Straddled vehicle

Abstract

A straddled vehicle, including: an engine, which includes a combustion chamber; a three-way catalyst, which is configured to purify exhaust gas exhausted from the combustion chamber; an upstream oxygen sensor, which is provided upstream of the three-way catalyst in a flow direction of the exhaust gas, and is configured to detect an oxygen concentration in the exhaust gas; a downstream oxygen sensor, which is provided downstream of the three-way catalyst in the flow direction of the exhaust gas, and is configured to detect the oxygen concentration in the exhaust gas; and a controller, which includes a processor, and a non-transitory storage medium containing program instructions, execution of which by the processor causes the controller to execute a detachment determination process of determining whether the three-way catalyst is detached at least based on a signal input as a signal of the downstream oxygen sensor.

Claims

1. A straddled vehicle, comprising: an engine, which includes a combustion chamber; a three-way catalyst, which is configured to purify exhaust gas exhausted from the combustion chamber; an upstream oxygen sensor, which is provided upstream of the three-way catalyst in a flow direction of the exhaust gas, and is configured to detect an oxygen concentration in the exhaust gas, a downstream oxygen sensor, which is provided downstream of the three-way catalyst in the flow direction of the exhaust gas, and is configured to detect the oxygen concentration in the exhaust gas; and a controller, which includes: a processor, and a non-transitory storage medium containing program instructions, execution of which by the processor causes the controller to control a fuel amount supplied to the combustion chamber to periodically increase and decrease the fuel amount, based on a signal input as a signal of an upstream oxygen sensor, and execute a detachment determination process of determining whether the three-way catalyst is detached based on both the signal input as the signal of the upstream oxygen sensor and the signal input as a signal of the downstream oxygen sensor, when the fuel amount is controlled to periodically increase and decrease based on the signal input as the signal of the upstream oxygen sensor, wherein the controller is configured to perform first feedback control, thereby controlling the fuel amount to periodically increase and decrease with a first cycle, based on the signal input as the signal of the upstream oxygen sensor, and to perform second feedback control, thereby controlling the fuel amount to periodically increase and decrease with a second cycle, based on the signal input as the signal of the upstream oxygen sensor, the second cycle in the second feedback control being longer than the first cycle in the first feedback control, and/or an amplitude of the fuel amount in the second feedback control being larger than the amplitude of the fuel amount in the first feedback control, and in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached based on both the signal input as the signal of the upstream oxygen sensor and the signal input as the signal of the downstream oxygen sensor, while the second feedback control is in execution.

2. The straddled vehicle according to claim 1, wherein, in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached based on an oxygen sensor delay time while the second feedback control is in execution, the oxygen sensor delay time being a time difference between a change of the signal input as the signal of the downstream oxygen sensor and a change of the signal input as the signal of the upstream oxygen sensor.

3. The straddled vehicle according to claim 2, wherein, in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached by comparing the oxygen sensor delay time while the second feedback control is in execution with a threshold.

4. The straddled vehicle according to claim 2, wherein in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached by comparing the oxygen sensor delay time while the second feedback control is in execution with a second oxygen sensor delay time, the second oxygen sensor delay time is a time difference between a change of the signal input as the signal of the downstream oxygen sensor and a change of the signal input as the signal of the upstream oxygen sensor, while fourth feedback control is in execution before the second feedback control, and the fourth feedback control is another control with which the controller periodically increases and decreases the fuel amount with a fourth cycle based on the signal input as the signal of the upstream oxygen sensor, the fourth cycle and the amplitude of the fuel amount in the fourth feedback control being identical to the second cycle and the amplitude of the fuel amount in the second feedback control.

5. The straddled vehicle according to claim 1, wherein, the controller is configured to execute a deterioration determination process of determining whether the three-way catalyst is deteriorated based on both the signal input as the signal of the upstream oxygen sensor and the signal input as the signal of the downstream oxygen sensor, while the second feedback control is in execution.

6. The straddled vehicle according to claim 1, wherein, the controller is configured to perform third feedback control, thereby controlling the fuel amount to periodically increase and decrease with a third cycle based on the signal input as the signal of the upstream oxygen sensor, the third cycle in the third feedback control is longer than the first cycle in the first feedback control, and/or the amplitude of the fuel amount in the third feedback control is larger than the amplitude of the fuel amount in the first feedback control, the third cycle in the third feedback control is different from the second cycle in the second feedback control, and/or the amplitude of the fuel amount in the third feedback control is different from the amplitude of the fuel amount in the second feedback control, and the controller is configured to execute a deterioration determination process of determining whether the three-way catalyst is deteriorated based on both the signal input as the signal of the upstream oxygen sensor and the signal input as the signal of the downstream oxygen sensor, while the third feedback control is in execution.

7. A straddled vehicle, comprising: an engine, which includes a combustion chamber; a three-way catalyst, which is configured to purify exhaust gas exhausted from the combustion chamber; an upstream oxygen sensor, which is provided upstream of the three-way catalyst in a flow direction of the exhaust gas, and is configured to detect an oxygen concentration in the exhaust gas, a downstream oxygen sensor, which is provided downstream of the three-way catalyst in the flow direction of the exhaust gas, and is configured to detect the oxygen concentration in the exhaust gas; and a controller, which includes: a processor, and a non-transitory storage medium containing program instructions, execution of which by the processor causes the controller to control a fuel amount supplied to the combustion chamber to periodically increase and decrease the fuel amount, based on a signal input as a signal of an upstream oxygen sensor, and execute a detachment determination process of determining whether the three-way catalyst is detached based on both the signal input as the signal of the upstream oxygen sensor and the signal input as a signal of the downstream oxygen sensor, when the fuel amount is controlled to periodically increase and decrease based on the signal input as the signal of the upstream oxygen sensor, wherein the controller is configured to perform first feedback control, thereby controlling the fuel amount to periodically increases and decreases with a first cycle, based on the signal input as the signal of the upstream oxygen sensor, and to perform second feedback control, thereby controlling the fuel amount to periodically increases and decreases with a second cycle, based on the signal input as the signal of the upstream oxygen sensor, the second cycle in the second feedback control being longer than the first cycle in the first feedback control, and/or an amplitude of the fuel amount in the second feedback control being larger than the amplitude of the fuel amount in the first feedback control, and in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached based on both the signal input as the signal of the upstream oxygen sensor and the signal input as the signal of the downstream oxygen sensor, while the first feedback control is in execution.

8. The straddled vehicle according to claim 7, wherein, when the signal input as the signal of the downstream oxygen sensor is changed while the first feedback control is in execution, the controller is configured to execute the detachment determination process of determining whether the three-way catalyst is detached based on an oxygen sensor delay time while the first feedback control is in execution, the oxygen sensor delay time being a time difference between the change of the signal input as the signal of the downstream oxygen sensor and a change of the signal input as the signal of the upstream oxygen sensor.

9. The straddled vehicle according to claim 8, wherein, in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached by comparing the oxygen sensor delay time while the first feedback control is in execution with a threshold.

10. The straddled vehicle according to claim 8, wherein in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached by comparing the oxygen sensor delay time while the first feedback control is in execution with a second oxygen sensor delay time, the second oxygen sensor delay time is a time difference between a change of the signal input as the signal of the downstream oxygen sensor and a change of the signal input as the signal of the upstream oxygen sensor, while fifth feedback control is in execution before the first feedback control, and the fifth feedback control is another control with which the controller periodically increases and decreases the fuel amount with a fifth cycle based on the signal input as the signal of the upstream oxygen sensor, the fifth cycle and the amplitude of the fuel amount in the fifth feedback control being identical to the first cycle and the amplitude of the fuel amount in the first feedback control.

11. The straddled vehicle according to claim 7, wherein, in the detachment determination process, the controller is configured to determine whether the three-way catalyst is detached based on a number of changes of the signal input as the signal of the upstream oxygen sensor during a first time period in which the first feedback control is in execution, and a number of changes of the signal input as the signal of the downstream oxygen sensor during the first time period.

12. A straddled vehicle comprising: an engine, which includes a combustion chamber; a three-way catalyst, which is configured to purify exhaust gas exhausted from the combustion chamber; an upstream oxygen sensor, which is provided upstream of the three-way catalyst in a flow direction of the exhaust gas, and is configured to detect an oxygen concentration in the exhaust gas; a downstream oxygen sensor, which is provided downstream of the three-way catalyst in the flow direction of the exhaust gas, and is configured to detect the oxygen concentration in the exhaust gas; and a controller, which includes: a processor, and a non-transitory storage medium containing program instructions, execution of which by the processor causes the controller to control a fuel amount supplied to the combustion chamber to periodically increase and decrease the fuel amount, based on a signal input as a signal of an upstream oxygen sensor, and execute a detachment determination process of determining whether the three-way catalyst is detached based on at least the signal input as a signal of the downstream oxygen sensor, when the fuel amount is controlled to periodically increase and decrease based on the signal input as the signal of the upstream oxygen sensor, wherein, the controller further includes a downstream oxygen sensor interface connected to the downstream oxygen sensor and an upstream oxygen sensor interface connected to the upstream oxygen sensor, the controller is so configured that, when the downstream oxygen sensor is detached from the straddled vehicle, a value of a signal input to the downstream oxygen sensor interface is maintained at a predetermined first value, and when the upstream oxygen sensor is detached from the straddled vehicle, a value of a signal input to the upstream oxygen sensor interface is maintained at a predetermined second value, and the controller is further configured to execute a second detachment determination process of determining that the three-way catalyst is detached, when the value of the signal input to the downstream oxygen sensor interface is maintained at the first value, or when the value of the signal input to the downstream oxygen sensor interface is maintained at the first value and the value of the signal input to the upstream oxygen sensor interface is maintained at the second value.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a side view of a straddled vehicle of First Embodiment of the present teaching.

(2) FIG. 2 shows graphs for explaining Second to Eighth Embodiments of the present teaching.

(3) FIG. 3 shows graphs for explaining Second to Eighth Embodiments of the present teaching.

(4) FIG. 4 shows graphs for explaining Second to Eighth Embodiments of the present teaching.

(5) FIG. 5 shows graphs for explaining Ninth and Tenth Embodiments of the present teaching.

(6) FIG. 6 shows graphs for explaining Ninth and Tenth Embodiments of the present teaching.

(7) FIG. 7A is a graph for explaining Eleventh Embodiment of the present teaching, and FIG. 7B is a graph for explaining Twelfth Embodiment of the present teaching.

(8) FIG. 8 is a control block diagram of the straddled vehicle of First Embodiment of the present teaching.

DESCRIPTION OF EMBODIMENTS

(9) The following will describe a straddled vehicle 1 of First Embodiment of the present teaching with reference to FIG. 1 and FIG. 8. In FIG. 1, the straddled vehicle 1 is a motorcycle. It is noted that the straddled vehicle of the present teaching is not limited to the motorcycle. The straddled vehicle 1 includes an engine 2 including a combustion chamber 3, an exhaust unit 4 connected to the engine 2, and a controller 8. The exhaust unit 4 includes a three-way catalyst 5 configured to purify exhaust gas exhausted from the combustion chamber 3, an upstream oxygen sensor 6 provided upstream of the three-way catalyst 5 in a flow direction of the exhaust gas, and a downstream oxygen sensor 7 provided downstream of the three-way catalyst 5 in the flow direction of the exhaust gas. The upstream oxygen sensor 6 and the downstream oxygen sensor 7 are configured to detect the oxygen concentration in the exhaust gas. The controller 8 is configured to execute a detachment determination process of determining whether the three-way catalyst 5 has been detached from the straddled vehicle 1 at least based on a signal input as a signal of the downstream oxygen sensor 7. The positions of the three-way catalyst 5, the upstream oxygen sensor 6, and the downstream oxygen sensor 7 are not limited to those shown in FIG. 1. As shown in FIG. 8, the controller 8 includes a processor 81, a non-transitory recording medium 82, an upstream oxygen sensor interface 83, and a downstream oxygen sensor interface 84. The upstream oxygen sensor interface 83 is electrically connected to the upstream oxygen sensor 6. The downstream oxygen sensor interface 84 is electrically connected to the downstream oxygen sensor 7.

(10) The following will describe Second to Ninth Embodiments of the present teaching with reference to graphs shown in FIG. 2 to FIG. 6. Straddled vehicles 1 of Second to Ninth Embodiments encompass all features of First Embodiment. FIG. 2 to FIG. 4 show graphs for describing Second to Seventh Embodiments, whereas FIG. 5 and FIG. 6 show graphs for describing Eighth and Ninth Embodiments. In the graphs shown in FIG. 2 to FIG. 6, UpO2 indicates the upstream oxygen sensor 6 and DnO2 indicates the downstream oxygen sensor 7. FIG. 2 to FIG. 6 include a graph showing changes over time of a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 when the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are not detached. The detachment determination processes described in Second to Ninth Embodiments are all detachment determination processes that are effective when the three-way catalyst 5 is detached but the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are not detached. In Second to Ninth Embodiments, a signal input to the controller 8 as a signal of the downstream oxygen sensor 7 is simply termed a signal of the downstream oxygen sensor 7, and a signal input to the controller 8 as a signal of the upstream oxygen sensor 6 is simply termed a signal of the upstream oxygen sensor 6. In Second to Ninth Embodiments, the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are O2 sensors. The controller 8 of each of Second to Eighth Embodiments determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7. The controller 8 of Ninth Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached not based on a signal of the upstream oxygen sensor 6 but based on a signal of the downstream oxygen sensor 7. In each of Second to Ninth Embodiments, the controller 8 is configured to perform feedback control of controlling a fuel amount supplied to the combustion chamber 3 based on a signal of the upstream oxygen sensor 6. The feedback control includes at least feedback control FB which is normal feedback control. The feedback control FB is equivalent to first feedback control of the present teaching.

(11) To begin with, Second and Third Embodiments will be described with reference to the graphs shown in FIG. 2 to FIG. 4. In Second and Third Embodiments, the feedback control includes feedback control FB of controlling the fuel amount in such a way that the cycle of increase and decrease of the fuel amount is longer than the cycle in the feedback control FB and/or the amplitude of increase and decrease of the fuel amount is larger than the amplitude in the feedback control FB. Furthermore, in Second and Third Embodiments, the feedback control includes feedback control FB of controlling the fuel amount in such a way that the cycle of increase and decrease of the fuel amount is longer than the cycle in the feedback control FB and/or the amplitude of increase and decrease of the fuel amount is larger than the amplitude in the feedback control FB. The controller 8 of Second Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. The controller 8 of Second Embodiment is configured to execute a deterioration determination process of determining whether the three-way catalyst 5 has been deteriorated, based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. On the other hand, the controller 8 of Third Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. The controller 8 of Third Embodiment is configured to execute a deterioration determination process of determining whether the three-way catalyst 5 has been deteriorated, based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. In other words, in Second and Third Embodiments, the feedback control for executing the detachment determination process is different from the feedback control for executing the deterioration determination process and the normal feedback control. In Second Embodiment, the feedback control FB is equivalent to third feedback control of the present teaching, and the feedback control FB is equivalent to second feedback control of the present teaching. In Third Embodiment, the feedback control FB is equivalent to the second feedback control of the present teaching, and the feedback control FB is equivalent to the third feedback control of the present teaching. The cycle and amplitude of increase and decrease of the fuel amount in the feedback control FB in Second Embodiment may be identical with the cycle and amplitude of increase and decrease of the fuel amount in the feedback control FB in Third Embodiment. Each of the graphs in FIG. 2 to FIG. 4 shows changes over time of the fuel amount, a signal of the upstream oxygen sensor 6, and a signal of the downstream oxygen sensor 7 when three feedback controls FB, FB, and FB are executed. FIG. 2 shows changes over time of the fuel amount, a signal of the upstream oxygen sensor 6, and a signal of the downstream oxygen sensor 7, when the three-way catalyst 5 is not detached and not deteriorated. FIG. 3 shows changes over time of the fuel amount, a signal of the upstream oxygen sensor 6, and a signal of the downstream oxygen sensor 7, when the three-way catalyst 5 is not detached but is deteriorated. FIG. 4 shows changes over time of the fuel amount, a signal of the upstream oxygen sensor 6, and a signal of the downstream oxygen sensor 7, when the three-way catalyst 5 is detached.

(12) The controller 8 of Second Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on an oxygen sensor delay time T that is a delay time of a change of a signal of the downstream oxygen sensor 7 from a change of a signal of the upstream oxygen sensor 6 while the feedback control FB is in execution in the detachment determination process. The controller 8 of Second Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than a threshold X1. In FIG. 2 to FIG. 4, the oxygen sensor delay time T is a time from a point at which the signal of the upstream oxygen sensor 6 becomes equal to a value A1 equidistant from a first voltage V1 and a second voltage V2 to a point at which the signal of the downstream oxygen sensor 7 becomes equal to the value A1. The controller 8 of Second Embodiment determines in the deterioration determination process that the three-way catalyst 5 has been deteriorated, when an oxygen sensor delay time T while the feedback control FB is in execution is shorter than a threshold X2. In FIG. 2 to FIG. 4, the oxygen sensor delay time T is a time from a point at which the signal of the upstream oxygen sensor 6 becomes equal to a value A1 equidistant from a first voltage V1 and a second voltage V2 to a point at which the signal of the downstream oxygen sensor 7 becomes equal to the value A1. The threshold X2 may be larger than or smaller than the threshold X1, or may be equal to the threshold X1. As shown in FIG. 3 and FIG. 4, a difference between the oxygen sensor delay time T when the three-way catalyst 5 is detached and the feedback control FB is in execution and the oxygen sensor delay time T when the three-way catalyst 5 is deteriorated and the feedback control FB is in execution is larger than a difference between the oxygen sensor delay time T when the three-way catalyst 5 is detached and the feedback control FB is in execution and the oxygen sensor delay time T when the three-way catalyst 5 is deteriorated and the feedback control FB is in execution. On this account, by executing the detachment determination process in the feedback control FB in which the cycle and amplitude of increase and decrease of the fuel amount are longer (larger) than those of the feedback control FB for executing the deterioration determination process, it is possible to improve the determination precision of the detachment determination process. As a modification of Second Embodiment, in the detachment determination process, the controller 8 may determine whether the three-way catalyst 5 has been detached by comparing the delay time T while the feedback control FB is in execution with the delay time T while the feedback control FB prior to the current detachment determination process is in execution.

(13) The controller 8 of Third Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on an oxygen sensor delay time T that is a delay time of a change of a signal of the downstream oxygen sensor 7 from a change of a signal of the upstream oxygen sensor 6 while the feedback control FB is in execution in the detachment determination process. The controller 8 of Third Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than a threshold X3. The controller 8 of Third Embodiment determines in the deterioration determination process that the three-way catalyst 5 has been deteriorated, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than a predetermined threshold. The predetermined threshold is larger than the threshold X3. The feedback control FB for executing the deterioration determination process is performed in known straddled vehicles, too. Because the feedback control FB for executing the detachment determination process is shorter (smaller) in cycle and amplitude of increase and decrease of the fuel amount than the feedback control FB for executing the deterioration determination process, it is possible to suppress the deterioration of drivability as compared to the known straddled vehicles. As a modification of Third Embodiment, in the detachment determination process, the controller 8 may determine whether the three-way catalyst 5 has been detached by comparing the delay time T while the feedback control FB is in execution with the delay time T while the feedback control FB prior to the current detachment determination process is in execution.

(14) Now, Fourth and Fifth Embodiments will be described with reference to the graphs shown in FIG. 2 to FIG. 4. In Fourth and Fifth Embodiment, the feedback control includes feedback control FB of controlling the fuel amount in such a way that the cycle of increase and decrease of the fuel amount is longer than the cycle in the feedback control FB and/or the amplitude of increase and decrease of the fuel amount is larger than the amplitude in the feedback control FB. The controller 8 of each of Fourth and Fifth Embodiments determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. In Fourth and Fifth Embodiments, the controller 8 is configured to execute a deterioration determination process of determining whether the three-way catalyst 5 has been deteriorated, based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. In other words, in Fourth and Fifth Embodiments, the feedback control for executing the detachment determination process is identical with the feedback control for executing the deterioration determination process. In Fourth and Fifth Embodiments, the feedback control FB is equivalent to the second feedback control. The controller 8 of each of Fourth and Fifth Embodiments determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on an oxygen sensor delay time T that is a delay time of a change of a signal of the downstream oxygen sensor 7 from a change of a signal of the upstream oxygen sensor 6 while the feedback control FB is in execution in the detachment determination process. The controller 8 of Fourth Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than a threshold X3. The controller 8 of Fourth Embodiment determines in the deterioration determination process that the three-way catalyst 5 has been deteriorated, when the oxygen sensor delay time T while the feedback control FB is in execution is not shorter than the threshold X3 and shorter than the threshold X2. The controller 8 of Fifth Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than an average of the oxygen sensor delay times T while the feedback controls FB prior to the current detachment determination process are in execution and a difference between the oxygen sensor delay time T and the average is larger than a reference value Y1. As a modification of Fourth and Fifth Embodiments, the controller 8 may execute the deterioration determination process and the detachment determination process based on a signal of the upstream oxygen sensor and a signal of the downstream oxygen sensor while the feedback control FB is in execution. In this modification, the feedback control FB is equivalent to the second feedback control.

(15) Now, Sixth to Eighth Embodiments will be described with reference to the graphs shown in FIG. 2 to FIG. 4. The controller 8 of each of Sixth to Eighth Embodiments determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on both a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 while the feedback control FB is in execution. In Sixth to Eighth Embodiments, the feedback control FB is executed irrespective of whether to execute the detachment determination process. The controller 8 of each of Sixth and Seventh Embodiments determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on an oxygen sensor delay time Ta that is a delay time of a change of a signal of the downstream oxygen sensor 7 from a change of a signal of the upstream oxygen sensor 6 while the feedback control FB is in execution in the detachment determination process. The controller 8 of Sixth Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than a threshold X4. In FIG. 2 to FIG. 4, the oxygen sensor delay time T is a time from a point at which the signal of the upstream oxygen sensor 6 becomes equal to a value A1 equidistant from a first voltage V1 and a second voltage V2 to a point at which the signal of the downstream oxygen sensor 7 becomes equal to the value A1. The controller 8 of Seventh Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the oxygen sensor delay time T while the feedback control FB is in execution is shorter than an average of the oxygen sensor delay times Ta while the feedback controls FB prior to the current detachment determination process are in execution and a difference between the oxygen sensor delay time T and the average is larger than a reference value Y2.

(16) The controller 8 of Eighth Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on the number of changes of the signal of the upstream oxygen sensor 6 during a first time period in which the feedback control FB is in execution and the number of changes of the signal of the downstream oxygen sensor 7 during the first time period. The controller 8 of Eighth Embodiment determines, in the detachment determination process, that the three-way catalyst 5 has been detached when the number of changes of the signal of the upstream oxygen sensor 6 during the first time period is larger than a threshold Z1 and the number of changes of the signal of the downstream oxygen sensor 7 during the first time period is larger than a threshold Z2. The first time period may be, for example, a period of several seconds. The number of changes of the signal of the upstream oxygen sensor 6 during the first time period may be, for example, the number of times when the signal of the upstream oxygen sensor 6 becomes at the second voltage V2 during the first time period, or the number of times when the signal of the upstream oxygen sensor 6 becomes at the value A1. The number of changes of the signal of the downstream oxygen sensor 7 during the first time period may be, for example, the number of times when the signal of the downstream oxygen sensor 7 becomes at the second voltage V2 during the first time period, or the number of times when the signal of the downstream oxygen sensor 7 becomes at the value A1. As shown in FIG. 3 and FIG. 4, between the same periods in which the feedback control FB is in execution, the number of changes of the signal of the downstream oxygen sensor 7 when the three-way catalyst 5 is detached tends to be larger than the number of changes of the signal of the downstream oxygen sensor 7 when the three-way catalyst 5 is deteriorated. On this account, it is less likely to mistake a case where the three-way catalyst 5 is deteriorated for a case where the three-way catalyst 5 is detached. It is possible to execute the detachment determination process without needing, for the detachment determination process, feedback control that is different from the normal feedback control.

(17) Now, Ninth and Tenth Embodiments will be described with reference to the graphs shown in FIG. 5 to FIG. 6. In Ninth and Tenth Embodiments, the controller 8 executes the detachment determination process by utilizing fuel cut control of temporarily stopping supply of fuel to the combustion chamber 3. In Ninth and Tenth Embodiments, the controller 8 executes the detachment determination process by utilizing at least a signal of the downstream oxygen sensor 7 when the feedback control FB is shifted to the fuel cut control. Each of the graphs in FIG. 5 and FIG. 6 shows changes over time of a signal of the upstream oxygen sensor 6 and a signal of the downstream oxygen sensor 7 when the feedback control FB is shifted to the fuel cut control. The graph of FIG. 5 shows changes over time of a flag of the fuel cut control, a signal of the upstream oxygen sensor 6, and a signal of the downstream oxygen sensor 7 when the three-way catalyst 5 is not detached. The graph of FIG. 6 shows changes over time of a flag of the fuel cut control, a signal of the upstream oxygen sensor 6, and a signal of the downstream oxygen sensor 7 when the three-way catalyst 5 is detached. The controller 8 of Ninth Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on a delay time T of a change of a signal of the downstream oxygen sensor 7 while the fuel cut control is in execution from a change of a signal of the upstream oxygen sensor 6 while the feedback control FB a or the fuel cut control is in execution. The controller 8 of Ninth Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the delay time T is shorter than a threshold X5. In FIG. 5 and FIG. 6, the delay time T is a time from a point at which the signal of the upstream oxygen sensor 6 becomes equal to a value A1 equidistant from a first voltage V1 and a second voltage V2 to a point at which the signal of the downstream oxygen sensor 7 becomes equal to the value A1. To be more specific, the delay time T is a time from a point at which the signal of the upstream oxygen sensor 6 becomes equal to the value A1 immediately before the signal becomes constant at the second voltage V2 to a point at which the signal of the downstream oxygen sensor 7 becomes equal to the value A1. While in FIG. 5 and FIG. 6 the signal of the upstream oxygen sensor 6 becomes equal to the value A1 during the fuel cut control, the signal of the upstream oxygen sensor 6 may become equal to the value A1 during the feedback control FB. The controller 8 of Tenth Embodiment determines, in the detachment determination process, whether the three-way catalyst 5 has been detached based on a delay time T of a change of a signal of the downstream oxygen sensor 7 while the fuel cut control is in execution from the start of the fuel cut control. The controller 8 of Tenth Embodiment determines in the detachment determination process that the three-way catalyst 5 has been detached, when the delay time T is shorter than a threshold X6. In FIG. 5 and FIG. 6, the delay time T is a time from the start of the fuel cut control to a time point at which the signal of the downstream oxygen sensor 7 becomes equal to a value A1 which is equidistant from the first voltage V1 and the second voltage V2. As a modification of Ninth and Tenth Embodiments, the controller 8 may determine in the detachment determination process whether the three-way catalyst 5 has been detached, by comparing the delay time Ty, To with the delay time T, T at the time of the shift from the feedback control FB to the fuel cut control prior to the current detachment determination process.

(18) The following will describe Eleventh Embodiment of the present teaching. A straddled vehicle 1 of Eleventh Embodiment encompasses all features of First Embodiment. The controller 8 of Eleventh Embodiment determines in the detachment determination process whether a signal that is input as a signal of the downstream oxygen sensor 7 is a signal that is input to the controller 8 not electrically connected to the downstream oxygen sensor 7. When it is determined that a signal input as a signal of the downstream oxygen sensor 7 is a signal input to the controller 8 not electrically connected to the downstream oxygen sensor 7, the controller 8 determines that the three-way catalyst 5 has been detached. Therefore, when a signal that is input as a signal of the downstream oxygen sensor 7 is a signal that is input when the downstream oxygen sensor 7 is detached from the straddled vehicle 1, the controller 8 determines that the three-way catalyst 5 has been detached. FIG. 7A shows an example of the signal that is input to the controller 8 as the signal of the downstream oxygen sensor 7 when the downstream oxygen sensor 7 is detached. The signal input to the controller 8 as a signal of the downstream oxygen sensor 7 when the downstream oxygen sensor 7 is detached is different from a signal input to the controller 8 as a signal of the downstream oxygen sensor 7 when the downstream oxygen sensor 7 is not detached. The same applies to the upstream oxygen sensor 6. For example, when the downstream oxygen sensor 7 is an O2 sensor, a signal that is at neither the first voltage V1 nor the second voltage V2 shown in FIG. 2 to FIG. 6 is input to the controller 8. The straddled vehicle 1 may be arranged so that, when the three-way catalyst 5 is detached from the straddled vehicle 1, the downstream oxygen sensor 7 is detached together with the three-way catalyst 5. In such a case, when the downstream oxygen sensor 7 is detached, the three-way catalyst 5 is assumed to be detached, too. Even if the straddled vehicle 1 is not structured in this way, when the three-way catalyst 5 is detached from the straddled vehicle 1, the downstream oxygen sensor 7 is likely to be detached, too. It is therefore possible to determine whether the three-way catalyst 5 has been detached based on a signal input as a signal of the downstream oxygen sensor 7.

(19) The following will describe Twelfth Embodiment of the present teaching. A straddled vehicle 1 of Twelfth Embodiment encompasses all features of First Embodiment. The controller 8 of Twelfth Embodiment determines in the detachment determination process whether a signal that is input as a signal of the upstream oxygen sensor 6 is a signal that is input to the controller 8 not electrically connected to the upstream oxygen sensor 6 and a signal that is input as a signal of the downstream oxygen sensor 7 is a signal that is input to the controller 8 not electrically connected to the downstream oxygen sensor 7. When it is determined that a signal input as a signal of the upstream oxygen sensor 6 is a signal input to the controller 8 not electrically connected to the upstream oxygen sensor 6 and a signal input as a signal of the downstream oxygen sensor 7 is a signal input to the controller 8 not electrically connected to the downstream oxygen sensor 7, the controller determines that the three-way catalyst 5 has been detached. Therefore, when a signal that is input as a signal of the upstream oxygen sensor 6 is a signal that is input when the upstream oxygen sensor 6 is detached from the straddled vehicle 1 and a signal that is input as a signal of the downstream oxygen sensor 7 is a signal that is input when the downstream oxygen sensor 7 is detached from the straddled vehicle 1, the controller 8 determines that the three-way catalyst 5 has been detached. FIG. 7B shows an example of a signal that is input to the controller 8 as a signal of the upstream oxygen sensor 6 and a signal that is input to the controller 8 as a signal of the downstream oxygen sensor 7, when the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are detached. The straddled vehicle 1 may be arranged so that, when the three-way catalyst 5 is detached from the straddled vehicle 1, the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are detached together with the three-way catalyst 5. In such a case, when the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are detached, the three-way catalyst 5 is assumed to be detached, too. Even if the straddled vehicle 1 is not structured in this way, when the three-way catalyst 5 is detached from the straddled vehicle 1, the upstream oxygen sensor 6 and the downstream oxygen sensor 7 are likely to be detached, too. It is therefore possible to determine whether the three-way catalyst 5 has been detached based on a signal input as a signal of the upstream oxygen sensor 6 and a signal input as a signal of the downstream oxygen sensor 7. Because both a signal input as a signal of the upstream oxygen sensor 6 and a signal input as a signal of the downstream oxygen sensor 7 are used, the precision of determination in the detachment determination process can be improved as compared to Eleventh Embodiment.

(20) Second to Twelfth Embodiment may be implemented in combination. In other words, the controller 8 may be arranged to have two or more of the detachment determination processes of Second to Twelfth Embodiments. For example, the controller 8 of each of Second and Third Embodiments may be arranged to perform the detachment determination process of Fourth Embodiment or Fifth Embodiment. For example, the controller 8 of each of Second to Fifth Embodiments may be arranged to perform the detachment determination process of any one of Sixth Embodiment to Eighth Embodiment. For example, the controller 8 of each of Second to Eighth Embodiments may be arranged to perform the detachment determination process of Ninth Embodiment or Tenth Embodiment. The controller 8 of each of Second to Tenth Embodiments may be arranged to perform the detachment determination process of Eleventh Embodiment or Twelfth Embodiment.

(21) The controller 8 of each of First to Twelfth Embodiments may be configured to further execute a detachment determination process of determining whether the three-way catalyst 5 has been detached based not on a signal input as a signal of the downstream oxygen sensor 7 but on a signal input as a signal of the upstream oxygen sensor 6. For example, the controller 8 may execute the detachment determination process of determining that the three-way catalyst 5 has been detached, when a signal that is input as a signal of the upstream oxygen sensor 6 is a signal that is input when the upstream oxygen sensor 6 is detached from the straddled vehicle 1. A controller of a straddled vehicle may be arranged to execute only this detachment determination process, although such an arranged is not encompassed in the present teaching.

(22) The controller 8 of each of First to Twelfth Embodiments may be arranged to further execute a detachment determination process of determining whether the three-way catalyst 5 has been detached based on a signal of a detection unit that is neither the upstream oxygen sensor 6 nor the downstream oxygen sensor 7. A controller of a straddled vehicle may be arranged to execute only this detachment determination process, although such an arranged is not encompassed in the present teaching. The detection unit may be a sensor exclusively used for the detachment determination process. The detection unit may be a sensor that is used for a process or control different from the detachment determination process. The detection unit used for a process or control different from the detachment determination process may be an intake pressure sensor. The detection unit exclusively used for the detachment determination process may be, for example, a camera which is configured to read a two-dimensional barcode provided on an outer surface of a catalyst unit that is detached together with the three-way catalyst. When one-dimensional barcode is provided in place of the two-dimensional barcode, the detection unit exclusively used for the detachment determination process may be a line sensor. The detection unit may be an exhaust gas temperature sensor configured to detect the temperature sensor of exhaust gas. The exhaust gas temperature sensor may be provided downstream or upstream of the three-way catalyst in a flow direction of the exhaust gas. The exhaust gas temperature sensor may be used exclusively for the detachment determination process, or may be used for a process or control different from the detachment determination process. The detection unit may be an exhaust gas pressure sensor configured to detect the pressure of exhaust gas. The exhaust gas pressure sensor may be provided downstream or upstream of the three-way catalyst in a flow direction of the exhaust gas. The exhaust gas pressure sensor may be used exclusively for the detachment determination process, or may be used for a process or control different from the detachment determination process.