A misfire detector device for use in an internal combustion engine

Abstract: In a misfire detector device for use in internal combustion engine, an electrical interrupter circuit on-off actuates a primary current flowing through a primary circuit of an ignition coil to induce a sparkplug voltage. A check diode is provided in a secondary circuit of the ignition coil to prevent a current flowing back to the ignition coil. The spark plug has a center electrode, a front end of which is projected from an insulator, and an outer surface area of the projected front end being 25 mm² or more. A sparkplug voltage detector circuit detects an attenuation time length of a sparkplug voltage waveform presented subsequent to a time period predetermined after an end of a spark action of the spark plug. On the basis of the attenuation time length, a distinction circuit determines whether a misfire occurs in a cylinder of an internal combustion engine.

Description: This invention relates to a misfire detector device for use in an internal combustion engine and which works by use of the fact that a spark-plug gap resistance is different between the case of a spark igniting an air-fuel mixture gas and in the case of the spark failing to ignite the air-fuel mixture gas injected in a cylinder of the internal combustion engine. With the demand of purifying emission gas and enhancing fuel efficiency of internal combustion engine, it has been necessary to detect firing condition in each cylinder of the internal combustion engine so as to protect the internal combustion engine against any type of misfire. In order to detect the firing condition in each of the cylinders it is known for an optical sensor to be installed within the cylinders, for a pressure-sensitive element to be attached to a seat pad of the spark-plug, or for ion current due to an ignition circuit to be measured. However, it is troublesome and time-consuming to install the optical sensor to each of the cylinders, thus increasing the installation cost, and at the same time, taking much time in carrying out check and maintenance. In addition, a high voltage withstanding diode is needed to introduce the ion current to a secondary circuit. Therefore, it is an object of the invention to provide a misfire detector device for use in an internal combustion engine which is capable of sufficiently precisely detecting a misfire by checking a spark-plug voltage waveform applied to the spark-plug installed to each cylinder of the internal combustion engine with a relatively simple structure. EP-A-0,519,588, which forms part of the state of the art under Article 54(3) EPC, discloses some misfire detection circuits having the same elements as are illustrated in Figs. 1 and 11 of the accompanying drawings. US-A-3,961,240, on which the preamble of claim 1 is based, discloses a misfire detector circuit which detects the voltage across the electrodes of a spark plug. The circuit indicates faulty ignition when the integral of the voltage exceeds a threshold, and distinguishes between two different faults based on the magnitude of the gradient of the voltage. According to the invention, there is provided a misfire detector circuit in combination with an internal combustion engine ignition system which comprises: a spark plug; an ignition circuit including a primary coil and a secondary coil; an interrupter circuit, the on-off action of which causes a primary current to flow through the primary coil so as to induce a voltage in the secondary coil for producing a spark between the electrodes of the spark plug; a series gap or check diode provided in the secondary circuit of said ignition circuit so as to prevent a current flowing back to said secondary coil, the misfire detector circuit comprising: a voltage detector circuit arranged to detect the voltage across the electrodes of said spark plug; and a distinction circuit arranged to determine on the basis of said detected voltage whether or not a spark between said electrodes has ignited an air-fuel mixture, characterised in that: said spark plug has a centre electrode, the front end of which projects from an insulator, and wherein the outer surface area of said projecting front end is 25mm 2 or more; said voltage detector circuit is arranged to detect an attenuation time length of attenuation characteristics of said detected voltage a predetermined time after the end of the spark action of the spark plug; and said distinction circuit is arranged to determine whether or not a spark has ignited the air-fuel mixture on the basis of said attenuation time length of said detected voltage. Preferably, said centre electrode has a middle axis including a nickel-alloyed clad and a heat-conductor core embedded in the clad, and a ratio of n/L, where L = the length of said middle axis projected from the front end of said insulator and n = a distance between the front end of said heat-conductor core and the front end surface of the middle axis, is determined such that an outer surface area of a projected portion of said middle axis residing between said front end surface of said middle axis and said front end of said heat-conductor core is less than half of the outer surface area of said projected portion of said middle axis. In one embodiment, said voltage detector is arranged to detect the attenuation time length of said detected voltage immediately after the spark action of said spark plug. In another embodiment, said misfire detection circuit further comprises a voltage charging circuit arranged to re-energize said primary coil so as to induce a voltage in said secondary coil for electrically charging a stray capacity inherent in said spark plug after the end of the spark action of the spark plug; and said voltage detector circuit is arranged to detect the attenuation time length of the detected voltage after said voltage charging circuit has charged the stray capacity. In a further embodiment, said misfire detection circuit further comprises a voltage charging circuit arranged to re-energize said primary coil so as to induce a voltage in said secondary coil for electrically charging a stray capacity inherent in said spark plug after the end of the spark action of the spark plug, when the engine runs at a low revolution rate; and said voltage detector circuit is arranged to detect, when the engine runs at said low revolution rate, the attenuation time length of the detected voltage after said voltage charging circuit has charged the stray capacity, and to detect, when the engine runs at a high revolution rate, an attenuation time length of the detected voltage immediately after the spark action of said spark plug. In a still further embodiment, said misfire detection circuit further comprises a voltage charging circuit arranged to re-energise said primary coil so as to induce a voltage in said secondary coil for electrically charging a stray capacity inherent in said spark plug after the end of the spark action of the spark plug, when the engine runs at a low revolution rate; said voltage detector circuit is arranged to detect, when the engine runs at said low revolution rate, the attenuation time length of the detected voltage after said voltage charging circuit has charged the stray capacity; and the voltage detector circuit is arranged to detect, when the engine runs at a high revolution rate, whether the detected voltage exceeds a predetermined reference level and the distinction circuit is arranged to determine, when the engine runs at a high revolution rate, whether or not a spark has ignited the air-fuel mixture on the basis of whether said detected voltage exceeds said reference level. The voltage charging circuit may be the interrupter circuit of the ignition system. This type of the misfire detector device is employed to a distributor or a distributorless ignition device. In this type of ignition device, an electrical energy stored the ignition circuit electrically charges the static capacity (10 ∼ 20 pF) inherent in the spark plug immediately after the spark terminates. The charged voltage forms a sparkplug voltage of 5 ∼ 8 kv when the internal combustion engine runs at a high revolution while forming a sparkplug voltage of 2 ∼ 3 kv when the internal combustion engine runs at a low revolution. The sparkplug voltage is rapidly discharged through the electrodes of the spark plug after the termination of the spark when the spark normally ignites the air-fuel mixture gas, since the combustion gas staying between the electrodes is ionized. When the spark fails to ignite the air-fuel mixture gas, the sparkplug voltage is slowly released through the secondary circuit because the gas staying between the electrodes is free from ionized particles. The attenuation time length of the attenuation characteristic of the charged voltage depends on the density of the ionized particles of the combustion gas staying between the electrodes. When the ionized particles of the combustion gas are present between the electrodes, the attenuation characteristic hinges on the outer area of the electrodes, and the attenuation characteristic becomes short with the enlargement of the outer area of the electrodes because of the increased intensity of the ion current. Therefore, whether or not misfire occurs in the cylinder of the internal combustion engine is determined by detecting an attenuation time length required for the sparkplug voltage to descend to a predetermined voltage level against the peak hold voltage after monitoring the sparkplug voltage between the check diode and the spark plug. In this instance, a descending ratio of the sparkplug voltage may be measured against a peak value of the peak hold voltage. Whether or not a misfire occurs is determined by detecting the attenuation characteristic of the sparkplug voltage charged in the stray capacity after the end of the spark action, and comparing the characteristics with data previously measured or calculated according to the running conditions. In this instance, the ion current smoothly flows between the electrodes when an exposed area of the center electrode exceeds 25 mm 2 which is usually smaller than that of an outer electrode. This enables to precisely detect the misfire by reducing the interruption of the ion current flow due to deviation of combustion swirls in a cylinder of the internal combustion engine. In the misfire detector device in which a distributor is needed for an ignition device, there is provided a series gap (e.g. rotor gap) between the ignition circuit and the spark plug so as to work as an air gap. This results in a relatively small electrical energy stored in the ignition circuit after the termination of the spark when the engine runs at a low revolution. The small electrical energy often restricts the sparkplug voltage level so as to make it difficult to precisely determine the attenuation characteristics of the sparkplug voltage. For this reason, the voltage charging circuit is provided to induce an enhanced level of the sparkplug voltage at a time predetermined after the end of the spark action only when the engine runs at a low revolution. The enhanced level of the sparkplug voltage is predetermined to be e.g. 5 ∼ 7 kv which is high enough to break down the series gap of the distributor, but not enough to break down the spark gap, and thus electrically charging the stray capacity inherent in the spark plug. Discharging time length of the charged capacity changes depending on whether or not ionized particles are present in the combustion gas staying in the spark gap when the spark ignites the air-fuel mixture gas in the cylinder of the internal combustion engine. The attenuation time length of the sparkplug voltage is detected after the spark is terminated in the same manner as previously mentioned to determine whether misfire occurs in the cylinder of an internal combustion engine. The sparkplug voltage is induced by on-off actuating the primary circuit of the ignition coil, or otherwise a certain level of the sparkplug voltage is induced in the secondary circuit by providing a discrete step-up coil. The sparkplug voltage is employed to electrically charge the stray capacity so as to detect the attenuation characteristics of the charged voltage in the spark plug electrode, the exposed front end of which has an outer surface area of 25 mm 2 or more. Meanwhile, the sparkplug voltage often becomes excessively enhanced after the termination of the spark so that an electrical discharge occurs between the electrodes of the spark plug when the engine runs at a high revolution with a high load. In this instance, the secondary voltage rapidly descends irrespective of the misfire since the voltage charged in the stray capacity is released at once. This makes it difficult to distinguish the misfire from the normal combustion only by detecting the attenuation characteristics of the sparkplug voltage. However, the enhanced voltage level of the sparkplug voltage is quite remarkable in distinguishing the misfire from the normal combustion after the end of the spark action when the engine runs at the high revolution with the high load. That is to say, the spark is likely to be sustained when the spark normally ignites the air-fuel mixture gas to ionize the particles in the combustion gas, so that the spark exhausts the electrical energy reserved in the ignition circuit after the end of the spark action to enhance the sparkplug voltage only by 3 ∼ 5 kv. As opposed to the enhanced voltage 3 ∼ 5 kv, the enhanced sparkplug voltage exceeds 10 kv when the misfire occurs in the cylinder of the internal combustion engine. Therefore, whether or not the misfire occurs is determined by detecting the enhanced level of the sparkplug voltage after the end of the spark action when the engine runs at the high revolution with the high load. With the exposed area of the elecrode being 25 mm 2 or more, its enlarged area makes it possible to excessively rise the temperature of the front end of the center electrode so as to cause a preignition. In order to avoid the preignition, the front end of the heat-conductor is placed in the proximity of the front end of the center electrode so as to facilitate the heat-dissipation through the heat-conductor. This enables to avoid the loss of the endurance and the decrease of amount of heat due to the enlarged area of the center electrode. With the center electrode in the side of negative polarity, the anode ions are attracted to the center electrode to draw the electric current by exchanging the charged particles in the combustion flame. In this instance, the cathode ions are considered to stay around the center electrode because the cathode ions are heavy and less mobile compared to the electrons. Consequently, the intensity of the current is determined by the mobility of the cathode ions. With the exposed area of the electrode being enlarged to be 25 mm 2 or more, the cathode ions are collected to the center electrode to increase the intensity of the current so as to clarify the attenuation characteristics. In the misfire detector device according to the invention, the exposed area of the center electrode has 25 mm 2 or more, so that the ion current flow is facilitated to insure the precise misfire detection irrespective of the swirl stream variation in the cylinder of the internal combustion. This also makes it possible to obviate the necessity of the optical sensor, the pressure-sensitive element and the high-voltage withstanding diode, thus enabling to provide a misfire detector device which is capable of precisely detecting the misfire in each cylinder of the internal combustion engine, and easy in mounting on the engine, superior in maintenance, simple in structure and readily reducable in practical use. In order that the invention may be more fully understood the following description is given, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic view of an ignition circuit in which an ignition detector is incorporated according to a first embodiment of the invention; Fig. 2 is an enlarged perspective view of a main part of a spark plug; Fig. 3 is a view of a wiring diagram of a sparkplug voltage detector circuit; Fig. 4 is a view of a sparkplug voltage waveform shown for the purpose of explaining how the sparkplug voltage detector circuit works; Fig. 5 is a view similar to Fig. 1 according to a second embodiment of the invention. Fig. 6 is of a sparkplug voltage waveform according to the second embodiment of the invention; Fig. 7 is a graph showing a relationship between an exposed area of middle axis and an ion current waveform; Fig. 8 is a graph showing a relationship between the exposed area of a middle axis and an ion current level; Fig. 9 is a graph showing a relationship between the exposed area of a middle axis and a misfire detection precision; Fig. 10 is a graph showing a relationship between the exposed area of a middle axis and a misfire detecting rate; Fig. 11 is a schematic view of an ignition circuit in which an ignition detector is incorporated according to a third embodiment of the invention; Fig. 12 shows a wiring diagram of a sparkplug voltage detector circuit according to the third embodiment of the invention; Fig. 13 is a view of a sparkplug voltage waveform shown for the purpose of explaining how the sparkplug voltage detector circuit works according to the third embodiment of the invention; Fig. 14 is a view similar to Fig. 11 according to a fourth embodiment of the invention; Fig. 15 is a schematic view of a sparkplug voltage waveform shown for the purpose of explaining how the sparkplug voltage detector circuit works according to the fourth embodiment of the invention; Fig. 16 shows a wiring diagram of a sparkplug voltage detector circuit according to the fourth embodiment of the invention; and Fig. 17 is a view of a voltage waveform shown for the purpose of explaining how the sparkplug voltage detector circuit works according to the fourth embodiment of the invention. Referring to Fig. 1 which shows an ignition detector 100 which is incorporated into an internal combustion engine, the ignition detector 100 according to a first embodiment of the invention has an ignition circuit 1 which includes a primary circuit 11 and a secondary circuit 12 with a vehicular battery cell (V) as a power source. The primary circuit 11 has a primary coil (L1) electrically connected in series with a switching device 41 and a signal generator 42, while the secondary circuit 12 has a secondary coil (L2) connected to a rotor 2a of a distributor 2. The distributor 2 has stationary segments (Ra), the number of which corresponds to that of the cylinders of the internal combustion engine. To each of the stationary segments (Ra), is an free end of the rotor 2a adapted to approaches so as to make a rotor gap 21 (series gap) with the corresponding segments (Ra). Each of the segments (Ra) is connected to a spark plug 3 by way of a sparkplug cable (H). The spark plug 3 has a center electrode 3a and an outer electrode 3b to form a spark gap 31 between the two electrodes 3a, 3b, across which spark occurs when energized. It is noted that a distributorless igniter in which no distributor is provided, may be used. In this instance, a one way diode or air gap may be employed instead of the rotor gap 21 of the distributor 2. The switching device 41 and the signal generator 42 forms an interrupter circuit 4 which detects a crank angle and a throttling degree of the engine to interrupt primary current flowing through the primary coil (L1) to induce a sparkplug voltage in the secondary coil (L2) of the secondary circuit 12 so that the timing of the spark corresponds to an advancement angle relevant to a revolution and a load which the engine bears. The interrupter circuit 4 serves as a voltage charging circuit which on-off actuates the primary coil (L1) to induce a charging voltage in the secondary circuit 12 either during establishing the spark between the electrodes 3a, 3b or during a predetermined time period after an end of the spark, thus leading to electrically charging stray capacity inherent in the spark plug 3. In this instance, a discrete voltage charging circuit may be provided independently of the interrupter circuit 4. As shown in Fig. 2, the spark plug 3 has a cylindrical metallic shell 33 to which the ground electrode 3b is welded. Within the metallic shell 33, an tubular insulator 35 is placed, an inner space of which serves as an axial bore 34. To a lower end of the center electrode 3a, is a middle axis 36 connected which is partly is projected from a front end of the insulator 35. The middle axis 36 is in the side of negative polarity, and having a nickel-alloyed clad 37 and a heat-conductor core 38 embedded in the clad 37. The clad 37 is made of pure nickel or a nickel alloy including 10 ∼ 20 wt% Cr, while the heat-conductor core 38 is preferably made of pure copper, silver or 0.25 wt% aluminum containing copper alloy. As indicated by a projected portion 39, an outer surface area (exposed area) of the middle axis 36 projected from the insulator 35 is 25 mm 2 or more. A ratio of n/L is determined such that an outer surface area of the projected portion 39 residing between a front end surface 39A of the middle axis 36 and a front end 38A of the heat-conductor core 38 is less than half of the outer surface area of the projected portion 39. Where n = a length between the front end surface 39A of the middle axis 36 and the front end 38A of the heat-conductor core 38, and L = a length of the middle axis 36. Meanwhile, an electrical conductor (sensor) 51 surrounds an extension part of the sparkplug cable (H) to define static capacity of e.g. 1pF therebetween so as to form a voltage divider circuit 5. The conductor 51 is connected to the ground by way of a condensor 52. To a common point between the conductor 51 and the condensor 52, is a sparkplug voltage detector circuit 6 electrically connected to which a distinction circuit 7 is connected. The condensor 52 has a static capacity of e.g. 3000 pF to serve as a low impedance element, and the condensor 52 further has an electrical resistor 53 (e.g. 2 MΩ) connected in parallel therewith so as to form a discharge path for the condensor 52. The voltage divider circuit 5 allows to divide the sparkplug voltage induced from the secondary circuit 12 by the order of 1/3000, which makes it possible to determine the time constant of RC path to be approximately 9 milliseconds to render an attenuation time length of the sparkplug voltage relatively longer (3 milliseconds) as described hereinafter. In this instance, the sparkplug voltage 30000 V divided to the level of 10 V is inputted to the sparkplug voltage detector circuit 6. The sparkplug voltage detector circuit 6 has a peak hold circuit 61, a voltage divider circuit 62 and a comparator 63 as shown in Fig. 3. To the peak hold circuit 61, are the input signal (A) of the signal generator 42 and the divided voltage of the voltage divider circuit 5 inputted. The voltage divider circuit 62 divides an output voltage from the peak hold circuit 61. The comparator 63 compares the output from the voltage divider circuit 5 with the divided voltage from the voltage divider circuit 62 in order to detect a holding time length of an output voltage, the level of which is more than a predetermined level among the divided voltage waveform of the sparkplug voltage. The distinction circuit 7 determines the misfire by detecting the holding time length longer than a certain period of time. With the structure thus far described, the signal generator 42 of the interrupter circuit 4 outputs pulse signals as shown at (A) in Fig. 4 in order to induce the primary current in the primary circuit 11 as shown at (B) in Fig. 4. Among the pulse signals, the pulses (a), (c) which have a larger width (h) energizes the spark plug 3 to establish the spark between the electrodes 3a, 3b. The pulses (a), (c) followed by the pulses (b), (d) delays by the time of 0.5 ∼ 1.5 ms (i). The pulses (b), (d) have a thin width to electrically charge the stray capacity inherent in the spark plug 3. In so doing, the time length during which the free end of the rotor 2a forms the rotor gap 21 with each of the segments (Ra), changes depending on the revolution of the engine. The pulse width (h) and the delay time (i) are determined shorter in a manner that the spark holds for 0.5 ∼ 0.7 ms when the engine is operating at high revolution (6000 rpm). With the actuation of the interruter circuit 4, the sparkplug voltage appears in the secondary coil (L2) of the secondary circuit 12 as shown at (C) in Fig. 2. Due to the high voltage (p) established following the termination of the pulse signals (a), (c), the spark begins to occur with an inductive discharge waveform (q) accompanied. In response to the rise-up pulse signals (b), (d), a counter-electromotive voltage accompanies a positive voltage waveform (r) flowing through the secondary circuit 12, thus making it possible to terminate the spark when the spark lingers. Due to an electrical energy stored in the ignition circuit 1 when the primary coil (L1) is energized, the secondary voltage is enhanced again to flow a voltage waveform (s) through the secondary circuit when the primary coil (L1) is deenergized. The enhanced voltage level is determined as desired by the delay time (i) and the width of the pulse signals (b), (d). The level of the voltage waveform (s) is 5 ∼ 7 kv, the magnitude of which is enough to break down the rotor gap 21, but not enough to establish a discharge between the electrodes 3a, 3b when the air-fuel mixture gas staying in the spark gap 31 is free from ionized particles. The discharge voltage in main from the stray capacity (usually 10 ∼ 20 pF) inherent in the spark plug 3, is released as shown at (D) in Fig. 4. The attenuation time length of the discharge voltage is distinguishable from the case in which the spark normally ignites the air-fuel mixture gas to the case in which the spark fails to ignite the air-fuel mixture gas injected in each cylinder of the internal combustion engine. That is to say, the misfire follows a slowly attenuating waveform (s1) as shown in Fig. 4, while the normal combustion follows an abruptly attenuating waveform (s2) as shown in Fig. 4. The sparkplug voltage detector circuit 6 detects a voltage waveform level of more than a reference voltage level (V) so as to deform the voltage waveform into square wave pulses t1 ∼ t4, each width of which is equivalent to an attenuation time length. The square wave pulses t1 ∼ t4 are inputted to the distinction circuit 7 so as to cause the circuit 7 to determine the misfire when the attenuation time length is more than 3 ms (1 ms) with the revolution of the engine as 1000 rpm (6000 rpm). The distinction circuit 7 further determines the misfire when the attenuation time length is more than the one decreasing in proportion to the engine revolution which falls between 1000 and 6000 rpm. In the first embodiment of the invention, the rotor gap 21 of the distributor 2 is used as a series gap. In the distributorless ignitor, a check diode is provided in the secondary circuit to acts as the series gap. When a discrete voltage charging circuit is employed, a step-up coil may be used instead of the ignition circuit 1 to induce a voltage (4 ∼ 5 kv) so as to energize the secondary circuit. When the exposed area of projected portion of the middle axis 36 is less than 25 mm 2 (which is not within the scope of the present invention), it is preferable that the sparkplug voltage is maintained positive by reversely connecting the ignition circuit 1 since the ionized particles in the air-fuel mixture gas allows the electric current to flow better when the middle axis 36 is kept positive than otherwisely connected. When the center electrode 3a is maintained positive polarity, the anode ions are attracted to the ground electrode 3b so that the exchange speed of the ions is facilitated by the outer surface area ratio (approx. 10 times) of the ground electrode 3b to the center electrode 3a. The exchange speed of the ions is dominated by the speed of the cathode ions because the light-weight electrons quickly moves towards the center electrode 3a. Although the exchange speed of the ions is dominated by the speed of the cathode ions, the exchange speed makes no substantial difference whether the middle axis 36 is maintained negative or positive when the outer surface area of the projected portion 39 exceeds 25 mm 2 (as in the present invention). When the middle axis 36 is maintained negative polarity, the cathode ions in the combustion flame are attracted to the middle axis 36 of the center electrodes 3a to permit a current flow so as to observe the attenuation characteristics of the sparkplug voltage waveform. In this instance, the heavy cathode ions are less mobile than the electrons, and are considered to stay around the middle axis 36 Therefore, it is effective to determine the outer surface area of the projected portion 39 to be 25 mm 2 or more when the middle axis 36 of the center electrode 3a is maintained negative polarity. Figs. 5 and 6 slow a second embodiment of the invention in which a check diode 13 is electrically connected between the rotor gap 21 of the distributor 2 and the secondary coil (L2) of the secondary circuit 12. The diode 13 allows electric current to flow from the secondary coil (L2) to the rotor gap 21 of the distributor 2, but prohibits the electric current to flow backward. With the pulse signals (A) which causes to induce the sparkplug voltage in the secondary circuit 12, the sparkplug voltage is enhanced again as mentioned hereinbefore when deenergized. The enhanced voltage electrically charges the stray capacity inherent in the spark plug 3 to make a potential difference between the ignition circuit 1 and the spark plug 3. In this instance, the check diode 13 prohibits the electric current to flow through the rotor gap 21 in the direction opposite to the spark which occurs from the center electrode 3a to the outer electrode 3b. Otherwise, the voltage waveform (s) shown in Fig. 6 reduces from 5 ∼ 7 kv to 3 ∼ 4 kv so as to deteriorate the precision on detecting the attenuation time length. With the provision of the check diode 13, the sparkplug voltage accompanies a slowly attenuating the voltage waveform (s3) as opposed to that accompanying the rapidly changing voltage waveform (s1) as shown in Fig. 6. In the sparkplug voltage detector circuit 6, the peak hold circuit 61 holds a peak voltage based on the stray capacity of the spark plug 3 with 1/3 of the peak voltage as the reference voltage (Vo) for example. The comparator 63 compares the reference voltage (Vo) with the output voltage waveform from the voltage divider circuit 5 so as to output square pulses t5, t6 as shown at (E) in Fig. 6. The square pulses t5, t6 are inputted to the distinction circuit 7 to determine whether the misfire occurs or not in the cylinder of the internal combustion engine. Fig. 7 shows a relationship between the exposed area (S) of the projected portion 39 and the ion current waveform derived immediately after the end of the spark action. The relationship is obtained by carrying out the experiment test with the spark plugs mounted on 2000 cc, four-cylinder and four-cycle engine. The three types of the spark plugs has the exposed area (S) of 10 mm 2 , 25 mm 2 and 50 mm 2 (10 mm 2 being outside the scope of the present invention). The results teach that the ion current increases with the enlargement of the exposed area (S) of the projected portion 39, and thus distinguishing the noise to clarify the peak of the voltage waveform so as to easily detect the ion current. Fig. 8 shows a relationship between the exposed area (S) of the projected portion 39 and the mean peak level of the ion current waveform derived immediately after the end of the spark action. When the exposed area (S) exceeds 25 mm 2 (S > 25 mm 2 ), the intensity of the ion current exceeds 8 µA. Considering that the noise level of the ion current detecting circuit is several µA, the ion current is precisely detected when the exposed area (S) exceeds 25 mm 2 . Fig. 9 shows a relationship between the exposed area (S) of the projected portion 39 and the misfire detecting rate. The results indicates that when the exposed area (S) is less than 25 mm 2 (S < 25 mm 2 ), and so outside the scope of the present invention, the peak level of the ion current is too low to distinguish the noise so that the misfire detecting rate quickly deteriorates. Fig. 10 shows temperature measurement results of the projected portion 39 of the middle axis with the spark plug mounted on the engine which runs 3000 rpm at full throttle. Regarding the ratio of n/L, the results indicate that the temperature of the front end of the middle axis 36 excessively rises to cause the pregnition when the outer surface area of the projected portion 39 above the heat-conductor core 38 exceeds the half of the outer surface area of the projected portion 39. Referring to Fig. 11 which shows a distributorless type of an ignition detector 200 in which no distributor is needed, and incorporated into a internal combustion engine according to a third embodiment of the invention, the ignition detector 200 has an ignition circuit 201 which includes a primary circuit 211 and a secondarycircuit 212 with a vehicular battery cell (Va) as a power source. The number of the ignition circuit 201 provided in the third embodiment corresponds to that of the cylinders of the internal combustion engine. The primary circuit 211 has a primary coil (L11) electrically connected in series with a switching device 241 and a signal generator 242, while the secondary circuit 212 has a secondary coil (L22) and a check diode 213 connected in series with each other. A sparkplug cable (Hca) connects the diode 213 to the spark plug 3 installed in each cylinder of the internal combustion engine. The spark plug 3 has the center electrode 3a and an outer electrode 3b to form a spark gap 31 between the two electrodes 3a, 3b, across which spark occurs when energized. The spark plug 3 has the same structure, and the center electrode 3a is in the side of negative polarity as described in the first embodiment of the invention (see Fig. 2). The switching device 241 and the signal generator 242 forms an interrupter circuit 204 which detects a crank angle and a throttling degree of the engine to interrupt primary current flowing through the primary coil (L11) to induce a sparkplug voltage in the secondary coil (L22) of the secondary circuit 212 so that the timing of the spark corresponds to an advancement angle relevant to a revolution and load which the engine bears. Meanwhile, an electrical conductor 251 surrounds an extension line of the sparkplug cable (Hca) to define static capacity of e.g. 1 pF therebetween so as to form a voltage divider circuit 205. The conductor 251 is connected to the ground by way of a condensor 252. To a common point between the conductor 251 and the condensor 252, is a sparkplug voltage detector circuit 206 electrically connected to which a distinction circuit 207 is connected. The condensor 252 has static capacity of e.g. 3000 pF to serve as a low impedance element, and the condensor 252 further has an electrical resistor 253 (e.g. 3 MΩ) connected in parallel therewith so as to form a discharge path for the condensor 252. The voltage divider circuit 205 allows to divide the sparkplug voltage induced from the secondary circuit 212 by the order of 1/3000, which makes it possible to determine the time constant of RC path to be approximately 9 milliseconds to render an attenuation time length relatively longer (2 ∼ 3 milliseconds) as described hereinafter. In this instance, the sparkplug voltage 30000 V divided to a level of 10 V is inputted to the sparkplug voltage detector circuit 206. As shown in Fig. 12, the sparkplug voltage detector circuit 206 has a peak hold circuit 261 which is adapted to be reset at the time determined by the signal generator 242 in order to hold an output voltage generated from the voltage divider circuit 205. The spark voltage detector circuit 206 further has a divider circuit 262 which divides an output from the peak hold circuit 261, and having a comparator 263 which generates pulse signals by comparing an output from the divider circuit 262 with that of the voltage divider circuit 205. Into the distinction circuit 207, is a microcomputer incorporated which compares output pulse singals with data previously determined by calculation and experiment so as to determine whether or not the misfire occurs in the cylinder of the internal combustion engine. With the structure thus far described, the signal generator 242 on-off actuates the switching device 241 to output pulse signals (a) as shown at (A) in Fig. 13 in order to induce a secondary voltage in the secondary coil L22 as shown at (B) in Fig. 13 in which a termination of the pulse signals (a) accompanies a high voltage waveform (p) to initiate the spark occurring across the electrodes 3a, 3b, and accompanying a low inductive discharge (q) following the high voltage waveform (p). Upon running the engine at a low revolution, the low inductive discharge (q) which forms a sparkplug voltage waveform sustains for approximately 2 ms, and disappears with an exhaustion of an electrical energy stored in the ignition circuit 201. The exhaustion of the electrical energy culminates the sparkplug voltage in 2 ∼ 3 kv. Upon running the engine at a high revolution, the low inductive discharge (q) which forms the sparkplug voltage waveform sustains for approximately 1 ms, and disappears with the exhaustion of the electrical energy stored in the ignition circuit 201. The exhaustion of the electrical energy culminates the sparkplug voltage in 5 ∼ 8 kv. A sparkplug voltage waveform between the diode 213 and the spark plug 3 is derived in main from the discharge of the stray capacity (usually 10 ∼ 20 pF) inherent in the spark plug 3 after the spark terminates. An attenuation time length of the sparkplug voltage waveform differs between the case in which the spark normally ignites the air-fuel mixture gas and the case in which the spark fails to ignite the air-fuel mixture gas. That is, the discharge from the stray capacity is released through ionized particles of the combustion gas upon carrying out the normal combustion, so that the sparkplug voltage waveform rapidly attenuates as shown at solid lines (q1) of (C) in Fig. 13. The misfire makes the unburned gas free from the ionized particles, so that the discharge from the stray capacity leaks mainly through the spark plug 3. The sparkplug voltage waveform slowly attenuates as shown at phantom lines (q2) of (C) in Fig. 13. In the meanwhile, an average value of the spark sustaining time length is determined according to operating conditions obtained from calculation and experiment based on the revolution, the workload of the engine and the design of the ignition system. The signal generator 242 is adapted to carry out the reset and peak hold timing of the peak hold circuit 261 by approximately 0.5 ms later following the expiration of the average value of the spark sustaining time length. The peak hold circuit 261 holds a charged voltage of the stray capacity inherent in the spark plug 3, while the divider circuit 262 divides the charged voltage. With 1/3 of the charged voltage as a reference voltage (v1), the comparator 263 compares the reference voltage (v1) with the output voltage waveform from the voltage divider circuit 205. The comparator 263 generates a shorter pulse (t1) as shown (D) in Fig. 13 when the spark normally ignites the air-fuel mixture gas, while generating a wider pulse (t2) as shown (E) in Fig. 13 when the misfire occurs. The pulses (ti), (t2) are fed into the distinction circuit 207 so as to cause the circuit 207 to determine the misfire when the attenuation time length exceeds 3 ms upon running the engine at the low revolution (1000 rpm), while determining the misfire when the attenuation time length exceeds 1 ms upon running the engine at the high revolution (6000 rpm). The distinction circuit 207 further determines the misfire when the attenuation time length exceeds the one decreasing in proportion to the engine revolution which falls within an intermediate speed range between 1000 rpm and 6000 rpm. Fig. 14 shows a fourth embodiment of the invention in which like reference numerals in Fig. 14 are identical to those in Fig. 11. A main portion in which the fourth embodiment differs from the third embodiment is that a distributor 202 is provided according to the fourth embodiment of the invention. In the fourth embodiment of the invention in which only a single ignition circuit is necessary as designated at numeral 201 as the same manner in Fig. 11, the secondary coil (L22) of the secondary circuit 212 is connected directly to a rotor 202a of the distributor 202. The distributor 202 has stationary segments (Rs), the number of which corresponds to that of the cylinders of the internal combustion engine. To each of the stationary segments (Rs), is an free end of the rotor 202a adapted to approaches so as to make a rotor gap 221 (series gap) with the corresponding segments (Rs). Each of the segments (Rs) is connected to the spark plug 3 by way of the sparkplug cable (Hca). The spark plug 3 has a center electrode 3a and an outer electrode 3b to form a spark gap 231 between the two electrodes 3a, 3b, across which spark occurs when energized. The spark plug 3 has the same structure, and the center electrode 3a is in the side of negative polarity as described at the first embodiment of the invention shown in Fig. 2. The interrupter circuit 204 which is formed by the switching device 241 and the signal generator 242 serves as a voltage charging circuit according to the fourth embodiment of the invention. Upon running the engine at a relatively low revolution less than 3000 rpm, the enhanced level of the sparkplug voltage is such a degree as to limit the voltage level charged in the stray capacity of the spark plug 3 by way of the series gap 221 after the spark terminates, thus rendering it impossible to precisely determine the attenuation characterics of the sparkplug voltage. In this instance, it is advantageous to independently induce an increased level of the secondary voltage based on the voltage charging circuit. The voltage charging circuit is adapted to selectively on-off actuates the primary coil (L11) so as to induce a charging voltage in the secondary circuit 12 either during establishing the spark between the electrodes 3a, 3b or during a predetermined time period immediately after an end of the spark, thus leading to electrically charging the stray capacity inherent in the spark plug 3. The voltage charging circuit is actuated only upon running the engine at a relatively low revolution of less than 3000 rpm. Upon running the engine at the high revolution exceeding 3000 rpm, it is needless to activate the voltage charging circuit since the secondary voltage is excited to reach 5 ∼ 8 kv enough to positively break down the series gap 221. A range which the voltage charging circuit is actuated is appropriately determind depending on a type of the internal combustion engine, and adjusted by operating conditions such as the load of the engine, temperature of cooling water and the vehicular battery cell (Va). The ignition detector 200 is operated in the same manner as described in the third embodiment of the invention, upon running the engine at the high revolution exceeding 3000 rpm. Upon running the engine at the relatively low revolution of less than 3000 rpm, the ignition detector 200 is operated as follows: The signal generator 242 of the interrupter circuit 204 outputs pulse signals in order to induce the primary current in the primary circuit 211 as shown at (A) in Fig. 15. Among the pulse signals, the pulse (a) which has the larger width (h) energizes the spark plug 3 to establish the spark between the electrodes 3a, 3b. The pulse (a) followed by the pulses (b) delays by the time (i) of 1.5 ∼ 2.5 ms. The pulse (b) has a small width (j) to electrically charge the stray capacity inherent in the spark plug 3. In so doing, the time length during which the free end of the rotor 202a forms the rotor gap 221 with each of the segments (Rs), changes depending on the revolution of the engine. The pulse width (h) and the delay time (i) are preferably determined relatively shorter (1.5 ms) in a manner that the spark sustains for 0.5 ∼ 0.7 ms when the engine is running within a range of the intermediate revolution. With the actuation of the interruter circuit 204, the sparkplug voltage appears in the secondary coil (L22) of the secondary circuit 212 as shown at (C) in Fig. 15. Due to the high voltage (p) established following the termination of the pulse signal (a), the spark discharge begins to occur across the electrodes 3a, 3b, and accompanying an inductive discharge waveform (q) until the spark terminates. In response to the rise-up pulse signal (b), a counter-electromotive voltage accompanies a positive voltage waveform (r) flowing through the secondary circuit 212. Due to an electrical energy stored in the ignition circuit 201 when the primary coil (L11) is energized, the sparkplug voltage is enhanced again to draw a voltage waveform (s) through the secondary circuit 212 when the primary coil (L11) is deenergized. The enhanced voltage level is determined as desired by the delay time (i) and the width (j) of the pulse signal (b). The level of the voltage waveform (s) is determined to be 5 ∼ 7 kv, the intensity of which is enough to break down the rotor gap 221, but not enough to establish a discharge across the electrodes 3a, 3b when substantially no ionized particles stay in the spark gap 31. The discharge voltage in main from the stray capacity (usually 10 ∼ 20 pF) inherent in the spark plug 3, is released as shown at (C) in Fig. 15. The attenuation time length of the discharge voltage distinguishes the case in which the spark normally ignites the air-fuel mixture gas from the case in which the spark fails to ignite the air-fuel mixture gas injected in each cylinder of the internal combustion engine. That is to say, the misfire follows a slowly attenuating waveform (s2) of (C) as shown in Fig. 15, while the normal combustion follows an abruptly attenuating waveform (s1) of (C) as shown in Fig. 15. Whether or not the misfire occurs is determined by detecting the attenuation time length required for the peak voltage level to drop as described at the third embodiment of the invention shown in Fig. 12. It is noted that a check diode may be electrically connected between the rotor 202a of the distributor 202 and the secondary coil (L22) of the secondary circuit 212. The check diode allows electric current to flow from the secondary coil (L22) to the rotor 202a of the distributor 202, but prohibits the electric current to flow backward. The check diode prevents an excessively charged voltage 5 ∼ 7 kv from inadvertently flowing backward to the ignition circuit 201 by way of the series gap 221. This enables to avoid an abrupt rise-up voltage in the ignition circuit so as to contribute to a precise misfire detection. The misfire is thus far detected on the basis of the attenuation time length by holding the sparkplug voltage at the predetermined time, it is however noted that the misfire may be determined by detecting the sparkplug voltage level changed after the elapse of the predetermined time. Fig. 16 shows a wiring diagram of a sparkplug voltage detector according to a fourth embodiment of the inventionin which like reference numerals in Fig. 16 are identical to those in Fig. 12. Numeral 8 designates a step-up level detector circuit which detects a stepped-up level of the sparkplug voltage after the end of the spark action. The step-up level detecter circuit 8 has a comparator 8a to compare a predetermined reference voltage (Vo) with a peak voltage value held by the peak hold circuit 261 so as to generate output pulses. The output pulses are fed into a auxiliary distinction circuit 9 which determines the misfire depending on the level of the output pulses. Fig. 17 shows a waveform of the sparkplug voltageupon running the engine at full revolution (5000 rpm) with a high load. An enhanced voltage level of the sparkplug voltage is only 3 ∼ 5 kv as shown at (q3) of (C) in Fig. 17 when the spark normally ignites the air-fuel mixture gas. The sparkplug may rise to 10 kv or more as shown at (q4) of (C) in Fig. 17 when the spark fails to ignite the air-fuel mixture gas. The subsequent spark causes to abruptly descend the rise-up sparkplug voltage as shown at (q5) of (C) in Fig. 17. The abruptly descended waveform (q5) makes it difficult to distinguish the attenuation characteristics of the normal combustion from that of the misfire. As opposed against this instance, it is possible to positively distinguish the normal combustion from the misfire upon running the engine at the high revolution by directly detecting the enhanced level of the sparkplug voltage to decide whether or not the enhanced level exceeds the predetermined reference voltage (Vo: e.g. 10kv). According to the third and fourth embodiments of the invention, the same results are obtained as represented by Fig. 7 through Fig. 10 of the first and second embodiments of the invention. While the invention has been described with reference to the specific embodiments, it is understood that the description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by skilled artisan without departing from the scope of the invention as defined in the following claims.

Claims: A misfire detector circuit (100,200) in combination with an internal combustion engine ignition system which comprises: a spark plug (3); an ignition circuit (1,201) including a primary coil (L1,L11) and a secondary coil (L2,L22); an interrupter circuit (4,204), the on-off action of which causes a primary current to flow through the primary coil so as to induce a voltage in the secondary coil (L2,L22) for producing a spark between the electrodes (3a, 3b) of the spark plug (3); a series gap (21,221) or check diode (12,213) provided in the secondary circuit (12,212) of said ignition circuit (1,201) so as to prevent a current flowing back to said secondary coil (L2,L22), the misfire detector circuit (100,200) comprising: a voltage detector circuit (6,206) arranged to detect the voltage across the electrodes (3a,3b) of said spark plug (3); and a distinction circuit (7,207) arranged to determine on the basis of said detected voltage whether or not a spark between said electrodes has ignited an air-fuel mixture, characterised in that: said spark plug has a centre electrode (3a), the front end of which projects from an insulator, and wherein the outer surface area of said projecting front end is 25mm 2 or more; said voltage detector circuit (6,206) is arranged to detect an attenuation time length of attenuation characteristics of said detected voltage a predetermined time after the end of the spark action of the spark plug; and said distinction circuit (7,207) is arranged to determine whether or not a spark has ignited the air-fuel mixture on the basis of said attenuation time length of said detected voltage. A misfire detector circuit and an internal combustion engine ignition system according to claim 1, wherein said misfire detector circuit includes voltage divider circuit (5,205) for dividing the voltage across said electrodes (3a,3b), and said voltage detector circuit (6) uses said divided voltage to detect said attenuation time length. A misfire detector circuit and an internal combustion engine ignition system according to claim 2 wherein said voltage divider circuit (5,205) includes an electrical conductor (51,251) surrounding an extension part of the spark plug cable (H) and is connected to ground by a condenser (52,252). A misfire detection circuit and an internal combustion engine ignition system according to either one of the preceding claims 2 or 3, wherein said voltage detector circuit (6,206) includes a peak hold means (61,261) for holding a peak voltage of said divided voltage, said peak voltage is compared with said divided voltage, and said distinction circuit (7,207) makes its determination on the basis of this comparison. A misfire detection circuit and an internal combustion ignition system according to any one of the preceding claims, wherein said centre electrode has a middle axis (36) including a nickel-alloyed clad (37) and a heat-conductor core (38) embedded in the clad (37), and a ratio of n/L, where L = the length of said middle axis (36) projected from the front end of said insulator (35) and n = a distance between the front end (38A) of said heat-conductor core (38) and the front end surface (39A) of the middle axis (36), is determined such that an outer surface area of a projected portion (39) of said middle axis (36) residing between said front end surface (39A) of said middle axis (36) and said front end (38A) of said heat-conductor core (38) is less than half of the outer surface area of said projected portion (39) of said middle axis. A misfire detection circuit and an internal combustion engine ignition system according to any one of the preceding claims, wherein the electrical connection of the spark plug is such that said projecting front end is on the side of negative polarity. A misfire detection circuit and an internal combustion engine ignition system according to any one of the preceding claims, wherein said voltage detector circuit (206) is arranged to detect the attenuation time length of said detected voltage immediately after the spark action of said spark plug (3). A misfire detection circuit and an internal combustion engine ignition system according to any one of claims 1 to 6, wherein: said misfire detection circuit (100,200) further comprises a voltage charging circuit arranged to re-energize said primary coil (L1,L11) so as to induce a voltage in said secondary coil (L2,L22) for electrically charging a stray capacity inherent in said spark plug (3) after the end of the spark action of the spark plug; and said voltage detector circuit (6,206) is arranged to detect the attenuation time length of the detected voltage after said voltage charging circuit has charged the stray capacity. A misfire detection circuit and an internal combustion engine ignition system according to any one of claims 1 to 6, wherein: said misfire detection circuit (200) further comprises a voltage charging circuit arranged to re-energize said primary coil (111) so as to induce a voltage in said secondary coil (L22) for electrically charging a stray capacity inherent in said spark plug (3) after the end of the spark action of the spark plug, when the engine runs at a low revolution rate; and said voltage detector circuit (206) is arranged to detect, when the engine runs at said low revolution rate, the attenuation time length of the detected voltage after said voltage charging circuit has charged the stray capacity, and to detect, when the engine runs at a high revolution rate, an attenuation time length of the detected voltage immediately after the spark action of said spark plug (3). A misfire detection circuit and an internal combustion engine ignition system according to any one of claims 1 to 6, wherein: said misfire detection circuit (200) further comprises a voltage charging circuit arranged to re-energise said primary coil (L11) so as to induce a voltage in said secondary coil (L22) for electrically charging a stray capacity inherent in said spark plug (3) after the end of the spark action of the spark plug, when the engine runs at a low revolution rate; said voltage detector circuit (206) is arranged to detect, when the engine runs at said low revolution rate, the attenuation time length of the detected voltage after said voltage charging circuit has charged the stray capacity; and the voltage detector circuit (206) is arranged to detect, when the engine runs at a high revolution rate, whether the detected voltage exceeds a predetermined reference level (Vo) and the distinction circuit (207) is arranged to determine, when the engine runs at a high revolution rate, whether or not a spark has ignited the air-fuel mixture on the basis of whether said detected voltage exceeds said reference level. A misfire detection circuit and an internal combustion engine ignition system according to any one of claims 8 to 10, wherein said voltage charging circuit is said interrupter circuit (4,204) of said ignition system (1,201).

Inventors: KAGAWA JUNICHI, MATSUBARA YOSHIHIRO, MIYATA SHIGERU, KAGAWA, JUNICHI, MATSUBARA, YOSHIHIRO, MIYATA, SHIGERU
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1993
Main classification: F02P17
IPC codes: F02P17, H01T13