Spark Ignition Timing Control System for an Internal Combustion Engine With Retarding Means Responsive to Transmission Gear Shifting

The present invention relates generally to a spark ignition timing control system for a spark ignition internal combustion engine for controlling spark advance corresponding to the engine operating condition. More specifically, the invention relates to a spark ignition timing control system for controlling spark advance in response to an engine knock sensor signal and transmission gear shifting.

Spark ignition internal combustion engines in automotive vehicle use a basic spark timing setting which is advanced with manifold vacuum and engine speed in accordance with a predetermined spark timing schedule. The basic setting, the speed advance, and the amount of vacuum advance, are selected to provide spark timing sufficiently retarded from the timing that creates objectionable engine knock. In establishing the basic setting and the advance characteristics, account is taken of the variations of knocking quality of the fuels likely to be used, the various engine operating conditions likely to be encountered, likely engine deterioration, and other factors that may require greater ignition retard to avoid engine knock. It has not been possible for the conventional spark advance controls to provide the more efficient and responsive engine operation and other advantages that could be obtained if engine knock at undesirable levels was precluded.

With regard to the more effective spark advance controls capable of application to spark ignition internal combustion engines to preclude operation at undesirable levels of engine knock, one approach had been made, which was described in U.S. Pat. No. 4,002,155 to John L. Harned et al, issued on Jan. 11, 1977. Harned et al shows an engine and engine spark timing control with a knock limiting circuit which includes a knock detector for sensing engine knock and a digital programmed spark timing controller. The knock detector produces a detector signal. The number of individual ringing vibrations exceeding a reference signal during a predetermined amount of engine crankshaft rotation is counted. When the number of such counts exceeds a predetermined crankshaft rotation, engine spark timing is retarded. If the number of such counts is less than a predetermined number during the predetermined crankshaft rotation, engine spark is advanced.

In such control systems, feedback control lags due to a lag of response in the system. Although such lag of response may be ignored in the normal and stable driving condition, this lag will possibly cause engine knocking upon shifting-up the transmission gear, particularly in the relatively high or full load condition. Namely, when an automatic transmission shifts up from second gear to top gear or from top gear to over-drive gear, the engine speed is abruptly dropped to the areas possibly causing engine knocking. Therefore, a spark timing control is required which can follow abrupt acceleration or deceleration of the engine and can retard spark timing in response to upshifting of the automatic transmission gear.

Therefore, it is an object of the present invention to provide a spark ignition timing control system which is responsive to upshifting of the transmission gear to retard the spark advance in order to prevent the engine from knocking due to an abrupt drop of the engine speed.

Another object of the invention is to provide a spark ignition timing control which includes means for detecting the engine operating condition in which the transmission gear shifting-up occurs.

To accomplish the above-mentioned and other objects, there is provided a spark ignition timing control, according to the present invention, in which is included means for detecting the automatic transmission gear shifting up. The upshifting detecting means produces a signal indicative of a predetermined retard angle of spark advance in order to retard the spark advance for a given period of time immediately after the transmission gear shifts up.

In the preferred construction, the spark advance is not retarded even when the transmission gear is shifted up, while the engine is in a cold engine condition. This is intended to avoid degrading drivability caused by retarding spark advance and possible knocking in a cold engine.

A further object of the invention is to provide a method for preventing knock in the engine, which otherwise may occur, which method includes detection of the automatic transmission gear shifting and retarding spark advance at a given angle and for a given period of time immediate after the gear is shifted up.

The object is accomplished by a method provided by the present invention, in which an engine operating condition in a vehicle wherein the automatic transmission gear shifting is detected by an intake vacuum sensor, a throttle angle sensor and/or engine speed sensor etc. The retarding angle and duration of the retarded spark advance responsive to gear shifting-up is selected so that it can prevent the engine from causing knock and from degrading drivability.

The present invention will become more fully understood from detailed description given herebelow and from the accompanying drawings. The specific embodiment illustrated on the drawings and described is not to be understood as limitative of the invention but only for the purpose of explanation.

In the drawings:

FIG. 1 is a schematic diagram of a preferred embodiment of a spark ignition timing control system according to the present invention;

FIG. 2 is a block diagram of a knock detecting circuit in the control system of FIG. 1;

FIG. 3 is a circuit diagram showing detailed circuit structure of an input section in the knock detecting circuit of FIG. 2;

FIG. 4 is a circuit diagram showing detailed circuit structure of a smoothing section in the knock detecting circuit of FIG. 2;

FIG. 5 is a circuit diagram showing detailed circuit structure of a comparator section in the knock detecting circuit of FIG. 2;

FIG. 6 is a time chart illustrating functions of the smoothing section and comparator section and waveforms of the outputs thereof;

FIG. 7 is a circuit diagram showing detailed circuit structure of a detecting section in the knock detecting circuit of FIG. 2;

FIG. 8 is a time chart illustrating functions of the detecting section of FIG. 7;

FIG. 9 is a circuit diagram of an arithmetic circuit of FIG. 1;

FIG. 10 is a circuit diagram showing detail of the timer circuit in the shifting-up detector of FIG. 1;

FIG. 11 is a time chart illustrating the function of the timer of FIG. 10.

FIG. 12 is a block diagram of the drive circuit of FIG. 1;

FIG. 13 is a circuit diagram illustrating detailed circuit structure of the frequency/voltage converter in the drive circuit of FIG. 12;

FIG. 14 is a circuit diagram illustrating detailed circuit structure of the advance control section in the drive circuit of FIG. 12;

FIG. 15 is a circuit diagram of a voltage/current converter associated with the advance control section of FIG. 14;

FIG. 16 is a time chart showing functions of the advance control section of FIG. 14;

FIG. 17 is a graph showing the variation of intake vacuum in relation to engine speed;

FIG. 18 is a graph showing the variation of engine speed in a condition of full-throttle acceleration;

FIG. 19 is a graph showing the variation of spark advance according to full-throttle acceleration of the engine; and

FIG. 20 is a graph showing the magnitude of engine knocking in relation to engine speed and transmission gear position.

Now, referring to the preferred embodiment of a spark ignition timing control system of the present invention, which is illustrated generally in FIG. 1, the system comprises a knock detector 100, an arithmetic circuit 200, a shifting-up detector 300, a drive circuit 400 and an ignition device 500 including a distributor 502 and an ignition coil 504. The knock detector 100 is associated with a knock sensor 101.

Incorporation is made by reference of U.S. Pat. No. 4,002,155 with respect to the knock detector 100 inclusive of the knock sensor 101. A vibration sensor for detecting the magnitude of engine vibration and producing a sensor signal having a value proportional to the detected engine vibration magnitude, can be used as the knock sensor 101. Vibration sensors for detecting engine vibrations are per se well known and an appropriate one may be selected for use in the present spark ignition timing control system. Also, other suitable sensors can be used for detecting knock on in the engine. The sensor signal S.sub.1 indicative of detected magnitude of engine knocking is fed to a knock detecting circuit 102.

The knock detecting circuit 102 together with the knock sensor 101 constitutes the knock detector 100. The knock detecting circuit 102 detects a specific frequency range of the sensor signal S.sub.1 which is representative of engine knocking and has a signal value proportional to the magnitude of engine knocking. The specific frequency range of the sensor signal S.sub.1 showing the engine knocking condition is defined depending on each individual engine and may be preset in the knock detecting circuit 102. When the specific frequency range of sensor signal S.sub.1 is detected, the knock detecting circuit 102 produces a knocking pulse signal S.sub.2 having a frequency proportional to the amplitude of the sensor signal S.sub.1, and thus, which is representative of the magnitude of engine knocking.

The arithmetic circuit 200 is responsive to the knocking pulse signal S.sub.2 fed from the knock detecting circuit 102 to produce a low level output S.sub.3. The low level output S.sub.3 of the arithmetic circuit 200 decreases at a predetermined rate. The arithmetic circuit 200 otherwise, produces an output S.sub.4 increasing at a predetermined rate. Here, the increasing rate of the arithmetic circuit output S.sub.4 is higher than the decreasing rate of the low level output S.sub.3. The arithmetic circuit outputs S.sub.3 and S.sub.4 are fed to drive circuit 400.

The drive circuit 400 is responsive to the arithmetic circuit outputs S.sub.3 and S.sub.4 to advance or retard the spark advance in order to control spark timing to maintain the engine in a trace or light knocking condition. The drive circuit 400 produces a control signal S.sub.5 which causes an advance or retardation of the angle of spark advance depending on the arithmetic circuit outputs S.sub.3 and S.sub.4. In practice, the drive circuit 400 produces control signal S.sub.5 to retard spark advance in response to the low level output S.sub.3 and otherwise, to advance the spark advance. The control signal S.sub.5 is fed to the distributor 502 associated with the ignition device 500. The ignition device 500 controls charge timing to the ignition coil 504 to control spark ignition timing.

According to the present invention, the shifting-up detector 300 is provided in the above-illustrated closed loop control circuit for feedback controlling spark ignition timing. The shifting-up detector 300 detects an abrupt drop of engine speed due to the shifting-up of the automatic transmission gear position, for example from second gear to third gear or from third gear to fourth gear, and produces a shift-up signal S.sub.6 for a predetermined period of time. The shift-up signal S.sub.6 is fed to the arithmetic circuit 200. The arithmetic circuit 200 is responsive to the shift-up signal S.sub.6 to produce the low level output S.sub.3 for a duration substantially corresponding to the period of time the shift-up signal S.sub.6 is inputted.

For detecting the gear shift-up in an automatic transmission, engine or engine coolant temperature, intake vacuum pressure, throttle valve angle position and engine speed are detected. When (1) the engine is not in a cold engine condition, (2) the absolute intake vacuum pressure drops to a predetermined value or the throttle valve is in the full-throttle position, and (3) engine speed is above a predetermined value, the shifting-up detector 300 produces the shift-up signal S.sub.6. A engine or engine coolant temperature switch 301 (hereinafter more generally termed an engine temperature switch) is mounted to the engine cylinder block (not shown) to detect the engine or engine coolant temperature and is connected to a vehicle battery 302 in series. The engine temperature switch 301 turns off (opens) while the engine temperature is maintained below a predetermined temperature, for example 40.degree. C. An intake vacuum switch 303 is connected in series to the engine temperature switch 301 and turns on (closes) while the absolute vacuum flowing downstream of a throttle valve 304 in an air intake passage 305 is below a predetermined value, for example, 100 mmHg.

In the preferred embodiment, a diaphragm switch 306 is used for the intake vacuum switch 303. The diaphragm switch 306 comprises a diaphragm housing 307 defining therein two chambers 308 and 309 separated by a diaphragm 310. The chamber 308 is connected to the air intake passage 305 downstream of the throttle valve 304 via a vacuum passage 311. A set spring 312 is disposed within the chamber 308 for providing a set pressure urging the diaphragm 310 toward the chamber 309 at a given pressure. The chamber 309 is opened to the ambient air via an inlet 313. A stem-like movable member 314 is fixed to the diaphragm 310 at one end thereof and extends through the chamber 309. The free end of the movable member 314 opposes a projecting contact switch member 315 of a vacuum switch 316 which is connected with the vehicle battery 302 via the engine or engine coolant temperature switch 301 in series.

A full-throttle switch 317 is connected to the throttle valve 304 via a per se well known mechanical linkage in order to turn on when the throttle valve 304 is placed in an angle position exceeding a predetermined open angle. The full-throttle switch is connected in series to the vehicle battery 302 via the engine temperature switch 301 and in parallel to the vacuum switch 316. An engine speed switch 318 comprises a movable switch member 319 adapted to vary the position thereof depending on the engine speed and a pair of stationary switch members 320 and 321 respectively connected with the vacuum switch 316 and the full-throttle switch 317. The stationary switch members 320 and 321 are located at positions where the movable switch member 319 comes into contact at respectively predetermined engine speeds, for example, 2,000 r.p.m. and 3,400 r.p.m. The predetermined engine speeds are corresponded to shifting-up points for shifting-up the automatic transmission gear under partial and full load conditions. The shifting-up switch 318 is connected to a timer 322 which is turned on for a predetermined period of time in response to closing of the shifting-up switch 318. The timer 322 produces the shift-up signal S.sub.6 when the switch 318 is closed on one of the contacts 320 or 321.

The further detailed circuit structure of the blocks illustrated in FIG. 1 will be described herebelow with reference to FIGS. 2 to 24 together with the function of the system as set forth.

FIGS. 2 to 8 illustrate the detail of the knock detecting circuit 102. As shown in FIG. 2, the knock detecting circuit 102 comprises an input section 110, a smoothing section 130, a comparator section 150 and a detecting section 170. The input section 110 is connected to the knock sensor 101 in order to receive the sensor signal S.sub.1. The input section 110 takes out background noise contained in the sensor signal S.sub.1, amplifies the signal level and rectifies the signal.

It should be noted, although the description herebelow is given for the input section for half-wave rectification, it is possible to use a circuit which can effect full-wave rectification. Also, it is possible to use a band-pass filter for picking up the specific frequency range of the sensor signal which is indicative of engine knocking. As shown in FIG. 3, the input section 110 has a filter 111 comprising resistors R.sub.101, R.sub.102, R.sub.103 and a capacitor C.sub.101. The filter 111 takes out the background noise in the sensor signal S.sub.1. The resistor R.sub.102 of the filter 111 is connected to a positive input of an operational amplifier OP.sub.101 of a half-wave rectifier 112. The operational amplifier OP.sub.101 consists of an amplifier circuit 113 together with resistors R.sub.104, R.sub.105 and R.sub.106. The amplifier circuit 113 is connected with a half-wave rectifier circuit 114 constituted by a diode D.sub.101 and a resistor R.sub.107. The amplifier circuit 113 amplifies the sensor signal passed through the filter 111 at a predetermined value. The half-wave rectifier circuit 114 rectifies the amplified sensor signal to take out any negative component in the signal and to produce a rectifier output S.sub.7, as shown in FIG. 6. The rectifier output S.sub.7 is again amplified in an amplifier circuit 115 which comprises an operational amplifier OP.sub.102 and resistors R.sub.108, R.sub.109 and R.sub.110.

Here, since the knock sensor 101 has a resonating frequency corresponding to possible engine knocking vibration, the sensor signal S.sub.1 level becomes higher when engine knock occurs. In FIG. 6, there is shown an example wherein engine knock occurs at the periods P.sub.1 and P.sub.2.

The rectifier output S.sub.7 amplified through the amplifier circuit 115 is fed to a negative input of an operational amplifier OP.sub.103 of the comparator section 150. Also, input section 110 is connected to the positive input of the operational amplifier OP.sub.103 via the smoothing section 130. As shown in FIG. 4 the smoothing section 130 comprises a resistor R.sub.111 and a capacitor C.sub.102 for smoothing the rectified output S.sub.7 as represented by S.sub.8 in FIG. 6. The smoothed signal S.sub.8 is amplified by an amplifier circuit 131 which comprises an operational amplifier OP.sub.104 and resistors R.sub.112 and R.sub.113. The comparator section 150 includes the operational amplifier OP.sub.103 and resistors R.sub.115 and R.sub.116. The resistor R.sub.115 is interposed between the smoothing section 130 and the positive input of the operational amplifier OP.sub.103. The operational amplifier OP.sub.103 produces pulse signal S.sub.9 which is normally maintained at a high level and, when the value of rectified output S.sub.7 becomes greater than that of the smoothed signal S.sub.8, is lowered from a period of time while the output S.sub.7 is above the signal S.sub.8, as shown in FIG. 6.

In FIG. 7, the detector section 170 receives the pulse signal S.sub.9 fed from the comparator section 150. The pulse signal S.sub.9 is inputted through a diode D.sub.102 to a negative terminal of an operational amplifier OP.sub.105 of an integrating circuit 170 via a resistor R.sub.117. The operational amplifier OP.sub.105 and the resistor R.sub.117 constitute the integrating circuit with a capacitor C.sub.103. The positive terminal of the operational amplifier OP.sub.106 is supplied with a fixed potential level. The integrating circuit 170 further includes a relay switch 171. If the relay switch 171 is open, the negative-going component of the above pulse signal S.sub.9 is integrated with respect to time. The integrating circuit 170 produces integrator output S.sub.10 having a step-like waveform as shown in FIG. 8. On the other hand, a spark command signal S.sub.11 is fed back from the ignition device 500 to an input terminal 172. The spark command signal S.sub.11 is differentiated in a differentiator 173 consisting of a capacitor C.sub.104 and a resistor R.sub.118 with respect to time, and is amplified in an operational amplifier OP.sub.106. Thus, the differentiated signal S.sub.12 having a positive-going component of a fixed width each time the spark command signal S.sub.11 is produced, is produced in the differentiator 173, as shown in FIG. 8. The relay switch 171 is responsive to the positive-going component to close, i.e. each time spark ignition is effected the capacitor C.sub.103 is short-circuited to reset the integrator. Therefore, the integrator output S.sub.10 instantaneously returns to a predetermined level, as shown in FIG. 8. The integrator output S.sub.10 is inputted to a negative input terminal of an operational amplifier OP.sub. 107. The operational amplifier OP.sub.107 compares the integrator output value with the value of a reference signal S.sub.ref supplied to a positive input terminal of the comparator OP.sub.107 from a power source 174. The reference signal S.sub.ref has a constant voltage level obtained by dividing resistors R.sub.119 and R.sub.120. The operational amplifier OP.sub.107 outputs a comparator signal S.sub.13 of relatively low level when the integrator output S.sub.10 exceeds the reference signal S.sub.ref, i.e. during the time T.sub.1 to T.sub.2 in FIG. 8. Thus low level comparator signal S.sub.13 is indicative of the engine knocking condition and serves as knocking pulse signal S.sub.2.

In the particular embodiment, although the reference signal S.sub.ref is shown as being at a constant level, it may be embodied otherwise so as to meet the driving condition of the engine if the resistors R.sub.119 and R.sub.120 are variable depending on the engine driving condition.

As shown in FIG. 9, the arithmetic circuit 200 includes a monostable-multivibrator 202 including an operational amplifier OP.sub.201, a capacitor C.sub.201, resistors R.sub.201 to R.sub.203, and a diode D.sub.201, at its forward stage. The monostable-multivibrator 202 is triggered when the comparator output S.sub.13 changes from high level to low level to output a trigger signal S.sub.14, as shown in FIG. 16. The monostable-multivibrator 202 is connected to an integrator 204 via diodes D.sub.202 and D.sub.203. The integrator 204 is constituted by an amplifier OP.sub.202, a capacitor C.sub.202, and resistors R.sub.204 to R.sub.207. The integrator 204 is connected through two diodes D.sub.202 and D.sub.203, which are disposed with opposing polarities, to the operational amplifier OP.sub.201. Thus the integrator 204 can be set with time constants for two integrating directions independently of each other. Namely, the downward time constant is determined by the resistor R.sub.204 and the capacitor C.sub.202, whereas the upward time constant is determined by the resistor R.sub.205 and the capacitor C.sub.202. A limiting circuit 205 consists of diodes D.sub.204 and D.sub.205, and resistors R.sub.208 to R.sub.211, and limits the operational range of the integrator 204 between 0 and +V.sub.cc of a power supply. Thus when the above-mentioned trigger signal S.sub.14 is applied to the integrator 204, the integrator output S.sub.15 varies in value according to the frequency and signal value of the trigger signal S.sub.14, as shown in FIG. 16. The integrator output S.sub.15 rises gradually so long as the trigger signal S.sub.14 is at a relatively low level, which is limited at an upper value determined by resistors R.sub.210 and R.sub.211, and which rapidly falls when the trigger signal S.sub.14 goes high. As will be clear from FIG. 16, if the respective time constants are selected so that the rising speed is less than the falling speed, and if high levels of the trigger signal S.sub.14 occur very frequently, the integrator output S.sub.15 will fall stepwise and is limited to a lower level defined by dividing resistors R.sub.208 and R.sub.209.

The time interval for which the trigger signal S.sub.14 stays high is a fixed time duration determined by the monostable multivibrator 202 including the amplifier OP.sub.201. The integrated value in the integrator 204 in the downward direction for this high level trigger signal S.sub.14 is constant. This constant value is preferably selected so as to correspond to a 0.5.degree. spark retard angle. Since the magnitude of the integrator output S.sub.15 corresponds to the frequency of occurrence of high levels of the trigger S.sub.14, it can be used as an adjustment value for the spark timing.

The integrator 204 is connected to a polarity inversion circuit 206 consisting of resistors R.sub.212 to R.sub.219 and an operational amplifier OP.sub.203. This circuit inverts the polarity of the incoming integrator output S.sub.15 in order to match the same to the signal of the equal advance angle control section of the drive circuit 400, adjusts the level of the integrator output S.sub.15 and outputs an inverter output which inverter output serves as the arithmetic circuit output S.sub.3 or S.sub.4 shown in FIG. 16.

To the integrator 204 of the arithmetic circuit 200, the timer 322 of the shifting up detector 300 for detecting transmission gear shifting up is connected through a diode D.sub.301. As shown in FIG. 1, the engine temperature switch 301 is turned on (closed) when the detected temperature is above the predetermined temperature. At an engine driving condition where the automatic transmission gear position shifts up the, e.g., from second gear to top gear or from top gear to over-drive gear, the absolute pressure of the intake vacuum and the engine speed reach their respective threshold values. In the preferred embodiment, the automatic transmission shifting up point is set 2,000 r.p.m. of engine speed under -100 mmHg of partial load condition and 3,400 r.p.m. of engine speed under full load condition. Therefore, the intake vacuum switch 303 of the preferred embodiment is adapted to turn on (close) when detected vacuum pressure in the air intake passage 305 downstream of the throttle valve 304 is above -100 mmHg. The set pressure of the set spring 312 of the diaphragm switch 306 is adjusted so that the diaphragm 310 is deformed toward the vacuum switch 316 to push the switch member 315 with the movable member 317 when the intake vacuum pressure greater than -100 mmHg is detected. The vacuum switch 316 is connected to the stationary switch member 320 of the engine speed switch 318. The stationary switch member 320 is adapted to contact with the movable switch member 319 when the detected engine speed is 2,000 r.p.m. Therefore, when the intake vacuum is greater than -100 mmHg and the engine speed is increased to 2,000 r.p.m., the power from the vehicle battery 302 is applied to the timer 322.

On the other hand, a full load condition on the engine is detected by the full-throttle switch 317. The full-throttle switch 317 is connected to the throttle valve 304 with a suitable link mechanism so that it may turn on when the throttle valve 304 is fully open. The full-throttle switch 317 is connected to the stationary switch member 321 of the engine speed switch 318. The movable switch member 319 contacts with the stationary switch member 321 when the engine speed is increased to 3,400 r.p.m. Therefore, the battery power is also applied to the timer 322 when the throttle valve 304 is fully opened and the engine speed reaches 3,400 r.p.m.

Referring to FIG. 10, there is shown a circuit structure of the timer 322. The timer 322 comprises a first differentiator 323, a first inverter 324, a second differentiator 325 and a second inverter 326. The first differentiator 323 includes a resistor R.sub.301 and a capacitor C.sub.301 and differentiates the engine speed switch output to produce differentiator output S.sub.17, as shown in FIG. 11. The first inverter 324 receives the differentiator output S.sub.17, which inverter 324 comprises a resistor R.sub.302, a diode D.sub.301 and a transistor Q.sub.301. The invertor 324 inverts and shapes the differentiator output S.sub.17 to produce negative rectangular pulse S.sub.18 in FIG. 11. The negative rectangular pulse S.sub.18 is again differentiated through the second differentiator 325 which comprises a resistor R.sub.303 and a capacitor C.sub.302. A differentiator output S.sub.19 is again inverted to a positive rectangular pulse serving as the shift-up signal S.sub.6. The shift-up signal S.sub.6 has a duration corresponding to a predetermined retard angle. The second inverter 326 thus includes a resistor R.sub.304, a diode D.sub.302 and a transistor Q.sub.302 which is turned off in response to the differentiator output S.sub.19 to produce the constant duration of shift-up signal S.sub.6.

The shift-up signal S.sub.6 is fed to the integrator 204 of the arithmetic circuit 200 via the diode D.sub.301, as set forth. The shift-up signal S.sub.6 is inputted to the negative input terminal of the operational amplifier OP.sub.202. The integrator output S.sub.15 representative of a predetermined retard angle is thus produced in a period of time corresponding to the duration of the shift-up signal S.sub.6, instead of taking the trigger signal S.sub.14 from the monostable multivibrator 202 into account.

The arithmetic circuit output S.sub.3 or S.sub.4 is fed to the drive circuit 400 which comprises a frequency/voltage convertor 410 and a spark advancer 430, as shown in FIG. 12. The arithmetic circuit output S.sub.3 or S.sub.4 is fed to the spark advancer 430 to advance or retard the spark advance angle. To the spark advancer 430, a feedback signal S.sub.11 is fed from a ignition device 500 via the frequency/voltage converter 410.

FIG. 13 shows a frequency/voltage converter 410 which produces an analog signal S.sub.20 having a value proportional to the engine speed based on the frequency of occurrence of the feedback signal S.sub.11. A monostable multivibrator 412, formed by a capacitor C.sub.401, resistors R.sub.401 to R.sub.404, a diode D.sub.401, and an operational amplifier OP.sub.401, converts the spark command signal as feedback signal S.sub.11 to a pulse signal S.sub.21 having a constant duration. This pulse signal S.sub.11 is converted to an analog signal S.sub.20 having a value proportional to the frequency of the pulse signal by a smoothing circuit 414 consisting of resistors R.sub.405 to R.sub.415, a capacitor C.sub.402 and operational amplifiers OP.sub.402 and OP.sub.403. The analog signal S.sub.20 indicative of the engine speed and is fed to the spark advancer 430.

As shown in FIG. 14, the spark advancer 430 includes a differentiating circuit 432 consisting of transistors Q.sub.401 to Q.sub.403, resistors R.sub.416 to R.sub.419, and a capacitor C.sub.403. This differentiating circuit 432 is also connected to the ignition device 500 to receive the feedback signal S.sub.11. The differentiating circuit 432 differentiates the feedback signal S.sub.11 from the distributor 502 of the ignition device 500 with respect to time, renders the transistor Q.sub.403 conductive at each rising edge of the feedback signal S.sub.11, and short-circuits a capacitor C.sub.404 for resetting purposes. In response to short-circuiting the capacitor C.sub.404, the transistor Q.sub.403 is rendered nonconductive. Then, the capacitor S.sub.404 is charged with an electric current fed from a voltage/current converter 436. The voltage/current converter 436 receives the analog signal S.sub.20 fed from the frequency/voltage converter 410. Thus, the value of the electric current S.sub.23 is proportional to the engine speed. At this time, the potential at a terminal 438 varies as shown in FIG. 16.

The voltage/current converter 436 has the structure shown in FIG. 15 in which the analog signal S.sub.20 proportional to engine speed is supplied through a resistor R.sub.420 to a plus input terminal of a differential amplifier OP.sub.404. Here assuming the potential at the output terminal 440 is V.sub.440 and the resistors R.sub.421 to R.sub.424 being of the same resistance value, the potential applied to the positive input terminal of the differential amplifier OP.sub.404 is (S.sub.20 +V.sub.440)/2, because of the feedback circuit including the differential amplifier OP.sub.405 and resistor R.sub.423, and the potential applied to the negative terminal of the differential amplifier OP.sub.404 is half of the output value of the differential amplifier OP.sub.404. The differential amplifier OP.sub.404 operates such that the output thereof becomes S.sub.20 +V.sub.440 under the presence of the feedback circuit. Thus the voltage across the resistor R.sub.424 is S.sub.20 (=S.sub.20 +V.sub.440 -V.sub.440). The electric current flowing through this resistor R.sub.424, i.e. the output current from the terminal 440 is S.sub.20 /R.sub.424 which is always proportional to the potential of the analog signal S.sub.20 which in turn is proportional to the engine speed. Accordingly, the output current from the terminal 440 is controlled by the analog signal value S.sub.20.

Returning to FIG. 14, the charging speed of the capacitor C.sub.404 is determined by the engine speed and takes the equal advance angle integration wave as illustrated in FIG. 16 as the potential at the terminal 438.

This waveform from the terminal 438 is inputted to a plus input terminal of an operational amplifier OP.sub.406 to a minus input terminal of which is inputted the signal S.sub.3 or S.sub.4. The operational amplifier OP.sub.406 outputs a retard angle signal S.sub.24 of negative-going pulses whose pulse widths represent the intervals when the signal 438 is at a lower level than the arithmetic circuit output S.sub.3, the retard angle signal S.sub.24 being as shown in FIG. 16. If it is arranged that the equal advance integration waveform at the terminal 438 is saturated at a crank angle of 30.degree., as shown in FIG. 16, the width of the negative-going pulse of the retard angle signal S.sub.24 will not exceed a crank angle of 30.degree.. Alternatively, it may be arranged that the retard angle of spark advance is never allowed to exceed 30.degree. even if erroneous operation takes place, thereby preventing stalling of the engine.

The retard angle signal S.sub.24 and the spark command signal S.sub.11 are inputted to an AND gate G.sub.401 which outputs the control signal S.sub.5 shown in FIG. 16 defined by the logical product of both input signals. This control signal S.sub.5 is fed to the ignition coil 504 through the distributor 502 shown in FIG. 1, thereby causing sparks at respective adjusted time points, and suppressing the occurrence of knocking.

FIG. 17 shows a relationship between the engine speed and the intake vacuum as an example. As apparent from FIG. 17, the automatic transmission is shifted from second gear to third or top gear at the engine speed 2,000 r.p.m. under partial load condition. Under partial engine load condition, the engine speed 2,000 r.p.m. corresponds to -100 mmHg of intake vacuum. On the other hand, the automatic transmission is shifted from second gear to third or top gear at the engine speed 3,400 r.p.m. under full load conditions. FIG. 18 shows variation of engine speed at the transition condition in which the automatic transmission is shifted from second gear to third or top gear. The example shown in FIG. 18 is a variation of engine speed under full load or full throttle condition. When the transmission gear position is shifted from second gear position to third or top gear position, the engine speed drops from 3,400 r.p.m. to 2,000 r.p.m., abruptly.

FIG. 19 shows the variation of spark ignition timing in response to the shifting of transmission gear position from second gear to top gear. In FIG. 19, the variation characteristic illustrated by the broken line shows the variation of the spark ignition timing under conventional spark ignition control system. On the other hand, the characteristic illustrated by the solid line in FIG. 19 is the improved spark ignition timing in which the spark advance is retard at a controlled value under control of the control system of the present invention. FIG. 20 shows the magnitude of knocking caused in the conventional spark ignition timing control. As seen from FIG. 20, under the conventional control, the magnitude of knocking is increased from a no-knock condition to a light knock condition in response to shifting up from second gear to third gear of the automatic transmission under full load condition.

According to the present invention, increasing of knocking magnitude is prevented by retarding spark advance in response to shifting up of the automatic transmission gear position. Therefore, the invention fulfills the objects and advantages sought therefor.