Combustion state detecting apparatus for internal combustion engine

A combustion state detecting apparatus for an internal combustion engine is imparted with facility for determining discriminatively the causes for non-occurrence of combustion. The apparatus includes a spark plug (4) for generating a spark discharge upon application of a high voltage generated by an ignition coil (1) in response to an ignition signal (X1) to thereby fire an air-fuel mixture within a cylinder of the internal combustion engine, an ion current detecting means (6A) for detecting as ion current detection signals (X2a; X2) an ion current corresponding to an amount of ions produced within the cylinder immediately after combustion of the air-fuel mixture, a signal detecting means (7A; 7B; 9) for comparing the ion current detection signal (X2a) outputted from the ion current detecting means (6A) with a first reference voltage (Vth1) to thereby output a first decision signal (X3) while comparing the ion current detection signal (X2) with a second reference voltage (Vth2) to thereby output a second decision signal (X4) while invalidating output of the second decision signal (X4) during a predetermine time period from a time point at which the comparison of the ion current detection signal (X2) with the second reference voltage (Vth2) is started, and an estimating logic unit (9, 10; 112) for estimating a cause for nongeneration of a combustion signal on the basis of output statuses of the first decision signal (X3) and the second decision signal (X4).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a combustion state detecting apparatus for
 an internal combustion engine, which apparatus is designed for detecting
 the combustion state of an air-fuel mixture within a cylinder or cylinders
 of the engine by detecting an ion current making appearance upon
 combustion of the air-fuel mixture. More particularly, the invention is
 concerned with a combustion state detecting apparatus for the internal
 combustion engine, which apparatus is imparted with a function or facility
 for estimating the causes for non-generation or disappearance of a
 combustion signal based on the ion current.
 2. Description of Related Art
 For having better understanding of the concept underlying the present
 invention, background techniques thereof will first be reviewed in some
 detail.
 FIG. 5A is a circuit diagram showing schematically an arrangement of a
 hitherto known or conventional combustion state detecting apparatus for an
 internal combustion engine (hereinafter also referred to simply as the
 engine), which apparatus is designed for detecting the combustion state
 within an engine cylinder or cylinders on the basis of an ion current
 produced upon combustion of air-fuel mixture. Referring to the figure,
 reference numeral 1 denotes an ignition coil which includes a primary
 winding 11 having a high-voltage end connected to a positive electrode of
 a power supply source 3 such as an onboard battery, while the low-voltage
 end of the primary winding 11 is connected to a collector electrode of a
 power transistor 2 which serves as a switching element for turning on/off
 a primary current flowing through the primary winding 11. The power
 transistor has an emitter electrode connected to the ground potential. On
 the other hand, a secondary winding 12 of the ignition coil 1 has a
 high-voltage end connected to an electrode of a spark plug 4 disposed
 within a cylinder of the engine while a low-voltage end of the secondary
 winding 12 is connected to a bias circuit 5 which is designed to apply a
 bias voltage of positive polarity to the spark plug 4 through a wiring
 conductor.
 Further, referring to FIG. 5A, reference numeral 6 denotes an ion current
 detecting circuit which is designed for detecting by way of the bias
 circuit 5 an ion current making appearance upon combustion of the air-fuel
 mixture and flowing through an inter-electrode gap of the spark plug 4 to
 thereby convert the ion current into a voltage signal. More specifically,
 the ion current detecting circuit 6 is implemented in the form of a
 conventional current mirror circuit constituted by a series connection of
 transistors Q1a and Q1b and a transistor Q2 which are connected in
 parallel between a positive voltage source (i.e., voltage source of plus
 or positive polarity) +V and the ground potential. A resistor R1 is
 inserted between the ground potential and the collector of the transistor
 Q2. A current analogous to the ion current flows through the resistor R1
 to undergo a voltage conversion, whereby an ion current detection voltage
 signal (hereinafter referred to as the ion current detection signal) X2 is
 produced.
 The ion current detection signal X2 is supplied to a decision circuit 7
 which is designed for shaping the ion current detection signal X2 detected
 by the ion current detecting circuit 6 into a pulse signal, which then
 undergoes a processing for deciding occurrence of combustion of the
 air-fuel mixture, as a result of which a pulse-like decision signal X3 is
 outputted from the decision circuit 7 to be supplied to an ECU (Electronic
 Control Unit) 10.
 More specifically, for shaping the ion current detection signal X2
 resulting from the voltage conversion mentioned above into a pulse signal,
 the decision circuit 7 includes a comparator circuit composed of a
 comparator CP1 for comparing the level of the ion current detection signal
 X2 with a reference voltage Vth4, an integrating circuit composed of a
 resistor R2 and a capacitor C2 for eliminating noise components N1 and N2
 superposed on the pulse-like ion current outputted from the comparator
 CP1, and a delay circuit composed of a comparator CP2. Parenthetically, it
 should be mentioned that a pull-up resistor R3 connected to the output
 terminal of the comparator CP2 serves for pulling up the output voltage
 level thereof.
 Next, referring to a signal waveform diagram shown in FIG. 5B, description
 will be made of operations of the conventional combustion state detecting
 apparatus in conjunction with normal combustion, a first type of
 non-combustion event (e.g. due to absence of fuel supply) and a second
 type of non-combustion event (e.g. due to failure of generation of high
 voltage for firing).
 1. Normal combustion
 Upon rising of the ignition signal X1 applied to the base of the power
 transistor 2 under the control of the ECU 10, the current flowing through
 the primary winding 11 of the ignition coil 1 is interrupted, as a result
 of which a high voltage E is induced in the secondary winding 12 of the
 ignition coil 1, whereby a spark discharge is caused to take place within
 the inter-electrode gap of the spark plug 4.
 The ion current generated due to the combustion of the air-fuel mixture
 within the engine cylinder in which the spark discharge has taken place is
 inputted to the ion current detecting circuit 6 by way of the bias circuit
 5 to undergo the current-to-voltage conversion in the current mirror
 circuit, whereby the ion current detection signal X2 is outputted from the
 ion current detecting circuit 6.
 At this juncture, it should be mentioned that the ion current detection
 signal X2 outputted from the circuit 6 contains in addition to the
 intrinsic ion current component due to the combustion of the air-fuel
 mixture a noise component N1 produced upon rising of the ignition signal
 and the noise component N2 making appearance upon termination or
 extinction of the spark discharge. Accordingly, these noise components N1
 and N2 have to be eliminated before outputting the decision signal X3 for
 deciding the combustion event on the basis of the ion current.
 Thus, before eliminating the noise components N1 and N2 through the medium
 of the delay circuit, the ion current detection signal X2 is inputted to
 the comparator CP1 constituting the comparator circuit for comparing the
 levels of the signal components of the ion current detection signal X2
 with the reference voltage Vth4. Since each of the noise components N1 and
 N2 is intrinsically in the form of a spike-like signal, these components
 are shaped into pulses each having an extremely short time duration.
 Thus, even when the pulse-like noise components N1 and N2 are inputted to
 the CR integrating circuit constituting a part of the delay circuit to
 thereby raise the charge voltage of the capacitor C2 up to or beyond the
 reference voltage Vth5 preset at the comparator CP2, the pulse-like noise
 components Ni and N2 will assume low level before the charge voltage of
 the capacitor C2 reaches the reference voltage Vth5 because the time
 duration of the noise components is short of the CR time constant.
 Consequently, no decision signal will be outputted from the comparator CP2
 in response to the noise components N1 and N2.
 On the other hand, when the ion current component undergone the
 pulse-shaping operation is inputted to the integrating circuit, the
 capacitor C2 is charged to a level equal to or exceeding the reference
 voltage Vth5 after lapse of a predetermined time, because the time
 duration of the pulse-like ion current component is greater than the CR
 time constant, as a result of which the output of the comparator CP2
 becomes high, whereby the decision signal X3 indicating that the
 combustion has taken place normally, i.e., normal combustion, is
 outputted.
 At this juncture, the time taken for the charge voltage of the capacitor C2
 to exceed the reference voltage Vth5 will be defined as the mask period
 only for convenience of description. Then, the noise components making
 appearance upon rising of the ion current and termination or extinction of
 the spark discharge, respectively, can be eliminated during the mask
 period.
 2. Non-combustion event due to absence of fuel supply
 When the fuel supply is absent, i.e., unless the air-fuel mixture is
 normally charged into the engine cylinder, the ion current due to the
 combustion of the air-fuel mixture can not naturally flow. Of course, upon
 rising of the ignition signal X1 as well as upon extinction of the spark
 discharge occurring within the intra-electrode gap of the spark plug 4,
 the noise components N1 and N2 make appearance, which would be outputted
 as the ion current detection signal X2. However, because the noise
 components N1 and N2 are eliminated by the delay circuit described
 previously in conjunction with the normal combustion, the decision signal
 X3 attributable to these noise components can not be generated.
 3. Non-combustion event due to failure of generation of secondary voltage
 by the ignition coil
 As can easily be appreciated from the foregoing description, unless the
 high-voltage is induced in the secondary winding of the ignition coil 1
 due to e.g. breakage of the primary winding thereof, neither the noise
 component N1 due to application (rising) of the ignition signal X1 nor the
 noise component N2 upon extinction of the spark discharge can be produced.
 Consequently, the decision signal X3 is not outputted.
 As will now be understood from the foregoing, with the conventional
 combustion state detecting apparatus of the structure described above, it
 is certainly possible to detect the combustion events, i.e., combustion
 and non-combustion of the air-fuel mixture within the cylinder(s) of the
 internal combustion engine. However, it is impossible to determine
 discriminatively or identify the cause for occurrence of the
 non-combustion event. In other words, additional detecting means or
 facilities have to be provided for specifying the cause for occurrence of
 the non-combustion event, which will however incur increase of
 manufacturing cost and overhead inclusive of processing time, giving rise
 to problems.
 SUMMARY OF THE INVENTION
 In the light of the state of the art described above, it is an object of
 the present invention to provide a combustion state detecting apparatus
 for an internal combustion engine, which apparatus is capable of
 estimating the causes for nongeneration of the combustion signal.
 In view of the above and other objects which will become apparent as the
 description proceeds, there is provided according to a general aspect of
 the present invention a combustion state detecting apparatus for an
 internal combustion engine, which apparatus includes a spark plug for
 generating a spark discharge upon application of a high voltage generated
 by an ignition coil in response to an ignition signal to thereby fire an
 air-fuel mixture within a cylinder of the internal combustion engine, an
 ion current detecting means for detecting as ion current detection signals
 an ion current corresponding to an amount of ions produced within the
 cylinder immediately after combustion of the air-fuel mixture, a signal
 detecting means designed for comparing the ion current detection signal
 outputted from the ion current detecting means with a first reference
 voltage to thereby output a first decision signal while comparing the ion
 current detection signal with a second reference voltage to thereby output
 a second decision signal while invalidating output of the second decision
 signal during a predetermine time period from a time point at which the
 comparison of the ion current detection signal with the second reference
 voltage is started, and an estimating means for estimating a cause for
 nongeneration of a combustion signal on the basis of output statuses of
 the first decision signal and the second decision signal.
 By virtue of the arrangement of the combustion state detecting apparatus
 for the engine described above, there can be obtained advantageous effect
 that the cause for the non-combustion event can be detected at an earlier
 time.
 In a preferred mode for carrying out the invention, the signal detecting
 means may be comprised of a first detecting unit for detecting noise
 signal components on the basis of result of the comparison of the ion
 current detection signal with the first reference voltage, and a second
 detecting unit for detecting an ion current component due to the
 combustion of the air-fuel mixture on the basis of result of the
 comparison of the ion current detection signal with the second reference
 voltage set lower than the first reference voltage.
 With the arrangement of the combustion state detecting apparatus described
 above, detection of occurrence and non-occurrence of combustion events can
 be realized with enhanced accuracy and reliability while avoiding adverse
 influence of external noises and the like.
 In another preferred mode for carrying out the invention, the estimating
 means may be so designed as to set a predetermined time period extending
 from a time point at which output of the first decision signal is started
 after firing of the air-fuel mixture as a detection period for the first
 decision signal while setting a predetermined period extending from a time
 point at which the detection period for the first decision signal is
 terminated as a detection period for the second decision signal so that
 the first and second decision signals can be detected discriminatively
 from each other.
 By detecting the first and second decision signals distinctively from each
 other, the signals for specifying the cause(s) for non-occurrence of the
 combustion event can be detected stably with high reliability, to another
 advantage.
 In yet another preferred mode for carrying out the invention, the
 estimating means may be so designed as to set a signal detection quiescent
 period over a predetermined time period which succeeds immediately to
 interruption of the ignition signal.
 Owing to the arrangement mentioned above, the signals for specifying the
 cause(s) for non-occurrence of the combustion event can be detected
 stably, to a further advantage.
 In still another preferred mode for carrying out the invention, the
 estimating means may be so designed as to output noise component signals
 only during an output period of the ignition signal.
 With the arrangement mentioned above, setting of the timing for outputting
 the noise component signals can be much facilitated, to yet another
 advantage.
 In a further preferred mode for carrying out the invention, the estimating
 means may be so designed as to limit a detection period for the noise
 component signals to within the output period of the ignition signal.
 With the above-mentioned arrangement, setting of the timing for detecting
 the noise component signals can equally be facilitated, to an advantage.
 In a yet further preferred mode for carrying out the invention, the ion
 current detecting means and the signal detection means may be built in the
 ignition coil.
 With the arrangement mentioned above, exchange or retrofit of relevant
 part(s) or device(s) for disposing of the non-combustion event (i.e.,
 non-occurrence of the combustion event) can be facilitated.
 The above and other objects, features and attendant advantages of the
 present invention will more easily be understood by reading the following
 description of the preferred embodiments thereof taken, only by way of
 example, in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will be described in detail in conjunction with what
 is presently considered as preferred or typical embodiments thereof by
 reference to the drawings. In the following description, like reference
 characters designate like or corresponding parts throughout the several
 views.
 Embodiment 1
 FIG. 1A is a circuit diagram showing schematically a configuration of the
 combustion state detecting apparatus for an internal combustion engine
 according to a first embodiment of the present invention. In the figure,
 like reference symbols as those used in FIG. 5 denote components same as
 or equivalent to those described hereinbefore by reference to FIG. 5.
 Now referring to FIG. 1A, reference character 6A denotes an ion current
 detecting circuit according to the first embodiment of the invention. In
 this ion current detecting circuit 6A, there is additionally provided a
 transistor Q3 having a base electrode connected to that of the transistor
 Q2, wherein the collector of the transistor Q3 is grounded by way of a
 resistor R11, while the emitter of the transistor Q3 is connected to a
 voltage source of plus polarity (+V). As can be seen in the figure, the
 ion current detecting circuit 6A is realized in the form of a current
 mirror circuit. The resistor R11 serves as a current-to-voltage converting
 element for converting the ion current flowing therethrough into an ion
 current detection voltage signal (hereinafter referred to as the ion
 current detection signal) X2a.
 Reference character 7A denotes a first decision circuit which includes a
 comparator CP3 for comparing the level of the ion current detection signal
 X2a with a reference voltage Vthl to thereby shape the ion current
 detection signal X2a into a pulse signal which is outputted as a first
 decision signal X3. At this juncture, it should be mentioned that the
 noise current generated upon ignition operation is ordinarily in the order
 of several hundreds microamperes, and thus the noise current assumes a
 greater value than the ion current making appearance upon combustion of
 the air-fuel mixture within the engine cylinder. Such being the
 circumstances, the reference voltage Vth1 is set to a higher level than
 that of the reference voltage Vth2 which will be described hereinafter in
 consideration of the fact that the first decision circuit 7A which
 includes no mask circuit can output a pulse signal in response to
 insignificant signals such as external noise generated in accompanying the
 ignition or firing operation. Thus, according to the concept of the
 present invention incarnated in the instant embodiment; the reference
 voltage Vth1 for the comparator CP3 is set higher than not only the
 reference voltage Vth2 but also the voltage level of the small signal such
 as the external noise and the like with a view to suppressing the
 erroneous output of the first decision circuit 7A.
 In this conjunction, it should be added that a decision circuit of the
 structure similar to that of the decision circuit 7 of the conventional
 combustion state detecting apparatus (see FIG. 5A) is also employed in the
 combustion state detecting apparatus according to the instant embodiment
 of the invention. This decision circuit will be referred to as the second
 decision circuit and designated by reference character 7B. Further, the
 decision signal outputted from the second decision circuit 7B on the basis
 of the ion current detection signal X2 will be referred to as the second
 decision signal X4 only for convenience of description.
 Turning back to FIG. 1A, reference numeral 9 denotes a logical-OR circuit
 having inputs connected to the outputs of the first decision circuit 7A
 and the second decision circuit 7B, respectively, for logically ORing the
 first decision signal X3 and the second decision signal X4 to thereby
 output an output signal X5 which is then supplied to the ECU 10.
 Incidentally, the first decision circuit 7A of the combustion state
 detecting apparatus according to the instant embodiment of the invention
 is so designed as to output intactly the output of the comparator circuit
 without passing it through the delay circuit. Consequently, the noise
 components N1 and N2 as well as the ion current component shaped into
 pulse signals, respectively, by the comparator CP3 are outputted from the
 first decision circuit 7A.
 Next, referring to a signal waveform diagram shown in FIG. 1B, description
 will be made of operations of the combustion state detecting apparatus
 according to the first embodiment of the invention in conjunction with
 normal combustion, a first type of non-combustion event due to a defect or
 fault of a fuel-supply/combustion system and a second type of
 non-combustion event due to a defect or fault of an ignition control
 system.
 Parenthetically, operation of the second decision circuit 7B is similar to
 that of the decision circuit described hereinbefore in conjunction with
 the conventional combustion state detecting apparatus by reference to FIG.
 5A. Accordingly, repeated description of the second decision circuit 7B
 will be unnecessary.
 1. Normal combustion
 Upon combustion of the air-fuel mixture within the engine cylinder in
 succession to application of the ignition signal X1, only the pulse-shaped
 ion current component is outputted from the second decision circuit 7B as
 the second decision signal X4 with the noise components N1 and N2 having
 been eliminated. The second decision signal X4 is applied to one of the
 input terminals of the OR circuit 9
 On the other hand, outputted from the first decision circuit 7A is the
 first decision signal X3 which is composed of the pulse-shaped ion current
 component and the noise components N1 and N2 also shaped into pulses. The
 first decision signal X3 is then applied to the other input terminal of
 the OR circuit 9. As a result of this, outputted from the OR circuit 9 as
 the output signal X5 is a logical sum signal which is composed of the
 first decision signal X3 supplied from the first decision circuit 7A and
 the second decision signal X4 supplied from the second decision circuit 7B
 in a time-serial sequence. The output signal X5 of the OR circuit 9 is
 then inputted to the ECU 10.
 Upon reception of the output signal X5, the ECU 10 detects the first and
 second decision signals X3 and X4 during a first decision signal detection
 period set in a manner described below to thereby make decision that the
 combustion has taken place normally.
 Referring to FIG. 1B, the time point T1 at which the ignition signal X1 is
 turned off (i.e., trailing or falling edge of the ignition signal X1) is
 defined as the detection start time point for the first decision signal
 X3, while a time point T3 corresponding to lapse of a predetermined time
 period L1 from the output start time point T2 of the first decision signal
 X3 is defined as the detection end time point for the first decision
 signal X3 while defining the time point T3 as the detection start time
 point for the second decision signal X4. Further, the time point
 corresponding to lapse of a predetermined time period L2 from the
 detection end time point T3 is defined as the detection end time point T4
 for the second decision signal X4.
 In this conjunction, it should be added that since there may arise
 possibility of ignition noise ascribable to ignition in the other
 cylinder(s) being superposed immediately after interruption of the
 ignition signal for the cylinder now concerned in the case where the
 simultaneous ignition scheme is adopted. Accordingly, a detection
 quiescent period during which the signal detecting operation remains
 quiescent is provided over a predetermined time period L3 starting from
 the interruption of the ignition signal X1 (i.e., turning-off of the power
 transistor 2).
 2. Non-combustion event due to fault of fuel-supply/combustion system
 When the fuel-supply/combustion system of the engine suffers some fault,
 the ion current due to the fuel combustion is not produced. Thus, no ion
 current component makes appearance in the ion current detection signals
 X2a and X2. However, the noise component N1 produced upon rising of the
 ignition signal X1 and the noise component N2 produced upon termination or
 extinction of the spark discharge can make appearance in the ion current
 detection signals X2a and X2, as a result of which the noise components N1
 and N2 contained in the ion current detection signal X2a undergone the
 pulse shaping operation of the comparator CP3 are outputted from the first
 decision circuit 7A as the first decision signal X3, which is then applied
 to one input of the OR circuit 9. By contrast, the noise components N1 and
 N2 contained in the ion current detection signal X2 are eliminated by the
 delay circuit incorporated in the second decision circuit 7B.
 Consequently, the second decision signal X4 applied to the other input of
 the OR circuit 9 contains no noise components. Thus, the output signal X5
 indicative of the noise components N1 and N2 is outputted from the OR
 circuit 9 to be inputted to the ECU 10.
 The ECU 10 detects the first and second decision signals X3 and X4 during
 the first decision signal detection period set as mentioned previously in
 synchronism with the falling edge (trailing edge) of the ignition signal
 X1. In that case, the signal will be detected during the first detection
 period for the first decision signal X3 while no signal will be detected
 during the second detection period for the second decision signal X4.
 Thus, decision can be made that the non-combustion event is ascribable to
 a fault in the fuel-supply/combustion system.
 As typical one of the faults in the fuel-supply/combustion system, there
 may be mentioned breakage (inter-coils short-circuit fault) of the
 secondary winding 12 of the ignition coil 1, breakage of a high-voltage
 conductor or cord used for interconnecting the spark plug 4 and the
 secondary winding 12 of the spark plug 4, occurrence of a smoldering state
 in the spark plug 4, fuel injection failure due to defect of the fuel
 supply system, e.g. fuel injector and/or the like, as can be seen from the
 table shown in FIG. 3.
 3. Non-combustion event due to fault in the ignition control system
 When no ignition or firing takes place due to breakage of the primary
 winding 11 of the ignition coil 1, the ignition signal X1 supplied to the
 input circuitry of the primary winding 11 of the ignition coil 1 exerts no
 influence to the output circuitry inclusive of the secondary winding 12 of
 the ignition coil 1. Consequently, there makes appearance no output signal
 X5 to be inputted to the ECU 10. As a result of this, the ECU 10 detects
 no signal of any significance during the first detection period for the
 first decision signal X3 and the second detection period for the second
 decision signal X4. Thus, decision is made that occurrence of the
 non-combustion event is due to some fault in the ignition control system.
 As typical ones of the fault of the ignition control system, there may be
 mentioned nongeneration of the ignition signal due to fault of the ECU 10,
 breakage of the ignition signal conductor, defect of the power transistor
 2 serving for turning on/off the current supply to the ignition coil 1 in
 response to the ignition signal, breakage of the primary winding of the
 ignition coil 1, breakage of the ion current conductor, generation of no
 output signal from the combustion state detector not shown or the like.
 Embodiment 2
 In the case of the combustion state detecting apparatus for the internal
 combustion engine according to the first embodiment of the invention, the
 normal combustion, the non-combustion events and the faults which lead to
 occurrence of such non-combustion event can be discriminatively decided or
 determined on the basis of detection of the first and second decision
 signals X3 and X4 based on the noise component N1 produced at the
 discharge end time point and the ion current produced in accompanying the
 combustion. The combustion state detecting apparatus according to a second
 embodiment of the present invention is so designed that the normal
 combustion and the fault(s) bringing about the non-combustion event can be
 discriminatively determined or decided on the basis of the first decision
 signal X3A generated upon rising of the ignition signal X1 and the ion
 current produced in accompanying the combustion.
 FIG. 2A is a circuit diagram showing schematically a structure of the
 combustion state detecting apparatus for the internal combustion engine
 according to the second embodiment of the present invention. In FIG. 2A,
 components similar or equivalent to those described hereinbefore by
 reference to FIG. 1A are denoted by like reference characters. The second
 embodiment of the invention differs from the first embodiment in the
 respects that an AND circuit 112 is provided for logically ANDing the
 output of the comparator CP3 constituting a part of the first decision
 circuit 7A and the ignition signal X1 inputted by way of a buffer 111 and
 that the OR circuit 9 is so connected as to logically ORing the first
 decision signal X3A outputted from the AND circuit 112 and the second
 decision signal X4 outputted from the second decision circuit 7B.
 Next, referring to a signal waveform diagram shown in FIG. 2B, description
 will be made of operations of the combustion state detecting apparatus
 according to the second embodiment of the invention in conjunction with
 the normal combustion, the first type of non-combustion event due to a
 defect or fault of the fuel-supply/combustion system and the second type
 of non-combustion event due to a defect or fault of the ignition control
 system. Incidentally, the faults of the fuel-supply/combustion system as
 well as faults of the ignition system are essentially same as those
 described hereinbefore in conjunction with the first embodiment of the
 invention. Besides, the operations of the first and second decision
 circuits 7A and 7B are similar to those of the combustion state detecting
 apparatus according to the first embodiment. Accordingly, repeated
 description thereof will be unnecessary.
 1. Normal combustion
 So long as the normal combustion is taking place within the engine
 cylinder, the noise components N1 and N2 and the ion current produced in
 accompanying the combustion are detected as the ion current detection
 signal X2.
 Consequently, the first decision signal X3 is inputted to one of the input
 terminals of the AND circuit 112 from the comparator CP3 as in the case of
 the combustion state detecting apparatus according to the first embodiment
 of the invention. On the other hand, inputted to the other input terminal
 of the AND circuit 112 by way of the buffer 111 is the ignition signal X1.
 Consequently, the AND circuit 112 outputs the first decision signal X3A
 resulting from the pulse shaping of the noise component N1 generated upon
 rising of the ignition signal X1 in synchronism with the application of
 the ignition signal X1. The first decision signal X3A is applied to one of
 input terminals of the OR circuit 9.
 On the other hand, applied to the other input terminal of the OR circuit 9
 is the second decision signal X4. Thus, the logical sum OR of the first
 decision signal X3A and the second decision signal X4 is outputted from
 the OR circuit 9 as the output signal X5 which is then supplied to the ECU
 10. In this way, when the signal corresponding to the second decision
 signal X4 is detected after lapse of a predetermined time from the time
 point at which the first decision signal X3A was detected in synchronism
 with application of the ignition signal X1, then it is decided that the
 normal combustion has taken place.
 2. Non-combustion event due to fault in fuel-supply/combustion system
 When the non-combustion event occurs due to absence of the fuel supply, no
 ion current can flow through the inter-electrode gap of the spark plug.
 Consequently, the second decision signal X4 is not generated. However,
 since the noise component N1 produced upon application of the ignition
 signal X1 is outputted, the first decision signal X3A is supplied to the
 OR circuit 9 from the AND circuit 112 in synchronism with rising of the
 ignition signal X1, as a result of which the first decision signal X3A is
 outputted from the OR circuit 9 as the output signal X5 which is then
 supplied to the ECU 10.
 Thus, when only the signal (i.e., output signal X5) produced in synchronism
 with rising of the ignition signal X1 is detected by the ECU 10, it is
 decided that the non-combustion event due to a fault in the
 fuel-supply/combustion system has occurred.
 3. Non-combustion event due to fault in the ignition control system
 It is assumed, by way of example, that no ignition or firing takes place
 due-to breakage of the primary winding 11 of the ignition coil 1. In that
 case, the ignition signal X1 applied to the input circuitry for the
 primary winding 11 of the ignition coil 1 exerts no influence to the
 output circuitry provided in association with the secondary winding 12 of
 the ignition coil 1. Consequently, the output signal X5 is not supplied to
 the ECU 10.
 In this case, neither the first decision signal X3A due to the application
 of the ignition signal X1 nor the second decision signal X4 due to the
 combustion is detected, and thus decision is made such that the
 non-combustion event is ascribable to no appearance of the high voltage
 across the secondary winding 12 of the ignition coil 1, which means that
 the ignition system suffers some fault.
 Absence of the output signal during the detection period of the second
 decision signal X4 indicates occurrence of non-combustion. In that case,
 the type of fault can be discriminatively identified in dependence on
 whether the first decision signal X3 is outputted or not, as summarized in
 the table shown in FIG. 4.
 More specifically, when the first decision signal X3 is outputted, this
 means that the high secondary voltage for the ignition or firing is
 generated. Thus, as the fault, there can be estimated the inter-layer
 short-circuit fault of the secondary winding 12 of the ignition coil 1,
 breakage of high-voltage cord, smoldering of the spark plug 4, some defect
 in the fuel-supply/combustion system or the like.
 In the combustion state detecting apparatus in which the combustion state
 detector composed of the first and second decision circuits 7A and 7B is
 built in the ignition coil 1, absence of the first decision signal X3A
 indicates that some fault is taking place in the ECU 10, the ignition
 signal conductor or the ignition coil 1, as can be seen in FIG. 4.
 In this conjunction, it is to be added that when the first decision signal
 X3A is not outputted from the AND circuit 112, fault diagnosis can be
 performed for the ECU 10 and the ignition signal conductor by monitoring
 the ignition signal with the ECU 10. When this diagnosis results in no
 abnormality of the ignition signal, then it can be discriminatively
 determined that the ignition coil 1 is suffering a fault.
 Many modifications and variations of the present invention are possible in
 the light of the above techniques. It is therefore to be understood that
 within the scope of the appended claims, the invention may be practiced
 otherwise than as specifically described.