a body frame (F) of a motorcycle in which the engine (E) is installed comprises a head tube (7) which supports a front fork (5) such that the front fork (5) is steerable with the front fork (5) supporting a front wheel (WF) such that the front wheel (WF) is rotatable, and a down tube (9) extending rearward from the

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