JPH06213042A - Exhaust gas sensor system for internal combustion engine and oxygen-level signal supply process - Google Patents

Abstract

PURPOSE: To accurately detect oxygen levels regardless of components of exhaust gas or the like and to shorten the reaction time for instantly detecting oxygen levels by providing an upstream exhaust gas oxygen sensor, a downstream exhaust gas oxygen sensor, and a controller for adjusting operation parameters. CONSTITUTION: Exhaust gas oxygen(EGO) sensors 38, 36 interconnected with an exhaust system are disposed upstream and downstream of a catalytic converter 32. Specifically, an exhaust gas sensor system comprises the EGO sensor 38 arranged upstream of the catalytic converter 32, the HEGO sensor 36 arranged downstream of the catalytic converter 32, and a controller 74. The controller 74 receives signals indicating an upstream oxygen level and a down oxygen level from the upstream HEGO sensor 38 and the downstream HEGO sensor 36, respectively. Then, in order to adjust operation parameters of a gas mixture or the like supplied to an engine 22, the controller 74 provides a signal that can be used by the engine 22. Thus, the engine 22 can use a controlled signal to adjust the operation parameters such as an air fuel ratio.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates generally to electronic engine controllers using exhaust gas oxygen sensors and feedback controllers for engine operation. In particular, the present invention relates to a sensor system having multiple exhaust gas oxygen (“EGO”) sensors interconnected with an exhaust system upstream and downstream of a catalytic converter.

[0002]

BACKGROUND OF THE INVENTION Many automobiles include an internal combustion engine and an exhaust system that provides the plumbing that carries heated combustion gases away from the engine. Exhaust gas temperatures range from ambient temperatures when the engine has not been run recently to over 400 ° C.

A typical exhaust system may include an EGO sensor assembly and a catalytic converter. Catalytic converters facilitate the conversion of hydrocarbons, carbon monoxide and nitrogen oxides to less harmful compounds. EGO sensors are often located "upstream" of the catalytic converter.
The terms "downstream" and "upstream" are relative terms used to indicate the relative position along the vehicle's exhaust pipe. The term "downstream" means that particles in the exhaust gas are "upstream."
The position along the exhaust pipe that arrives later in time than the position.

In many currently available air-fuel control systems in vehicles, an EGO sensor located upstream of the catalyst provides an air-fuel feedback signal to the closed loop mixture delivery system in the engine. However, the upstream EGO sensor may be "blocked" by compounds such as lead and silicon. Such components may be present in the raw exhaust gas. This may occur, for example, because the driver uses "leaded" gasoline for a "leadless" gasoline only engine. If it is blocked in this way, EGO
The sensor may not be able to accurately determine the oxygen concentration level in the exhaust gas.

The output characteristics of the upstream EGO sensor may change with time. Moreover, under certain operating conditions, the upstream EGO sensor may not be able to substantially equilibrate the exhaust gas flowing thereabout. Such conditions may be due to, for example, engine load and erroneous distribution of mixture between cylinders in the engine. as a result,
The EGO sensor will indicate an "offset error".

Furthermore, many EGO sensors only work effectively when the temperature of the sensor is within a certain range. Of course, the temperature of the sensor is affected by the temperature of the adjacent exhaust gas. To help the EGO sensor make accurate measurements over a wide range of exhaust gas temperatures, the EGO sensor assembly often includes an electrical heater physically adjacent to, or near, the EGO sensor. Such a heated exhaust gas oxygen sensor is a kind of EGO sensor and is often called HEGO. Upon activation, the heater warms the sensor to allow more accurate measurements, thus reducing the effects of temperature fluctuations in the exhaust gas passing through the exhaust pipe of the vehicle.

There is a conventional technique for controlling the air-fuel ratio of an internal combustion engine. For example, Claoka U.S. Pat. No. 4,708,777.
Discloses an air-fuel ratio feedback control system responsive to an EGO sensor. The EGO sensor is maintained at a predetermined temperature by the feedback from the sensor heater.

[0008]

Therefore, it has been attempted in some prior art systems to maintain a constant air-fuel ratio operating point independent of exhaust gas temperature. However, in addition to maintaining a constant closed-loop air-fuel ratio operating point regardless of exhaust gas temperature and engine operating conditions, it is also desirable to have an EGO sensor that can accurately detect oxygen levels regardless of exhaust gas components and inhibitory effects. At the same time, it is also desirable to have a system in which the oxygen level is immediately detected to reduce the reaction time of the air-fuel controller. In this way, the feedback control allows the controller to more precisely adjust the operation of the internal combustion engine.

Further, since the EGO sensor assembly is generally mass-produced and installed in many vehicles, even if the cost of one component of the assembly is slightly reduced, the accumulated amount for the year is considerable. Therefore, the EGO sensor system should not have an excessive number of parts or a high manufacturing cost. Further, it is important that the sensor assembly be reliable.

[0010]

The present invention is an EG for an internal combustion engine.
It is an O sensor system. It has a catalytic converter on the engine exhaust pipe and on the exhaust pipe downstream of the engine. The system includes an oxygen sensor upstream of the catalytic converter and an oxygen sensor and controller downstream of the catalytic converter.
The controller receives signals from the upstream and downstream oxygen sensors indicating upstream and downstream oxygen levels, respectively. The controller then provides a signal that the engine can use to adjust operating parameters such as the mixture supplied to the engine.

In another embodiment, the invention is a method of providing an oxygen level signal to a mixture controller of an internal combustion engine. The method includes the steps of detecting a downstream oxygen level, detecting an upstream oxygen level, and providing a controlled signal to a fuel distribution system based on the upstream and downstream oxygen levels.

[0012]

1 to 14, an embodiment of the present invention is shown as an oxygen sensor system 20 with signal modification for use in an internal combustion engine 22. As shown in FIG.
Is an engine block 24 having an internal cylinder (not shown) for combustion, a mixture delivery system 26, and an exhaust system 28.
Is included.

The exhaust system 28 includes an exhaust pipe 30 that carries exhaust gas from the engine 22 and a three-way catalytic converter 32. In one embodiment, the mixture delivery system 26 includes a mixer distributor 34 and an oxygen sensor system 20.
The sensor system 20 includes a downstream HEGO sensor 36 downstream of both the engine 22 and the catalytic converter 32,
It includes an upstream HEGO sensor 38 upstream of the catalytic converter 32 (but downstream of the engine 22) and a HEGO control assembly 40. Mixer distributor 34 receives signals from control assembly 40 to physically supply the mixture to the engine cylinders.

Each EGO sensor 36, 38 includes the same components, and the downstream sensor 36 will be described to show its basic operation. The sensor 36 includes the first and second output conductors 4
4, 46, including a heater 48 having a sensing tip 42 interconnected to it and also first and second conductors 50, 52. Please refer to FIGS. Conductors 44, 4
An oxygen level signal (representing the oxygen concentration of the exhaust gas adjacent to the sensing tip 42) is sent to the control assembly 40 via 6.

First and second conductors 50 of the heater 48,
52 is interconnected with a resistance heating element 54. Sensing tip
The part 42 is housed in a protective canister 58 and the assembly is removed.
It is screwed into the trachea 30. The sensing tip 42 is an exhaust pipe
Oxygen level in exhaust gas in contact with gas flowing in 30
Effectively measure and measure the differential voltage along the output leads 44, 46.
Gives an oxygen level signal in the form. The tip 42 is typically two
Zirconia oxide ZrO ₂It is composed of

Control assembly 40 receives oxygen level signals from upstream and downstream EGO sensors 36,38. In response, the assembly 40 sends a mixture control signal to the mixer distributor 34 to affect the richness or leanness of the mixture supplied to the cylinders of the engine 22.

The downstream HEGO sensor 36 acts as a feedback device. The sensor 36 is effectively "protected" by the catalytic converter 32 and the exhaust gas reaches the downstream sensor 36 after being substantially chemically equilibrated by the catalytic converter 32 (and the catalytic converter 32 is polluted by lead and the like). Preventing material from reaching the downstream sensor 36). as a result,
Air-fuel offset error is reduced. Therefore, the sensor 36 can equilibrate the chemicals in the exhaust gas in its vicinity, and the downstream sensor 36 will produce a signal that more accurately represents the oxygen level concentration in the exhaust gas.

In contrast, the upstream sensor 38 produces a signal that responds more quickly to changes in exhaust gas composition. However, while the dynamic response is faster than the downstream sensor 36, the upstream sensor 38 may not be "protected" by the catalytic converter 32 and produce a signal with an offset error.

Accordingly, control assembly 40 receives signals from both upstream and downstream sensors 38,36. When the composition of the exhaust gas changes substantially, both upstream and downstream sensors 36, 38 tend to change their oxygen level signals. In response to such a dynamic signal, the control assembly 40 immediately adjusts the mixture control signal to the upstream sensor 38.
To substantially correspond to the changed signal from. Next, when the downstream sensor 36 reacts to the change in the composition of the exhaust gas,
The control assembly 40 can further modify the mixture control signal sent to the mixture distributor 34 according to the signal of the downstream sensor. When both the upstream and downstream sensor signals reach substantially steady state, the controller 40 "conditions" the mixture control signal to substantially correspond to the slower but generally more accurate signal from the downstream sensor 36. To do so.

Therefore, in many cases, the downstream sensor 36
Provides a more precise exhaust gas oxygen concentration representation than the upstream sensor 38 (although with a slower response time). However, fluctuations in the temperature of the downstream sensor 36 can substantially affect the accuracy of its signal. Therefore, the heater 48 warms the sensor 36 to reduce the influence of the exhaust gas temperature fluctuation.
A heater for warming the upstream sensor 38 may be arranged if desired.

A power signal is sent from control assembly 40 to heater 48 via conductors 50, 52 to activate heater 48. The control assembly 40 selectively activates the heater 48 of the sensor 36 to maintain the sensor 42 within the proper temperature range.

Graph 58 of FIG. 4 shows a typical oxygen level signal provided by HEGO sensor 38 as a function of air / fuel ratio delivered from system 26 to engine 22.
When the air-fuel ratio is lower than 14.5, the sensor 36 indicates 0.
It produces a substantially high voltage above 8V and a low voltage substantially below 0.2V when the air-fuel ratio is higher than 15. Therefore, a relatively small change in the air-fuel mixture causes a dramatic change in the sensor voltage (or "oxygen level signal").

The output of the sensor 36 is the controller 4
It is often passed to the mixture delivery system 34 after being processed by the in-zero comparator. Depending on whether the HEGO sensor voltage is higher or lower than a reference "set point" (or "control point") voltage, such as 0.45V, the signal from the comparator is (1). Larger value (ie "1") or (2). It can be a small value (that is, "0").

Many air-fuel control systems that use EGO or HEGO sensors tend to control the air-fuel ratio to values that are too high ("lean") when the exhaust gas temperature is too low. Conversely, the controlled air-fuel ratio may be too low ("rich") if the sensor is heated beyond its operating range.

For example, graph 60 of FIG. 5 shows experimentally derived data on how the closed loop control point of the sensor changes as a function of exhaust gas temperature. If the exhaust temperature change is less than 100 ° F, the control point changes well above 0.1. Thus, for example, oxygen sensor 36 and control assembly 40 adjust the air / fuel ratio of engine 22 to 14.65 when the exhaust temperature is approximately 640 ° F and 14/65 when the exhaust temperature is approximately 700 ° F. It can be adjusted to .56.

A change in the set point--an indication of the oxygen sensor assembly what mixture is appropriate--is determined by the engine 22.
May substantially affect the operation of the. FIG. 6 shows experimentally derived data for the catalytic converter hydrocarbon and nitrogen oxide conversion efficiencies and closed loop air-fuel ratios as a function of temperature. Line 62 shows the hydrocarbon conversion efficiency of the converter as temperature and thus air-fuel ratio 66 changes, and line 64 shows the nitrogen oxide conversion efficiency of the converter. Only mixtures near the specified equilibrium 68 point provide substantially optimum efficiency for reducing hydrocarbons and nitrogen oxides.

Therefore, in order to maintain the efficient operation of the converter 32, it is important to maintain the air-fuel mixture precisely. It is important to provide an accurate oxygen level concentration signal to the system 34 to maintain an accurate air / fuel ratio. The oxygen level signal produced by the oxygen sensor 36 may have a substantial impact on the air-fuel ratio and thus on the operation of the fuel distribution system 34 and the efficiency of the catalytic converter 32.

As shown in FIGS. 1 and 7, in the embodiment, a temperature sensor 70 is included inside the downstream sensor 36. The temperature sensor 70 provides a temperature level signal to the control assembly 40 via one or more conductors 71. Control assembly 40
Includes a microprocessor 72 that receives inputs from the downstream and upstream sensors 36, 38, and a temperature sensor 70 to provide a controlled signal to the engine 22 and a start signal to a heater 54 inside the downstream sensor 36.

FIG. 8 shows an embodiment of the temperature sensor 70.
The temperature sensor 70 comprises a thermocouple 76 disposed inside the canister 58 and adjacent to the sensor tip 56. Under this configuration, the thermocouple 76 provides the control assembly 40 with an accurate temperature level signal regarding the operating temperature of the adjacent sensing tip.

Another embodiment of the temperature sensor 70 is shown in FIG.
Shown in. The temperature sensor 70 includes an extension pipe 80 mounted on the tip of the sensor 36, a compression joint 82 in the exhaust pipe 30, and an elongated thermocouple 84 fitted between the extension pipe 80 and the joint 82.
It is composed by. Again, the thermocouple 84 provides an electrical output that depends on ambient temperature. The compression joint 82 and tube 80 hold the thermocouple 84 in place in the exhaust tube 30 adjacent the tip 56 of the sensor 36.

Yet another device 86 for detecting the temperature in the immediate vicinity of the sensor 36 is shown in FIG. The device 86 includes a known resistor 90 with a heater 70 and a voltage detector 92.
And a known voltage source such as an automobile battery 88 connected in series. Heater 70 and known resistor 9
The voltage supplied from the automobile battery 88 is divided by 0. The voltage across known resistor 90, as measured by detector 92, is substantially directly proportional to the resistance of heater 70. It has been found that the resistance of the heater 70 reflects the temperature of the sensor 36. Therefore, the control assembly 40 can receive a signal from the voltage detector 92 that is indicative of the temperature of the sensor 36. Note, however, that if vehicle battery 88 is selected as the voltage source, the temperature associated with a particular resistance will be a function of battery voltage.

In yet another embodiment, the sensor 36
Rather than use a direct measurement of temperature, the control assembly 4
0 receives inputs for engine variables such as speed and load. From this and the length of time that the engine 22 has been running, the microprocessor 72 "maps" the inputs regarding the engine parameters experienced by the table in its memory.
The expected temperature of the sensor 36 can then be evaluated.

The voltage output of the thermocouples 76, 84 can be converted into a digital signal proportional to temperature and read by the microprocessor 72 in the control assembly 40. Microprocessor 72 compares the sensor temperature to the desired temperature. When the actual temperature and the desired temperature are different, the activation signal is given to the heater 48. The activation signal may increase or decrease the duty cycle of the current supplied to the heater 48. As a result, the actual sensor temperature can be adjusted and maintained within the desired range.

Device 100 for controlling the current to the heater 48
Is shown in FIG. The signal from the temperature sensor 70 is sent to the analog / digital converter 102, and the output therefrom is sent to the microprocessor 72. Microprocessor 72 provides an output to digital to analog converter 104,
From there, a control signal is sent to the heater control circuit 106.
The heater control circuit 106 turns on / off the heater 48 according to a specified duty cycle to affect the sensed temperature which is the analog output of the temperature sensor 70.

The heater control circuit 106 is shown in FIG. The control circuit 106 includes a switching transistor 108 connected in series with the automobile battery 88 and the heating element 54. An analog signal provided from digital / analog converter 104 to switching transistor 108 turns transistor 108 and thus heater 48 on and off as desired.

The control signals sent to the switching transistor 108 are shown in FIGS. The microprocessor 72 detects from the output of the temperature sensor when the actual temperature of the sensor 36 is lower than the desired temperature. If the actual temperature is substantially above the desired temperature, then substantially no signal is provided to switching transistor 108. If the actual temperature is slightly below the desired temperature, the switching transistor 108 will be on for a short time. If the actual temperature is substantially below the desired temperature,
The duty cycle of the switching transistor 108 becomes long.

(1). The duty cycle of the transistor 108 and heater 48 voltage and (2). Graph 110 showing the relationship between the desired temperature and the difference between the actual (measured) temperature
Is shown in FIG. As can be seen, if the difference between the desired operating temperature of sensor 36 and the actual temperature of sensor 36 (as measured by temperature sensor 70) exceeds a predetermined amount, heater 48 voltage will continue. An example graph of the duty cycle of the heater 48 is shown in FIG. 14 as the temperature of the sensor 36 continues to drop below the desired operating temperature.

Therefore, the temperature sensor 70 returns to the control assembly 40 to maintain the downstream sensor 36 within a predetermined temperature range. Of course, the same feedback system can be used by the heater associated with the downstream sensor 38 to maintain the temperature of the sensor 38 within a predetermined range or "window".

The advantage of using the temperature sensor to avoid the offset of the downstream HEGO sensor 36 is that such a system allows the operation of the downstream sensor 36 to change battery voltage, heater resistance (between various heaters) and exhaust gas. It is relatively insensitive to temperature differences. Furthermore, such a method does not have the offset error associated with the open loop adjustment scheme.

Further, depending on the application, the control assembly 40
Is an error signal such as engine speed, engine load, coolant temperature, ambient temperature, engine operating time, and / or the oxygen level signal sent from the downstream sensor 36 via the conductor and should be adjusted upward or downward. One or more inputs relating to the oxygen level sensor temperature may be received. The control assembly 40 then adjusts the activation of the heater 48 to cause the sensor 36 to warm or cool (resulting in an increase or decrease in the heat provided by the heater 48) so that the output of the sensor 36 is adjusted. . Therefore, the increase or decrease of the operation of the heater 48 can correct the operation of the sensor 36 and the change of the oxygen level signal.

The invention has been described with reference to examples. However, various changes and modifications can be made without departing from the true scope and spirit of the invention. The true scope and spirit is set forth by the claims and their equivalents to be construed in light of the specification.

RELATED APPLICATION The present invention is another US patent application published on the same date and including the same inventor as the present invention, an oxygen sensor system with signal modification,
Is related to. The disclosure of this US patent application is hereby incorporated by reference.

[Brief description of drawings]

1 is a schematic diagram of an oxygen sensor system connected to the exhaust system of an internal combustion engine.

2 is a side view of the HEGO sensor assembly of FIG.

3 is a partial cross-sectional view of the HEGO sensor assembly of FIG.

FIG. 4 is a graph showing experimentally measured output voltage of the HEGO sensor assembly of FIG. 1 as a function of engine air-fuel ratio.

5 is a graph showing experimentally measured control points of the HEGO sensor assembly of FIG. 1 as a function of exhaust gas temperature.

6 is a graph showing experimentally measured changes in conversion efficiency and engine closed loop air-fuel ratio of the catalytic converter of FIG. 1 as a function of temperature.

FIG. 7 is a schematic diagram of the embodiment shown in FIG.

FIG. 8 is an HE that can be used in the present invention shown in FIG.
The partial cross section figure of the combination of GO sensor and temperature sensor.

9 is a partial cross-sectional view of another HEGO sensor and temperature sensor that can be used in the present invention shown in FIG.

FIG. 10 is a schematic diagram of a temperature sensor that can be used in the present invention shown in FIG. 7.

11 is a schematic diagram of a driving device of the sensor heater shown in FIG. 1. FIG.

12 is a schematic diagram of an embodiment of the heater control circuit shown in FIG.

13 is a graph showing the variation of the duty cycle of the heater as a function of the difference between the desired temperature and the actual temperature of the oxygen sensor shown in FIG.

FIG. 14 is a graph showing the duty cycle of the heater shown in FIG. 1 when the actual sensor temperature drops below the desired temperature.

[Explanation of symbols]

 20 Oxygen sensor system 22 Engine 24 Engine block 26 Mixture delivery system 28 Exhaust system 30 Exhaust pipe 32 Three way catalytic converter 34 Mixture distributor 36 Downstream HEGO sensor 38 Upstream HEGO sensor 40 Control device 42 Sensing tip 44 Output lead wire 46 Output lead wire 48 heater 50 conducting wire 52 conducting wire 54 resistance heating element 56 sensor tip portion 58 protective canister 70 temperature sensor 71 conducting wire 74 controller 76 thermocouple 80 extension tube 82 compression joint 84 thermocouple 86 temperature detecting device 88 automobile battery 90 resistance 92 voltage detector 98 Microprocessor 102 Analog / digital converter 104 Digital / analog converter 106 Heater control circuit 108 Switching transistor 110 Automotive battery

 ─────────────────────────────────────────────────── ———————————————————————————————————————————————————————————————————————————————–——————————————————————————— Introducer Lewis John Shelley, Taylor, Michigan, USA 24420

Claims (9)

[Claims] 1. An exhaust gas sensor system for an internal combustion engine having an exhaust pipe and a catalytic converter thereon, the system comprising an upstream exhaust gas oxygen sensor on the exhaust pipe upstream of the converter for providing an upstream oxygen level signal. A downstream exhaust gas oxygen sensor on the exhaust pipe downstream of the converter for providing a downstream oxygen level signal, and for receiving the upstream and downstream sensor oxygen level signals and providing a controlled signal in response thereto, An exhaust gas sensor system consisting of a combination of a controller used for an engine and adjusting operating parameters. 2. The apparatus according to claim 1, further comprising a temperature sensor for detecting the temperature of the downstream sensor and generating a temperature level signal in response thereto, and a heater for warming the downstream exhaust gas oxygen sensor. A heater controller that receives the temperature level signal, compares the detected temperature with a predetermined standard, and sends an activation signal to the heater when the detected temperature is different from the predetermined standard. Gas sensor device. 3. The apparatus of claim 2, wherein upon receipt of a dynamic signal from the upstream and downstream sensors, the controller substantially relies on the upstream oxygen level signal to provide the controlled signal, the upstream signal. And an exhaust gas sensor device wherein the controller substantially relies on the downstream oxygen level signal to provide the controlled signal upon receipt of a steady state signal from the downstream sensor. 4. The apparatus of claim 1, wherein the activation signal provided by the heater controller is a percentage of time that the heater is on when the measured temperature varies widely from the predetermined standard. An exhaust gas sensor device having a duty cycle signal that increases the temperature. 5. An exhaust gas sensor system for an internal combustion engine having an exhaust pipe and a catalytic converter thereon, the system comprising an upstream exhaust gas oxygen sensor on the exhaust pipe upstream of the converter for providing an upstream oxygen level signal. A downstream exhaust gas oxygen sensor on the exhaust pipe downstream of the converter for providing a downstream oxygen level signal, an upstream heater warming the upstream exhaust gas oxygen sensor, and a downstream heater warming the downstream exhaust gas oxygen sensor, A temperature sensor that detects the temperature of the downstream sensor adjacent to the downstream exhaust gas oxygen sensor and generates a temperature level signal in response thereto, and a temperature sensor that receives the temperature level signal and uses the detected temperature as a predetermined standard. Comparing, if the temperature is different from the predetermined standard, a duty cycle signal is generated and the measured temperature is A heater controller for increasing the proportion of time for turning on the downstream heater by the duty cycle signal when widely varying from a predetermined standard,
A controller for receiving the upstream and downstream sensor oxygen level signals to provide a controlled signal, wherein the controller receives the dynamic signal from the upstream and downstream sensors and relies on the upstream sensor signal to provide the controlled signal. An exhaust gas sensor system comprising a controller that relies on the downstream sensor signal to provide the controlled signal when receiving steady state signals from the upstream and downstream sensors. 6. A step of providing an oxygen level signal to a fuel delivery system of an internal combustion engine, said internal combustion engine comprising a catalytic converter on an exhaust pipe, the step comprising the next step, namely downstream of said catalytic converter. Detecting the oxygen level, detecting the upstream oxygen level upstream of the catalytic converter, and providing a controlled signal to the fuel delivery system based on the upstream and downstream oxygen levels. 7. The step of providing an oxygen level signal according to claim 6, wherein the downstream oxygen level is detected by an oxygen sensor adjacent to the heater, the step further comprising the step of sensing the temperature downstream of the catalytic converter. Then
Comparing the downstream temperature to a predetermined standard and activating the heater if the temperature is substantially below the predetermined standard;
An oxygen level signal supply process consisting of 8. The process of claim 7, further comprising producing a controlled signal related to the upstream and downstream oxygen levels when the upstream and downstream oxygen levels substantially change, the upstream and downstream oxygen levels being Providing an oxygen level signal comprising the step of producing a controlled signal related to said downstream oxygen level when a substantially steady state is reached. 9. The process according to claim 8, wherein the step of activating the heater comprises a step of activating the heater for a first predetermined period and stopping and activating for a second predetermined period, the downstream of the catalytic converter. An oxygen level signal supplying step of increasing the first predetermined period with respect to the second predetermined period when the temperature drops with respect to the predetermined standard.