Method and system for internal combustion engine oxygen sensor heating control, synchronizing heater voltage detection with heater energization, and calculating power loss

An internal combustion engine has an exhaust system and an oxygen sensor fitted to the exhaust system including a sensor element and an electrically powered heater for heating it. The power supplied to the heater is controlled by determining a target value for it according to operational parameters of the engine, and by controlling the mean power in a time interval as follows: the voltage applied to the heater and the current flowing through it are detected, an intermittent voltage is supplied to the heater, and the duty ratio of the intermittent voltage is controlled so that the product of the heater voltage and the heater current is substantially equal to the target power, with the detection of the voltage supplied to the heater synchronized with the energization of the heater. A system is also described for implementing this method.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of controlling the heating of an 
oxygen sensor fitted to the exhaust system of an internal combustion 
engine for the purpose of controlling air-fuel mixture air/fuel ratio, and 
to a system for practicing the method. More particularly, the present 
invention relates to such a method and device for oxygen sensor heating 
control which synchronize the detection of an intermittent or pulse 
voltage applied to an electrical heater element of the oxygen sensor with 
the time interval in which said heater element is energized, thereby 
controlling more accurately the power to said heater element. 
The present patent application has been at least partially prepared from 
material which has been included in Japanese Patent Application No. Sho 
59-90682(1984), which was invented by the same inventors as the present 
patent application, and the present patent application hereby incorporates 
the text of that Japanese Patent Application and the claim or claims and 
the drawings thereof into this specification by reference; a copy is 
appended to this specification. 
It is known to fit an oxygen sensor to the exhaust system of an internal 
combustion engine. Such an oxygen sensor typically comprises a solid 
electrolyte or semiconductor, and varies a generated current or resistance 
in response to the concentration of oxygen in the exhaust gases of the 
engine. This electrical signal is fed to a control device which controls 
the amount of fuel provided to the engine in relation to the amount of air 
sucked thereinto, and is used for controlling the air/fuel ratio of the 
air-fuel mixture supplied to the engine by a feedback process. Various 
such forms of control device, which practice various methods of air-fuel 
mixture ratio control, are per se known. 
The output of the sensor element of such an oxygen sensor varies with 
temperature, and, particularly when the air/fuel ratio is lean and is in 
the range of 14.5 to 25, in order for the sensor element to accurately 
measure the oxygen concentration, said sensor element must be maintained 
at a temperature higher than a certain critical minimum active 
temperature. This maintenance of the temperature of the sensor element can 
be done by using a heater, and oxygen sensors with sensor element heaters 
have already been proposed, along with methods for operation of such 
heaters; for example in Japanese Patent Application No. 53-78476, which 
has been published as Japanese Patent Publication No. 54-13396. Further, 
in Japanese Patent Application No. 53-83120, which has been published as 
Japanese Patent Publication No. 54-21393, there has been proposed a method 
and a system for control of the eletrical power supplied to such an oxygen 
sensor element heater, in which the power is varied as a function of 
intake manifold pressure, of throttle opening, and of engine revolution 
speed, so as to ensure that the oxygen sensor element is kept at a 
temperature no lower than its minimum active temperature. 
The sensor element of such an oxygen sensor fitted to an exhaust system is 
of course heated up by the exhaust gases in the exhaust system, so the 
effect of a heater for the sensor element must be controlled to take 
account of the temperature of these exhaust gases. Now, in an internal 
combustion engine which is controlled by a throttle valve, the exhaust 
temperature is largely determined by the amount of air-fuel mixture 
supplied per engine piston stroke and by engine revolution speed, and if 
the air/fuel ratio of the air-fuel mixture is constant the amount of such 
mixture supplied is proportional to the rate of intake air flow. 
Therefore, in the above mentioned patent applications, the above are used 
as parameters, and the supply of electricity to the sensor element heater 
is varied depending on the engine load and the engine revolution speed. 
Thus, the exhaust temperature is considered to depend on the engine intake 
flow and engine revolution speed, and the values are determined 
experimentally in advance with reasonable accuracy. This method and system 
are adequate to keep the temperature of the sensor element of the oxygen 
sensor reasonably constant regardless of engine operational conditions. 
The heater element of such an oxygen sensor fitted to an exhaust system is 
typically of a pure resistive load type, using the Joule heating 
phenomenon for producing heating power, and typically such a heater 
element has a resistance which increases as its temperature increases. 
This causes fluctuations in the power dissipated in the heater element, 
with respect to the same voltage supplied thereto. One approach to 
tackling this problem is to supply a pulsed voltage signal to the heater 
element, and to alter the duty ratio of this pulsed voltage signal so as 
to keep the heater power, as measured by the product of the current 
through said heater element and the voltage across said heater element, at 
the desired control target value. In this case, however, the pulse nature 
of the voltage signal should be allowed for. 
Further, ideally the actual voltage across the heater element should be 
measured, but in practice the supply voltage to the control unit therefor 
may be used. In this case, however, the internal resistance of the vehicle 
battery, and in the wiring harness of the vehicle and so on, may mean a 
fluctuation in voltage of some hundreds of millivolts or so, between the 
times when the heater is on and the times when it is off. This can be a 
source of error if it is not allowed for. Also, the saturation voltage 
across the emitter and the collector of a control transistor for the 
heater element is a non linear function of the collector current, and so 
to correct for it could possibly involve rather complicated calculation. 
SUMMARY OF THE INVENTION 
Accordingly, it is the primary object of the present invention to provide a 
method and system for internal combustion engine oxygen sensor heating 
control, which control the power supplied to the heater of the oxygen 
sensor more accurately. 
It is a further object of the present invention to provide such a method 
and system for oxygen sensor heating control, which provide proper timing 
of the detection of the voltage across the heater element of the oxygen 
sensor. 
It is a further object of the present invention to provide such a method 
and system for oxygen sensor heating control, which provide proper timing 
of the detection of the current through the heater element of the oxygen 
sensor. 
It is a further object of the present invention to provide such a method 
and system for oxygen sensor heating control, which take account of the 
quick variation inherent in the pulse signal nature of the voltage signal 
supplied to the heater element of the oxygen sensor. 
According to the most general method aspect of the present invention, these 
and other objects are accomplished by, for an internal combustion engine 
comprising an exhaust system and an oxygen sensor fitted to said exhaust 
system comprising a sensor element and an electrically powered heater for 
heating said sensor element: a method for controlling the electrical 
supply to said heater, wherein: according to operational parameters of 
said engine, a target value for the power to be supplied to said heater is 
determined; and the mean power in a time interval is controlled as 
follows: the voltage applied to said heater and the current flowing 
through said heater are detected, an intermittent voltage is supplied to 
the heater, and the duty ratio of said intermittent voltage supply is 
controlled in such a way that the product of the heater voltage and of the 
heater current is substantially equal to said target power to be supplied; 
and wherein the detection of the voltage applied to said heater is 
synchronized with the energization of said heater; and according to the 
most general device aspect of the present invention these and other 
objects are accomplished by, for an internal combustion engine comprising 
an exhaust system and an oxygen sensor fitted to said exhaust system 
comprising a sensor element and an electrically powered heater for heating 
said sensor element: a system for controlling the electrical supply to 
said heater, comprising: a means for determining a target value for the 
power to be supplied to said heater, according to operational parameters 
of said engine; and a means for controlling the mean power in a time 
interval by detecting the voltage applied to said heater and the current 
flowing through said heater, while synchronizing said detection of the 
voltage applied to said heater with the energization of said heater, by 
supplying an intermittent voltage to said heater, and by controlling the 
duty ratio of said intermittent voltage supply in such a way that the 
product of the heater voltage and of the heater current is substantially 
equal to said target power to be supplied. 
According to such a method and such a system, because the detection of the 
voltage supplied to said heater is synchronized with the energization of 
said heater, the power supplied to the heater of the oxygen sensor is 
controlled more accurately, and proper timing of the detection of the 
voltage across the heater element of the oxygen sensor is provided. Thus, 
the quick variation inherent in the pulse signal nature of the voltage 
signal supplied to the heater element of the oxygen sensor is taken 
account of.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows in schematic view an internal combustion engine with an oxygen 
sensor of the above described kind, said engine incorporating the 
preferred embodiment of the oxygen sensor heating control system of the 
present invention, for performing the preferred embodiment of the oxygen 
sensor heating control method of the present invention. In this figure, 
the internal combustion engine 1 has a cylinder bore 2 within which a 
piston 3 reciprocates, said piston 3 being coupled in a per se 
conventional manner to a crankshaft, not shown, by a connecting rod, only 
partially shown; in fact the engine 1 has a plurality of such cylinders 
and pistons but only one of each of them can be seen in the figure. A 
combustion chamber 6 is defined above the piston 3 in the figure in the 
cylinder bore 2, between it and a cylinder head, and an intake port 5 
opens to this combustion chamber 6 via a valve aperture the opening and 
closing of which is controlled by an intake valve 4. A per se conventional 
spark plug 7 provides ignition for air-fuel mixture in the combustion 
chamber 6 when appropriately energized. Further, an exhaust port, not 
shown in the figure, opens to the combustion chamber 6 via a valve 
aperture the opening and closing of which is controlled by an exhaust 
valve, also not shown, and to this exhaust port there is connected an 
exhaust system, only a portion of an exhaust manifold 8 incorporated in 
which is shown. 
To the inlet port 5 there is connected the downstream end of an intake 
manifold 9, the upstream end of which is connected to the outlet of a 
surge tank 10. To the inlet of the surge tank 10 there is connected the 
downstream end of a throttle body 11, the upstream end of which is 
connected to the downstream end of an inlet tube 12. The upstream end of 
this inlet tube 12 is communicated to the outlet of an air cleaner 13, the 
inlet of which is left open to the atmosphere. In the throttle body 11 
there is mounted an intake butterfly valve 14 the opening and closing 
action of which for intake air amount control is linked to the foot 
depression movement of a throttle pedal for the engine 1, not shown, by a 
throttle pedal linkage also not shown. 
To the intake manifold 9 there is mounted a per se conventional fuel 
injection valve 15 which incorporates a solenoid 15a (not shown in FIG. 
1), and this fuel injection valve 15 is supplied with pressurized fuel 
(i.e. gasoline) by a fuel supply system which is not shown. The opening 
and closing action of this valve 15 is electronically controlled by a 
control device 16 which will be more particularly described hereinafter. 
Thus, the valve 15 squirts spirts of fuel into the intake manifold 9 the 
total volume of each of which depends on the opening and closing times 
thus provided for said fuel injection valve 15 by the control device 16. 
The control device 16 is supplied with actuating electrical energy from the 
battery 17 of the vehicle to which this engine 1 is fitted, via an 
ignition switch 31. To the distributor 18 of the engine 1 there is fitted 
a crank angle sensor 19, the electrical output signal of which is 
representative of the position of the crankshaft of the engine 1 and is 
dispatched to the control device 16. To the surge tank 10 of the engine 1 
there is fitted an intake pressure sensor 20, the electrical output signal 
of which is representative of the air pressure in the intake system of the 
engine 1 and is also dispatched to the control device 16. To the wall 8a 
of the exhaust manifold 8 of the engine 1 there is fitted an oxygen sensor 
21 to be more particularly described later, the electrical output signal 
of which is representative of the oxygen concentration in the exhaust 
gases flowing through said exhaust manifold 8 and is also dispatched to 
the control device 16; and the oxygen sensor 21 further has a heater 28 
(not shown in FIG. 1) as will be described later, supply of actuating 
electrical energy to which is provided from the control device 16. To the 
throttle valve 14 mounted in the intake system of the engine 1 there is 
fitted a throttle valve idling opening amount sensor 29 incorporating a 
switch 29a (not shown particularly in FIG. 1), the electrical output 
signal of which is also dispatched to the control device 16 and is 
representative of the opening amount of said throttle valve 14, being ON 
when said throttle valve 14 is opened by more than a predetermined amount 
and thus indicating engine operation at a level higher than idling level 
and being OFF when the throttle valve 14 is opened by less than said 
predetermined amount and thus indicating engine idling operation. To the 
starter 32 of the engine 1 there is fitted a starter switch 33, an 
electrical output signal from which is indicative of whether said starter 
32 is being actuated to crank said engine 1 or not and is also dispatched 
to the control device 16. And to the water jacket of the engine 1 there is 
fitted a water temperature sensor 35, the electric output signal of which 
is indicative of the temperature of the cooling water of said engine 1 and 
is also dispatched to the control device 16. Further a test switch 34 
optionally provides earthing for a terminal of the control device 16, and 
an output signal from said control device 16 is fed to a test alarm lamp 
36. 
Referring to FIG. 2, the oxygen sensor 21 fitted in the wall 8a of the 
exhaust manifold 8 comprises a sensor element 22 formed as a tube with one 
end closed and made of a solid electrolyte material such as zirconia which 
can transmit oxygen ions. The outside of this sensor element 22 has, laid 
on it, an outer electrode 23 formed as a porous thin conducting layer 
(this layer is not clearly separately shown in the figure because it is so 
thin as to be represented by a single line), and the inside of said sensor 
element 22 has, likewise laid on it, an inner electrode 24 likewise formed 
as a porous thin conducting layer (again, this layer is shown only by a 
single line in FIG. 2). The outer surface of the outer electrode 23 has an 
exhaust gas dispersion layer 25 also laid on it, said layer 25 being 
formed of porous ceramic. The sensor element 22, etc., are mounted within 
a casing and so on, not particularly described here because they are per 
se known, and are fixed into the wall 8a of the exhaust manifold 8 with 
their lower parts in FIG. 2 projecting into the interior of said exhaust 
manifold 8. And a shield 26 with a plurality of holes 27 formed therein is 
provided around said lower ends of the sensor element 22 etc. projecting 
into the exhaust manifold 8, so as to protect them from the impact of the 
rushing flow of exhaust gases in the exhaust manifold 8, while allowing 
said exhaust gases to impinge gently on the exhaust gas dispersion layer 
25 and the outer electrode 23 to reach the sensor element 22. During use 
of this oxygen sensor 21 as a current limiting type lean sensor, a certain 
voltage is applied by the control device 16 between the outer electrode 23 
and the inner electrode 24 as will be explained in detail hereinafter, so 
that as is per se well known the current between these electrodes 
increases approximately in proportion to the oxygen concentration in the 
exhaust gases flowing through the exhaust manifold 8, within certain 
limits. And, in order to keep the sensor element 22 etc. at the correct 
temperature for activation, an electrical heater 28 is provided for the 
oxygen sensor 21. This heater 28 is a per se known type of resistive 
heater, and the magnitude of the heating power instantaneously provided 
thereby is proportional to the product of the voltage and the amperage 
being provided by the control device 16 thereto. 
The function of the control device 16 is in partial outline as follows. 
From the data it receives relating to engine rotational speed from the 
crank angle sensor 19 and relating to intake manifold pressure from the 
intake manifold pressure sensor 20, it determines the volume of intake air 
which is being sucked into the combustion chamber in each intake stroke of 
the piston 3, and according thereto determines a theoretically proper 
amount of fuel to be mixed with this intake air to provide a proper and 
appropriate target value for the air/fuel ratio of the air-fuel mixture in 
the combustion chamber. And, during normal engine operation, as determined 
by when the engine 1 has been warmed up as is indicated by the output of 
the engine cooling water temperature sensor 35 and possibly as also 
determined by other criteria, based upon the actual value of the oxygen 
concentration in the exhaust gases in exhaust manifold 8 of the engine 1 
as detected by the oxygen sensor 21, information regarding which is 
dispatched therefrom to the control device 16, said control device 16 
makes a correction to this theoretical value in order to produce a value 
for the actual amount of fuel to be injected, so as to bring the air/fuel 
ratio to its target value by a form of per se known feedback control. 
Then, the control device 16 produces electrical output signals at 
appropriate crank angles and supplies them to the solenoid 15a of the fuel 
injector 15, so as to control the opening and closing of the fuel injector 
15 so as to inject this determined appropriate amount of fuel, in each 
injection spirt. On the other hand, when the engine 1 has not yet properly 
been warmed up as determined by the aforementioned criteria, no such 
feedback correction according to exhaust oxygen concentration of the 
calculated theoretically proper amount of fuel to be injected in order to 
provide a proper and appropriate target value for the air/fuel ratio of 
the air-fuel mixture in the combustion chamber is made, but instead the 
theoretically calculated value is directly used as a value of fuel to be 
injected, and accordingly the control of fuel injection is by a form of 
open loop control without any feedback. At this time the air/fuel ratio is 
controlled to be smaller than in the warmed up engine case when feedback 
is being utilized. 
Referring to FIG. 3, herein the internal structure of the control device 16 
is partially shown as an electrical circuit diagram, and also ancillary 
circuits relating thereto are shown. This control device 16 comprises a 
microcomputer 50, which may be for example of the Motorola 6801 type, and 
this microcomputer 50 is powered, like other parts of the circuitry of the 
control device 16, by a constant voltage Vcc supplied by a voltage 
regulator circuit 51 of a per se well known type, when and only when the 
ignition switch 31 of the vehicle is ON. This microcomputer 50 of this 
preferred embodiment has six inputs designated in the figure as I1 through 
I6 and seven outputs designated as O1 through O7. The inputs I1 through I6 
are connected as follows. The input I1 receives an ON signal when and only 
when the starter switch 33 is in the ON state. The input I2 receives an ON 
signal when and only when the ignition switch 31 of the vehicle is in the 
ON state. The input I3 receives an ON signal when and only when the test 
switch 34 is in the OFF state. The input I4 receives an ON signal when and 
only when the switch 29a incorporated in the throttle valve idling opening 
amount sensor 29 is in the OFF state, i.e. when and only when the engine 1 
is not idling. The input I5 receives the output of the crank angle sensor 
19, after this has been converted to a square wave by a wave shaping 
circuit 52. And the input I6 receives a pulse width signal from a RSTP 
terminal of an A/D converter (an analog-digital converter) 53 of a per se 
well known sort. Further, the outputs O1 through O7 are connected as 
follows. The signal from the output O1 is furnished to the base of a 
transistor 54 as a pulse signal, so as to control the power supplied to 
the heater 28 of the oxygen sensor 21 as will be explained hereinafter. 
The signal from the output O2 is furnished to the base of a transistor 55 
as a pulse signal, so as to control the solenoid 15a of the fuel injector 
15 for providing fuel injection. The signal from the output O3 is 
furnished to the base of a transistor 56 as a sensor diagnostic result 
signal, so as to selectively energize the test alarm lamp 36 according to 
the result of circuit testing, as will be explained hereinafter. The 
signal from the output O4 is furnished to a convert control terminal RSRT 
of the A/D converter 53 as a convert start signal. And the signals from 
the outputs O5 through O7 are furnished as channel control signals to the 
channel control terminals CH1 through CH3 respectively of said A/D 
converter 53. 
The transistor 54 receives the pulse signal from the output O1 of the 
microcomputer 50 at its base, and is thereby selectively switched ON so as 
to provide power via its collector to the heater 28 of the oxygen sensor 
21, when and only when said pulse signal from said output O1 is ON. This 
power for the heater 28 is provided directly from the battery 17 via the 
ignition switch 31, i.e. not via the voltage regulation circuit 51, and of 
course also via the wiring harness of the vehicle, which inevitably has a 
certain electrical resistance. The transistor 55 receives the pulse signal 
from the output O2 of the microcomputer 50 at its base, and is thereby 
selectively switched ON so as to provide power via its collector to the 
solenoid coil 15a of the fuel injector 15, when and only when said pulse 
signal from said output O2 is ON. And the transistor 56 receives the 
signal from the output O3 of the microcomputer 50 at its base, and is 
thereby selectively switched ON so as to provide power via its collector 
to the test alarm lamp 36, when and only when said signal from said output 
O3 is ON. And the reference numeral 57 denotes a differential amplifier: 
when the ignition switch 31 is ON, then a constant voltage Vcc is provided 
via the voltage regulation circuit 51, and drives the transistor 58 to 
supply a constant voltage to the sensor element 22 of the oxygen sensor 
21. 
The A/D converter 53 comprises a multiplexer, not particularly shown, and 
is powered by the constant voltage Vcc supplied by the voltage regulator 
circuit 51. This A/D converter 53 of this preferred embodiment has six 
inputs designated as I1 through I6, as well as a control terminal RSRT and 
an output terminal RSTP and channels CH1 through CH3. The inputs I1 
through I6 are connected as follows. The input I1 receives the reference 
voltage signal Vcc. The input I2 receives a voltage signal dropped from 
this reference voltage Vcc by a variable amount which depends upon the 
current through the sensor element 22 of the oxygen sensor 21 because of 
the resistor 59 as shown in the circuit diagram of FIG. 3. The input I3 
receives a voltage signal amplified by a differential amplifier 66 from 
the voltage across a load dropping resistor 60, thus detecting the value 
of the current passing through the heater 28 of the oxygen sensor 21. The 
input I4 receives a voltage signal proportional to the current value of 
the voltage Vi being supplied by the battery 17, according to the 
operation of a voltage divider circuit incorporating two resistors 61 and 
62. This voltage signal is of course not actually proportional to the 
voltage which is being applied across the heater 28, but as stated above 
rather is proportional to the voltage Vi being supplied to the control 
unit by the battery 17. The input I5 receives a voltage signal 
representative of the pressure in the surge tank 10 of the engine intake 
system from the intake pressure sensor 20. And the input I6 receives a 
voltage signal representative of the temperature of the cooling water of 
the engine 1 from the engine cooling water temperature sensor 35. 
Thus during operation by using a combination of the CH1 through CH3 signals 
from the microcomputer 50 a particular one of the input signals I1 through 
I6 is selected, and then, when the "start A/D convert" signal is 
dispatched by the microcomputer 50 (from its output O4) and is received at 
the RSRT terminal of the A/D converter 53, said A/D converter 53 performs 
the analog-digital conversion process and outputs a pulse width signal 
corresponding to the voltage of the selected input from its output 
terminal RSTP to the input I6 of the microcomputer 50. In particular, the 
microcomputer 50 receives pulse width signals from the A/D converter 53 
which are together representative of the voltage across the current 
detecting resistor 59 for the sensor element 22 of the oxygen sensor 21, 
said signals being received by said A/D converter 53 at its I1 and I2 
input terminals; and, by converting these pulse width signals into digital 
values and by subtracting one of them from the other, the microcomputer 50 
can obtain a digital value representative of said voltage across said 
sensor element 22. This value, which is representative of the oxygen 
concentration in the exhaust gases flowing through the exhaust manifold 8, 
is the value that the microcomputer 50 uses for performing the above 
described feedback control of the air/fuel ratio of the air-fuel mixture 
supplied to the engine 1, when appropriate. 
Now, the opertion of this preferred embodiment of the oxygen sensor heating 
control system of the present invention, while performing the preferred 
embodiment of the oxygen sensor heating control method of the present 
invention, will be explained with reference to FIGS. 4, 5, and 6, which 
are flow charts of the operation of certain parts of the program stored in 
the microcomputer 50. 
The flow chart of FIG. 4 shows the opertion of an initialization and base 
subroutine which is caused to be executed by the microcomputer 50 when the 
ignition switch 31 is turned on. The initialization part of this 
subroutine performs various operations such as register initialization and 
I/O port definition and so on, while the base part of this subroutine 
performs various operations such as calculating the fuel injection amount 
for feedback and the fuel injection amount cooling water temperature 
correction coefficient and so on. 
The flow chart of FIG. 5 shows the opertion of an interrupt subroutine for 
controlling supply of electrical energy to the heater 28 of the oxygen 
sensor 21; and this subroutine is caused to be executed by the 
microcomputer 50 at fixed intervals, which are for example of the order of 
tens of milliseconds. 
In this FIG. 5 subroutine, first, in the step 1, certain registers are 
saved. 
Next, in the step 2, the heater control transistor 54 is turned ON, i.e. an 
ON signal is output to the base of the transistor 54 from the output O1 of 
the microcomputer 50. This starts to supply power to the heater 28, i.e. 
turns said heater 28 ON. 
Next, in the step 3, the present count value of a free running timer 
attached to the microcomputer 50 is read, and is stored as T1, the ON time 
for the transistor 54. 
Next, in the step 4, the value Vi of the battery voltage is determined by, 
as described above, selecting the input I4 of the A/D converter 53 (see 
FIG. 3), which receives a voltage representative of this battery voltage 
Vi. In this case, the A/D converter 53 sends an output pulse signal 
representative of the battery voltage Vi to the microcomputer 50. It 
should be particularly noted that this detection of battery voltage Vi is 
done in synchronization with the power supply to the heater 28 having 
started, said starting of power supply having been performed in the step 2 
of this subroutine. 
In the next step 5, the value Ih of the current through the sensor element 
heater 28 is determined, likewise to the step 4 by selecting the input I3 
of the A/D converter 53, and by thus reading into the microcomputer 50 a 
pulse signal corresponding to the voltage drop across the resistor 60 on 
said input I3 of the A/D converter 53. Again, it should be particularly 
noted that this detection of battery voltage Vi is done in synchronization 
with the power supply to the heater 28 having started, said starting of 
power supply having been performed in the step 2 of this subroutine. 
Next, in the step 6, first the current values of the intake manifold 
pressure Pm and the engine revolution speed Ne are determined by the 
microcomputer 50: the intake manifold pressure Pm is determined in a 
similar way to the determination of the battery voltage Vi and of the 
heater current Ih in the steps 4 and 5 by the microcomputer 50 selecting 
the input I5 of the A/D converter 53, and the engine revolution speed Ne 
is determined by calculating the time interval between successive pulses 
from the crank angle position sensor 19 supplied to the input terminal I5 
of the microcomputer 50. Next, by consultation of a two way look up table 
of values stored in the ROM (read only memory) of the microcomputer 50, a 
proper and appropriate value for the amount Wh of electrical power to be 
supplied to the heater 28 of the oxygen sensor 21 is determined. The 
values of Wh in this look up table in the ROM are determined in advance by 
experiment, and generally decrease as the intake pressure increases and as 
the engine revolution speed increases. It should be noted that this 
required or target power value Wh is the actual amount of power to be 
dissipated in the element of the heater 28, and excludes any power loss in 
the wiring harness, etc. 
Next, in the step 7, the current average actual power consumption or 
dissipation Wa in the heater 28 is calculated, according to the equation 
Wa=Vi*Ih*Dh, where Vi and Ih are the actual current heater voltage and 
amperage as just calculated as above, and Dh is the current duty factor of 
the pulse electrical signal which is being supplied to the heater 28, said 
duty factor Dh having been calculated in the step 11 of the last iteration 
of this FIG. 5 subroutine some tens of milliseconds ago, and having been 
stored. 
Next, in the step 8, a value W1 for the power loss in the wiring harness 
and so on is determined, as being some function of the current average 
power consumption Wa, just determined in the step 7. The relationship 
between the power loss W1 and the average power Wa dissipated in the 
heater 28 may be investigated beforehand, in order to determine this 
function. In some cases, the relationship between W1 and Wa may be stored 
in the ROM memory of the microcomputer 50 in the form of a table, as 
schematically illustrated in FIG. 7 of the drawings, in which the power 
loss W1 is shown on the vertical axis and the total power Wa supplied is 
shown on the horizontal axis. In such a case, in this step 8, the value W1 
is looked up from this table according to the current value of Wa. In 
other cases, in which the relationship between the power loss W1 and the 
heater power Wa is not so complicated, their relationship may be expressed 
in the program of FIG. 5 (when it is considered in more detail) as a 
formula for calculation, and in such a case in this step 8 the calculation 
of this formula is performed for obtaining the value W1 based upon the 
current value of Wa. 
Next, in the step 9, the duty ratio Dh of the power pulse signal to be 
supplied to the heater element 28 of the oxygen sensor 21, in order to 
obtain the correct desired (average) power supply value Wa for the power 
actually dissipated in said heater 28, is calculated as the ratio of the 
sum of the desired power Wa and the power W1 that will be lost, to the 
power which would be dissipated in the heater element 28 if a continuous 
supply of power from the battery 17 were provided on the same basis as the 
instantaneous present one thereto--i.e. the product of the battery voltage 
Vi and the present heater current Ih. I.e., the equation used is 
Dh=(Wa+W1)/(Vi*Ih). This duty ratio Dh thus decreases with increase in Ih, 
the current in the heater 28. It will be understood that this calculation 
is not absolutely accurate, since the value of the lost power W1 that is 
used therein is that produced by using the value of Dh that pertained to 
the last iteration of this FIG. 5 subroutine and not the value of Dh just 
calculated; but this will give no problem, since the value of Dh alters 
fairly slowly, and the value of W1 in any case only affects the value 
calculated for Dh with a fairly low gearing or sensitivity, and thus by a 
homing in process, as the FIG. 5 subroutine is executed many times per 
second, the substantially correct result will be attained. 
Next, in the step 10, from the heater control period t and the duty ratio 
Dh calculated as above, the length of time Ton that the heater 28 is to be 
energized is calculated as t.Dh. And next, in the step 11, the time point 
T2 at which the heater 28 should be deenergized is determined, as being 
T1+Ton. 
Next, in the step 12, this time T2, at which the transistor 54 should be 
turned OFF and the heater 28 should be deenergized, is stored in a "time 
compare register". 
Finally, in the step 13, the registers which were saved in the step 1 are 
restored; and then the subroutine returns. 
The flow chart of FIG. 6 partially shows the operation of a time compare 
interrupt subroutine. In this subroutine, first a decision is made as to 
whether the transistor off time T2, stored in the time compare register as 
explained in the step 13, has arrived or not. It should be understood that 
the time counter is upcounted at fixed time intervals. If the time point 
T2 has not yet arrived, then the flow of control is transferred to various 
other interrupt decisions and actions, as schematically indicated by the 
double dotted lines and boxes; but, if the time point T2 for switching the 
heater power supply transistor 54 has in fact arrived, then the flow of 
control is transferred to a block which turns said heater control 
transistor 54 OFF by outputting to its base an OFF signal from the output 
O1 of the microcomputer 50. This stops supplying power to the heater 28, 
i.e. turns said heater 28 OFF. And then finally the subroutine returns. 
Thus, referring to FIG. 8, which is a timing chart showing the duty ratio 
control performed by the interrupt routine, and which shows against time 
the battery voltage Vi, the current Ih received by the heater 28, the 
ON/OFF signal to the heater control transistor 54, the Vi input timing, 
the Ih input timing, the timing of the calculation of duty ratio, the 
timing of the transistor-OFF signal, and the count value counted by the 
timer, the operation of the shown preferred embodiment of the present 
invention will be further clarified. 
Now, some mathematical and calculation considerations with regard to the 
process explained above will be made. These considerations are helpful but 
not essential for understanding of the concept of the present invention. 
First, let Ra be the total resistance of the circuit which includes the 
oxygen sensor heater 28, let Rh be the resistance of the heater element 
itself, let Rw be the resistance of the wiring harness, and let Rtr be the 
resistance of the transistor 54; then Ra=Rh+Rw+Rtr. Now, Rh is a function, 
say f', of the oxygen sensor heater temperature Th and the current through 
said heater; so Rh=f'(Th,Ih). A typical characteristic for the variation 
of the heater resistance with its temperature is as shown in FIG. 9, in 
which the heater temperature Th is shown along the horizontal axis and the 
heater resistance Rh is shown along the vertical axis. Also, the wiring 
harness resistance Rw is small and substantially constant. Further, the 
resistance Rtr of the transistor 54 is approximately equal to Vce/Ih, 
where Vce is the saturation voltage from collector to emitter of the 
transistor 54. FIG. 10 shows a typical characteristic for the variation of 
the saturation voltage from collector to emitter of the transistor 54, in 
said figure said saturation voltage Vce of said transistor 54 being shown 
on the vertical axis and the collector current Ic of said transistor 54 
being shown on the horizontal axis. 
Thus, from the overall power Wa=Ra * Ih.sup.2 =(Rh+Rw+Rtr) * Ih.sup.2, we 
get Vi * Ih=Rh * Ih.sup.2 +Rw * Ih.sup.2 +Vce * Ih.sup.2 
Rh * Ih.sup.2 =(Vi-Vce) * Ih-Rw * Ih.sup.2 
=((Vi-Vce)-Rw * Ih) * Ih 
Vi * Ih=((Vi-Vce)-Rw * Ih) * Ih 
Vi=(Vi-Vce)-Rw * Ih 
The control target power Wa in the case when the total power supplied to 
the heater element is to be controlled by duty ratio control is 
Wa=Vi * Ih * D=((Vi-Vce) * Ih-Rw * Ih.sup.2) * D 
so Wa+(Vce * Ih+Rw * Ih.sup.2) * D=Vi * Ih * D 
The duty ratio D for heater power control in this case is expressed by the 
following equation: 
D=(Wa+(Vce * Ih+Rw * Ih.sup.2) * D)/Vi * Ih 
Power loss Wl=(Vce * Ih+Rw * Ih.sup.2) * D=f(Vce*Ih*D) 
therefore D=(Wa+Wl)/Vi * Ih=(Wa+f(Vce,Ih,D))/Vi * Ih 
The result of experimentally obtained actual heater power consumption Wa 
relative to the total power consumption is shown in FIG. 11, which is a 
graph in which said actual heater power consumption Wh is shown on the 
vertical axis and total consumed power is shown on the horizontal axis. 
The lost power Wl=Wa-Wh=f(Wa)=f(Vi * Ih * Dh) 
Next, put Wa=Wh-Wl=Wh, and we get Pa=Vi * Ih * Dh-f(Vi,Ih,Dh); 
and then Dh=(Wa+Wl)/Vi * Ih=(Wa=f(Vi,Ih,Wh)/Vi * Ih 
Here, because the Dh term appears on both sides, the previously computed 
value of Dh may be used on the right hand side, and we may take the 
equation as approximately valid. 
Therefore the power loss Wl=f(Wa'), where Wa' is the product of Vi and Ih 
(for this iteration of the FIG. 5 subroutine) and Dh (for the last 
iteration of the FIG. 5 subroutine). 
Thus, by getting Wl as a function of Wa, it merely has to be added to Wa to 
determine the heater supply power. This can thus be done relatively 
simply, without any involved calculation of Wl. 
It should be noted that, in this duty ratio control, the cycle period is 
typically of the order of some tens of milliseconds, and so an 
analog/digital converter or A/D converter such as a serial/sequential 
comparator, which can perform an A/D conversion in microseconds or tens of 
microseconds, with a high speed multiplexer, may be used. If on the other 
hand an A/D converter requiring some milliseconds for A/D conversion is 
used, then by synchronizing the multiplexer channel specification to the 
heater cycle, the timing for reading the voltge applied across the oxygen 
sensor heater can always be matched with energization of said heater. 
Thus, according to the method and system of the present invention, because 
the detection of the voltage supplied to the heater 28 is synchronized 
with the energization of said heater 28, the power supplied to the heater 
28 is controlled more accurately, and proper timing of the detection of 
the voltage across this heater 28 is provided. Thus, the quick variation 
inherent in the pulse signal nature of the voltage signal supplied to the 
heater element 28 is taken account of. Also, the power supplied to the 
heater 28 is controlled more accurately, in a simple and economical way 
without the performance of complicated calculations and without the 
provision of a complicated circuit. Thus, allowance is made for the losses 
due to the internal resistance of the battery 17 and in the wiring harness 
of the vehicle and so on. The allowance for these losses is practicable, 
even if they are not proportional or simply related to heater power. And 
these losses may be determined experimentally beforehand, so that the 
necessary corrections may be made by look up in a simple manner by the use 
of a simple and economical device. 
Although the present invention has been shown and described with reference 
to the preferred embodiment thereof, and in terms of the illustrative 
drawings, it should not be considered as limited thereby. Various possible 
modifications, omissions, and alterations could be conceived of by one 
skilled in the art to the form and the content of any particular 
embodiment, without departing from the scope of the present invention. For 
example, although in the shown preferred embodiment the parameters 
according to which the fuel injection amount for the engine, and the 
amount of heater power provided for the oxygen sensor element heater, were 
engine intake manifold pressure and engine revolution speed, the present 
invention is not limited to this choice of parameters, and for example 
engine intake air flow and engine revolution speed could be utilized 
instead; other variations, such as throttle opening, are also possible for 
the chosen parameters. Therefore it is desired that the scope of the 
present invention, and of the protection sought to be granted by Letters 
Patent, should be defined not by any of the perhaps purely fortuitous 
details of the shown preferred embodiment, or of the drawings, but solely 
by the scope of the appended claims, which follow.