JPH0568652B2 - - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heater control device that heats an oxygen sensor provided in an exhaust system for controlling the air-fuel ratio of an air-fuel mixture supplied to an engine.

Background Art The oxygen concentration and air-fuel ratio of the exhaust gas of an internal combustion engine have a good correlation in the air-fuel ratio that is larger than the stoichiometric air-fuel ratio, that is, in the region of a lean mixture. Therefore, the oxygen concentration of the exhaust gas in this region is measured. This makes it possible to accurately detect the exhaust gas air-fuel ratio. An oxygen sensor for measuring exhaust gas oxygen concentration in such a region includes a gas permeability measurement electrode provided on the side of the exhaust gas to be measured, a gas permeability counter electrode provided on the side of a reference gas having a known oxygen concentration, for example, the atmosphere, and a gas permeability measurement electrode provided on the side of the exhaust gas to be measured. A sensor is used that includes a bottomed cylindrical sensor element made of a solid electrolyte, such as stabilized zirconia, between electrodes. When a current is passed between the electrodes in such an oxygen sensor, oxygen can be moved in one direction through the electrolyte, but the microporous diffusion resistance layer transports less oxygen than the oxygen delivery capacity of the air permeable measuring electrode. By coating this permeability measuring electrode with , it is possible to maintain its current at approximately a certain value over a certain applied voltage range. This current value is called the limiting current value, and since it changes linearly almost in proportion to the oxygen concentration,
Oxygen concentration can be continuously detected from changes in this current value. On the other hand, in this oxygen sensor,
In order to output a current value proportional to the oxygen concentration of exhaust gas with a constant applied voltage, it is necessary to heat the sensor element to approximately 650° C. or higher and maintain it in an active state.

For this reason, a heater is installed in the center hole of the oxygen sensor to maintain the oxygen sensor at an active temperature. However, when the oxygen sensor temperature exceeds 850℃, that is, the heater temperature exceeds 1100℃, the heater heating element turns off. There are problems such as thermal deterioration and melting.

Therefore, the power supplied to the heater is controlled in relation to engine operating parameters such as engine speed, load, and intake air flow rate. If the amount of power supplied to the heater increases to a value corresponding to the state where the intake air flow rate is small when the air flow rate is low, the heater temperature may reach 1100° C. or higher. Additionally, if the heater is de-energized as soon as the idle switch that detects the idling opening of the throttle valve is turned on while the engine has a large intake air flow rate, the oxygen sensor temperature will decrease as the exhaust gas temperature decreases. Immediately after the temperature drops below 650° C. and the intake air flow rate is high and the intake air flow rate is low, it becomes difficult to accurately detect the oxygen concentration.

DISCLOSURE OF THE INVENTION It is an object of the present invention to avoid overheating of the heater while
An object of the present invention is to provide a control device for an oxygen sensor heater that can maintain an oxygen sensor within an appropriate temperature range even during a transient period.

To achieve this objective, according to the present invention, an oxygen sensor is provided in the exhaust system to detect the oxygen concentration in the exhaust gas of the engine, and the smaller the intake air flow rate, the more the supply to the heater that heats the oxygen sensor increases. In a control device for an oxygen sensor heater that controls high power, when the intake air flow rate of the engine changes from a high state to a low state, the intake air flow rate after the change is changed after a predetermined period has elapsed since the change. The amount of power supplied to the heater is set so that the amount of power supplied to the corresponding heater becomes the amount of power supplied to the heater.

The larger the intake air flow rate of the engine before the change in operating conditions, the more the oxygen sensor is sufficiently heated by the exhaust gas, and the heater heats up to increase the supplied power after the change in operating conditions. There is a high risk that the temperature will rise above the point at which problems such as deterioration occur (critical temperature). In the present invention, power supplied to the heater,
In other words, when the amount of heat generated by the heater changes from a state where the intake air flow rate is large to a state where it is small, the amount of power supplied to the heater corresponding to the intake air flow rate after the change after a predetermined period has elapsed from the time of the change. However, it is possible to maintain the heater at the activation temperature while preventing the heater from exceeding the critical temperature.

When the operating state of the engine changes from a state where the intake air flow rate is large to a state where the intake air flow rate is small, it is preferable that the power supplied to the heater is gradually increased to a steady value after the change in the operating state. As a result, the power supply changes smoothly, and changes in the heater temperature can be suppressed.

Advantageously, the power supplied to the heater is controlled by the duty ratio of the heater drive pulse.

Example The present invention will be described with reference to the embodiments shown in the drawings.

FIG. 1 shows an oxygen sensor 10 used in the present invention, in which a bottomed cylindrical oxygen ion conductive solid electrolyte 12 made of zirconia has an air permeable platinum thin film electrode 14 as an anode and an air permeable platinum thin film electrode 14 as a cathode on its inner and outer surfaces. A DC voltage is applied between lead wires 18 and 20, which are each covered with a gas-permeable platinum thin film electrode 16 and connected to these electrodes 14 and 16. A porous ceramic layer 22 is provided on the outer surface of the cathode 16 as a diffusion resistance layer. In order to heat the sensor element 24 thus formed, a tubular ceramic heater 28 having an air hole 26 in the center communicating with the atmosphere penetrates the insulating bushing 30 and protrudes into the sensor element 24, and is supplied with power via lead wires 32, 34. be done. The sensor element 24 is housed in a casing 38 having a number of holes 36 and projects into the exhaust pipe through a wall 40 of the exhaust passage, for example the exhaust pipe.

FIG. 2 is a block diagram of an electronically controlled fuel injection device that also serves as a heater control device. An air flow meter 44 is provided in the intake passage to detect the intake air flow rate, a rotational speed sensor 46 detects the rotational speed of the engine, and a throttle sensor 48 detects the throttle opening. In the electronically controlled fuel injection computer 50, the input port 52, the output port 54, and the ROM 5
6, RAM 58, and MPU 60 are connected to each other via address data bus 62. The analog output of the air flow meter 44 is buffer 6.
4 and A/D (analog/digital converter) 6
6 to input port 52, and the output pulses of rotational speed sensor 46 and throttle sensor 48 are sent to input port 52 via buffers 68 and 70, respectively. Output voltage of battery 72
Vb is sent to the input port 52 via the A/D 74, and the output current of the oxygen sensor 10 is I/V (current/
voltage converter) 76, amplifier 78, and A/D8
0 to input port 52. Battery 72 is connected to ground via heater 28, power transistor 82, and resistor 84. The power transistor 82 receives an on/off control signal from the output port 54, and receives a terminal voltage of the resistor 84.
Vr is sent to input port 52 via A/D 86. Down counter 88 is set with data from output port 54 and resets SR flip-flop 90 when the count decreases to zero.
The Q terminal of the SR flip-flop 90 is connected to the amplifier 92.
is sent to the fuel injection valve 94 of the intake port. The clock pulse of CLOCK96 is MPU60,
It is sent to the down counter 88 and the S terminal of the SR flip-flop 90. down counter 88
Data corresponding to the fuel injection amount is set in , and the fuel injection 94 is held open only while the SR flip-flop 90 is set.

FIG. 3 shows the relationship between the intake air flow rate Ga and the basic power Po of the heater 28. Intake air flow rate Ga
Since the exhaust gas temperature rises as the amount increases, the basic power Po decreases as the intake air flow rate Ga increases.

FIG. 4 shows the relationship between the intake air flow rate Ga and the basic duty ratio Do of the drive pulse of the heater 28. The power supplied to the heater 28 is controlled by turning on and off the power transistor 82 shown in FIG. 2, that is, by the duty ratio of the drive pulse. The voltage Vh applied to the heater 28 also changes as the voltage of the battery 72 changes, but the duty ratio D for supplying the basic power Po to the heater 28 when Vh is at the reference voltage is defined as the basic duty ratio Do.

The actual power P supplied to the heater 28 in FIG. 2 is calculated from the following equation.

P=Vh・Ih=(Vb−Vt−Vr)・Vr/Rr Where, Vh: Applied voltage of heater 28 Ih: Applied current of heater 28 Vb: Voltage of battery 72 Vt: Voltage across transistor 82 Vr: Resistor 84 Rr: the resistance value of the resistor 84. Note that Vt and Rr are constant, and Vb and Vr are detected by A/Ds 74 and 86, respectively.

Due to fluctuations in the voltage Vb of the battery 82, even if the duty ratio D is the same, the power supplied to the heater 28 P
changes. Therefore, power supply correction coefficient α=Po/
By correcting the duty ratio D to D·α, the power P supplied to the heater 28 can be made accurate.

FIG. 5 shows the relationship between the intake air flow rate Ga and the parameter correction coefficient β. The intake air flow rate of the engine is measured. In Figure 5, the range of Ga20g/sec is defined as a state where the intake air flow rate is large, and this range is
Divide into Ga1, Ga2, ... regions in order from the smallest Ga. In each region during the period until the throttle valve reaches the idle opening degree, that is, until the idle switch of the throttle sensor 48 is turned on.
Measure the total time of Ga1, Ga2, ... (time counter counts Ta1, Ta2, ...).In this way, when the idle switch is turned on, that is, the engine operating state changes from a state where the intake air flow rate is large. When the state changes to small, Ta1, Ta2,...
. . . The maximum value Tax and the area Gax where the maximum value Tax is located are determined, and the parameter correction coefficient β is determined from Gax and Tax. Parameter correction coefficient β
The larger Gax is in the region of Ga, the more
The larger Tax is, the smaller the value is set, and the duty ratio D of the drive pulse is corrected to D·β/Bo.
The oxygen sensor 10 is sufficiently heated by the exhaust gas as Gas and Tax are larger, and the heater 28 is heated to avoid thermal deterioration of the heater 28 when the idle switch changes from off to on.
It is necessary to keep the amount of heat generated in 8 small. As shown in Figure 5, the parameter correction coefficient β is
By defining it as a function of , thermal deterioration and melting of the heater 28 can be prevented. Furthermore, even after the idle switch is turned on from off, the predetermined amount of heat generated by the heater 28 is maintained.
This prevents the oxygen sensor 10 from becoming inactive due to a decrease in exhaust gas temperature.

FIG. 6 shows the change over time of the parameter correction coefficient β. β is set as shown in FIG. 5 when the idle switch is turned on, and gradually increases to 1 during the subsequent idling opening period as shown in FIG. 6. In this way, the duty ratio D is gradually returned to a steady value, the power supplied to the heater 28 changes smoothly, and sudden changes in the heater temperature are prevented.

FIG. 7 illustrates the waveform of the drive pulse for the heater 28. (a) shows the waveform during the steady period when Ga is large, (b) shows the waveform during the idling period during steady state, and (c) shows the waveform during the transient period immediately after the idle switch is turned on. In the steady periods of (a) and (b), the duty ratio D is the fourth
The duty ratio D is defined as shown in the figure, but as a result of being modified by β in the transient period (c), the duty ratio D is
The duty ratio D is larger than that in case (b) and gradually shifts to the duty ratio D in case (b).

FIG. 8 is a flowchart of the heater control routine. This routine is executed as a 4msec time interrupt routine. The heater 28 is energized by the count number Dx corresponding to the duty ratio D during the count number Tci corresponding to the period T of the drive pulse (for example, 48 msec). The duty ratio D is calculated for each period of the drive pulse, and if the idle switch is off (step 128), and the intake air flow rate is
When the time change of Ga ΔGaga is small (step
134), the duty ratio D is defined according to Fig. 4,
Within a predetermined time after the idle switch is turned on, the duty ratio D is modified according to β defined in FIG.

Each step in FIG. 8 will be explained in detail. In addition, Tc, Dx as values of the period counter and energization counter,
and Ta1, Ta2, . . . are set to initial values Tci, Dxi and 0, respectively, by an initial setting routine executed immediately after the engine switch is turned on. In step 100, the intake air flow rate Ga is RAM
Write to 58. In step 102, Ga and a predetermined value are set.
Gac (in Fig. 5, Gac corresponds to 20 g/sec), and if it is GaGac, that is, if the period is in a state where the intake air flow rate is large, the step is
Proceed to step 104, and if Ga<Gac, that is, if the period is in a state where the intake air flow rate is small, proceed to step 104.
Proceed to 106. In step 104, the Ga region at that time is
The counter value Tan corresponding to Gan is increased by 1. In step 106, it is determined whether or not the conditions for executing heater control are met. If the determination is positive, the process proceeds to step 108, and if not, the process proceeds to step 118. As a condition for executing the heater control, for example, the cooling water temperature is 60° C. or higher, that is, the warm-up operation has already been completed. step
At 108, the value Tc of the period counter is decreased by 1. In step 110, Tc is compared with 0, and Tc
If Tc>0, that is, the cycle has elapsed, the process proceeds to step 120; if Tc>0, that is, the cycle has not yet elapsed, the process proceeds to step 112.
In step 112, the value Dx of the energization counter is decreased by 1. In step 114, Dx is compared with 0,
If Dx is 0, that is, if the energization time of the heater 28 has passed, 0 is assigned to Dx in step 116, and then the heater is turned off in step 118.
If Dx>0, that is, if the heater 28 is within the energization time, the process advances to step 150 and the heater 28 is turned on.
Turn on. In step 120, the voltage Vb of the battery 72 and the current Ih of the heater 28 are written into the RAM 58. Note that Ih is calculated from Vr/Rr. In step 122, the actual power P supplied to the heater 28 is calculated from P=(Vb-Vt-Vr).Ih. step
In step 124, the power supply correction coefficient α is calculated from α=Po/P. In step 126, the duty ratio D is calculated from Do·α, and the corresponding value of D is set as Dx. step 128
Now determine whether the idle switch is on or off,
If it is on, that is, if the engine has a small intake air flow rate, the process proceeds to step 130; if it is off, the process proceeds to step 148. In step 130, the value of the setting flag F for β is determined, and if F=1, that is, if β has already been set, the process proceeds to step 140.
If F=0, that is, if β has not yet been set, the process advances to step 134. step 132
Now, compare the absolute value of the amount of change ΔGa in the intake air flow rate Ga per predetermined time |ΔGa| and the predetermined value C, |
If ΔGa|C, that is, if it is a transient period, the process proceeds to step 134, and if |ΔGa|<C, the process proceeds to step 148. In step 134, Ta1, Ta2,
Find the maximum value Tax among ... and the area Gax where the maximum value Tax is located. Step 136 is Tax
The parameter correction coefficient β is calculated based on and Gax according to the graph of FIG. In step 138, the setting flag F of β is set, and Ta1, Ta2,
Assign 0 to .... In step 140, β is increased by one. In step 142, β is compared with a predetermined value βo, and if ββo, the process advances to step 146, and β
<If Bo, proceed to step 148. In step 144, the setting flag F of β is reset. step
In step 146, D·β/βo is set as a new D, and Dx is corrected to a value corresponding to this D. In step 148, Tc to Tci
Substitute. Step 150 turns on heater 28.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of an oxygen sensor with a heater, Figure 2
The figure shows a block diagram of an electronically controlled fuel injection system that also serves as a heater control device. Figure 3 is a graph showing the relationship between the intake air flow rate and the basic power of the heater. Figure 4 shows the basic duty of the intake air flow rate and the heater drive pulse. 5 is a graph showing the relationship between the intake air flow rate and the parameter correction coefficient, and FIG. 6 is a graph showing the change in the parameter correction coefficient over time.
FIG. 7 is a diagram showing the waveform of the heater drive pulse, and FIG. 8 is a flowchart of the heater control routine. 10...Oxygen sensor, 28...Heater, 44...
...Air flow meter, 48...Throttle sensor, 72...Battery, 82...Power transistor, 84...Resistor.

Claims (1)

[Claims] 1 An oxygen sensor that detects the oxygen concentration in the exhaust gas of the engine is installed in the exhaust system, and the smaller the intake air flow rate, the greater the power supplied to the heater that heats the oxygen sensor.Control of the oxygen sensor heater. In the device, when the intake air flow rate of the engine changes from a large state to a small state, the amount of power supplied to the heater becomes equal to the intake air flow rate after the change after a predetermined period of time has elapsed since the change. A control device for an oxygen sensor heater, characterized in that the amount of power supplied to the heater is set.