Device and method for evaluating the heat resistivity of spark plug

Abstract: In a device for evaluating heat resistivity of a spark plug, an ignition spark control circuit is provided to miss out an ignition spark across the electrodes of a spark plug mounted on an internal combustion engine at predetermined intervals. A counter circuit is provided to count the number of times the ignition spark is suppressed by the ignition spark control circuit. A self-ignition detection circuit is provided to detect self-ignition caused by a hot-spot on the spark plug when the ignition spark is suppressed by the ignition spark control circuit. A self-ignition counter circuit is provided to count the number of self-ignitions detected by the self-ignition detection circuit. A calculation circuit is provided to calculate the occurrence rate of self-ignition from the spark plug on the basis of the number of suppressed ignition sparks counted by the counter circuit and the number of detected self-ignitions counted by the self-ignition counter circuit. A self-ignition timing detection circuit is provided to detect the engine timing at which self-ignition is detected by the self-ignition detection circuit. A preignition timing prediction circuit is provided to predict an ignition timing when preignition of the internal combustion engine would occur on the basis of the occurrence rate of self-ignition determined by the calculation circuit and on the timing of the self-ignition detected by the self-ignition timing detection circuit.

Description: This invention relates to a method for evaluating the heat resistivity of a spark plug and a device use in evaluating the same which makes use of the ignition characteristics shown by a spark ignition type internal combustion engine. In general, it is necessary to select some heat value for a spark plug which enables an internal combustion engine to operate normally under all conditions, since the operating heat range of the engine depends on the heat value in which the spark plug works normally in the spark ignition type internal combustion engine. A method of detecting preignition has been utilized to obtain an allowance of the heat range up to an upper limit as a way of estimating the thermal resistivity of the spark plug. So-called preignition is one of the ignition characteristics shown by the internal combustion engine in which an air-fuel vapour mixture is ignited prematurely by a hot spot (in general on a surface of the insulator) of the spark plug before a discharge spark occurs across the electrodes of the spark plug. This type of preignition is uncontrollable. A method involving detecting preignition is a means to check the heat resistivity of a spark plug. In this method, preignition is generally induced by advancing the ignition timing to increase the heat load imposed on the spark plug, and thus raises its temperature so as to measure a thermal allowance of the spark plug. The advantage of this method is that it directly obtains the allowance for thermal resistivity of the spark plug being measured. However, when the ignition timing is advanced so far as to cause preignition, this induces a rapid temperature rise within a combustion chamber of the internal combustion engine, and the repetitive occurrences of preignition poses the problem of doing serious damage to the internal combustion engine. In such an internal combustion engine in which the ignition timing is constrained in the proximity of the knocking limit by a knocking control system or the like, the allowed advancement of the ignition timing is very restricted so that the allowance for the thermal resistivity is not directly obtained since the operating heat value is determined using a lower heat rating of the spark plug than that supposedly used. As well as the preignition method, a postignition method has been utilized which estimates a heat compatibility (heat value) without changing the ignition timing. Postignition is another ignition characteristic of an engine in which the air-fuel vapour mixture is ignited after the discharge spark occurs across the electrodes of the spark plug. Postignition is further divided into two types. One is controlled ignition as caused by the normal ignition spark, and another is ignition uncontrolled by the ignition spark (self-ignition). The latter type of uncontrollable ignition is generally caused by a hot surface on which deposits have built up, on the spark plug and the combustion chamber of the internal combustion engine. The postignition method is generally concerned with the latter type of the uncontrollable ignition which utilizes ignition from a hot spot of the spark plug so as to check the heat resistivity for the spark plug. In the postignition method, when the ignition spark which would occur at a normal advancement timing of the ignition is suppressed at regular intervals, and the assumption is made that ignition does not occur due to the deposits on the hot spot of the spark plug or the like, a rate of postignition occurrence can be determined on the basis of the occurrences of the postignition so as to measure a degree of the thermal allowance of the spark plug. The advantage of postignition method is that the heat value can be estimated without advancing the ignition, thus obviating the serious damage to the internal combustion engine as done in the preignition method. However, in this type of postignition method, the thermal allowance of the spark plug obtained is determined whether or not the heat value of the spark plug is appropriate only for the rate of postignition that occurred. postignition occurrence rate at ignition timing of 20° BTDC ignition timing in which preignition actually occurs (heat value) spark plug A 33.8% 31° BTDC spark plug B 70.2% 31° BTDC In the spark plugs A, B of different internal structure, but same heat value as shown in TABLE 1, the rate of the postignition occurrence differs between the spark plugs A, B when the ignition timing is measured in terms of 20° BTDC (Before Top of Dead Center) because the former rate is 33.8%, and the latter 70.2%. By way of illustration, when the heat value of the spark plug is determined with this type of postignition method and the rate of postignition occurrences below 50%, the spark plug B is mistaken for having a lower heat value although the spark plugs A, B are of the same heat value. DE-A-3 921 616 and US-A-5 005 547 each disclose a device which retards the spark pulse to the spark plug in one cylinder of an engine. By delaying the spark, post-ignition can be detected and counted, for example by ionic current, optical fibre, or other means such as the rate of change of rotational speed or torque of the engine. Post ignition gives a warning that pre-ignition may be liable to occur if the engine gets any hotter. Consequently, the device retards the ignition timing of all of the cylinders, or takes other measures to cool the engine and prevent damage caused by pre-ignition. Therefore, it is one of the objects of the invention to provide a method for evaluating the thermal resistivity of a spark plug and a device for use in evaluating the same which is capable of measuring the heat resistivity of a spark plug without doing serious damage to an internal combustion engine, and without changing the ignition timing at which a discharge spark occurs across electrodes of the spark plug. It is also one of the objects of the invention to provide a method for use in evaluating the thermal resistivity of a spark plug and a device for evaluating the same which is capable of predicting the ignition timing at which preignition would occur with a given spark plug. Further, it is one of the objects of the invention to provide a method for evaluating the heat resistivity of a spark plug and a device for evaluating the same which is capable of correctly selecting a heat value compatible with the spark plug by estimating its heat resistivity. According to the invention, there is provided a method for evaluating the heat resistivity of a spark plug, comprising: (a) a first step of detecting self-ignition in an internal combustion engine when an ignition spark across electrodes of the spark plug is suppressed; (b) a second step of determining the occurrence rate of self-ignition in the internal combustion engine by counting the number of self-ignitions detected in the first step and the number of suppressed ignition sparks; (c) a third step of predicting the ignition timing when preignition of the internal combustion engine will occur on the basis of the occurrence rate of self-ignition and the detected timing of self-ignition in the internal combustion engine; and (d) a fourth step of evaluating the heat resistivity allowance of the spark plug on the basis of the ignition timing at which preignition is predicted in the third step. According further to the invention, there is provided a method for evaluating the heat resistivity of a spark plug in which detection of self-ignition in the internal combustion engine is based on ionic current. According still further to the invention, there is provided a device for use in evaluating the heat resistivity of a spark plug, comprising: (a) an ignition spark control means provided to suppress an ignition spark across the electrodes of a spark plug at predetermined intervals; (b) a counter means provided to count the number of ignition sparks which are suppressed by the ignition spark control means; (c) a self-ignition detection means provided to detect self-ignition from the spark plug when the ignition spark is suppressed by the ignition spark control means; (d) a self-ignition counter means provided to count the number of self-ignitions detected by the self-ignition detecting means; (e) a calculation means provided to calculate the occurrence rate of self-ignition from the spark plug on the basis of the number of suppressed ignition sparks counted by the counter means and the number of detected self-ignitions counted by the self-ignition counter means; (f) a self-ignition timing detection means provided to detect the timing of each self-ignition which is detected by the self-ignition detecting means; and (g) a preignition timing predicting means provided to predict the ignition timing when preignition of the internal combustion engine would occur on the basis of the occurrence rate of self-ignition calculated by the calculation means and of the timing of the self-ignition detected by the self-ignition timing detection means. According stillmore to the invention, there is provided a device for use in evaluating heat resistivity of a spark plug, including a heat resistivity estimation means to obtain the heat resistivity allowance based on the ignition timing at which an ignition spark occurs across electrodes of the spark plug, and the ignition timing at which preignition from the spark plug is predicted to occur by the preignition timing predicting means. With the structure of the invention, the ignition timing at which preignition would occur is predicted on the basis of the detected timing of self-ignition and the occurrence rate of self-ignition determined from the number of suppressed ignition sparks. On the basis of the ignition timing at which preignition is predicted, an allowance degree for the heat resistivity of the spark plug is estimated, thus making it possible to estimate the heat resistivity of the spark plug without changing the ignition timing and doing serious damage to the internal combustion engine. With the ignition spark control means, it is possible to suppress the ignition spark at regular intervals, and to count the number of ignition sparks suppressed using the ignition spark suppression counter circuit. With the aid of the self-ignition detecting circuit, self-ignition is detected when the ignition spark is suppressed, and the number of detected self-ignitions is counted by the self-ignition counter. With the provision of the calculation circuit the occurrence rate of self-ignition from the spark plug is calculated on the basis of the number of suppressed ignition sparks and the number of detected self-ignitions. The timing of the self-ignition is detected by the self-ignition timing detecting circuit. By means cf the preignition timing prediction circuit, it is possible to predict the ignition timing when preignition in the internal combustion engine would occur on the basis of the occurrence rate of self-ignition determined by the calculation circuit and the detected timing of the self-ignition detected by the self-ignition timing detection circuit. This makes it possible to estimate the heat resistivity of the spark plug without changing the ignition timing and doing serious damage to the internal combustion engine. With the provision of the heat resistivity estimation circuit, it is possible to obtain a heat resistivity allowance based on an ignition timing at which an ignition spark occurs across electrodes of the spark plug, and an ignition timing at which the preignition occurs from the spark plug as forecast by the preignition timing prediction circuit. A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which :- Fig. 1 is a block diagram according to an embodiment of the invention; and Fig. 2 is a flow chart showing a method of evaluating the heat resistivity of a spark plug according to an embodiment of the invention. Referring to Fig. 1, which shows a postignition measuring device 1 for evaluating the heat resistivity of a spark plug 2, the postignition measuring device 1 is to select a heat value compatible with the spark plug 2 by measuring a thermal allowance of the heat resistivity of the spark plug 2. The postignition measuring device 1 includes an ignition spark detecting circuit 3, an ignition spark counter circuit 4, an ignition timing determining circuit 5, an ignition control circuit 6, an ionic current detecting circuit 7, an engine revolution sensor 8, a crank angle sensor 9 and a postignition determining circuit 10. The spark plug 2, across which a high voltage is applied by means of an ignition coil 12, is mounted on a cylinder of an experimental test engine 11. The ignition spark detecting circuit 3 detects when an ignition spark occurs by detecting a secondary voltage signal (high voltage signal) which is applied to the spark plug 2 by way of the ignition coil 12. The ignition spark counter circuit 4 counts the occurrence of ignition sparks detected by the ignition spark detecting circuit 3. The ignition timing determining circuit 5 determines the predetermined time at which to establish a discharge spark across the electrodes of the spark plug 2 (the ignition timing). The ignition control circuit 6 controls the ignition timing, at the same time, serving as a misfire controlling means to suppress the ignition spark once when the number of occurrences of the ignition spark reaches a certain value or after a certain interval of time. In this instance, it is observed that the ignition spark may be suppressed e.g. every 1.2 seconds. The ionic current detecting circuit 7 includes a high voltage diode, and works as a self-ignition detecting means which detects whether or not a postignition self-ignition occurs by detecting whether or not an ionic current is present between the electrodes of the spark plug 2. The engine revolution sensor 8 is of well-known structure which detects the speed of revolution (NE) of an experimental test engine 11, and feeds a revolution signal to the postignition determining circuit 10. The crank angle sensor 9 is also of well-known structure which detects the crank angle G (crank position) of the experimental test engine 11, and feeds a crank angle signal to the postignition determining circuit 10. The postignition determining circuit 10 includes a misfire counter circuit 21, a self-ignition counter circuit 22, an occurrence rate calculation circuit 23, a self-ignition timing detection circuit 24, a preignition timing calculation circuit 25 and a heat resistivity evaluation circuit 26. The misfire counter circuit 21 counts the number ignition sparks suppressed by the ignition control circuit 6. The self-ignition counter circuit 22 counts the number of postignitions detected by the ionic current detecting circuit 7. The occurrence rate calculation circuit 23 calculates the occurrence rate (H) of postignition on the basis of the number (c) of ignition sparks suppressed counted by the misfire counter circuit 21, and the detected number (p) of the postignitions counted by the self-ignition counter circuit 22. The self-ignition timing detection circuit 24 detects the timing at which postignition occurs by measuring the time from the actual timing set by the ignition timing determining circuit 5 until the ionic current is detected by the ionic current detection circuit 7 (the hot-spot ignition delay time). The preignition timing calculation circuit 25 is provided to predict the ignition timing (P) at which preignition would occur. The ignition timing (P) is calculated based on an average value (t) of the postignition timing and on the occurrence rate (H) of the postignition calculated by the occurrence rate calculation circuit 23. The heat resistivity evaluation circuit 26 is provided to evaluate the heat resistivity (compatible heat value) of the spark plug 2 by comparing the ignition timing (P) calculated by the preignition timing calculation circuit 25 with the thermal allowance for heat resistivity (F) (e.g., 10°-20°). With the structure of the postignition measuring device 1 thus far described the postignition measuring device 1 is operated in accordance with a flow chart shown in Fig. 2 which depicts a method for evaluating the heat resistivity of the spark plug 2. In this instance, the postignition is measured at every 2.5 CA of the crank angle, and is carried out after a flying interval (e.g., 30 sec) to thermally stabilize the spark plug 2 when the test is resumed under different conditions. STAGE 1 Firstly, initialization is carried out at stage 1. STAGE 2 Then, ignition sparks across the electrodes of the spark plug 2 are detected by the ignition spark detection circuit 3, and the number of ignition sparks is counted by the ignition spark counter circuit 4. STAGE 3 It is determined whether the number of the ignition sparks counted amounts to a predetermined value. Also, it is determined whether a predetermined time (e.g. 1.2 sec) has passed. When the answer in both cases is in the negative, the procedure returns to stage 2. STAGE 4 Upon answering in the affirmative at stage 3, the procedure passes to stage 4 in which an ignition spark is suppressed by the ignition control circuit 6 and the number of the ignition sparks suppressed is counted by the misfire counter circuit 21. STAGE 5 In stage 5 (first step), it is recognized whether an ionic current is detected by the ionic current detection circuit 7. When the answer is in the negative, the procedure returns to stage 2. STAGE 6 Upon answering in the affirmative at stage 5, the procedure passes to stage 6 to measure the hot-spot ignition delay time, which is the period from the actual timing (T) set by the ignition timing determining circuit 5 until the ionic current is detected by the ionic current detection circuit 7. STAGE 7 Based on the speed of revolution (NE) of the engine, a crank angle is determined which corresponds to the hot-spot ignition delay time. The crank angle is calculated by the ignition timing (T) when the ionic current is detected. STAGE 8 It is recognized whether the detected ionic current is due to an entry of exhaust gas from another cylinder when an exhaust valve of the test cylinder of the experimental test engine 11 is opened. That is, it is recognized whether the crank angle detected by the crank angle sensor 9 is before or after 120°-140° after the top dead center (TDC) at the time when the ionic current is detected. Upon answering in the affirmative, the procedure returns to stage 2 without resorting to the self-ignition counter circuit 22. When answering in the negative at stage 8, the procedure advances to stage 9 (second step). STAGE 9 Based on the number (c) of suppressed ignition sparks counted by the misfire counter circuit 21 and on the detected number (p) of postignitions counted by the self-ignition counter circuit 22, the occurrence rate of postignition (H) is calculated as follows : H = p/c STAGE 10 In stage 10, the hot-spot delay time is used to produced the average value (t) of the postignition occurrence timing. STAGE 11 Then, it is determined whether the number of the sample is statistically significant. Namely, it is determined whether the occurrence number (p) of postignition has reached a predetermined value (e.g. between 300 and 500 times). When answering in the negative, the procedure returns to stage 2. Upon answering in the affirmative, the procedure advances to stage 12. STAGE 12 In stage 12 (third step), based on the average value (t) of the postignition timing detected by the self-ignition timing detection circuit 24 of the preignition timing calculation circuit 25 and on the occurrence rate (H) of postignition calculated by the occurrence calculation circuit 23, the ignition timing (P) is predicted in accordance with the following equation. P = a x T + b x t + c x H Where, P = the ignition timing at which the preignition is predicted to occur, T = a term including the actual ignition timing used when the postignition test is carried out, t = a term including the postignition occurrence timing measured when the postignition test is carried out, H = a term including the postignition occurrence rate attained when the postignition test is carried out, a, b and c = constants. STAGE 13 Then, based on the ignition timing (P) calculated by the preignition timing calculation circuit 25 of the heat resistivity evaluation circuit 26 and on the thermal allowance of the predetermined heat resistivity F (e.g. 10°-20°), the heat resistivity (compatible heat value) is calculated by the following expression. P - T ≥ F Where, P = the ignition timing at which preignition is predicted to occur, T = the actual ignition timing used (experimental test advancement of ignition) when the postignition test is carried out, F = a thermal allowance for the heat resistivity (e.g., 10°). STAGE 14 Then, it is determined whether the expression P - T ≥ 10° is satisfied. Namely, it is determined whether the heat value of the spark plug 2 is compatible based on the ignition timing (P) at which preignition is predicted to occur. STAGE 15 & 16 Upon replying to stage 14 in the affirmative, it is determined in stage 16 (fourth step) that the thermal allowance of the spark plug 2 is satisfactory under the present conditions. When answering in the negative, it is determined in stage 15 (fourth step) that the thermal allowance of the spark plug 2 is insufficient under the present conditions. By this method it can thus be determined whether or not the spark plug being tested is suitable for use with the particular engine under test at the particular running condition of the engine. TABLE 2 shows the postignition occurrence rate, the ignition timing at which preignition occurs, and the predicted preignition timing based on the postignition occurrence timing and the postignition occurrence rate. For this purpose, an experimental test was carried out with spark plugs A, B mounted on four types of engine E1, E2, E3 and E4. The spark plugs A, B are of different internal structure, but of the same heat value which means that the spark plugs A, B have the same ignition timing at which preignition occurs. Various dimensions of engines E1, E2, E3 and E4 used in the experimental test are indicated in TABLE 3, and dimensions of the spark plugs A, B are indicated in TABLE 4. The engines E1, E2 are of a natural intake type (normally aspirated , NA) for an automotive vehicle, and the engine E3 with a turbocharger is for a light automotive vehicle. The engine E4 is of an air-cooled type for a two-wheel vehicle. The leg length in TABLE 4 indicates a dimension from a front end of a firing portion of the spark plug insulator to a shouldered seat of the metallic shell of the spark plug. "Diametrical difference" indicates the dimension between the outer periphery of the centre electrode and the inner surface of the axial bore of the firing portion of the insulator. The center electrode includes a composite electrode structure consisting of a copper core clad by an alloyed metal (Ni-1.5 Si-1.5 Cr-2 Mn). engine (Ig . T=ignition timing) spark plug postignition occurrence rate ignition timing in which preignition actually occurs ignition timing in which anticipated preignition occurs E1 (5000rpm) (Ig . T=32° BTDC) A 33.8 % 31° BTDC 31.6° BTDC B 70.2 % 31° BTDC 30.1° BTDC E2 (3500rpm) (Ig . T=20 ° BTDC) A 38.0 % 44° BTDC 45.4° BTDC B 75.6 % 44° BTDC 44.3° BTDC E3 (7300rpm) (Ig . T=20° BTDC) A 31.6 % 33° BTDC 32.4° BTDC B 87.6 % 33° BTDC 31.6° BTDC E4 (9000rpm) (Ig . T=25° BTDC) A 53.4 % 36° BTDC 36.4° BTDC B 99.8 % 36° BTDC 35.3° BTDC engine E1 engine E2 engine E3 engine E4 displacement (cm 3 ) 1997 2500 659 124 bore × stroke (mm) 81 × 95 84 × 75 68 × 60.5 56.5 × 49.5 compression ratio (ε) 9.4 10.0 8.0 9.4 number of valve/cylinder 4 4 4 2 max. output (PS/rpm) 145/6000 192/5900 64/7500 14/10000 max. torque(kgm/rpm) 17.8/4500 25.0/4700 9.4/4000 1.0/9000 intake system NA NA TC NA spark plug A spark plug B leg length (mm) 16 17 diametrical difference 0.135 0.094 constituent of insulator 94.8 % alumina by weight 94.8 % alumina by weight center electrode composite structure composite structure As shown in TABLE 2, it is found that the postignition occurrence rate differs significantly between the four engines E1, E2, E3 and E4. The ignition timing of the preignition occurrence calculated from taking the postignition occurrence rate and the timing of the postignition occurrence into consideration, shows that it is confirmed that this ignition timing is substantially the same as the ignition timing of the preignition occurrence actually measured by gradually advancing the ignition. This means that the heat value inherent in the spark plug can be measured by the postignition method. Namely, it is possible to predict the ignition timing of preignition occurrence by taking into consideration the ignition timing of the preignition occurrence calculated from the postignition occurrence rate and from the timing of the postignition occurrence. After checking 102 spark plugs of 36 types with regard to the engines E1, E2, E3 and E4, a slight difference of ±2° CA is observed in terms of crank angle between the ignition timing at which preignition actually occurs and the ignition timing measured according to the present postignition method. Therefore, the postignition method obviates the possibility of inviting errors at the time when selecting the compatible heat value of the spark plug only by the postignition occurrence rate, and thus makes it possible to measure precisely the compatible heat value of the spark plug.

Claims: A method for evaluating the heat resistivity of a spark plug comprising: (a) a first step (S5) of detecting self-ignition in an internal combustion engine when an ignition spark across electrodes of the spark plug is suppressed; (b) a second step (S9) of determining the occurrence rate of self-ignition in the internal combustion engine by counting the number of self-ignitions detected in the first step and the number of suppressed ignition sparks; (c) a third step (S12) of predicting the ignition timing when preignition of the internal combustion engine will occur on the basis of the occurrence rate of self-ignition and the detected timing of self-ignition in the internal combustion engine; and (d) a fourth step (S15,S16) of evaluating the heat resistivity allowance of the spark plug on the basis of the ignition timing at which preignition is predicted in the third step. A method according to claim 1, wherein detection of self-ignition in the internal combustion engine in the first step (S5) is based on ionic current. A method according to claim 1, including the step of determining whether the spark plug (2) is compatible with the internal combustion engine (11) being used and with the operation conditions of the engine. A device (1) for use in evaluating the heat resistivity of a spark plug (2), comprising; (a) an ignition spark control means (6) provided to suppress an ignition spark across the electrodes of a spark plug (2) at predetermine intervals; (b) a counter means (21) provided to count the number of ignition sparks which are suppressed by the ignition spark control means (6); (c) a self-ignition detection means (7) provided to detect self-ignition from the spark plug (2) when the ignition spark is suppressed by the ignition spark control means (6); (d) a self-ignition counter means (22) provided to count the number of self ignitions detected by the self-ignition detecting means (7); (e) a calculation means (23) provided to calculate the occurrence rate of self-ignition from the spark plug (2) on the basis of the number of suppressed ignition sparks counted by the counter means (21) and the number of detected self-ignitions counted by the self-ignition counter means (22); (f) a self-ignition timing detection means (24) provided to detect the timing of each self-ignition which is detected by the self-ignition detecting means (7); and (g) a preignition timing predicting means (25) provided to predict the ignition timing when preignition of the internal combustion engine would occur on the basis of the occurrence rate of self-ignition calculated by the calculation means (23) and of the timing of the self-ignition detected by the self-ignition timing detection means 24). A device for use in evaluating heat resistivity of a spark plug according to claim 4, including a heat resistivity estimation means (26) to obtain the heat resistivity allowance based on the ignition timing at which an ignition spark occurs across electrodes of the spark plug (2), and the ignition timing at which preignition from the spark plug is predicted to occur by the preignition timing predicting means (25).

Inventors: ISHIKAWA MASAHIRO, MATSUBARA YOSHIHIRO, ISHIKAWA, MASAHIRO, MATSUBARA, YOSHIHIRO
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1995
Main classification: F02P17
IPC codes: F02P17, F02P5, F02P9, H01T15