Air-fuel ratio controller of internal combustion engine

An air-fuel (A/F) controller of an internal combustion engine, which computes an average value of a plurality of duty ratios of the voltage supplied to the heater according to the engine speed and the engine load periodically detected during a period from a predetermined moment up to the present, and fixes said average value as the present duty ratio of the voltage to be supplied to the heater, since the A/F ratio sensor is heated not only by the heater but also by exhaust gas. The A/F ratio controller smoothes the variation of voltage to be supplied to the heater by applying the computed average value as the present duty ratio of the supply voltage which is to be supplied to the heater. Accordingly, even when the driving condition is in high-speed, variation of the temperature borne by the heater is mild and smooth, and thus, the temperature of the oxygen-concentration detecting element does not rise suddenly. Furthermore, the A/F ratio controller corrects the above-mentioned present duty ratio of the voltage to be supplied to the heater according to the supply voltage to be supplied to the heater.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for controlling air-fuel (A/F) 
ratio of an internal combustion engine, more particularly, to an A/F ratio 
controller which corrects voltage to be supplied to the heater for heating 
the A/F ratio sensor according to the engine-driving condition. 
2. Description of the Prior Art 
When operating an internal combustion engine, in particular, one which 
drives a vehicle engine provided with a ternary catalyzer for purifying 
exhaust gas, the A/F ratio of exhaust gas must be strictly held at the 
theoretical A/F ratio. Today, there is such a specific A/F ratio 
controller available for use, which executes feedback control of A/F ratio 
by means of an A/F ratio sensor which sharply varies the level of output 
by applying the theoretical A/F ratio in order that the actual A/F ratio 
can approximate the theorectical A/F ratio. 
Nevertheless, since the A/F ratio sensor of the abovecited A/F ratio 
controller can only measure the theoretical A/F ratio, actually, this 
controller cannot execute feedback control of A/F ratio covering an 
extensive range. To compensate for such disadvantage, recently, a 
preceding art presents a system for controlling the A/F ratio using an A/F 
ratio sensor which is capable of measuring not only the theoretical A/F 
ratio, but can also continuously measure the A/F ratio from the rich to 
the lean degree according to the volume of specific component like oxygen 
present in the exhaust gas. This A/F ratio sensor incorporates an 
oxygenconcentration detecting element composed of ion-conductive solid 
electrolyte and a heater which activates the element. Unless held at the 
predetermined temperature by means of a heater, the oxygen-concentration 
detecting element of the A/F ratio sensor is it cannot function correctly 
by itself. FIG. 1 is the graphical chart designating the relationship 
between the temperature of the oxygen-concentration detecting element and 
the deviation of signals outputted from the above-cited A/F ratio sensor 
(.DELTA.A/F). As is clear from this chart, independent of differential 
values of temperature borne by the oxygen-concentration detecting element 
against the predetermined reference level, deviation is generated by 
signals outputted from the A/F ratio sensor. 
On the other hand, depending on the engine driving condition, the 
temperature of exhaust gas varies, and thus, the temperature of the A/F 
ratio sensor set to the exhaust-gas tube also varies. To compensate for 
this conventionally, the caloric value of the heater is controlled 
according to the load and the number of the rotational of the engine. 
Nevertheless, although the temperature of exhaust gas instantly responds 
to the engine driving condition, the temperature of the A/F ratio sensor 
does not instantly respond to the exhaust gas temperature. Conventional 
A/F ratio sensors cannot maintain the temperature of the 
oxygen-concentration detecting device at the predetermined value since 
they merely apply the variation of the load and the number of the rotation 
of the engine to the control of the heater. Consequently, error is easily 
generated in the signal outputted from the A/F ratio sensor, and as a 
result, the A/F ratio controller cannot precisely control the A/F ratio. 
SUMMARY OF THE INVENTION 
The invention has been achieved for fully solving the problems mentioned 
above. 
The primary object of the invention is to provide a novel A/F ratio 
controller which can constantly maintain the temperature of the 
oxygen-concentration detecting element of the A/F ratio sensor at a 
predetermined value and precisely control the A/F ratio. 
The second object of the invention is to constantly maintain the 
temperature of the oxygen-concentration detecting element at the 
predetermined value by applying the actual duty ratio of the heater for 
heating the oxygen-concentration detecting element of the A/F ratio 
sensor, where the actual duty ratio of the heater is substantially 
composed of the average value of the duty ratios of voltage to be supplied 
to the heater, computed in relation to the engine speed and the load of 
the engine, which are periodically detected during a period from a moment 
before the predetermined period of time to the present. 
The third object of the invention is to correct the duty ratio of the 
voltage to be supplied to the heater in accordance with the voltage 
outputted from the power-supply source of the heater. 
The above and further objects and features of the invention will more fully 
be apparent from the following detailed description with accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The reference numeral 1 shown in FIG. 2 designates the engine. 
Radiated-water temperature sensor 2 detects temperature of radiated water. 
Crank-angle sensor 3 detects the number of the rotation of the engine 1. 
Fuel injector 4 feeds fuel to the engine 1. Throttle valve 5 adjusts the 
volume of air flowing through air-inlet tube. Pressure sensor 6 detects 
absolute pressure inside of the air-inlet system. The A/F ratio sensor 8 
installed in the exhaust-gas tube 7 detects the A/F ratio by analyzing 
specific components present in the exhaust gas. The A/F ratio sensor 8 is 
provided with an oxygen-concentration detecting element and a heater which 
heats this element to a predetermined temperature value. Absorbed-air 
temperature sensor 9 detects temperature of the absorbed air. Control 
circuit 10 receives signals outputted from radiated-water temperature 
sensor 2, crankangle sensor 3, pressure sensor 6, A/F ratio sensor 8, and 
absorbed-air temperature, 9, to control operation of the fuel-injector 4. 
Substantially, the control circuit 10 is composed of a microcomputer. The 
reference numeral 11 designates a battery. 
FIG. 2 designates a D-J format A/F ratio controller. The A/F ratio 
controller shown in FIG. 2 computes the basic injection pulse time on the 
bases of at least the value delivered from the pressure sensor 6 and the 
data, obtained from the crank angle sensor 3, designating the number of 
the rotation of the engine 1. The control circuit 10 executes corrections 
and transitory corrections of the computed values by referring to signals 
from the radiated-water temperature sensor 2 and the absorbed-air 
temperature sensor 9, while it also executes feedback correction of the 
computed values by applying the A/F ratio sensor 8, and. Finally, the 
control circuit 10 determines the fuel-injection pulse time. 
FIG. 3 is a detailed block diagram of the control circuit 10. Central 
processing unit (CPU) 16 executes computations and operations for 
controlling the A/F ratio controller. ROM 17 stores programs. RAM 18 
provisionally stores data. Power is constantly delivered to RAM 19 so that 
it can continuously retain data. Analog-digital (A/D) converter 12 
converts the analog signal into a digital signal. The A/F ratio sensor 
control circuit 13 controls signals outputted from the A/F ratio sensor 8 
in order that the sensor itself can output correct signals proportional to 
the actual A/F ratio. 
The switching circuit 14 turns power supplied from the battery 11 ON and 
OFF. The power from the battery 11 is then delivered to the heater for 
heating the oxygen-concentration detecting element built in the A/F ratio 
sensor 8. I/O port 15 is the terminal which receives and outputs data. Bus 
20 transfers data to and from respective elements of the control circuit 
10. Signals outputted from the radiated-water temperature sensor 2, 
pressure sensor 6, battery 11, absorbed-air temperature sensor 9 via 
output terminals and signals outputted from the A/F ratio sensor 8 through 
A/F ratio sensor control circuit 13 are delivered to the A/D converter 12, 
and the A/F ratio sensor control circuit 13. Signals outputted from the 
crank-angle sensor 3 are delivered to I/O port 15. Fuel injector 4 
receives a control signal from the CPU 16 via I/O port 15. The switching 
circuit 14 is controlled by the CPU 16. 
FIG. 4 is the flowchart designating the sequential procedure of the duty 
control operation executed by the CPU 16 against the switching circuit 14. 
First, in step 200, the CPU 16 reads the number of the rotation of the 
engine 1 from the signal outputted by the crank-angle sensor 3. Next, in 
step 201, the CPU 16 reads the engine-load parameter composed of either 
the pressure inside of the air-inlet tube, or aperture degree of throttle, 
or the absorbed-air volume per a certain rotation of the engine 1. ROM 17 
preliminarily stores a data map (shown in FIG. 5) designating the duty 
ratio according to the number of the rotation of the engine 1 and the load 
applied to the air-inlet tube. Next, in step 202, the CPU 16 reads the 
duty ratio according to the number of the rotation of the engine 1 an the 
load applied to the air-inlet tube already identified. The CPU 16 then 
computes the basic ratio, i.e., the ratio between time "t.sub.on " needed 
for feeding power to the heater and time "t.sub.off " designating the 
period to stop the power supply to the heater by executing interpolatory* 
computations (see FIG. 6). The reference character V.sub.B designates the 
power voltage of the battery 11. Next, in step 203, the CPU 16 computes 
the average value of the basic duty ratio which is computed every specific 
period of time (where the average value covers a period from a 
predetermined moment up to the present), and then, the CPU 16 coverts the 
computed average value into the duty ratio for driving the heater. 
However, even though the duty ratio remains constant, if the battery 
voltage V.sub.B varies, the power level supplied to the heater also varies 
itself. To securely read this, when step 204 is underway, the CPU 16 reads 
the actual level of the battery voltage V.sub.B. Then, in step 205, the 
CPU 16 corrects the duty ration according to the value of the battery 
voltage V.sub.B. Next, in step 206, the CPU 16 drives the switching 
circuit 14 in order that the duty ratio can be the corrected one and the 
power supply to the heater can be turned ON and OFF. 
FIG. 7 is the graphical chart designating the variation of the duty ratio 
and the variation of the temperature of oxygen-concentration detecting 
element when the engine load varies. The broken line of FIG. 7 designates 
the computed basic duty ratio and the variation of temperature of the 
oxygen-concentration detecting element when control operation is executed 
in accordance with the basic duty ration. The temperature of the 
oxygen-concentration detecting element does not remain constant. On the 
other hand, since the preferred embodiment of the invention executes the 
control operation on the basis of the average value of the computed duty 
ratio as shown by the solid line of FIG. 7, the oxygen-concentration 
detecting element can maintain a constant temperature. 
FIG. 8 is a flowchart designating the sequential procedure of the A/F ratio 
control operation to be executed in accordance with programs stored in ROM 
17. First, in step 100, the CPU 16 reads the number of the rotation of the 
engine 1 from the signal outputted from the crank-angle sensor 3. Next, in 
step 101, the CPU 16 reads the pressure inside of the air-inlet tube from 
the signal outputted from the pressure sensor 6. Next, in step 102, the 
CPU 16 reads temperature of radiated water from the signal outputted from 
the radiatedwater* temperature sensor 2. In step 103, the CPU 16 reads the 
temperature of absorbed air the from signal outputted from the 
absorbed-air temperature sensor 9. Next, in step 104, the CPU 16 computes 
the basic fuel injection pulse width on the basis of the number of the 
rotation of the engine 1 and pressure inside of the air-inlet tube. The 
CPU 16 then corrects the pulse width by checking the radiated-water 
temperature and the absorbed-air temperature. Next, in step 105, the CPU 
16 reads the signal outputted from the A/F ratio sensor 8. Next, in step 
106, the CPU 16 corrects the fuel injection pulse width on the basis of 
the deviation between the objective A/F ratio and the actual A/F ratio. 
Finally, in step 107, the CPU 16 drives fuel injector 4 by applying the 
corrected fuel injection pulse width. 
As this invention may be embodied in several forms without departing from 
the spirit of the essential characteristics thereof, the present 
embodiment is therefore illustrative and not restrictive, since the scope 
of the invention is defined by the appended claims rather than by the 
description preceding them, and all changes that fall within the meets and 
bounds of the claims, or equivalence of such meets and bounds thereof are 
therefore intended to be embraced by the claims.