Apparatus for Controlling Air-Fuel Ratio of Internal Combustion Engine

1. Field of the Invention

This invention relates to an apparatus for controlling the air-fuel ratio of an internal combustion engine.

2. Prior Art

Devices for detecting the air-fuel ratio of an internal combustion engine have previously been known. Such devices comprise a wide range air-fuel ratio sensor including an oxygen sensor section which generates an electromotive force in response to the difference between atmospheric pressure and the oxygen concentration in exhaust gas discharged from the engine, and an oxygen pump section which provides a pump-driving current to feed and discharge oxygen to and from the exhaust gas used for comparison with the atmospheric pressure, the flow of the pump-driving current being controlled so that the output voltage of the oxygen sensor attains a predetermined value, whereby the air-fuel ratio of the engine is detected with the magnitude of the pump-driving current (see, for example, Japanese Utility Model Public Disclosure No. 18659/1987). Thus, the control of the air-fuel ratio of the engine is carried out by using such a device to detect the air-fuel ratio. The above-mentioned device can continuously measure the air-fuel ratio over a wide range from rich to lean.

In a conventional apparatus for controlling air-fuel ratio, as mentioned above, the pump-driving current is fed so that the output voltage of the oxygen sensor section is kept constant, and the air-fuel ratio is detected by measuring the pump-driving current. If any trouble or failure occurs in the oxygen sensor section, the oxygen pump section, or a connector or the like associated therewith, the precise pump-driving current does not flow, even when the apparatus operates to establish a flow of the pump-driving current sufficient to cause the voltage of the oxygen sensor section to reach the predetermined value. Alternatively, if the current is able to flow normally, it may possibly happen that an excessive current flows as a result of insufficient electromotive force in the oxygen pump section, and thus correct information on the air-fuel ratio cannot be obtained. This has led to the problem that, if the feedback control of the air-fuel ratio is conducted in response to the operation of the wide range air-fuel ratio sensor when the latter is malfunctioning, the air-fuel ratio will be greatly affected such as to produce an excessively rich or lean mixture, resulting in poor performance, deterioration of the exhaust gas quality and possibly even stalling of the engine.

It is therefore an object of the present invention to solve the above-mentioned problems and to provide an apparatus for controlling the air-fuel ratio of an internal combustion engine, said apparatus being arranged to avoid any possibility of a lowering of the engine performance, deterioration in the quality of the exhaust gas, or stalling of the engine.

According to the present invention, there is provided an apparatus for controlling the air-fuel ratio of an internal combustion engine comprising a failure detecting means for detecting any failure in a wide range air-fuel ratio sensor when the pump-driving voltage which causes the pump-driving current to flow remains outside a predetermined range of values for a period of more than a predetermined duration during the operation of the sensor, and a feedback interrupting means for breaking the feedback control loop of the air-fuel ratio when such a failure is detected by the failure detecting means.

With such an arrangement, the feedback interrupting means operates to suspend the feedback control of the air-fuel ratio when a failure is detected by the failure detecting means, whereby any marked lowering of the engine performance or marked deterioration in the quality of the exhaust gas may be prevented.

The invention, as well as other objects and advantages, will become more apparent from a reading of the detailed description of the preferred embodiment presented below.

In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an apparatus in accordance with the present invention;

FIG. 2 is a block diagram of an electronic control section incorporated in the apparatus of the invention;

FIG. 3 is a block diagram of an apparatus for detecting the air-fuel ratio incorporated in the apparatus of the invention; and

FIGS. 4 and 5 are flow charts showing the operation of the apparatus of the invention.

Referring to FIG. 1, the air sucked through an air cleaner 1 is fed to a combustion chamber 8 in an engine block 7 through a suction passage 12 including a throttle valve 3, a surge tank 4, a suction port 5 and a suction valve 6. Provided in the suction passage 12 is a negative pressure sensor 48 which is connected to an electronic control unit 40. The throttle valve 3 is interlocked with an accelerator pedal 13 provided in the driver's compartment of the vehicle. The combustion chamber 8 is defined by a cylinder head 9, a cylinder block 10 and a piston 11, and the exhaust gas generated therein by the combustion of the fuel-air mixture is discharged to the atmosphere through an exhaust valve 15, an exhaust port 16, an exhaust manifold 17 and an exhaust pipe 18. A bypass passage 21 is provided to communicate the upstream portion of the throttle valve 3 and the surge tank 4, and a valve 22 for controlling the rate of bypass flow is also provided to control the cross-sectional area of the bypass passage 21, thereby maintaining the engine speed at a constant value during idling operation. An intake-air temperature sensor 28 is provided in the suction passage 12 to detect the temperature of the intake air, and a throttle position sensor 29 serves to detect the degree of opening of the throttle valve 3. Further, a water temperature sensor 30 is mounted on the cylinder block 10 to detect the temperature of the cooling water, and a device 31 for detecting the air-fuel ratio is mounted at the junction of the exhaust manifold 17 and connected to a battery E through a switch 79 to detect the air-fuel ratio at the junction. A crank angle detecting sensor 32 is provided to detect the crank angle and rotational speed of the crank by measuring the rotation of a shaft 34 of a distributor 33 connected to the crank shaft of the engine block 7. A reference numeral 36 in FIG. 1 designates a transmission.

The outputs of the intake-air temperature sensor 28, the throttle position sensor 29, the water temperature sensor 30, a battery 37, a negative pressure sensor 48, the air-fuel ratio detecting device 31 and the crank angle sensor 32 are fed to an electronic control unit 40. Fuel injection valves 41 corresponding to each of the cylinders are disposed adjacent to each suction port 5, and a pump 42 serves to supply fuel from a fuel tank 43 through a fuel passage 44 to the fuel injection valve 41. The electronic control unit 40 calculates the rate at which fuel is injected by utilizing as parameters the input signals from the respective sensors and delivers to the fuel injection valve 41 an electric pulse having a width corresponding to the calculated fuel injection rate, whereby the fuel injection valve is opened in accordance with the pulse width representing the fuel injection rate.

The electric control unit 40 also controls the bypass flow control valve 22 and an ignition coil 46, the secondary side of which is connected to the distributor 33.

The system of the electonically controlled injection type engine shown in FIG. 1 is a D-Jetronix (speed density) type fuel injection system in which a basic injection pulse time period is calculated at least on the basis of the output values of the negative pressure sensor 48 and the engine rotation detecting sensor 32, and the basic injection pulse time period it then subjected to correction on the basis of the signal from the intake-air temperature sensor 28, transitory correction, feedback correction effected by the air-fuel sensor and so forth, whereby the fuel injection allowed through the fuel injection valve 41 is given a target air-fuel ratio.

FIG. 2 is a block diagram showing the detail of the electronic control unit 40. The control unit 40 comprises a CPU (central processing unit) 56 including a microprocessor for effecting operation and control, a ROM (read-only memory) 57 for providing a program for the correction process as mentioned below and other programs for the bypass flow control process and so forth, a first RAM 58 for temporarily storing data obtained during the operation, a second RAM 59 which serves as a non-volatile memory supplied with power from an auxiliary power source and adapted to hold necessary data in its memory even when the engine is not in operation, A/D (analog-to-digital) converter 60, an I/O (input/output) device 61 and a bus 62. The outputs of the throttle position sensor 29, the negative pressure sensor 48, the intake-air temperature sensor 28, the water temperature sensor 30, as well as the outputs 38, 39 of the air-fuel ratio detecting device 31 and the outputs of the battery 37, are supplied to the A/D converter 60. The output of the crank angle sensor or rotational speed sensor 32 is also supplied to the I/O device 61. The bypass flow control valve 22, the fuel injection valve 41 and the ignition coil 46 are supplied with inputs from the CPU 56 through the I/O device 61.

An example will be described below of the way in which the fuel supply system is controlled using the above-described electronic control unit 40 to calculate a target air-fuel ratio and, after correcting the target air-fuel ratio, to provide a corrected target air-fuel ratio. The program for executing such a process is stored in the ROM 57.

FIG. 3 shows an arrangement of the air-fuel ratio detecting device 31 comprising a wide range air fuel ratio sensor 80 and an air-fuel ratio detecting circuit 81. The wide range air-fuel ratio sensor 80 includes a solid-electrolyte oxygen sensor section 82 for generating electromotive force in accordance with the difference between the atmospheric pressure and the oxygen concentration of the engine exhaust gas and a solid-electrolyte oxygen pump section 83 for flowing a pump-driving current so that the output voltage of the oxygen sensor section 82 attains a predetermined value. The air-fuel ratio detecting circuit 81 includes a circuit 84 for detecting the differential value representing the difference between the reference value and the electromotive force of the oxygen sensor 82, a circuit 85 for supplying a pump-driving current i.sub.p, a current-voltage converting circuit 86, a pump-driving voltage absolute value circuit 87 and a voltage amplifying circuit 88.

The operation of the air-fuel ratio detecting device 31 shown in FIG. 3 is described below. The differential value detecting circuit 84 detects the difference between the output of the oxygen sensor section 82 and the reference voltage and supplies this differential signal to the pump-driving current supplying circuit 85 which, in turn, supplies a pump-driving current i.sub.p in accordance with the differential signal to the oxygen pump section 83. Thus, oxygen is supplied and feedback control is conducted so that the output of the oxygen sensor section 82 is changed to correspond to the reference value. The amount of oxygen conveyed by the pump-driving current i.sub.p corresponds to the air-fuel ratio. Then, the pump-driving current is converted by the conversion circuit 86 to a voltage which is, in turn, amplified by the amplifier circuit 88 and supplied as an air-fuel ratio signal 39 to the electronic control unit 40. The absolute value of the pump-driving voltage is determined by the absolute value conversion circuit 87 and the resultant pump-driving voltage signal 38 is also supplied to the electronic control unit 40.

The operation of the air-fuel ratio control apparatus of FIG. 1 will be described below by reference to the flowchart shown in FIG. 4. At steps 101 to 103, the conditional parameters of the engine, such as the engine speed, the negative pressure in the suction pipe, the water temperature, and the intake-air temperature, are read out. At step 104, the basic pulse width for driving the fuel injection valve 41 is computed in accordance with the engine speed and suction pipe pressure read out in steps 101 - 102. At step 105, the basic pulse width is adjusted in accordance with the conditional parameters such as values for the water temperature, intake-air temperature, etc. At step 106, a decision is made as to whether or not the wide range air-fuel ratio sensor 80 is normally operative. If any failure is detected, the operation is advanced to step 111 at which the fuel injection valve 41 is driven in accordance with the pulse width calculated at and before step 105. At step 106, if the air-fuel ratio sensor 80 is decided to be in a normal state, the air-fuel ratio signal 39 is read at step 107, the target air-fuel ratio is calculated at step 108, a compensation coefficient in respect of the fuel pulse width is calculated in accordance with the deviation of the real air-fuel ratio from the target ratio at step 109, the pulse width is adjusted by the calculated compensation coefficient at step 110, and the fuel injection valve 41 is driven in accordance with the adjusted pulse width at step 111.

The failure decision routine in step 106 will now be described by reference to the flowchart shown in FIG. 5. At step 201, a heater (not shown) of the wide range air-fuel ratio sensor 80 is energized for a predetermined period of time after starting and a check is made to determine whether or not the wide range air-fuel ratio sensor 80 is active. If after a predetermined lapse of time the sensor is not active, this is decided as being as sensor failure, though not a substantial failure, and the operation advances to step 205 and thus to step 111 of FIG. 4. If the air-fuel ratio sensor is active after the predetermined lapse of time in step 201, a decision is then made at step 202 as to whether or not the pump-driving voltage is within a predetermined range, such as, for example, a range of .+-.3V, and if the answer is yes, a timer is set to a predetermined period of time (100 msec, for example) in step 203. If the pump-driving voltage is within the predetermined range, the timer is always reset so that it does not become zero, and the operation is advanced to steps 204 and 206 to reset the sensor following failure, and feedback control is then performed in step 107 to step 110. If the pump-driving voltage is out of the predetermined range, however, the timer is counted down to zero and timer is up at step 204, whereby the sensor is set following the failure and the feedback control is omitted so as to proceed to step 111.

As described above, according to the present invention, any failure of the wide range air-fuel ratio sensor is monitored by checking whether or not the value of the pump-driving voltage of the oxygen pump section thereof is within a predetermined range. If the value is out of the range, feedback control is suspended. Thus such problems as lowering of engine performance and exhaust gas quality, and stalling of the engine due to the improper feedback control that would result from an abnormal output of the wide range air-fuel ratio sensor can be avoided.

Having described preferred embodiments of the invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the concepts of the invention.