Combustion state detector apparatus for an internal combustion engine

A combustion state detector apparatus for an internal combustion engine has a current detection device for detecting a current flowing between the ignition plug and a ground, and an ignition driving device for driving the ignition plug on the basis of an ignition instruction signal from an electronic control device of the internal combustion engine. An independent unit including a current detection device and an ignition driving device, is provided for each cylinder of the internal combustion engine. With respect to each cylinder, the apparatus determines whether a misfire has occurred on the basis of the current flowing between the ignition plug and the ground at the time of combustion. The apparatus is also capable of detecting abnormalities and/or wire breakages in a signal system provided for signal transmission between each independent unit and the electronic control device of the internal combustion engine.

INCORPORATION BY REFERENCE 
The disclosure of Japanese Patent Application No. HEI 9-168786 filed on 
Jun. 25, 1997 including the specification, drawings and abstract is 
incorporated herein by reference in its entirety. 
FIELD OF THE INVENTION 
The present invention relates to a combustion state detector apparatus for 
an internal combustion engine that detects a combustion state in a 
combustion chamber of the internal combustion engine on the basis of a 
current that flows between the ground and an ignition plug disposed in the 
combustion chamber. 
BACKGROUND OF THE INVENTION 
Japanese Patent Application Laid-Open No. HEI 4-148074 and 4-148076 
describes an ignition coil, an ion current detector unit and a switching 
element integrated into a single unit for all the cylinders of an internal 
combustion engine so as to achieve a compact construction and reduce the 
number of component parts required, thereby improving reliability. 
Japanese Patent Application Laid-Open No. 4-148076 describes a detecting 
device and a switching device integrated into a single unit for each 
cylinder of an internal combustion engine so as to achieve a compact 
construction and reduce the number of component parts required, thereby 
improving reliability. 
In the detection of a misfire in an internal combustion engine, a typical 
diagnostic system outputs a misfire diagnostic signal and turns on a 
misfire diagnostic lamp. However, since the misfire diagnostic lamp can be 
turned on due to various factors, it is difficult to identify the cause of 
a misfire diagnosis even when the diagnostic lamp is tuned on. For 
example, when the misfire diagnostic lamp is turned on due to a connector 
contact failure, the cause is difficult to identify if the connector 
contact failure is one that has a low reproducibility. In the technology 
described in Japanese Patent Application Laid-Open No. HEI 4-148074, since 
various components are integrated into a single misfire detection unit for 
all the cylinders, it is difficult to determine which cylinder is 
experiencing the misfire. Therefore, the entire misfire detection unit for 
all the cylinders needs to be replaced. Moreover, this technology is 
unable to detect a wire breakage or a connector contact failure. The 
technology described in Japanese Patent Application Laid-Open No. HEI 
4-148076 is able to check component parts separately for each cylinder 
when a misfire is detected, but is not able to detect a wire breakage or a 
connector contact failure. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
combustion state detector apparatus for an internal combustion engine 
capable of detecting a misfire in each cylinder of the internal combustion 
engine and of identifying a detection factor such as a wire breakage or a 
connector contact failure, in addition to a misfire. 
According to the invention, there is provided a combustion state detector 
apparatus for an internal combustion engine, including the engine 
including a plurality of combustion chambers, comprises a central control 
unit, a plurality of independent satellite control units, and each of the 
satellite control units corresponds to a respective one of the combustion 
chambers and is coupled to the central control unit. Each satellite 
control unit includes an ignition plug disposed in the respective one of 
the combustion chambers, a current detection means for detecting a current 
flowing between the respective ignition plug and a ground and transmitting 
signals to the central control unit, and ignition driving means for 
driving the respective ignition plug. The central control unit detects a 
combustion state of the engine on the basis of the current detection 
signals received from the satellite control units and outputs ignition 
instruction signals to the ignition driving means of each of the satellite 
control units to control the driving of the ignition plugs. With this 
construction, the sending and the receiving of signals between the central 
control device and the current detection device can be performed 
separately for each cylinder of the internal combustion engine, so that it 
becomes easy to determine a misfire and other failure factors and identify 
a cylinder with the misfire or failure. 
In the combustion state detector apparatus according to the invention, the 
current detection device may detect a voltage signal corresponding to an 
ion current occurring when the ignition plug is driven on the basis of the 
ignition instruction signal from the central control device. Furthermore, 
the current detection device may include a signal conversion device for 
converting the detected voltage signal corresponding to the ion current, 
into a current signal. Therefore, it becomes possible to input into the 
central control device a signal corresponding to an ion current occurring 
when the ignition plug is driven. 
Furthermore, the combustion state detector apparatus according to the 
invention may further include a construction wherein the signal conversion 
device has an output offset device for causing an output from the signal 
conversion device to the central control device to be within a 
predetermined range, and wherein the central control device monitors an 
output signal from the signal conversion device and determines whether the 
output signal is within the predetermined range. Therefore, if the output 
signal is not within the predetermined range, the apparatus determines 
that a failure has occurred, for example, a breakage of a wire connected 
to the electronic control device, a connector contact failure or the like. 
Further, the combustion state detector apparatus according to the invention 
may further include a construction wherein, with respect to each cylinder, 
if a first value of the output signal at a first time and a second value 
of the output signal at a second time after the first time from a 
respective one of the satellite control means are within second 
predetermined range, respectively and wherein, when the first and second 
values are both within the respective second range and a difference 
between a third value of the output signal at a third time after the 
second time and the second value is less than a predetermined amount, the 
central control means determines that a misfire has occurred when the 
first value and the second value are within a third predetermined range 
and, when the first and second values are both within the respective 
second range and the difference between the third value and the second 
value is less than the predetermined amount, the central control means 
determines that a malfunction other than a misfire has occurred when the 
first value and the second value are within a fourth predetermined range. 
Further, the central control device may output a diagnostic signal in 
accordance with a result of the determination made by the electronic 
control device. Therefore, it becomes possible to determine whether a 
misfire or a failure has occurred with respect to each cylinder. In 
addition, the record of misfires and failures can be stored in a 
diagnostic apparatus. 
Further, in the combustion state detector apparatus according to the 
invention, the central control device may stop fuel supply to the cylinder 
that has the misfire or the unit failure, when the electronic control 
device determines that a misfire has occurred or that a unit failure has 
occurred. Therefore, it becomes possible to prevent, for example, an 
inflow of unburned gas to a catalytic device provided in an exhaust 
passage of the internal combustion engine, and therefore prevent 
overheating of the catalytic device, whereby the catalytic device can be 
protected and emission degradation can be prevented.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
A preferred embodiment of the present invention will be described in detail 
hereinafter with reference to the accompanying drawings. FIG. 1 is a block 
diagram illustrating the construction of a combustion state detector 
apparatus for an internal combustion engine according to an embodiment of 
the invention. According to this embodiment, an internal combustion engine 
has four cylinders #1-#4. The ignition coil/igniter units for the 
cylinders #1-#4 have the same construction. Therefore, the following 
description will be made mainly in conjunction with an ignition 
coil/igniter unit 1 for the cylinder #1. 
Referring to FIG. 1, an ignition coil 10 has a primary winding 10a and a 
secondary winding 10b. An ignition plug 12 is disposed in a combustion 
chamber of the internal combustion engine (not shown). The primary winding 
10a of the ignition coil 10 is connected to a switching element 11. When 
the gate of the switching element 11 receives an ignition instruction 
signal IGt1 from an electronic control unit (hereinafter, referred to 
simply as "ECU") 30 of the internal combustion engine, via an external 
connection terminal 1a, the switching element 11 turns on so that a 
primary current I1 from a terminal +B (external connection terminal) 
connected to a battery power source (not shown) flows through the primary 
winding 10a of the ignition coil 10. The ECU 30 is provided as an 
electronic control device according to this embodiment and will be 
described below. 
A current passage on the secondary coil 10b side of the ignition coil 10 
though which a secondary current I2 circulates is formed by the ignition 
plug 12, the secondary winding 10b of the ignition coil 10, a Zener diode 
13 and another Zener diode 14. The Zener diode 14 is connected in a 
forward direction relative to the direction of the flow of the secondary 
current I2 (secondary circulating current). The Zener diode 13 is 
connected to charge a capacitor 15 connected as an ion current detecting 
power source in parallel to the Zener diode 13. A resistor 16 is connected 
in parallel to the Zener diode 14. 
An ion current is detected as follows. An ion current IION flows from the 
capacitor 15 through the secondary winding 10b of the ignition coil 10 to 
the ignition plug 12. Further, the ion current IION flows from the 
inversion (-) terminal of an operational amplifier 20 through an ion 
current detecting resistor 17. The ion current IION is detected by the ion 
current detecting resistor 17. A resistor 21 connected between the 
inversion (-) terminal and an output terminal of the operational amplifier 
20 is an amplifying resistor for setting a gain of the operational 
amplifier 20. 
The voltage signal outputted from the operational amplifier 20 on the basis 
of the ion current IION is converted into a current signal by a V/I 
converting (voltage-to-current conversion) circuit 22. The output side of 
the V/I converting circuit 22 is connected to an external connection 
terminal 1b for connection to the ECU 30. The output side of the V/I 
converting circuit 22 is also connected to a negative offset current 
source 23 for causing an offset current I0 (described below) to constantly 
flow in order to detect a line or wire breakage. The other terminal of the 
offset current source 23 is connected to a GND terminal (external 
connection terminal) for grounding. Connected also to the GND terminal are 
the emitter of the switching element 11, the negative terminal of the 
Zener diode 14, one of the terminals of the resistor 16, and the 
non-inversion (+) terminal of the operational amplifier 20. 
The external connection terminal 1b of the ignition coil/igniter unit 1 of 
the cylinder #1 is connected to an external connection terminal 30a of the 
ECU 30. The external connection terminal 30b is connected to a fixed power 
supply (5 V) by a pull-up resistor 31. Due to the fixed power supply (5 V) 
through the external connection terminal 30a, a current i flows through 
the external connection terminal 1b of the ignition coil/igniter unit 1 of 
the cylinder #1. The voltage signal v based on the current i is passed 
through a noise mask circuit 32, and then its peak value is held at 
set/reset timing by a peak hold circuit 33. The signal is then 
A/D-converted (analog-to-digital converted) by an A/D converter 34. The 
A/D-converted voltage signal is inputted to a microcomputer 36. The 
voltage signal v inputted to the ECU 30 is also processed by a low pass 
filter (LPF) 35 connected in parallel to the noise mask circuit 32 and the 
peak hold circuit 33, whereby high frequency components are removed from 
the voltage signal v. The filtered voltage signal v is converted by the 
A/D converter 34, and the converted signal is inputted to the 
microcomputer 36. 
The microcomputer 36 is constructed as a logic operating circuit formed by 
a CPU (central processing unit) that executes various operations, a ROM 
storing various control programs, a RAM and a backup RAM for storing 
various data, input/output circuits, bus lines connecting therebetween, 
and the like. 
FIG. 2 is a timing chart indicating the transition of various output 
signals in the combustion state detector apparatus of the embodiment. 
The ignition instruction signal IGt1 outputted by the ECU 30 to the 
ignition coil/igniter unit 1 of the cylinder #1 is turned to an ON level 
at a time point t1 and to an OFF level at a time point t3, that is, an 
ignition timing (see the chart (a) in FIG. 2). The transition of output 
signals outputted when the combustion state is normal in the internal 
combustion engine will be described with reference to the charts (a), (b) 
and (c) of FIG. 2. When the combustion state in the internal combustion 
engine is normal, the current i flows through the external connection 
terminal 1b side of the ignition coil/igniter unit 1 of the cylinder #1 
(hereinafter, the current i will sometimes be termed "ION current 
signal"), based on the current signal outputted from the operational 
amplifier 20 via the V/I converting circuit 22 and the offset current I0 
outputted from the offset current source 23. The ion current signal, 
superimposed with the constant offset current I0 (mA), is superimposed 
with an ignition-on noise signal as indicated in the chart (b) in FIG. 2 
(time t1-t2) immediately after a primary current I1 starts to flow through 
the primary winding 10a of the ignition coil 10 at the ON timing (time 
point t1) of the ignition instruction signal IGt1 indicated in the chart 
(a) of FIG. 2. During the time period t1-t2, the ion current signal 
becomes a maximum current Imax (mA). At the OFF timing (time point t3) of 
the ignition instruction signal IGt1, a secondary current I2 starts to 
flow through the secondary winding 10b of the ignition coil 10, and 
discontinues (at time point t4) after the ignition plug 12 has been 
energized for ignition. After the discontinuation of the secondary current 
I2, magnetism lingers in the core of the ignition coil 10. A lingering 
magnetism noise signal based on the effect of the lingering magnetism is 
superimposed on the ion current signal (time t4-t5). The ion current 
signal is further superimposed with an ion current signal based on actual 
ignition (time t5-t6). During the time period (t4-t6), the ion current 
signal becomes the maximum current Imax (mA). 
Besides the ion current signal occurring while the combustion state of the 
internal combustion engine is normal, a peak hold circuit output processed 
through the noise mask circuit 32 and the peak hold circuit 33 in the ECU 
30 is set at a timing which is 0.5 ms after ignition, that is, 0.5 ms 
after the time point t3 indicated in FIG. 2, and is reset at a timing 
which corresponds to 70 CA (crank angle) after the top dead center (AFTD), 
that is, which is slightly after the time point t6 at which the transition 
or change of the ion current signal comes to an end. A peak hold circuit 
output is read at the reset timing (see (c) of FIG. 2) in the peak hold 
circuit 33. The read peak hold circuit output is A/D-converted by the A/D 
converter 34. The thus-obtained value is set as A/D3 (indicated by an 
arrow in FIG. 2). Presetting is made such that the peak hold circuit 
output corresponding to the maximum current Imax (mA) of the ion current 
signal becomes 4 V and so that the peak hold circuit output corresponding 
to the offset current I0 (mA) becomes 1 V. 
The transition of the output signals when a misfire occurs in combustion of 
the internal combustion engine will be described with reference to the 
charts (a) and (d) of FIG. 2. If a misfire occurs, the ion current signal 
(superimposed with the offset current I0 (mA)) is superposed only with an 
ignition-on noise signal as indicated in the chart (d) (time t1-t2) 
although the ignition instruction signal IGt1 outputted from the ECU 30 to 
the ignition coil/igniter unit 1 of the cylinder #1 of the internal 
combustion engine is turned to the ON and OFF levels as indicated in the 
chart (a). During the time period t1-t2, the ion current signal becomes 
the maximum current Imax (mA). 
The transition of the output signals when a wire breaks will be described 
with reference to the charts (a) and (e) of FIG. 2. When a breakage occurs 
in the wire connecting to the +B terminal, the GND terminal or any other 
external connection terminals of the ignition coil/igniter unit 1 of the 
cylinder #1, the ion current signal remains at 0 mA without the offset 
current I0 (mA) or the ignition-on noise signal being superimposed (see 
the chart (e)) although the ignition instruction signal IGt1 outputted 
from the ECU 30 to the ignition coil/igniter unit 1 of the cylinder #1 of 
the internal combustion engine is turned to the ON and OFF levels as 
indicated in the chart (a). 
The procedure of the diagnosis control executed by the CPU of the 
microcomputer 36 in the ECU 30 employed in the combustion state detector 
apparatus according to this embodiment of the invention will be described 
with reference to the flowchart of FIG. 3. FIGS. 2 and 4 will be also 
referred to in the following description. FIG. 4 shows a map indicating 
the relationship between the A/D converted values, A/D1 and A/D2, and the 
determination regions, wherein region A is a range of voltage that can be 
taken by A/D1 and A/D2 when the ignition coil/igniter units 1-4 of the 
cylinders #1-#4 are normal, and region B is a range of voltage that can be 
taken by A/D1 and A/D2 at the time of a failure of the detection system, a 
breakage of the wire connecting to the +B terminal, or the GND terminal or 
the external connection terminals for the ion current signal, or at the 
time of a circuit failure in the ignition coil/igniter units 1-4 of the 
cylinders #1-#4, and region D is a range of voltage that can be taken by 
A/D1 and A/D2 at the time of a failure in the input system, or at the time 
of a breakage of a wire connecting to the external connection terminal la 
to which the ignition instruction signal IGt1 is inputted, or at the time 
of a failure in the ECU 30, and range E is a range of voltage that can be 
taken by A/D1 and A/D2 at the time of a normal state or at the time of a 
misfire. The diagnosis control routine illustrated in FIG. 3 is repeatedly 
executed by the CPU every time the ECU 30 outputs the ignition instruction 
signals IGt1-IGt4 to the ignition coil/igniter units 1-4 of the cylinders 
#1-#4. 
Referring to the flowchart of FIG. 3, in step S101, the CPU determines 
whether A/D1 and A/D2 are in a region other than the region B. A/D1 and 
A/D2 are A/D-converted values provided by the processing of the ion 
current signal by the LPF 35 and the A/D-conversion of the signal by the 
AID converter 34. A/D1 is read at a timing which is 0.2 ms after the 
energization of the primary winding 10a of the ignition coil 10 is started 
by the turning of the ignition instruction signal IGt to the ON level, 
that is, 0.2 ms after the time point t1 indicated in FIG. 2. A/D2 is read 
at a timing which is 1.0 ms after the energization of the primary winding 
10a of the ignition coil 10 is started by the turning of the ignition 
instruction signal IGt to the ON level, that is, 1.0 ms after the time 
point t1 indicated in FIG. 2. In FIG. 2, A/D1 and A/D2 are indicated by 
arrows. If the determination condition in step S101 are established, that 
is, if it is determined that A/D1 and A/D2 are within a region other than 
the range B, the operation proceeds to step S102, where the CPU calculates 
a magnitude C (mA) of the ion current signal by subtracting A/D2 from A/D3 
(see FIG. 2). 
Subsequently in step S103, the CPU determines whether the value C 
calculated in step S102 is less than a preset value C0, that is, whether a 
misfire has occurred. If a misfire has occurred, the ion current signal 
becomes equal to the offset current I0 (mA) between the set timing and the 
reset timing of the peak hold circuit 33 as indicated in the chart (d) of 
FIG. 2, that is, C is less than C0. The affirmative determination in step 
S103 is followed by step S104. In step S104, the CPU increments a misfire 
counter Mi in order to determine in the subsequent step whether a misfire 
has occurred a predetermined number of times. Subsequently in step S105, 
the CPU determines whether the count value of the misfire counter Mi is 
greater than a predetermined count value k. If the count value of the 
misfire counter Mi is greater than the predetermined count value k (YES in 
step S105), the operation proceeds to step S106, where the CPU determines 
whether A/D1 and A/D2 are in the region E in the map shown in FIG. 4, that 
is, a second predetermined region which is defined around a center point 
at which A/D1 is the voltage value corresponding to the maximum current 
Imax of the ion current signal and A/D2 is the voltage value corresponding 
to the offset current I0 of the ion current signal. If A/D1 and A/D2 are 
within the region E (YES in step S106), the operation proceeds to step 
S107. Since the affirmative determination in step S106 indicates that 
combustion has not occurred but a misfire has occurred, the CPU outputs, 
in step S107, a misfire diagnostic signal to promote a diagnosis, so that 
an operation of turning on a diagnostic lamp in the diagnostic apparatus 
(not shown) is performed. 
Conversely, if A/D1 and A/D2 are not in the region E (NO in step S106), the 
operation proceeds to step S108, where the CPU determines whether A/D1 and 
A/D2 are in the region D, that is, a third predetermined region which is 
defined around a center point at which A/D1) and A/D2 are the voltage 
values corresponding to the offset current I0 of the ion current signal. 
If A/D1 and A/D2 are in the region D (YES in step S108), the operation 
proceeds to step S109. Since the affirmative determination in step S108 
indicates that a failure has occurred in the input system, the CPU 
outputs, in step S109, an input system failure diagnostic signal, so that 
an operation of lighting a diagnostic lamp in the diagnostic apparatus 
(not shown) is performed. 
If it is determined in step S101 that A/D1 and A/D2 are in the region B (NO 
in step S101), or if it is determined in step S108 that A/D1 and A/D2 are 
not in the region D (NO in step S108), that is, if the determinations in 
step S103 and step S105 are affirmative and A/D1 and A/D2 are in the 
region A, that is, a fourth predetermined region that is other than the 
regions E and D, then the operation proceeds to step S110. Since the 
result of determination made before step S110 indicates that an output 
characteristic abnormality, including a wire breakage and the like, has 
occurred in at least one of the ignition coil/igniter units 1-4 of the 
cylinders #1-#4, the CPU outputs, in step S110, a unit failure diagnostic 
signal, so that an operation of lighting a diagnostic lamp in the 
diagnostic apparatus (not shown) is performed. The operation in step S107, 
S109 or S110 is followed by step S111, where the CPU performs an operation 
of cutting fuel to the cylinder that is experiencing a misfire or a 
failure. This routine is then ended. If it is determined in step S103 that 
C is equal to or greater than CO (NO in step S103), or if it is determined 
in step S105 that the count value of the misfire counter Mi is equal to or 
less than the predetermined count value k (NO in step S105), it is 
regarded that the ignition state is normal as indicated in the chart (b) 
of FIG. 2, and then the routine is ended. 
As is apparent from the foregoing description, the combustion state 
detector apparatus according to this embodiment includes a current 
detector device formed by the capacitor 15, the ion current detecting 
resistor 17, the operational amplifier 20 and the like for detecting a 
current that flows between the ground and the ignition plug 12 disposed in 
a combustion chamber of the internal combustion engine, a signal converter 
device formed by the V/I converting circuit 22 for converting the current 
detected by the current detector device into an ion current signal that 
can be processed by the ECU 30, and an ignition driving device formed by 
the switching element 11 and the like for driving the ignition coil 10 on 
the basis of the ignition instruction signal IGt from the ECU 30. The 
current detector device, including the signal converter device, and the 
ignition driving device, including the ignition coil 10, are integrated 
into units separately for the individual cylinders #1-#4 of the internal 
combustion engine so that the integrated units are independent of one 
another. 
Therefore, the ignition driving device formed by the switching element 11 
and the like drives the ignition coil 10 on the basis of the ignition 
instruction signal IGt from the ECU 30, and the current detector device 
formed by the capacitor 15, the ion current detecting resistor 17, the 
operational amplifier 20 and the like detects a current that flows between 
the ignition plug 12 and the ground. The signal converter device formed by 
the V/I converting circuit 22 converts the detected current into an ion 
current signal that is to be inputted to the ECU 30. The current detector 
device, the signal converter device, and the ignition driving device 
including the ignition coil 10 are integrated into units corresponding on 
a one-to-one basis to the individual cylinders #1-#4 of the internal 
combustion engine. The units for the cylinders #1-#4 are independent of 
each other. With this construction, signals are transmitted and received 
between the ECU 30 and the units separately for each of the cylinders 
#1-#4, so that it is easy to identify a cylinder that is experiencing a 
misfire or a failure. Consequently, this embodiment makes it possible to 
design a system wherein a failure can easily be determined separately for 
each cylinder and wherein component parts or the like of the unit for a 
cylinder with a failure can be replaced without disturbing the units for 
normal cylinders. In the combustion state detector apparatus for an 
internal combustion engine according to this embodiment, the signal 
converter device formed by the V/I converting circuit 22 includes the 
offset current source 23, that is, an output offset circuit for causing 
the outputs to the ECU 30 to be within a predetermined range. The ECU 30 
monitors the ion current signal outputted from the signal converter 
device, and performs a failure diagnosis regarding a failure in a 
signal-conducting wire, separately from a misfire diagnosis, and then 
outputs a diagnostic signal separately for each of the cylinders #1-#4 of 
the internal combustion engine. That is, since the signal converter 
device, including the offset current source 23 and the V/I converting 
circuit 22, causes the magnitude of the output to the ECU 30 to be within 
a predetermined range if the signal converter device is normal, the ECU 30 
can perform the failure diagnosis regarding a failure in the 
signal-conducting wires or the signal converter device, separately from 
the misfire diagnosis, by monitoring the ion current signal from the 
signal converter device and determining whether magnitude of the ion 
current signal is within the predetermined range or not. Therefore, the 
ECU 30 is able to output a failure diagnostic signal regarding the 
signal-conducting wires or the signal converter device, separately from a 
misfire diagnostic signal, for each of the cylinders #1-#4 of the internal 
combustion engine. Consequently, it becomes possible to identify a 
cylinder that is experiencing a failure and identify a failure factor. 
In the combustion state detector apparatus according to this embodiment, 
the ECU 30 performs a diagnostic determination separately for each 
cylinder as follows. If a change in the ion current signal from the signal 
converter device formed by the V/I converting circuit 22 and the like is 
in the first predetermined range, the ECU 30 determines that a misfire has 
occurred when the output signal change is in the second predetermined 
range, and determines that an output signal input abnormality has occurred 
when the output signal change is in the third predetermined range, and 
determines that a failure in the unit has occurred when the output signal 
change is in the fourth predetermined range. In short, the combustion 
state detector apparatus of this embodiment performs a determination 
regarding the occurrence of a misfire, a failure in the input system, and 
a failure in the unit, separately for each cylinder, thereby making it 
possible to identify a failure factor and the cylinder experiencing a 
failure. The combustion state detector apparatus outputs a diagnostic 
signal in accordance with the result of this determination. 
According to this embodiment, if it is determined that a misfire has 
occurred in a cylinder of the internal combustion engine or a unit failure 
has occurred in a unit formed by the current detector device, the signal 
converter device and the ignition driving device, the combustion state 
detector apparatus stops the fuel supply to the cylinder experiencing the 
misfire or the unit failure. More specifically, since the ECU 30 sends 
signals to and receives signals from the units separately for the 
individual cylinders, it is possible to identify a cylinder that is 
experiencing a misfire or a unit failure and to stop the fuel supply to 
that cylinder. Consequently, it becomes possible to prevent an inflow of 
unburned gas to a catalytic device provided in an exhaust passage of the 
internal combustion engine, and therefore prevent overheating of the 
catalytic device, whereby the catalytic device can be protected and the 
emission degradation can be prevented. 
While the present invention has been described with reference to what is 
presently considered to be a preferred embodiment thereof, it is to be 
understood that the invention is not limited to the disclosed embodiment 
or constructions. To the contrary, the invention is intended to cover 
various modifications and equivalent arrangements included within the 
spirit and scope of the invention.