SLIP SUPPRESSIVE DRIVE CONTROL SYSTEM FOR AUTOMOTIVE VEHICLE WITH REDUCTION OF POWER OUTPUT OF INTERNAL COMBUSTION ENGINE

Abstract: A slip suppressive drive control system derives an optimal wheel slippage for slip control mode operation on the basis of an actual engine speed, a vehicle speed and the wheel speed of a driving wheel. A target engine speed is derived based on the derived optimal wheel slippage so that optimal wheel slippage is obtained at the target engine speed. The system is responsive to wheel slippage which is derived on the basis of the vehicle speed and wheel speed on the driving wheel, to perform slip control mode operation by performing fuel cut-off to shut down fuel supply for the engine in order to adjust the actual engine speed to the target engine speed.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a control system for adjusting driving torque to be exerted on an automotive driving wheel for presenting the latter from causing wheel slip which is otherwise caused due to loss of road traction. More specifically, the invention relates to a slip suppressive drive control system which can reduce power output of an automotive internal combustion engine in response to excessive wheel slippage for recovery of road traction of wheel. Description of the Background Art One of the typical construction of slip control system for adjusting driving torque to be applied to a road wheel, has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 58-38347. The disclosed slip control system derives a wheel slippage on the basis of difference of rotation speed of driving and driven wheels. The slip control system performs fuel cut-off for reducing engine output when wheel slippage is detected. In such type of slip control, hunting in slip control fuel cut-off mode and normal fuel supply mode tends to be caused in certain vehicle driving condition. Namely, when wheel slippage is caused, fuel cut-off mode operation is performed. Fuel supply system is maintained in fuel cut-off mode while wheel slippage is greater than a predetermined wheel slippage threshold. As soon as wheel slippage drops below the wheel slippage threshold, the fuel supply system is switched into fuel supply mode. By this, engine output is instantly resumed to exert driving torque on the road wheel. This again causes wheel slippage greater than the wheel slippage threshold. By repeating fuel cut-off and fuel resumption, jerking of vehicular body is caused to degrade vehicle riding comfort. On the other hand, Japanese Patent First Publication (Tokkai) Showa 58-202142 discloses another type slip control system, in which a brake is used to decelerate the road wheel in response to wheel slippage. In such system pressurized brake fluid is required to be constantly supplied. For this purpose, the system needs a pressure medium source which is maintained in operation while the vehicle is driven. This clearly increases the cost for fabricating the system. Japanese Patent First Publication (Tokkai) Showa 59-18251 discloses a still another type of slip control system, in which a throttle valve angular position is adjusted for adjusting driving torque to be exerted on the driving wheels. In this system, the throttle open angle is reduced in response to the wheel slippage for reducing engine load and thereby reducing the engine output. In order to achieve satisfactorily quick response in response to the wheel slippage, a precise and high response actuator is required in place of the conventional mechanical linkage system, for achieving quick response in adjustment of the throttle valve angular position. This clearly causes high cost. A slip control system according to the preamble of claim 1 is disclosed in EP-A-0 240 174. It comprises first sensor means for monitoring the rotation speed of the driving wheel, second sensor means for monitoring the rotation speed of a driven wheel, and a means for detecting wheel slippage on the basis of the data from said sensors. Additionally, the latter means calculates changes in the wheel slippage. As soon as the wheel slippage and/or the change of the wheel slippage decreases a respective threshold, the fuel supply to the engine is cut off. It is the object of the present invention to provide a simple slip control system having stable control characteristics. The slip control system of the present invention comprises the just mentioned two sensor means and the means for wheel slippage detection, and additionally it comprises: third sensor means for monitoring the engine revolution speed to produce engine speed indicative data; and fifth means being responsive to said fourth means for initiating a slip control mode operation, as soon as the wheel slippage greater than the predetermined wheel slippage threshold is detected, for initiating a slip control mode operation in which a target engine speed achieving a predetermined allowable wheel slippage is derived on the basis of said engine speed data, said driving wheels speed representative data and said vehicle speed representative data, and for controlling the engine revolution speed towards the target engine speed. The system of the present invention stably maintains the wheel slippage at the optimal wheel slippage level without hunting in switching operation mode between a slip control mode and a normal mode. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic block diagram of the first embodiment of a slip control system according to the present invention; Fig. 2 is a block diagram of a circuitry of the first embodiment of the slip control system of Fig. 1; Fig. 3 is a flowchart of a main routine of a slip suppressive drive control; Fig. 4 is a flowchart of a sub-routine which is triggered in the slip suppressive drive control main routine of Fig. 3, which sub-routine governs fuel cut-off operation in slip control mode operation; Fig. 5 is a flowchart of a routine for controlling fuel injection; Fig. 6 is a flowchart of a routine for deriving duty cycle of fuel cut-off operation in view of catalytic converter temperature; Fig. 7 is a timing chart showing variation of fuel injection counter in fuel cut-off control; Fig. 8 is a timing chart showing switching timing of fuel cut-off and fuel supply; Fig. 9 is a chart showing time ratio of fuel cut-off period and fuel supply period in relation to an engine speed; Fig. 10 is a time chart showing fuel cut-off timer ratio in catalytic converter temperature dependent fuel cut-off control; Fig. 11 is a chart showing variation of wheel speed, in which wheel speed variation upon wheel slippage occurs on driving wheel; Fig. 12 is a chart showing variation of engine speed upon occurrence of wheel slip on driving wheel; Fig. 13 is a block diagram of another embodiment of slip suppressive drive control system, according to the present invention; Fig. 14 is flowchart of a transmission shift control routine in the second embodiment of the slip suppressive drive control system of Fig. 13; Fig. 15 is flowchart of a modified transmission shift control routine in the second embodiment of the slip suppressive drive control system of Fig. 13; and Fig. 16 is a chart showing schedule of variation of transmission gear ratio. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1, the first embodiment of a slip suppressive drive control system, according to the present invention, is applied for a fuel injection internal combustion engine. As is well known, the engine 10 is associated with an induction system 12 which includes an induction pipe 14, a throttle chamber 16 and an intake manifold 18. A throttle valve 20 is disposed in the throttle chamber 16 for adjusting an intake air path area and whereby adjusting intake air flow rate. The intake air flow rate as an engine load representative data Q is monitored by an air flow meter 22. The air flow meter 22 produces an air flow rate indicative signal S Q . The air flow meter 22 is connected to a control unit 100. The engine 10 is also associated with an exhaust system 24 for exhausting an exhaust gas. The exhaust system 24 includes an exhaust pipe 26 and a catalytic converter 28. The catalytic converter 28 is provided for purification of exhaust gas for anti-polution purpose. At a position downstream of the catalytic converter 28, a catalytic converter temperature sensor 30 for monitoring exhaust gas temperature Tex to produce a catalytic converter temperature indicative signal S Tex . The catalytic converter temperature sensor 30 is connected to the control unit 100 to input the catalytic converter temperature indicative signal S Tex . As shown in Figs. 1 and 2, the control unit 100 is also connected to fuel injection valves 102 disposed within the intake manifold 18 for injecting controlled amount of fuel at a controlled timing. The control unit 100 is also connected to an ignition coil 104 for triggering the latter at a controlled timing derived in synchronism with the engine revolution cycle. The control unit 100 derives a fuel injection control signal and a spark ignition control signal for controlling operation of the fuel injection valves 102 and the ignition coil 104. In order to derive the fuel injection control signal and the spark ignition control signal, the control unit 100 is connected to a crank angle sensor 106. As is well known, the crank angle sensor 106 monitors engine revolution cycle to produce a crank reference signal ϑ ref at every predetermined engine revolution cycle position, e.g. 70° before top-dead-center (BTDC) of each engine cylinder, and a crank position signal ϑ pos at every given angle, e.g. 2° of angular displacement of the engine revolution cycle. The control unit 100 is further connected to a first wheel speed sensor 108 which monitors rotation speed Vw₁ of a driving wheel to produce a driving wheel speed indicative signal S Vw1 , and a second wheel speed sensor 110 which monitors rotation speed Vw₂ of a driven wheel to produce a driven wheel to produce a driven wheel speed indicative signal S Vw2 . The control unit 100 derives wheel slippage λ on the basis of the driving wheel and driven wheel speed indicative signal values Vw₁ and Vw₂. The control unit 100 performs slip suppressive drive control so as to maintain wheel slippage λ at a value smaller than a predetermined wheel slippage threshold λ ref . As shown in Fig. 2, the control unit 100 comprises an input/output interface 112, CPU 114, ROM 116 and RAM 118. An analog-to-digital (A/D) converter 120 is provided in the input/output interface 112 for converting the intake air flow rate indicative signal S Q and the catalytic converter temperature indicative signal S Tex which are input in forms of analog signals, into digital signals. The input/output interface 112 is also provided with an engine speed deriving section 122 which derives an engine speed data N on the basis of frequency of occurrences of the crank reference signal ϑ ref or the crank position signal ϑ pos . As is well known, the engine speed data N can be derived by calculating reciprocal of an interval of occurrences of crank reference signal ϑ ref , by counting the crank position signal ϑ pos for a given period or by measuring period for given number of crank position signal ϑ pos . The input/output interface 112 is further provided with a fuel injection control section 126 and a spark ignition control section 128. The fuel injection control section 126 includes a Ti register 130, to which a fuel injection amount data Ti is set for controlling fuel injection amount and fuel injection timing. On the other hand, the spark ignition control section 128 includes a ADV register 132, to which a spark ignition timing data ADV is set for controlling spark ignition timing. The input/output interface 112 further includes a wheel speed derivation section 134 which receives the driving wheel speed indicative signal S Vw1 and the driven wheel speed indicative signal S Vw2 in a form of pulse signals and derives driving wheel speed data Vw₁ and drive wheel speed data Vw₂. Further detail of the slip suppressive drive control system will be discussed herebelow with slip suppressive fuel supply control process which will be described in detail with reference to Figs. 3 to 6. In brief of the control operation, the control unit 100 derives the wheel slippage λ. The control unit 100 performs slip control mode fuel injection control when the wheel slippage λ becomes greater than the wheel slippage threshold λ ref . In the slip control mode operation, a target engine speed N cut is derived on the basis of the engine speed data N, the driving wheel speed data V w1 and the driven wheel speed data V w2 , so that an optimal wheel slippage λ opt can be obtained. In the slip control mode operation, fuel cut-off is cyclically performed at a predetermined frequency for decelerating the engine toward the target engine speed N cut . In practice, fuel cut-off in the slip control mode is performed in two modes, i.e. partial cut-off mode in which fuel cut-off is performed for one of two groups of fuel injection valves 102, and a full cut-off mode in which fuel cut-off is performed for both groups of fuel injection valves 102. In either case, fuel-cut and fuel resumption are performed alternatively and cyclically. Ratio of a period, in which fuel is cut-off and which is thus hereafter referred to as fuel cut period and a period, in which fuel is supplied and which will be hereafter referred to as fuel supply period is variable depending upon catalytic converter temperature T ex as monitored by the catalytic converter temperature sensor 30. Fuel injection control process will be discussed herebelow with reference to Fig. 3. The main routine shown in Fig. 3 is executed periodically at a predetermined timing. Immediately after starting execution, the engine speed data N is read out at a step 1002. Then, at a step 1004, the driven wheel speed data V w2 is read from the wheel deriving section 134 of the input/output interface 112. At a step 1006, the driving wheel speed data V w1 is read from the wheel speed deriving section 134. Since the driven wheel is driven through the vehicle body which is driven by the driving wheel, the driven wheel speed generally corresponds to the vehicular speed. Therefore, the driven wheel speed is taken as the vehicle speed representative data. Based on the driven wheel speed data Vw₂ and the driving wheel speed data Vw₁ read at the steps 1004 and 1006 wheel slippage λ is derived at a step 1008. In practice, the wheel slippage λ is derived from the following equation: λ = (Vw₂ - Vw₁)/Vw₁ The wheel slippage λ derived at the step 1008 is compared with the wheel slippage threshold λ ref , at a step 1010. When the wheel slippage λ is smaller than the wheel slippage threshold λ ref , then fuel injection control is performed in normal control mode. When normal control mode is ordered, fuel cut-off mode indicative flags FL cut1 and FL cut2 are reset at a step 1012. After resetting the fuel cut-off mode indicative flags FL cut1 and FL cut2 , process goes END. On the other hand, if the wheel slippage λ as checked at the step 1010, the target engine speed N cut is derived on the basis of the engine speed data N, the driving wheel speed data Vw₁ and the driven wheel speed data Vw₂, at a step 1014. When the power transmission of the vehicle is manual transmission, the target engine speed N cut can be calculated by the following equation: N cut = N x Vw₂/Vw₁ x 1.2 From the foregoing equation, the target engine speed N cut corresponds the wheel slippage of 20%. Therefore, in this case, predetermined optimal wheel slippage λ opt becomes 20%. Though the shown embodiment is set the optimal wheel slippage λ opt at 20%, the set optimal wheel slippage may be set any value. After deriving the target engine speed N cut at the step 1014, sub-routine of Fig. 4 is triggered at a step 1016. Immediately after triggering the sub-routine of Fig. 4, a fuel cut timer value TM₁ of a fuel cut timer 140 in RAM 118 is checked at a step 2002. The fuel cut timer 140 is set at a value representative of a fuel cut-off period for first group of fuel injection valve 102. Therefore, the fuel is cut-off for a predetermined fuel cut-off period for the first group of the fuel injection valve 102 corresponding to an initial value set in the fuel cut timer value TM₁. When the fuel cut timer value TM₁ is not zero as checked at the step 2002, the fuel cut timer value TM₁ is decremented by 1, at a step 2004. When the fuel cut timer value TM₁ is zero as checked at the step 2002 or after decrementing the fuel cut timer value TM₁ at the step 2004, a fuel cut timer value TM₂ of a fuel cut timer 142 in RAM 118, is checked at a step 2006. The initial value set in the fuel cut timer 142 represents the fuel cut-off period for the second group of fuel injection valves 102. If the fuel cut timer value TM₂ is not zero as checked at the step 2006, the fuel cut timer value TM₂ is decremented by 1, at a step 2008. Through the sets 2002 to 2008, the fuel cut timer values TM₁ and TM₂ are decremented to zero from the initial value over fuel cut cycles, as shown in Fig. 7. When the fuel cut timer value TM₂ as checked at the step 2006 is zero or after decrementing the fuel cut timer value TM₂ at the step 2008, the fuel cut timer value TM₁ is checked whether it is equal to half of a set fuel cut cycle period T at a step 2010. As seen from Fig. 10, the fuel cut cycle period T includes fuel cut period and fuel supply period in each fuel cut-off cycle. If the fuel cut timer value TM₁ is equal to T/2 as checked at a step 2010, the fuel cut timer value TM₂ of the fuel cut timer 142 is set at the set fuel cut-off cycle period T, at a step 2012. If the fuel cut timer value TM₁ as checked at the step 2010 or after setting the fuel cut timer value TM₂ is set at a value T, a difference ΔN between the engine speed data N and the target engine speed N cut is calculated at a step 2014. Then, the fuel cut timer value TM₁ is again checked at a step 2016. When the fuel cut timer value TM₁ as checked at the step 2016 is not zero, the difference ΔN derived at the step 2014 is checked at a step 2018. If the difference ΔN is greater than zero, which means that the engine speed N is higher than the target engine speed N cut , as checked at the step 2018, the difference ΔN is compared with a predetermined engine speed difference criterion N₁ at a step 2020. As seen from Fig. 9, the engine speed difference criterion N₁ is set at a value to discriminate the engine speed condition requiring long or short fuel cut period in the fuel cut-off cycle period. Namely, as shown in Figs. 9 and 10, the fuel cut period is set at a period T₁ which is equal to a fuel supply period t₁ in the fuel cut-off cycle period T when the difference ΔN is smaller than or equal to the engine speed difference criterion N₁. On the other hand, when the difference ΔN is greater than the engine speed difference criterion N₁, the fuel cut period is set at T₂ which is shorter than the period T₁. When the fuel cut period is set at T₂, the fuel supply period is expanded to t₂. The fuel cut period T₁ will be hereafter referred to as "even fuel cut period rate representative value". The fuel cut period T₂ will be hereafter referred to as "shorter fuel cut period rate representative value". When the difference ΔN as checked at the step 2020 is smaller than or equal to the engine speed difference criterion N₁, a fuel recovery timer value T REC of a fuel recovery timer 144 in RAM 118 is set at a predetermined value T₁, at a step 2022. The even fuel cut period rate representative value T₁ represents a fuel cut period in the fuel cut-off cycle period T. On the other hand, when the difference ΔN is greater than the engine speed difference criterion N₁ as checked at the step 2020, the fuel recovery timer value T REC is set at a shorter fuel cut period rate representative value T₂, at a step 2024. After setting the fuel recovery timer value T REC at the step 2022 or 2024, the fuel cut timer value TM₁ is set at the set fuel cut-off cycle period T, at a step 2026. After setting the fuel cut timer value TM₁ to the fuel cut-off cycle period T at the step 2026, the difference ΔN is compared with a predetermined engine speed difference criterion N₂ which is set at a greater value than the engine speed difference criterion N₁ at a step 2028. As seen from Fig. 9, the engine speed difference criterion N₂ is greater than the engine speed difference criterion N₁, which criterion N₂ represents a criterion to discriminate the engine driving condition whether the engine driving condition is to perform partial fuel cut-off to shut down fuel supply for one of the two fuel injection value groups, or to perform full fuel cut-off to shut down fuel supply for both groups of fuel injection valves. As shown, in the partial fuel cut-off mode, fuel cut-off is performed alternatively in two groups of fuel injection valves. When the difference ΔN is greater than the engine speed difference criterion N₂ as checked at the step 2028, the fuel cut timer value TM₁ is set at a value T₃ which is greater than the fuel cut timer value TM₂ by 1, at a step 2030. Thereafter, the fuel cut timer value TM₂ is set at a value T₄ which is a sum value of the fuel cut timer value TM₁ and T/2, at a step 2032. Then, the fuel cut timer value TM₁ is compared with the fuel recovery timer value T REC at a step 2034. When the fuel cut timer value TM₁ is greater than the fuel recovery timer value T REC as checked at the step 2034, the fuel cut mode indicative flag FL cut1 is set at a step 2036. By setting the fuel cut mode indicative flag FL cut1 , fuel cut-off operation for the first group of fuel injection valves 102 is ordered. On the other hand, when the fuel cut timer value TM₁ is smaller than or equal to the fuel recovery timer value T REC as checked at the step 2034, the fuel cut mode indicative flag FL cut1 is reset at a step 2038. By resetting of the fuel cut mode indicative flag FL cut1 , fuel supply is ordered. Subsequently, the fuel cut timer value TM₂ is compared with the fuel recovery timer value T REC at a step 2040. When the fuel cut timer value TM₂ is greater than the fuel recovery timer value T REC as checked at the step 2040, the fuel cut mode indicative flag FL cut2 is set at a step 2042. By setting the fuel cut mode indicative flag FL cut2 , fuel cut-off operation for the first group of fuel injection valves 102 is ordered. On the other hand, when the fuel cut timer value TM₂ is smaller than or equal to the fuel recovery timer value T REC as checked at the step 2040, the fuel cut mode indicative flag FL cut2 is reset at a step 2044. By resetting of the fuel cut mode indicative flag FL cut2 , fuel supply is ordered. After one of the steps 2042 and 2044, process returns to the main routine of Fig. 3. On the other hand, when the fuel cut timer value TM₁ is not zero as checked at the step 2016, process jumps to the step 2028. When the difference ΔN is smaller than or equal to zero as checked at the step 2018, process jumps to the step 2034. Similarly, when the difference ΔN is smaller than or equal to the engine speed difference criterion N₂, process also jumps to the step 2034. As shown in Fig. 8, the fuel cut mode representative flags FL cut1 and FL cut2 are maintained at set condition as long as the fuel cut timer value TM₁ and TM₂ are greater than the fuel recovery timer value T REC . Therefore, through the steps 2034 to 2042, fuel cut and fuel supply are controlled. Fig. 5 shows routine for governing fuel injection as governed by the fuel cut mode representative flags FL cut1 and FL cut2 . Immediately after starting execution, the fuel cut mode representative flag FL cut1 is checked at a step 3002. If the fuel cut mode representative flag FL cut1 is not set as checked at the step 3002, fuel injection is enabled. Therefore, fuel injection pulse is set for fuel injection through first group of the fuel injection valve 102, at a step 3004. After outputting the fuel injection pulse at the step 3004 or when the fuel cut mode representative flag FL cut1 is set as checked at the step 3002, the fuel cut mode representative flag FL cut2 is checked at a step 3006. If the fuel cut mode representative flag FL cut2 is not set as checked at the step 3006, fuel injection is enabled. Therefore, fuel injection pulse is set for fuel injection through first group of the fuel injection valve 102, at a step 3008. After outputting the fuel injection pulse at the step 3008 or when the fuel cut mode representative flag FL cut2 is set as checked at the step 3006, process goes END. Fig. 6 shows a routine for setting the fuel cut period depending upon the catalytic converter temperature monitored by the catalytic converter temperature sensor 30. At a step 4002, the catalytic converter temperature data T ex is read out. The catalytic converter temperature data T ex is compared with a catalytic converter temperature threshold value Tex ref , at a step 4004. When the catalytic converter temperature data T ex is smaller than or equal to the catalytic converter temperature threshold value Tex ref as checked at the step 4004, the fuel cut-off cycle period value T, the even fuel cut period rate indicative value T₁ and the shorter fuel cut period rate indicative value T₂ are set at predetermined standard values, at a step 4006. On the other hand, when the catalytic converter temperature data T ex is greater than the catalytic converter temperature threshold value Tex ref as checked at the step 4004, the fuel cut-off cycle period value T, the even fuel cut period rate indicative value T₁ and the shorter fuel cut period rate indicative value T₂ are set at predetermined greater values, at a step 4008. The operation of the slip suppressive engine control will be further discussed with reference to timing charts in Figs. 11 and 12. Assuming wheel slip occurs at a time t₀ in Fig. 11, the wheel speed Vw₁ (shown by solid line) of the driving wheel is abruptly accelerated to become much higher than the wheel speed Vw₂ (shown by broken line) of the driven wheel. This is caused by reduction of the load at the driving wheel. Namely, since the engine is connected to the driving wheel via a power train including the power transmission, reduction of the load on the driving wheel causes acceleration of the engine to increase the engine speed N, as shown in Fig. 12. Further assuming the wheel slippage λ increases across the wheel slippage threshold λ ref at a time t₁, fuel injection control mode is switched from normal mode to slip control mode, at the time t₁. In the slip control mode operation, the target engine speed N cut is set as illustrated by broken line in Fig. 12. As seen, the target engine speed N cut is increased linearly. Therefore, the actual engine speed N is increased according to increasing of variation of the target engine speed N cut . Because the actual engine speed N can be controlled to maintain the optimal wheel slippage, hunting of switching of control mode can be successfully prevented. In addition, by varying the setting of the fuel cut-off cycle depending upon the catalytic converter temperature, overheating of the catalytic converter can be successfully prevented. This maintains effective operation of the catalytic converter. Fig. 13 shows another embodiment of the slip suppressive drive control system, according to the present invention. The shown embodiment of the slip suppressive drive control system is applied for an automotive vehicle which is facilitated a stepless automatic power transmission 202 which transmit engine output to driving wheels 204 via an output shaft 206 of the power transmission, a differential gear unit 208 and drive shafts 210. As seen from Fig. 13, the shown embodiment is directed to the automotive vehicle which has front engine, front wheel drive layout. Therefore, the driving wheels 204 are front wheels in the shown embodiment. The rear wheels 212 are driven wheel which rotates freely according to vehicular travel. Therefore, the rotation speed of the driven wheel constantly reflects the vehicle speed. An automotive internal combustion engine 214 is associated with the stepless automatic power transmission 202 for outputting the driving torque. In order to perform control for controlling operations of the engine 214 and the transmission 202, a microprocessor based control unit 300 is provided. The control unit 300 is connected to a crank angle sensor 302 for receiving crank reference signal or crank angle signal as an engine speed representative signal. The control unit 300 is also connected to an air flow meter 304, a driving wheel speed sensor 306 and driven wheel speed sensors 308 and 310. On the other hand, the control unit 300 is connected to a gear shift actuator 312 associated with the stepless automatic power transmission 202 for adjusting the transmission gear ratio. The control unit 300 is also connected to a fuel injection valves 314 and an ignition coil 316. In addition, the control unit 300 is connected to a transmission neutral range switch 317 which is associated with a transmission selector lever (not shown) and designed to be turned ON when the selector lever is shifted into neutral range position. As schematically illustrated, the control unit 300 includes an input interface 318, an arithmetic circuit 320 which may include CPU, RAM and ROM and so forth, and an output interface 322. Operation of the shown embodiment of the slip suppressive drive control system will be discussed herebelow with reference to Fig. 14. It should be appreciated that the shown routine is directed for transmission gear ratio selection control in relation to slip suppressive engine output control. In the shown embodiment, the engine control for suppressing wheel slip is performed according to the process set forth with respect to the former embodiment. Therefore, in order to avoid redundant discussion and thus avoid confusion, the following discussion with respect to the second embodiment will be concentrated to the transmission shift control in relation to slip suppressive engine control and neglect detailed discussion about the engine speed control. In the operation shown in Fig 14, selected transmission control parameters, such as engine speed data N, the driving wheel speed indicative data Vw₁, the driven wheel speed indicative data Vw₂ and so forth, are read out at a step 5002. At a step 5004, the transmission neutral position switch signal is checked whether the transmission is in drive position (D position) or neutral position (N position). When the transmission selector position as checked at the step 5004 is N position, target transmission gear ratio in N position, is derived at a step 5006. On the other hand, when the transmission selector position is D position as checked at the step 5004, the target transmission gear ratio in D position is derived at a step 5008. Then, actual transmission gear ratio is derived at a step 5010. Based on the target transmission gear ratio and the actual transmission gear ratio, gear shift command is derived for adjusting the transmission gear ratio toward the target gear ratio, at a step 5012. After deriving the gear shift command at the step 5012, the fuel cut mode representative flag FL cut1 and/or FL cut2 are checked at a step 5014. When the fuel cut mode representative flag as checked at the step 5014 is not set, the gear shift command is output at a step 5016. On the other hand, when the fuel cut mode representative flag is set as checked at the step 5014, process jumps the step 5016 and directly goes END. Therefore, while the slip control mode operation is performed according to the process illustrated in Figs. 3 to 6, the gear shift command is not output so that the transmission gear ratio is fixed. Process and parameter for deriving the target transmission gear ratio, actual transmission gear ratio, and gear shift command for feedback controlling the transmission gear ratio to the target transmission gear ratio has been disclosed in the United States Patent 4,597,308, issued on July 1, 1986. The U. S. Patent identified above has been assigned to the common assignee to the present invention. The disclosures of the U. S. Patent will be herein incorporated by reference for the sake of disclosure. Fig. 15 shows modified transmission control routine to be performed in the slip suppressive drive control. In this modified routine, gear shift command derived at the step 5012 is modified when the slip control mode control operation for the engine is performed. Therefore, when the fuel cut mode representative flag is not set as checked at the step 5014, the gear shift command derived at the step 5012 is output at the step 5016. On the other hand, when the fuel cut mode representative flag is set as checked at the step 5014, gear shift command value is modified at a step 5018. Process of modifying the gear shift command value at the step 5018 will be discussed herebelow with reference to Fig. 16. Fig. 16 shows gear shift pattern in the stepless automatic power transmission. In the normal driving condition, the transmission gear ratio command orders stepping down of the gear ratio from the gear ratio A to the gear ratio B in response to abrupt acceleration demand which is detected by abrupt change of throttle valve open angle or by abrupt increase of the air flow rate. According to stepping down ordering transmission gear ratio command, the actual gear ratio in the transmission is shifted. Thereafter, the gear ratio is gradually shifted toward the higher gear ratio according to increasing of the vehicle speed along line E in Fig. 16. When wheel slippage occurs at the gear ratio C, the engine speed increases abruptly. This causes higher rate gear shifting as shown by line D in Fig. 16. If such higher rate gear ratio shifting is allowed, effect of fuel cut-off for decelerating the driving wheel speed becomes smaller. Therefore, in the preferred process, gear ratio shifting is modified to vary along the arrow F. On the other hand, in case of the process of Fig. 14, since the transmission gear ratio is fixed at the ratio at the occurrence of the wheel slip as illustrated by line H. Therefore, in either case, the transmission gear ratio can be maintained low enough to make fuel cut-off effective for decelerating the wheel speed and thus recover wheel traction. While the present invention have been disclosed in terms of the specific embodiments, the invention can be implemented in various embodiments other than that illustrated hereabove. These embodiments implementing the invention should be appreciated to be involved with the scope of the invention, principle of which is set out in the appended claims. For example, it may be beneficial for introduce histerisis technologies in setting the target engine speed to differentiate the values for switching from normal mode to slip control mode and from slip control mode to normal mode. On the other hand, though the shown embodiment of Fig. 13 is applied for the stepless automatic power transmission, it is, of course, possible to apply the same shift control technology to ordinary fixed gear ratio type power transmissions.

Claims: A slip control system for an automotive vehicle, comprising: first sensor means (108) for monitoring the rotation speed of a driving wheel which is driven by an automotive internal combustion engine (10), and for producing driving wheel speed representative data; second sensor means (110) for monitoring the vehicle speed to produce vehicle speed representative data; and fourth means (100) for detecting wheel slippage greater than a predetermined wheel slippage threshold on the basis of said driving wheel speed representative data and said vehicle speed representative data; characterized by third sensor means (106, 122) for monitoring the engine revolution speed to produce engine speed indicative data; and fifth means (100) being responsive to said fourth means for initiating a slip control mode operation, as soon as the wheel slippage greater than the predetermined wheel slippage threshold is detected, for initiating a slip control mode operation in which a target engine speed achieving a predetermined allowable wheel slippage is derived on the basis of said engine speed data, said driving wheels speed representative data and said vehicle speed representative data, and for controlling the engine revolution speed toward the target engine speed. A slip control system as set forth in claim 1, wherein said fifth means (100) controls fuel supply for the engine (10) in said slip control mode operation to adjust the engine speed toward said target engine speed. A slip control system as set forth in claim 2, wherein said fifth means (100) periodically performs fuel cut-off in slip control mode operation. A slip control system as set forth in one of the claims 1 - 3, wherein said second sensor means (110) monitors rotation speed of a driven wheel for producing said vehicle speed representative data. A slip control system as set forth in claim 4, wherein said fourth means (100) derives said wheel slippage based on a difference between the driving wheel speed and the driven wheel speed. A slip control system as set forth in claim 3, wherein said fifth means (100) derives a ratio of said fuel cut period versus said fuel supply period in a predetermined fuel cut-off control cycle period. A slip control system as set forth in claim 6, which further comprises sixth sensor means (30) monitoring a catalytic converter temperature to produce a catalytic converter temperature indicative signal, and said fifth means (100) adjusts said fuel cut-off control cycle period depending upon said catalytic converter temperature indicative signal value. A slip control system as set forth in one of the claims 3 or 7, wherein said fifth means (100) is operative in said slip control mode in a first mode to perform fuel cut-off for limited number of engine cylinders and in a second mode to perform fuel cut-off for all of the engine cylinders, said fifth means selects one of said first and second modes depending upon the difference between said engine speed data and the target engine speed indicative value. A slip control system as set forth in one of the claims 3 or 8, wherein said fifth means (100) further adjusts a fuel cut period, in which fuel cut-off is performed, versus a fuel supply period, in which fuel is supplied, depending upon the difference between the engine speed data and the target engine speed indicative value. A slip control system as set forth in claim 9, wherein said fifth means (100) compares said difference with a first criterion to select said second mode when said difference is greater than said first criterion and otherwise select said first mode. A slip control system as set forth in claim 10, wherein said fifth means (100) compares said difference with a second criterion to set said fuel cut period at a longer first period when said difference is greater than a second criterion and otherwise set said fuel cut period at a shorter second period. A slip control system as set forth in claim 11, wherein said second criterion is set at smaller value than said first criterion. A slip control system as set forth in one of the claims 11 or 12, wherein said fifth means (100) compares said catalytic converter indicative signal value with a catalytic converter temperature criterion to set said fuel cut-off control period, said first period and said second period at predetermined smaller standard values when said catalytic converter temperature indicative signal value is smaller than said catalytic converter temperature criterion, and otherwise at longer values. A slip control system as set forth in claim 1, which further comprises a seventh means (312) for controlling gear shifting of a power transmission (202), said seventh means is responsive to said fifth means (300) operating in said slip control mode to fix gear position at a gear ratio at which wheel slippage greater than said wheel slippage threshold is detected. A slip control system as set forth in claim 1, which further comprises a seventh means (312) for controlling gear shifting of a power transmission (202), said seventh means is responsive to said fifth means (300) operating in said slip control mode to cause delay in gear shifting from a gear ratio at which wheel slippage greater than said wheel slippage threshold is detected to a desired gear ratio determined based on preselected transmission control parameters.

Inventors: ABO, TOSHIMI, FUJISHIRO, TAKESHI, OSHIAGE, KATSUNORI
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1988
Main classification: B60K28
IPC codes: B60K28, B60T8