DRIVING WHEEL SLIP CONTROL SYSTEM FOR VEHICLES

Abstract: A driving wheel slip control system for a vehicle detects an excessive slip state of at least one of the driving wheels of the vehicle wherein the slip of the driving wheel has a magnitude greater than a predetermined magnitude, and reduces engine output when the excessive slip state is detected, by repeating alternate fuel cut and fuel supply such that fuel supply is stopped for a first predetermined time period, and then effected for a second predetermined time period. During the second predetermined time period the engine is supplied with an amount of fuel smaller than one required for an operating condition of the engine in the excessive slip state. The first and second time period ratio and the fuel amount may depend upon engine parameters e.g. engine rotational speed and/or pressure in intake passage, or slip state. If the excessive slip state has a greater variation in magnitude than a certain value, fuel supply is continuously effected for a third predetermined time period longer than the first predetermined time period or all the time during the excessive slip state. When the driving wheel has left the excessive slip state, engine output is reduced below a value required for an operating condition of the engine.

Description: BACKGROUND OF THE INVENTION This invention relates to driving wheel slip control systems, and more particularly to improvements in or to such systems, which enable proper slip control when the driving wheels have shifted to an excessive slip state, and when they have left the excessive slip state. As recognized in general, a driving wheel of an automotive vehicle undergoes a slip when the vehicle is started to run or when it is accelerated, if the driving force of the driving wheel surpasses a frictional force developed between the tire of the driving wheel and the road surface [= the coefficient of friction between the tire and the road surface x load of the vehicle weight on the driving wheel (wheel load)]. The frictional force acts in the advancing or longitudinal direction of the vehicle. The magnitude of the slip may be represented by a slip rate λ which is expressed by the following equation: λ = (V W - V)/V W (I) where V W represents the circumferential velocity of the driving wheel, and V represents the speed of the vehicle. The frictional force F between the driving wheel tire and the road surface, which defines the upper limit of the effective driving force of the driving wheel, varies with the slip rate λ, as shown in Fig. 1. It will be noted from the figure that the frictional force F assumes the maximum value when the slip rate λ has a predetermined value λ₀. While the longitudinal frictional force F varies with slip rate λ as indicated by the solid line in the graph, the transverse frictional force, which acts in the transverse direction of the vehicle, varies with slip rate λ, as indicated by the broken line in the graph, such that it becomes smaller as the slip rate increases. The US-PS 4,432,430 discloses a wheel spinning control system for motor vehicles wherein in dependence of the extent of the slip either the turbo supercharging or the fuel supply to a specific cylinder is interrupted intermittently or continuously. This type of slip control leads to irregular operation of the internal combustion engine causing an uncomfortable feeling for the passenger of the motor vehicle. A slip prevention system has been proposed, e.g. by Japanese Patent Publication (Kokoku) No. 52-35837, which is based upon the above recognition of the relationship between longitudinal frictional force, transverse frictional force and slip rate, and which controls the slip rate such that the longitudinal frictional force may be the maximum so as to obtain the maximum driving efficiency of the vehicle, while the transverse frictional force may have a drop as small as possible so as to prevent a skid or sideslip of the vehicle. The above proposed control system is constructed such that the slip of the driving wheels is prevented from becoming excessive through control of the torque of the engine by turning on and off an ignition device of the engine of the vehicle or by allowing and inhibiting fuel supply from a fuel supply device to the engine. However, according to the proposed control system, immediately when either one of the driving wheels has shifted from a moderate slip state to an excessive slip state, fuel supply is suddenly interrupted, for instance, which results in a sudden drop in the torque or driving force of the engine, and on the other hand, when the driving wheel leaves the excessive slip state, fuel supply is suddenly resumed, resulting in a sudden increase in the torque or driving force of the engine. Thus, according to the proposed control system, the driver or passenger will feel a shock, i.e. degraded driveability both at the time of transition to the excessive slip state and at the time of transition from the excessive slip state. A solution to this problem would be to reduce the fuel supply to the engine instead of completely cutting off the fuel supply as above, so as to progressively reduce the torque of the engine. However, the slip rate variation of the driving wheels varies depending upon running conditions of the vehicle. Therefore, if the vehicle is in a running condition wherein the slip rate variation is large, mere decrease in the torque of the engine according to the solution would lead to insufficient control of the slip so that a desired slip rate will be attained with much delay, resulting in degraded driveability. While internal combustion engines in general have combustion characteristics varying with engine rotational speed, the above proposed control system does not contemplate slip control in accordance with engine rotational speed. Therefore, it would fail to properly control the combustion characteristics when the driving wheels have shifted to an excessive slip state. A result of this failure would be emission of unburnt fuel in large quantities, leading to so-called after-fire, i.e. combustion of unburnt fuel in the exhaust system of the engine. Further, if an exhaust purging catalyst device, particularly a three-way catalytic converter, is provided in the exhaust system, combustion of unburnt fuel in the exhaust system will cause an excessive rise in the temperature of the catalyst device, deteriorating the performance thereof. Furthermore, according to the proposed control system no contemplation is made of load on the engine in effecting the fuel supply control for slip control. For instance, in the case of controlling the air-fuel ratio to a leaner value in an excessive slip state, the amount of drop in the torque or driving force of an internal combustion engine to be caused by reducing the fuel amount supplied to the engine varies depending upon the magnitude of load on the engine. Therefore, in effecting slip control independently of load on the engine in the excessive slip state, if the target air-fuel ratio is set at a value conforming to a high load operating condition of the engine, slip control is effected to an excessive extent when the engine is operating in a low load operating condition, whereas if the target air-fuel ratio is set at a value conforming to a low load operating condition of the engine, slip control is effected to an insufficient extent when the engine is operating in a high load operating condition. This, therefore, makes it impossible to achieve desired values of slip rate over the entire load range, thereby failing to secure good driveability of the vehicle. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a driving wheel slip control system for a vehicle, which is capable of preventing a driving shock and promptly attaining a desired slip rate to thereby improve driveability when the driving wheel(s) of the vehicle has shifted to an excessive slip state, by properly controlling the torque of an engine of the vehicle in accordance with slip rate variation in particular. It is a further object of the invention to prevent after-fire as well as deterioration of the performance of the catalyst device in the engine exhaust system due to its excessive temperature rise, when the driving wheel(s) is in the excessive slip state. It is another object of the invention to prevent a driving shock and hence improve driveability when the driving wheel(s) has left the excessive slip state, by restraining a sudden increase in the torque of the engine. In order to attain the above objects, the present invention provides a driving wheel slip control system for a motor vehicle, comprising driving wheel speed sensor means for sensing the speed of at least one driving wheel of the vehicle and outputting a driving wheel speed, excessive slip detecting means for sensing an excessive slip state of said motor vehicle based on said at least one driving wheel speed, fuel supply control means for controlling the amount of fuel supplied to the engine of the vehicle. The system according to the invention is characterized in that said fuel supply control means includes engine output reducing means for reducing output from said engine in response to said excessive slip state by leaning means for leaning an air fuel ratio of an air-fuel mixture supplied to the engine, and fuel cut-off means for cutting off fuel supply for a respective time interval, said respective time interval being a specified multiple value of the time period between the beginning (TDC-signal) of successive intake strokes of the engine. The excessive slip detecting means outputs a slip value corresponding to the amount of excessive slip of said motor vehicle and said fuel cut-off means determines said multiple value in response to said slip value. Preferably, the engine output reducing means further includes means for causing said fuel supply means to repeat alternate fuel cut and fuel leaning, when said excessive slip state is detected by said excessive slip detecting means. Said fuel supply means are provided for stopping fuel supply for a first predetermined time interval. Preferably, said engine output reducing means comprises first leaning means for leaning the air-fuel ratio of an air-fuel mixture supplied to the engine when said slip value exceeds a first predetermined slip value, and fuel cut-off means for cutting off fuel supply for a first predetermined time interval subsequent to the operation of said first leaning means, said predetermined time interval being a specified multiple value of the time period between the beginning (TDC-signal) of successive intake strokes of the engine, said multiple value being determined in response to said slip value, when said slip value exceeds a second predetermined slip value wherein said second predetermined slip value is greater than said first predetermined slip value. The engine output reducing means further includes second leaning means for leaning the air-fuel ratio of an air-fuel mixture supplied for a second predetermined time interval when said slip value exceeds said second predetermined slip value. Said first and second leaning means increase the degree of leaning of the air-fuel ratio as said slip value increases. Preferably, said fuel cut-off means includes alternating means for causing said fuel supply control means to repeat alternate fuel cut and fuel supply in a manner such that fuel supply is cut off for a first predetermined time interval and fuel supply is then effected for a second predetermined time interval, when said motor vehicle has a slip value exceeding a predetermined slip value. The fuel supply control means supplies fuel to said engine in an amount smaller than the amount of fuel required for an operating condition of the engine assumed when the slip value of said motor vehicle exceeds said predetermined slip value during said second time interval. The amount of fuel supplied to said engine for a second predetermined time interval is set by a fuel decremental value determined in dependence on an operating condition of said engine assumed when the slip value of said motor vehicle exceeds said predetermined slip value, wherein said fuel decremental value is dependent on at least one of the rotational speed of said engine and pressure in an intake passage of said engine. The ratio between said first predetermined time interval and said second predetermined time interval is set to a value dependent on an operating condition of said engine assumed when the slip value of said motor vehicle exceeds said predetermined slip value. Preferably, the ratio between the first predetermined time interval and said second predetermined time interval is set to a value dependent on at least one of the rotational speed of said engine and pressure in an intake manifold of said engine. Alternatively, the ratio between said first predetermined time interval and said second predetermined time interval is set to a value dependent on a slip state of said at least one driving wheel assumed when the slip value exceeds said predetermined slip value. In this case it is preferred that the ratio between said first predetermined time interval and said second predetermined time interval is set to a value dependent on at least one of the slip value and a variation of the slip value. It is further preferred that the fuel supply control means repeats alternate fuel cut and fuel supply when the slip value exceeds said predetermined slip value after said slip value has a magnitude which is smaller than said predetermined slip value for less than a predetermined period of time. The engine output reducing means can comprise ignition timing retarding means for retarding ignition timing of said engine which is determined by at least one operating parameter of said engine when the slip value exceeds said second predetermined slip value. Preferably said fuel cut means includes first control means for repeating alternate fuel cut and fuel supply in a manner such that fuel cut is effected for a first predetermined time interval and then fuel is effected for a second predetermined time interval when said motor vehicle is in a first slip state which is during effecting slip control, and second control means for effecting continuous fuel cut for a third predetermined time interval longer than said first predetermined time interval when said motor vehicle is in a second slip state which follows a stable less slip condition. Alternatively the fuel cut-off system includes first control means for repeating alternate fuel cut and fuel supply in a manner such that fuel cut is effected for a first predetermined time interval and then fuel supply is effected for a second predetermined time interval subsequent to said first time interval, when said motor vehicle is in a first slip state which is during effecting slip control, and second control means for effecting continuous fuel cut as long as said motor vehicle is in a second slip state which follows a stable less slip condition. The first slip state is a state to which said motor vehicle shifts after staying for less than a predetermined period of time in a less slip state, and said second slip state is a state to which said motor vehicle shifts after staying for at least said predetermined period of time in said less slip state. Preferably said fuel supply control means includes furtherengine output reducing means associated with said excessive slip detecting means, and operable when the slip value returns to a value below said predetermined slip value for reducing output from said engine to a value smaller than a value required for an operating condition of said engine assumed immediately after the slip value has returned to a value below said predetermined slip value. The engine output reducing means may comprise mixture leaning means which leans a mixture supplied to said engine. Alternatively said further engine output reducing means comprises ignition timing retarding means for retarding ignition timing in said engine. It is preferred that said further engine output reducing means operates when the rotational speed at said engine is higher than a predetermined value immediately after the slip value returns to a value below said predetermined slip value, and that said further engine output reducing means operates for a third predetermined time interval after said slip value returns to a value below said predetermined slip value. In the driving wheel slip control system according to the invention it is preferred that said fuel supply control means further includes air fuel ratio correcting means for correcting an air-fuel ratio of a mixture supplied to said engine in response to engine operating parameters, and inhibiting means for inhibiting operation of said air-fuel ratio correcting means when said excessive slip state is detected. Preferably the fuel supply control means further includes engine operating parameter detecting means for detecting at least one engine operating parameter, and said engine output reducing means reduces output from said engine in a manner such that the reduction ratio depends on at least one engine operating parameter so that the reduction ratio changes in proportion to said output from said engine. The above and other objects, features and advantages of the invention will be more apparent from the ensuing detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the slip rate of a driving wheel tire of a vehicle and a frictional force between the tire and the road surface; Fig. 2 is a schematic diagram showing the construction of a vehicle equipped with a driving wheel slip control system according to the invention; Fig. 3 is a schematic diagram showing a fuel supply control system for controlling the torque of the driving wheels; Fig. 4 is a schematic circuit diagram showing an essential part of an electronic control unit (ECU) 35 in Fig. 1; Fig. 5 is a flowchart showing a control program for executing the slip control; Fig. 6 is a timing chart showing the relationship between a slip rate or a slip rate variation, output signals from the circuit of Fig. 4, and the slip control; Fig. 7 is a view showing tables of predetermined values X STB , X TRC , and predetermined time numbers N₀, N₁ plotted with respect to engine rotational speed Ne; Fig. 8 is a view showing a table of a predetermined value K TW0 of an engine coolant temperature-dependent fuel increasing coefficient K TW ; Fig. 9 is a view showing a variation of part of the control program of Fig. 5; Fig. 10 is a view showing tables of first and second predetermined time numbers N₀ and N₁ plotted with respect to slip rate λ; Fig. 11 is a view showing tables of first and second predetermined time numbers N₀ and N₁ plotted with respect to slip rate variation λ ˙; Fig. 12 is a view showing tables of predetermined values X STB and X TRC , and first and second predetermined time numbers N₀ and N₁; and Fig. 13 is a fragmentary flowchart showing a variation of part of the control program of Fig. 5. DETAILED DESCRIPTION The invention will now be described in detail with reference to the drawings showing an embodiment thereof. Referring first to Fig. 2, there is shown a vehicle 1 equipped with a driving wheel slip control device according to the invention. The vehicle 1 is a front wheel drive type with front wheels 11 and 12 as driving wheels which are driven by an internal combustion engine 31, and rear wheels 13 and 14 as driven or trailing wheels. Further, the invention could be similarly applied to a rear wheel drive type vehicle. Therefore, for the purposes of this application, trailing wheels are considered to be non-driving wheels. In other words, in a front wheel drive car, the trailing wheels would be the rear wheels, while in a rear wheel drive car, the trailing wheels (non-driving wheels) would be the front wheels. The driving wheels 11, 12 and the trailing wheels 13, 14 are provided, respectively, with driving wheel speed sensors 21, 22, and trailing wheel speed sensors 23, 24, the driving wheel speed sensors 21, 22 sensing the rotational speeds ωFL and ωFR of the respective left and right driving wheels, and the trailing wheel speed sensors 23, 24 sensing the rotational speeds ωRL and ωRR of the respective left and right driven wheels. Output signals from these sensors are delivered to an electronic control unit (hereinafter referred to as "ECU") 35. In the present embodiment, ECU 35 forms excessive slip detecting means, engine output reducing means, and second engine output reducing means. In ECU 35, the larger value of the driving wheel speeds ωFL and ωFR is selected as the driving wheel speed V W in equation I above and the speed ωRL or ωRR of the trailing wheel on the same side as the driving wheel whose speed is selected as V W is calculated and used as the vehicular speed V in equation I above. The slip rate λ is thus calculated in the following equation: Furthermore, the variation or differential value λ ˙ of the slip rate λ is also determined by ECU 35. In digital control, the variation λ ˙ may be calculated from difference in slip rate λ between adjacent processing cycles. In addition, a transmission 16 is interposed between the engine 31 and the driving wheels 11 and 12. The transmission 16 is equipped with a sensor, not shown, which transmits a transmission signal indicative of a gear ratio selected by the transmission 16 to ECU 35. In ECU 35, the torque of driving wheels 11 and 12 is controlled by controlling the output from the engine 31 by means of a fuel supply control system, described hereinbelow, thereby controlling the slip rate λ of the driving wheels 11 and 12. In Fig. 3, the fuel supply control system is shown, which controls the fuel supply to the engine 31, which may be a six cylinder type, for instance. Of course, the system would be operable on any type of engine. An intake manifold 32 is connected to the engine 31 to provide an intake passage thereto. A throttle valve 33' is disposed inside of a throttle body 33 which is positioned in intake manifold 32. Connected to the throttle valve 33' is a throttle valve opening (ϑth) sensor 34 which converts the opening of the throttle valve 33' into an electrical signal and transmits the signal to ECU 35. A fuel injection valve (fuel supply means) 36 is positioned between the engine 31 and the throttle body 33. A fuel injection valve 36 is positioned slightly upstream of the intake valve (or valves) of each cylinder. The fuel injection valve 36 is connected to a fuel pump and a fuel source, neither of which is shown, in a conventional manner. Fuel injection valve 36 is electrically connected to ECU 35 as well, and the valve opening time period of the valve is controlled by a signal from ECU 35. An absolute pressure (P BA ) sensor 38 is disposed at the end of a pipe 37 which communicates with the intake manifold 32 at a point downstream of throttle valve 33' and upstream of fuel injector 36. An absolute pressure signal, converted into an electrical signal by the absolute pressure sensor 38, is transmitted to ECU 35. An engine coolant temperature sensor (hereinafter referred to as "TW sensor") 39 is disposed on the cylinder block of the engine 31 to monitor the temperature of the engine coolant or engine cooling water. The TW sensor 39 is formed of a thermistor (or a like device) mounted in the peripheral wall of the engine cylinder which is filled with coolant and applies the detected coolant temperature signal to ECU 35. An engine speed sensor (hereinafter referred to as "Ne sensor") 40 is connected to a crankshaft or a camshaft, neither of which is shown, of the engine. The Ne sensor 40 outputs a crank angle position signal (hereinafter referred to as "TDC (top dead center) signal) at a predetermined crank angle position for every 120° rotation of the crankshaft of the engine in this example with a six cylinder type, the predetermined crank angle position being at a predetermined angle before top dead center (TDC) at the beginning of the intake stroke for each cylinder, and this TDC signal is transmitted to ECU 35. A three-way catalytic converter 42 is disposed in an exhaust pipe 41 and performs the purging of HC, CO and NOx components in the exhaust gases. On the upstream side of the catalytic converter 42, an O₂sensor 43 is disposed in exhaust pipe 41. Sensor 43 detects the concentration of oxygen in the exhaust gases and transmits an O₂ concentration signal to ECU 35. Furthermore, the driving wheel speed sensors 21 and 22, and trailing wheel speed sensors 23 and 24 are connected to ECU 35. Other sensors 44 are provided to detect other engine parameters and connected to ECU 35, the sensors 44 including the aforementioned sensor for detecting the selected gear ratio of transmission 16. ECU 35 includes an input circuit 35a for shaping the waveforms of input signals from various sensors (including driving wheel speed sensors 21 and 22, trailing wheel sensors 23 and 24, the gear ratio sensor of the transmission 16) to shift the voltage levels of sensor output signals to a predetermined level, to convert analog signals from sensors to digital signals and so forth. ECU 35 also includes a central processing unit (hereinafter referred to as "CPU") 35b, memory means 35c storing various operational programs which are executed in CPU 35b and operational results therefrom, etc., and an output circuit 35d which outputs a driving signal to fuel injection valve 36, and so forth. For example, whenever the TDC signal is inputted to ECU 35, the CPU 35b calculates the fuel injection time period T OUT for fuel injection valve 36 from the following equation based on engine parameter signals from the above various sensors which are fed through the input circuit 35a: T OUT = Ti x (K TW x K PA x K STB x K WOT x K LS x K AST x K PB x K O2 ) + (T ACC + T IDL ) (III) wherein Ti is a basic value of the fuel injection time period for fuel injection valves 36, and is determined as a function of engine rotational speed Ne and absolute pressure P BA in the intake pipe 32. K TW is an engine coolant temperature-dependent fuel increasing coefficient which is determined as a function of engine coolant temperature TW and applied at warming-up of the engine 31 for accelerating the warming-up as well as early securing an appropriate air-fuel ratio after the start of the engine. K PA is an atmospheric pressure-dependent correction coefficient. K STB is a correction coefficient provided by the invention, applied for slip control and is determined in accordance with slip rate λ and differential value or rate λ ˙ of slip rate λ, as hereinafter described. K WOT is a mixture-enriching coefficient applied at wide-open-throttle or full opening of the throttle valve, K LS is a mixture-leaning coefficient applied at full closing of the throttle valve, and K AST is a fuel increasing coefficient applied immediately after the start of the engine. K PB is a predicted load-dependent correction coefficient which is determined in accordance with a rate of change of the intake pipe absolute pressure P BA , and K O2 is an O₂-feedback correction coefficient which is determined in response to the output of the O₂ sensor. T ACC and T IDL are correction variables, the former being an acceleration fuel increasing coefficient applied at acceleration of the engine, and the latter being an idle correction coefficient applied at idle of the engine. Then, CPU 35b feeds through output circuit 35d a driving signal to fuel injection valve 36 to open valve 36 for a time period T OUT as set forth above. For example, in order to cut fuel to a cylinder, the driving signal from CPU 35b would simply be inhibited. Fig. 4 shows the configuration of the principal part of CPU 35b of Fig. 3. In Fig. 4, an average vehicular speed arithmetic circuit 49 receives trailing wheel speed signal inputs from trailing wheel speed sensors 23 and 24 which represent ωRL and ωRR, respectively. The circuit 49 calculates from the signal inputs the average value (ωRL + ωRR)/2. An average vehicular speed checking circuit 50 receives the average value (ωRL + ωRR)/2 and compares same with a predetermined value V MIN (e.g. 5 km/h) to determine whether the vehicular speed is in a very low region. The circuit 50 outputs a high level signal (hereinafter referred to as "H signal") when the former is lower than the latter, and a low level signal (hereinafter referred to as "L signal") when the former is higher than the latter, and applies the output signals to a driving wheel speed selecting circuit 51. The circuit 51 selects the smaller value of the two detected driving wheel speeds ωFL and ωFR when the H signal is supplied from the average vehicular speed checking circuit 50, i.e. when the vehicular speed is in the very low region (Lo-Select Mode), and the larger value of the two detected driving wheel speeds ωFL and ωFR when the L signal is supplied from the circuit 50, i.e. when the vehicular speed is not in the very low region (Hi-Select Mode). The selected detected driving wheel speed ωFL or ωFR is inputted to a slip rate calculating circuit 53 wherein it is applied to Equation I set forth above as the driving wheel speed V W . A trailing speed selecting circuit 52 selects one of the two detected speeds ωRL and ωRR of the left and right trailing wheels which is on the same side as one of the driving wheels whose detected speed value has been selected by the driving wheel speed selecting circuit 51. The selected detected trailing wheel speed ωRL or ωRR is inputted to the slip rate calculating circuit 53 wherein it is applied to Equation I set forth above as the vehicular speed V. The slip rate calculating circuit 53 operates on the outputs from the selecting circuits 51 and 52 to calculate a slip rate λ based upon Equation II. An output indicative of the calculated slip rate λ is applied to a differentiating circuit 54 which in turn calculates from the input slip rate λ a differential value λ ˙. A predetermined value setting circuit 60 is supplied with the signal indicative of the gear ratio from the gear ratio sensor of transmission 16 to set, based upon the input gear ratio, a correction coefficient k₁ and a correction variable C₁ which are applied to determine a first threshold value λ₁ of slip rate λ, a correction coefficient k₂ and a correction variable C₂ which are applied to determine a second threshold value λ₂ of slip rate λ, a correction coefficient r₁ and a correction variable F₁ which are applied to determine a first reference value λ ˙₁ of slip rate variation λ ˙, and a correction coefficient r₂ and a correction variable F₂ which are applied to determine a second reference value λ ˙₂ of slip rate variation λ ˙. The correction coefficients r₁ and r₂ and the correction variables F₁ and F₂ are further corrected in dependence on control lag from the time ECU 35 commands the fuel supply control system to operate to the time the latter actually starts to operate. A first reference speed calculating circuit 61 is supplied with the output indicative of vehicular speed V from the trailing wheel selecting circuit 52 as well as the output indicative of correction coefficient k₁ and correction variable C₁ from the predetermined value setting circuit 60 and calculates from the input signals a first reference value V R1 based upon the following equation: V R1 = k₁V + C₁ (IV) From the first reference speed value V R1 calculated as above can be determined the first threshold value λ₁ of slip rate based upon an equation λ₁ = (V R1 - V)/V R1 . Further, the first reference speed calculating circuit 61 compares the calculated first reference speed value V R1 with a first lower limit V C1 , and selects the larger value of the two values V R1 and V C1 . The larger value is outputted from the circuit 61 as the first reference speed value corresponding to the first threshold value λ₁. A second reference speed calculating circuit 62 operates, similarly to the first reference speed calculating circuit 61, on the output indicative of vehicular speed V from the trailing speed selecting circuit 52 and the output representing correction coefficient k₂ and correction variable C₂ from the predetermined value setting circuit 60 to calculate a second reference speed value V R2 based upon the following equation: V R2 = k₂V + C₂ (V) Similarly, the circuit 62 outputs the larger value of the second reference speed value V R2 and a second lower limit V C2 as the second reference speed value corresponding to the second threshold value λ₂ of slip rate. A first correction circuit 65 is supplied with the output indicative of selected trailing wheel speed from the trailing wheel speed selecting circuit 52 and the output indicative of correction coefficient r₁ and correction variable F₁ corresponding to the selected gear ratio from the predetermined value setting circuit 60. The circuit 65 calculates a first reference value λ ˙₁ of slip rate variation based upon the following equation: λ ₁ = r₁V + F₁ (VI) Similarly, a second correction circuit 66 is reponsive to the vehicular speed signal from the trailing wheel speed selecting circuit 52 and the output indicative of correction coefficient r₂ and correction variable F₂ corresponding to selected gear ratio from the predetermined value setting circuit 60, and calculates a second reference value λ ˙₂ of slip rate variation based upon the following equation: λ ₂ = r₂V + F₂ (VII) The correction coefficients k₁, k₂, r₁ and r₂ and correction variables C₁, C₂, F₁ and F₂ are so set that the first and second threshold values λ₁ and λ₂ and first and second slip rate variation reference values λ ˙₁ and λ ˙₂, which are determined from the coefficients and variables as above, satisfy the relationship λ₁ < λ₂, and λ ˙₁ < λ ˙₂ irrespective of the value of the selected gear ratio of transmission 16. An excessive λ ˙ decision circuit 55 is supplied with the output indicative of differential slip rate λ ˙ from the differentiating circuit 54 and the output indicative of second reference slip rate variation value λ ˙₂ from the second correction circuit 66 and compares between them. When the differential value λ ˙ is greater than the second reference value λ ˙₂, it applies H signal to an OR gate 56, and otherwise it applies L signal to OR gate 56. A first prediction control decision circuit 58 is supplied with the λ ˙ output from the differentiating circuit 54 and the λ ˙₁ output from the first correction circuit 66 and compares between them to apply H signal to an AND gate 59 when the differential value λ ˙ is greater than the first reference value λ ˙₁, and otherwise it generates L signal. A second prediction control decision circuit 63 is supplied with the output indicative of selected driving wheel speed V W from the driving wheel speed selecting circuit 51 and the output indicative of first reference speed value V R1 corresponding to first threshold value λ₁ from the first reference speed calculating circuit 61 to compare therebetween so that when the driving wheel speed V W is higher than the first reference speed value V R1 , it applies H signal to AND gate 59, and otherwise it applies L signal thereto. When supplied with two H signals from the first and second prediction control decision circuits 58 and 63, the AND gate 59 generates H signal which is outputted through OR gate 56. An excessive λ decision circuit 64 is supplied with the V W output from the driving wheel speed selecting circuit 51 and the output indicative of second reference speed value V R2 corresponding to second threshold value λ₂ from the first reference speed calculating circuit 62 to compare therebetween so that when the driving wheel speed V W is higher than the second reference speed value V R2 , it applies H signal to OR gate 56, and otherwise it applies L signal thereto. As described above, H signal is outputted through the OR gate 56 if any of the following conditions holds: (i) λ ˙ > λ ˙₂, (ii) λ ˙ > λ ˙₁ and (iii) λ > λ₂. The H signal through OR gate 56 acts as a trigger signal for causing execution of a control program shown in Fig. 5, hereinafter described in detail, to effect control of the fuel supply control system for prevention of excessive differential slip rate (excessive slip rate variaton) if the condition (i) holds, prediction control if the condition (ii) holds, and prevention of excessive slip rate if the condition (iii) holds, respectively. According to the invention, the fuel supply is cut (hereinafter referred to as "fuel cut") in response to the H signal through OR gate 56 in a manner hereinafter described in detail so as to reduce the torque of the driving wheels 11 and 12 and hence reduce the slip rate λ or the slip rate variation λ ˙, to thereby bring the slip rate λ to a desired value. The H signal through OR gate 56 will be hereinafter referred to as fuel cut signal (FCM signal) as appearing at (b) (1) of Fig. 6, and a driving region of the vehicle 1 in which the fuel cut signal is generated will be hereinafter referred to as fuel cut region, as shown at (c) (1) of Fig. 6. The outputs from the first and second prediction control decision circuits 58 and 63 in Fig. 4 are also applied to an OR gate 67 which generates H signal when at least either one of the outputs from the circuits 58 and 63 is H signal, that is, if either the condition λ ˙ > λ ˙₁ or the condition λ > λ₁ holds. The H signal from the OR gate 67 will be hereinafter referred to as standby signal (STB signal) as shown at (b) (1) of Fig. 6, a driving region of the vehicle 1 in which the standby signal is generated and the fuel cut signal is not generated will be referred to as standby region as at (c) (2) of Fig. 6, and any fuel-supplied driving region of the vehicles other than the standby region and the fuel cut region will be referred to as off-standby region as at (c) (3) of Fig. 6. The standby region corresponds to a driving region which the vehicle passes immediately before shifting to the fuel cut region from the off-standby region or immediately after leaving the fuel cut region [(a), (c) of Fig. 6], as will be understood from the condition λ ˙ > λ ˙₁ or λ > λ₁ corresponding to the standby region, and the condition λ ˙ > λ ˙₁ and λ > λ₁ corresponding to the fuel cut region, etc. According to the invention, this standby region is provided wherein the fuel quantity supplied to the engine 31 is controlled in an appropriate manner as hereinafter described in detail so as to control the air-fuel ratio of a mixture supplied to the engine to an appropriate value and hence control the output or driving force of the engine during transition from the ordinary fuel-supplied region to the fuel cut region or vice versa, because otherwise during transition between the fuel cut region and the ordinary fuel-supplied region there can take place an excessive rise in the temperature of the three-way catalytic converter 42 and after-fire as well as driving shock or degraded driveability. Referring again to Fig. 4, there are further provided first and second vehicular speed determining circuits 68 and 69. The first vehicular speed determining circuit 68 is supplied with the vehicular speed output V from the vehicular speed selecting circuit 52 and a reference signal representing a first predetermined vehicular speed value V₁ (e.g. 12 km/h). It compares between the two values and generates a first determined vehicular speed signal (FCM₁ signal) when V < V₁ holds. The second vehicular speed determining circuit 69 operates similarly to the circuit 68 to generate a second determined vehicular speed signal (FCM₂ signal) when the vehicular speed V is higher than a second predetermined vehicular speed V₂ (e.g. 20 km/h) higher than the first predetermined value V₁, both inputted thereto. Fig. 5 shows a control program for effecting slip control in dependence on the fuel cut signal and the standby signal outputted from the circuit of Fig. 4 and other engine operating parameters. The present program is executed whenever the TDC signal is inputted to the ECU 35. First, a step 501 calls for a determination as to whether the standby signal has been inputted. If the answer is No, i.e. if no standby signal has been inputted, which means the vehicle 1 is being driven in the off-standby region, the program proceeds to a step 502 wherein it is determined whether a second flag FLG FCT2 is equal to 0. This second flag FLG FCT2 is set to 1 at a step 519, referred to later, when the standby signal has been inputted, that is, when the vehicle 1 is being driven in either the standby region or the fuel cut region, whereas it is set to 0 at a step 516, referred to later, when the vehicle 1 is being driven in the off-standby region. If the answer to the question of step 502 is No, that is, if the second flag FLG FCT2 is equal to 1, which means the present loop is the first loop immediately after transition to the standby region, the program proceeds to a step 503. In step 503, a timer T.TRC formed e.g. by a down counter is started to count a predetermined time period tTRC (e.g. 2 sec), followed by a step 504 wherein it is determined whether the engine rotational speed Ne is higher than a predetermined value Ne TRC (e.g. 2300 rpm). If Ne > Ne TRC stands, the program proceeds to a step 505 wherein a third control variable CU FCT3 is set to a fifth predetermined number N₄ (e.g. 2), followed by a step 506 wherein the correction coefficient K STB for slip control is set to a predetermined mixture-leaning value X STB2 (e.g. 0.8) which is applied in the off-standby region, as shown at A of (d) (3) of Fig. 6. Then, the engine coolant temperature-dependent fuel increasing coefficient K TW is set to a predetermined value K TW0 at a step 506a, followed by the program proceeding to a step 511, hereinafter described. The predetermined value K TW0 is determined, for example, from a K TWO table, given, by way of example, in Fig. 8, in accordance with the engine coolant temperature TW. According to the Fig. 8 table, the predetermined value K TW0 is set in relation to three predetermined engine coolant temperature values TW₁ - TW₃ (e.g. -10°C, +20°C, and + 50°C, respectively). Specifically, it is set to a first value K TW01 (e.g. 1.00) when the engine coolant temperature TW is below TW₁ or above TW₃, and to a second value K TW02 (e.g. 0.90) when TW is TW₁ or more and below TW₂, and to a third value K TW03 (e.g. 0.95) when TW is TW2 or more and below TW₃, respectively. If the answer to the question of step 504 is No, that is, if Ne ≦ Ne TRC holds, the program proceeds to a step 507 wherein the third control variable CU FCT3 is set to 0, followed by setting the slip control correction coefficient K STB to 1.0 at a step 508 (B of (d)(3) of Fig. 6), and then the program proceeds to step 511, referred to above. If the answer to the question of step 502 is Yes, that is, if the second flag FLG FCT2 is 0, indicating that the vehicle was also in the off-standby region in the last loop, the program proceeds to a step 509 wherein it is determined whether the third control variable CU FCT3 is 0. If CU FCT3 is not 0, CU FCT3 is reduced by 1 at a step 510, and then the aforementioned step 506 is executed, followed by step 511 being executed. If CU FCT3 is 0, the aforementioned step 508 is executed, followed by execution of step 511. As will be understood from the above explanation, when the engine rotational speed Ne is high (Ne > Ne TRC ) immediately after the vehicle has shifted to the off-standby region, the slip control correction coefficient K STB is set to the predetermined value X STB2 which is smaller than a normal value of 1.0 over a time period corresponding to a number of TDC signals equal to the fifth predetermined number N₄ after transition to the off-standby region, as shown at (d) (1) of Fig. 6. By thus setting correction coefficient K STB , a lean mixture is supplied to the engine 31 at an instant immediately after fuel cut is terminated so that the output of engine 31 may be made smaller than a normal value, thereby preventing driving shock which would otherwise be caused by sudden recovery of engine torque at the termination of fuel cut. However, if the engine rotational speed Ne is low (Ne ≦ Ne TRC ) immediately after fuel cut termination, the above mixture leaning is inhibited in order to prevent engine stall. Alternatively of leaning the mixture by making smaller the correction coefficient KSTB as above, the ignition timing of the engine 31 may be retarded with substantially the same results as mentioned above, because as the ignition timing is retarded, the output of the engine is smaller. Fig. 9 shows an example of a manner of retarding the ignition timing. In Fig. 9, steps 901 and 902 can be applied in the flowchart of Fig. 5. All the steps in Fig. 5 but the steps 901 and 902 are applicable to the manner of retarding the ignition timing. In the step 901, the correction coefficient K STB is set to 1.0 and a retarding correction value ϑ STB is set to a predetermined value ϑ STB1 . The set retarding correction value ϑ STB is applied to the following equation to calculate an ignition advance angle ϑ ig : ϑ ig = ϑ igMAP + ϑ STB + ϑ CR (VIII) where the ignition advance angle is a crank angle before a reference crank angle position of a cylinder in which ignition is to be effected (e.g. TDC at the end of a compression stroke), ϑ igMAP is a basic ignition timing value, which is determined from a map stored in the memory means 35c in ECU 35 as a function of engine rotational speed Ne and a parameter representing load on the engine, e.g. absolute pressure in the intake pipe 32. ϑ CR is another retarding correction value which is determined as a function of intake air temperature, etc. The predetermined value ϑ STB1 has a minus value (e.g. 20°), and therefore the ignition timing value 0 ig is corrected by ϑ STB1 to a retard side. Details of the ignition timing control are disclosed, e.g. by U.S. Patent No. 4,702,211 assigned to the assignee of the present application. On the other hand, in the step 902 of Fig. 9, the correction coefficient K STB is set to 1.0, and the retarding correction value ϑ STB is set to another predetermined value ϑ STB2 (= 0) to inhibit retardation of the ignition timing. Referring again to Fig. 5, in the step 511, a determination is made as to whether the counted value T.TRC of the timer T.TRC set to start at step 503 is equal to 0. If the counted value T.TRC is not equal to 0, that is, the predetermined time period tTRC has not elapsed after the vehicle 1 shifted to the off-standby region, a third flag FLG FCT3 and a fourth flag FLG FCT4 are set to 1 at respective steps 512 and 513, and then the program proceeds to a step 516, hereinafter referred to. If the answer to the question of step 511 is Yes, that is, if the counted value T.TRC is equal to 0, indicating that the predetermined time period tTRC has elapsed after shifting to the off-standby region, the third and fourth flags FLG FCT3 and FLG FCT4 are set to 0 at respective steps 514 and 515, and then the program proceeds to the step 516. In step 516, the second flag FLG FCT2 is set to 0, and then the first flag FLG FCT1 is set to 0 at a step 517, followed by the program proceeding to a step 518 wherein the value of the correction coefficient K STB set at step 506 or at 508 is applied to Equation III given before to calculate the fuel injection time period T OUT , to effect fuel injection based upon the calculated injection time period T OUT , followed by termination of the program. If the answer to the question of step 501 is Yes, that is, if the standby signal has been inputted, indicating that the vehicle is in either the standby region or the fuel cut region, the program proceeds to a step 519 wherein the second flag FLG FCT is set to 1. Then, at a step 520, predetermined values X STB and X TRC of correction coefficient K STB to be applied in the standby region and the fuel cut region, respectively, and first and second predetermined numbers N₀ and N₁ have their values selected, respectively, from an X STB table, an X TRC table, an N₀ table, and an N₁ table, all stored in the memory means 35c of ECU 35, in accordance with engine rotational speed Ne. (a) - (d) of Fig. 7 show examples of these tables. According to Fig. 7, each table is divided into five engine speed regions, for each of which is provided a predetermined value of each of X STB , X TRC , N₀, and N₁. More specifically, four predetermined values Ne₁, Ne₂, Ne₃, and Ne₄ of engine rotational speed Ne (e.g. 2,300 rpm, 2,800 rpm, 3,300 rpm, and 4,800 rpm, respectively) are provided and define the respective engine speed regions, i.e. a region I corresponding to less than Ne₁, a region II to Ne₁ - Ne₂, a region III to Ne₂ - Ne₃, a region IV to Ne₃ - Ne₄, and a region V to Ne₄ and more, respectively. As predetermined value X STB to be applied in standby region, there are provided values X STB1 , X STB2 , X STB3 , X STB4 , and X STB5 (e.g. 0.50, 0.60, 0.80, 0.80, and 1.70, respectively), to be applied in respective regions I - V. Similarly, as predetermined values X TRC to be applied in fuel cut region, and first and second predetermined numbers N₀ and N₁, there are provided X TRC1 - X TRC5 (e.g. 0.35, 0.40, 0.40, 0.45, and 0, respectively) as X TRC , N₀₁ - N₀₅ (e.g. 1, 2, 3, 4, and 255, respectively) as N₀, and N₁₁ - N₁₅ (e.g. 1, 2, 3, 3, and 0, respectively) as N₁. These tables may be designed in various forms, depending upon the characteristics of the engine, the kind of the three-way catalytic converter 42, etc. Then, the program proceeds to a step 521 wherein it is determined whether the fuel cut (FCM) signal has been inputted. If the answer is No, that is, if the fuel cut signal has not been inputted, indicating that the vehicle 1 is being driven in the standby region, it is then determined at a step 522 whether the third flag FLG FCT3 is equal to 0. If the flag FLG FCT3 is equal to 0, the fourth flag FLG FCT4 is set to 0 at a step 523, while if the third flag FLG FCT3 is 1, the fourth flag FLG FCT4 is set to 1 at a step 524, followed by the program proceeding to a step 525. In the step 525, all the correction coefficients but atmospheric pressure-dependent correction coefficient K PA , as well as all the correction variables, which are applied to Equation III given above, are set to 1.0 and 0, respectively. That is, all these correction coefficients and correction variables are rendered virtually ineffective. As a result, it is avoided that any changes in the values of these correction coefficients and correction variables affect the fuel injection time interval T OUT , which makes it possible to control the air-fuel ratio of the mixture supplied to the engine 31 to an optimal value during standby region, in cooperation with setting of correction coefficients K STB and K TW to X STB and K TW0 , respectively, at steps 526 and 527, hereinafter described. By thus controlling the air-fuel ratio to an optimal value, emission of unburnt fuel can be reduced in the standby region, thereby preventing after-fire and degradation of the performance of the three-way catalytic converter 42 caused by temperature rise thereof. As stated above, the atmospheric pressure-dependent correction coefficient K PA is not set to 1. 0 at step 525, because also in the standby region, the air-fuel ratio can vary depending upon a change in the atmospheric pressure and therefore has to be corrected by means of the correction coefficient K PA even in the standby region. After setting all the correction coefficients and correction variables to virtually ineffective values at step 525, the program proceeds to step 526 wherein the slip control correction coefficient K STB is set to a value of predetermined value X STB which has been set in step 520 for standby region (at C of (d) (3) of Fig. 6). The ground for this setting of K STB is as follows: During transition between the off-standby region which is an ordinary fuel-supplied region and the fuel cut region wherein fuel supply is interrupted, the air-fuel ratio can largely vary, which can cause unstable combustion of the engine 31 and hence emission of unburnt fuel in large quantities in a low engine speed region in particular, resulting in excessive temperature rise of the three-way catalytic converter 42 and/or in after-fire. Further, the fuel quantity required by the engine 31 varies depending upon engine rotational speed Ne. Still further, in a low engine speed region, the suspension system of the vehicle can be subject to resonance, degrading the driveability. In order to prevent these unfavorable phenomena in the standby region, according to the invention, the slip control correction coefficient K STB is set to predetermined value X STB dependent upon engine rotational speed Ne, whereby excessive temperature rise of three-way catalytic converter and after-fire are prevented and the driveability is improved, over the entire engine speed range. It will be noted that in a high engine speed region (region in V Fig. 7) the predetermined value X STB is set to a value (1.7) more than 1.0 in the standby region to enrich the mixture. This is because the mixture enrichment increases the amount of unburnt fuel whose evaporation heat promotes cooling of the three-way catalytic converter 42, thus preventing excessive temperature rise thereof. Conversely, the predetermined value X STB may alternatively be set to a value less than 1.0 in a high engine speed region in the standby region, so as to lean the mixture, depending upon the kind of three-way catalytic converter and/or characteristics of the engine. The program then proceeds from step 526 to a step 527, wherein the engine coolant temperature-dependent fuel increasing coefficient K TW which has been set to 1.0 at step 525 is set to the aforementioned predetermined value K TW0 . By thus setting K TW , the mixture supplied to the engine 31 will be leaned in the standby region, which cooperates with the setting of all the correction coefficient and correction variables to virtually ineffective values at step 525 to prevent after-fire as well as degraded performance of the three-way catalytic converter caused by excessive temperature rise thereof, even at lower temperatures of the engine 31. Following the step 527, the aforementioned steps 517 and 518 are executed to effect fuel injection by applying the slip control correction coefficient K STB and engine coolant temperature-dependent fuel increasing coefficient K TW set at steps 526 and 527, respectively, followed by ending of the program. If the answer to the question of step 521, that is, if the fuel cut signal has been inputted, indicating that the vehicle 1 is being driven in the fuel cut region, the program proceeds to a step 528 wherein it is determined whether the fourth flag FLG FCT4 is equal to 0. If the fourth flag FLG FCT4 is equal to 0, the third flag FLG FCT3 is set to 1 at a step 529, and the first flag FLG FCT1 is set to 1 at a step 530. Then, fuel cut is effected at a step 531 (at D of (d)(3) of Fig. 6), followed by ending of the program. As stated before, the fourth flag FLG FCT4 is set to 0 when the counted value T.TRC of the timer T.TRC is equal to 0, as is clear from the aforesaid steps 511 and 515, etc. Therefore, when the vehicle has shifted to the fuel cut region after staying in the off-standby region for the predetermined time period tTRC or more, the answer to the question of the step 528 should be Yes, and then fuel cut is to be effected in a continuous manner, as hereinafter described. That is, in the case where the vehicle has shifted to the fuel cut region after a long time of stay in the off-standby region, it can be presumed that the vehicle is being accelerated from a state wherein the slip rate assumes a very small value, e.g. 0 or nearly 0 so that it can be predicted that the variation amplitude and variation speed of the slip rate λ will become greater unless no control is made of the fuel supply. Therefore, in such a case, fuel cut is continuoulsy effected to positively reduce the driving force or torque of the engine 31 so as to promptly bring the slip rate λ to a desired value. A different continuous fuel cut-effecting condition from the above described one may alternatively be provided. For example, such condition is that the throttle valve 33' has been opened from a fully closed state, or the variation rate of throttle valve opening ϑ TH has exceeded a predetermined value, and at the same time the driving wheel(s) 11 and/or 12 are/is in an excessive slip state. When this alternative condition holds, fuel cut may be effected in a continuous manner over a certain period of time, providing subtantially the same results as mentioned above. If the answer to the question of step 528 is No, that is, if the fourth flag FLG FCT4 is equal to 1, the program then proceeds to a step 532 wherein it is determined whether the first flag FLG FCT1 is equal to 0. As is clear from the aforementioned steps 517 and 530, this first flag is set to 1 during fuel cut in the fuel cut region, and set to 0 in the other regions. If the answer to the question of step 532 is Yes, that is, if the first flag FLG FCT1 is equal to 0, indicating that the present loop is the first loop immediately after transition to the fuel cut region, it is determined at a step 533 whether FCM₂ signal has been inputted, that is, whether the vehicular speed V is lower than the second predetermined value V₂. If FCM₂ signal has been inputted, that is, V < V₂ holds, the program proceeds to a step 534 wherein the second predetermined number N₁ selected at step 520 is increased by a fourth predetermined number N₃ (e.g. 1), followed by execution of a step 535. In the step 535, a determination is made as to whether FCM₁ signal has been inputted, that is, whether the vehicular speed V is lower than the first predetermined value V₁. If FCM₁ signal has been inputted, that is, if V < V₁ holds, the program proceeds to a step 536 wherein the first predetermined number N₀ selected at step 520 is decreased by a third predetermined number N₂ (e.g. 1), followed by the program further proceeding to a step 537. If then answer to the question of step 535 is No, that is, if FCM₁ signal has not been inputted, indicating that V ≧ V₁ holds, or if the answer to the question of step 533 is No, that is, if FCM₂ signal has not been inputted so that V ≧ V₂ stands, the program skips over the aforementioned step 536 to a step 537. As noted above, if V < V₂ holds, correction of the first and second predetermined numbers N₀ and N₁ is effected by deducting N₂ from the former and adding N₃ to the latter, and if V₂ ≦ V holds, correction of the second predetermined number N₁ alone is effected by adding N₃ to same, while no correction of N₀ and N₁ is effected if V ≧ V₁ holds. In the step 537, a first control variable CU FCT1 is set to the first predetermined number N₀ which has been selected at step 520 or corrected at step 536, followed by executing a step 538. If the answer to the question of step 532 is No, that is, if the first flag FLG FCT1 is equal to 1, indicating that the present loop is any loop following the first loop immediately after the vehicle has shifted to the fuel cut region, the program jumps to the step 538. That is, the above described steps 532 - 537 are executed only one time immediately after transition to the fuel cut region. The step 538 calls for a determination as to whether the first control variable CU FCT1 is equal to 0. If the control variable CU FCT1 is not equal to 0, the program proceeds to a step 539 wherein a second control variable CU FCT2 is set to the second predetermined number N₁ which has been selected at step 520 or corrected at step 534. Thereafter, 1 is deducted from the first control variable CU FCT1 set as above, followed by execution of the steps 530 and 531 to effect fuel cut [at E₁ of (d)(3) of Fig. 6], followed by the program being ended. If the answer to the question of step 538 is Yes, that is, if the first control variable CU FCT1 is equal to 0, it is determined at a step 541 whether the second control variable CU FCT2 is equal to 0. If CU FCT2 is not equal to 0, the program proceeds to a step 542 wherein 1 is deducted from the second control variable CU FCT2 . Then, the program proceeds to a step 543 wherein all the correction coefficients but atmospheric pressure-dependent correction coefficient K PA and all the correction variables are set to virtually ineffective values just in the same manner as the aforesaid step 525. Then, the slip control correction coefficient K STB is set to the predetermined value X TRC to be applied in the fuel cut region, selected at step 520 [at E₂ of (d)(3) of Fig. 6]. Then, the engine coolant temperature-dependent coefficient K TW is set to 1.0 at a step 545, followed by execution of the step 518 to effect fuel injection by applying these set correction coefficients and correction variables to Equation III, and by ending the program. If the anwswer to the question of step 541 is Yes, that is, if the second control variable CU FCT2 is equal to 0, the program proceeds to a step 546 wherein the first control variable CU FCT1 is set to the first predetermined number N₀, as was done in the step 537. This is followed by the steps 540, 530, and 531 to effect fuel cut, and by ending of the program. It will be learned from the above explanation that even when the vehicle is being driven in the fuel cut region, fuel cut is not effected in a continuous manner if the fourth flag FLG FCT4 is equal to 1, but fuel cut over a number of TDC signal pulses equal to the first predetermined number N₀ and interruption of fuel cut over a number of TDC signal pulses equal to the second predetermined number N₁ are repeatedly alternately effected [at E of (d)(3) of Fig. 6]. The fourth flag FLG FCT4 is set to 1 when the vehicle stayed in the off-standby region for a time period shorter than the predetermined time period tTRC before shifting to the fuel cut region, as is clear from the steps 511 and 513, etc., or when the vehicle has returned to the fuel cut region from the standby region after shifting to the standby region from the fuel cut region without shifting to the off-standby region, as is clear from the steps 529, 522 and 524, etc. In other words, the fourth flag FLG FCT4 is set to 1 when the slip control mode is changed at relatively short time intervals such that the slip rate λ has small variation amplitude and small variation rate. Therefore, on such occasions, it is preferable to effect slip control by alternating effected fuel cut and interruption of same over respective appropriate TDC signal pulse numbers, thereby preventing driving shock due to a sudden drop in the torque of the engine 31 which would be caused if fuel cut is effected in a continuous manner, and hence improving the driveability. Further, the air-fuel ratio of mixture supplied to the engine 31 is controlled to a leaner value than a normal value by means of the predetermined value X TRC set in dependence on engine rotational speed Ne at the step 520, providing substantially the same results as the aforedescribed control of setting the predetermined value X STB of correction coefficient K STB in dependence on engine rotational speed Ne in the standby region, that is, prevention of excessive temperature rise of three-way catalytic converter 42 and after-fire, and improved driveability. Also, the first predetermined time number N₀ and second predetermined time number N₁ which determine the ratio of TDC signal pulse numbers over which fuel cut and interruption of same are effected, respectively, are essentially set in dependence on engine rotational speed Ne, as stated above, thus also providing similar results to the above-mentioned results obtained by setting the predetermined value X STB for standby region and the predetermined value X TRC for fuel cut region. Although in the foregoing embodiment the first and second predetermined time numbers N₀ and N₁ are set in dependence on engine rotational speed Ne, this is not limitative, but they may be set in dependence on the slip state of the vehicle, e.g. slip rate λ or slip rate variation λ ˙. Specifically, the first and second predetermined time numbers N₀ and N₁ should be set such that the ratio therebetween has an appropriate value to the slip rate λ or the slip rate variation λ ˙, by taking into consideration the fact that as the slip rate λ or the slit rate variation λ ˙ is greater, the torque of the engine 31 should be reduced by a greater amount or at a greater rate. This alternative setting of N₀ and N₁ may make it possible to achieve appropriate air-fuel ratio control in a manner more directly responsive to the actual slip rate of the vehicle so that the driving force or torque of the engine can be suitably reduced to promptly bring the slip rate λ or the slip rate variation λ ˙ to a desired value. (a) and (b) of Fig. 10 show an example of tables of the first and second predetermined time numbers N₀ and N₁ plotted with respect to slip rate λ. According to the tables of Fig. 10, the slip rate λ is divided into five slip rate regions I - V, for each of which is a predetermined value of each of predetermined time numbers N₀ and N₁. Specifically, there are provided predetermined values of λ, i.e. λ₁, λ₂, λ₃, and λ₄ (e.g. 15%, 20%, 25%, and 30%, respectively), defining respective regions I, II, III, IV, and IV, for respective ones of which regions there are provided predetermined values N₀₁, N₀₂, N₀₃, N₀₄, and N₀₅ of N₀ (e.g. 1, 2, 3, 4, and 255, respectively), and predetermined values N₁₁, N₁₂, N₁₃, N₁₄, and N₁₅ (e.g. 1, 2, 3, 3, and 0, respectively). (a) and (b) of Fig. 11 show an example of tables of the first and second predetermined time numbers N₀ and N₁ plotted with respect to slip rate variation λ ˙. According to the tables of Fig. 11, the slip rate variation λ ˙ is divided into five slip rate regions I - V, for each of which is a predetermined value of each of predetermined time numbers N₀ and N₁. Specifically, there are provided predetermined values of λ ˙, i.e. λ ˙₁, λ ˙₂, λ ˙₃, and λ #SP·#₄ (e.g. 30% /sec , 40% /sec , 60% /sec , and 80% /sec , respectively), defining respective regions I, II, III, IV, and IV, for respective ones of which regions there are provided predetermined values N₀₁, N₀₂, N₀₃, N₀₄, and N₀₅ of N₀ (e.g. 1, 2, 3, 4, and 255, respectively), and predetermined values N₁₁, N₁₂, N₁₃, N₁₄, and N₁₅ (e.g. 1, 2, 3, 3, and 0, respectively). The tables of Fig. 10 or Fig. 11 may be applied in the step 520 in Fig. 5 in place of the tables in Fig. 7, to determine the values of N₀ and N₁. Furthermore, instead of setting the predetermined values X STB and X TRC , and the predetermined time numbers N₀ and N₁ in dependence on engine rotational speed Ne as in the embodiment described above, these values may be set in dependence on other operating parameters of the engine such as load on the engine, e.g. absolute pressure P BA in the intake pipe or the opening ϑ TH of the throttle valve, in addition to or in lieu of Ne. By so setting N₀ and N1 based upon load on the engine, the ratio between fuel cut period and fuel cut-interruption period can conform to engine load represented by the above parameter PBA or ϑ TH . This alternative setting is advantageous in overcoming the problem with the prior art that since the amount of drop in the driving force or torque of the engine 31 is different depending upon the magnitude of load on the engine, if the above predetermined values and/or predetermined time numbers are so set that the target air-fuel ratio is appropriate to a high load operating condition of the engine, the slip control will be effected to an excessive extent when the engine is operating in a low load operating condition, whereas if they are so set that the target air-fuel ratio is appropriate to a low load operating condition of the engine, the slip control will be effected to an insufficient extent when the engine is operating in a high load operating condition. Therefore, by setting the values X STB , X TRC , and N₀ and N₁ based upon engine load, it will be possible to achieve proper air-fuel ratio control and hence proper slip control throughout the entire engine load region, thereby securing satisfactory driveability. If the predetermined values and/or predetermined time numbers are set based upon both engine rotational speed Ne and intake pipe absolute pressure P BA , they may be set by means of maps with Ne and P BA as setting factors. (a) - (d) of Fig. 12 show examples of these tables. According to Fig. 12, each table is divided into five intake pipe absolute pressure regions, for each of which is provided a predetermined value of each of X STB , X TRC , N₀, and N₁. More specifically, four predetermined values P BA1 , P BA2 , P BA3 , and P BA4 of intake pipe absolute pressure P BA (e.g. 200mmHg, 350mmHg, 450mmHg, and 600mmHg, respectively) are provided and define respective intake pipe absolute pressure regions, i.e. a region I corresponding to less than P BA1 , a region II to P BA1 - P BA2 , a region III to P BA2 - P BA3 , a region IV to P BA3 - P BA4 , and a region V to P BA4 and more, respectively. As predetermined value X STB to be applied in standby region, there are provided values X STB1 , X STB2 , X STB3 , X STB4 , and X STB5 (e.g. 0.50, 0.60, 0.80, 0.80, and 1.70, respectively), to be applied in respective regions I - V. Similarly, as predetermined value X TRC to be applied in fuel cut region, and first and second predetermined time numbers N₀ and N₁, there are provided X TRC1 - X TRC5 (e.g. 0.35, 0.40, 0.40, 0.45, and 0, respectively) as X TRC , N₀₁ - N₀₅ (e.g. 1, 2, 3, 4, and 255, respectively) as N₀, and N₁₁ - N₁₅ (e.g. 1, 2, 3, 3, and 0, respectively) as N₁. The tables of Fig. 12 may also be applied in the step 520 of Fig. 5, in place of the tables of Fig. 7. Still further, the alternate fuel cut and fuel cut interruption may be effected in various manners, besides the manner described above. For example, the first and second predetermined time numbers N₀ and N₁ may be set in dependence on the speed of the trailing wheels. Moreover, the ratio between fuel cut period and fuel cut-interruption period may be a fixed value, instead of using TDC signal pulses. This can simplify the control system. Fig. 13 shows another example of slip control, which is identical with the control program of Fig. 5 in effecting slip control in dependence on generation of the fuel cut signal, the standby signal, etc. and in response to engine operating parameters. The control program of Fig. 13 is also executed whenever the TDC signal is inputted to the ECU 35. The control program of Fig. 13 is different only in part from that of Fig. 5, and therefore in Fig. 13, only the different portion and its adjacent common parts are shown, the steps of the common parts being designated by identical reference numerals with those in Fig. 5. The control program of Fig. 13 is distinguished from that of Fig. 5 in steps executed when the answer to the question of step 528 in Fig. 5 is Yes, i.e. when the vehicle remains in the off-standby region for the predetermined time period tTRC or more before shifting to the fuel cut region. When the answer to the question of step 528 is Yes, the program proceeds to a step 1301 wherein it is determined whether the third flag FLG FCT3 is equal to 1. If the answer is No, i.e. if the third flag FLG FCT3 is equal to 0, that means the present loop is the first loop immediately after the vehicle has shifted to the fuel cut region, the program proceeds to a step 1302 wherein the third control variable CU FCT3 is set to 0, and followed by a step 1303 wherein the third flag FLG FCT3 is set to 1. Thereafter the steps 530 and 521 in Fig. 5 are executed to effect fuel cut, followed by ending of the program. If the answer to the question of step 1301 is Yes, that is, if the third flag FLG FCT3 is equal to 1, indicating that the present loop is any loop subsequent to the first loop immediately after the vehicle has shifted to the fuel cut region, the program proceeds to a step 1304 wherein the third control variable CU FCT3 is increased by 1, and then it is determined at a step 1305 whether the third control variable CU FCT3 thus increased is greater than a sixth predetermined time number N₅. This sixth predetermined time number N₅ is set at a value greater than the first predetermined time number N₀, in dependence on engine rotational speed Ne, engine load, or the speed of trailing wheels, like the first and second predetermined time numbers N₀ and N₁. If the answer to the question of step 1305 is No, that is, if CU FCT3 ≦ N₅ holds, which means the number of loops executed after the transition to the fuel cut region is less than N5 + 1, the steps 530 and 531 are executed to continue fuel cut, followed by the program being ended. If the answer to the question of step 1305 is Yes, that is, if CU FCT3 > N5 holds, indicating that the number of loops executed after the transition to the fuel cut region exceeds the sum N₅ + 1, the fourth flag FLG FCT4 is set to 1 at a step 1306, and the first flag FLG FCT1 is set to 0 at a step 1307, and then the program proceeds to the step 532 in Fig. 5. Since the fourth flag FLG FCT4 has been set to 1 as noted above, the answer to the question of step 528 should be No in the next loop, and accordingly, the program should proceed to the step 532 as in the control program of Fig. 5. As stated above, if the vehicle shifts to fuel cut region after staying in the off-standby region for the predetermined time period t TRC or more, as distinct from the Fig. 5 program wherein fuel cut is continued as long as the vehicle remains in the fuel cut region, in the Fig. 13 program the fuel cut is continued only over a time interval corresponding to the number of TDC signals (= N₅ + 1) generated immediately after transition to the fuel cut region, and thereafter alternate fuel cut and fuel supply are repeatedly effected. Like the control manner of Fig. 5, the control manner of Fig. 13 makes it possible to positively reduce the driving force or torque of the engine 31 so as to promptly bring the slip rate λ to a desired value. The other steps than those shown in Fig. 13 are identical with the corresponding steps in Fig. 5, description of which are therefore omitted. The invention as described above can be summarized as follows: A driving wheel slip control system for a vehicle detects an excessive slip state of at least one of the driving wheels of the vehicle wherein the slip of the driving wheel has a magnitude greater than a predetermined magnitude, and reduces engine output when the excessive slip state is detected, by repeating alternate fuel cut and fuel supply such that fuel supply is stopped for a first predetermined time interval, and then effected for a second predetermined time interval. During the second predetermined time interval the engine is supplied with an amount of fuel smaller than one required for an operating condition of the engine in the excessive slip state. The first and second time interval ratio and the fuel amount may depend upon engine parameters e.g. engine rotational speed and/or pressure in intake passage, or slip state. If the excessive slip state has a greater variation in magnitude than a certain value, fuel supply is continuously effected for a third predetermined time interval longer than the first predetermined time interval or all the time during the excessive slip state. When the driving wheel has left the excessive slip state, engine output is reduced below a value required for an operating condition of the engine.

Claims: A driving wheel slip control system for a motor vehicle, comprising driving wheel speed sensor means (21, 22) for sensing the speed of at least one driving wheel (11, 12) of the vehicle (1) and outputting a driving wheel speed (w FL, w FR); excessive slip detecting means (in 35) for sensing an excessive slip state of said motor vehicle (1) based on said at least one driving wheel speed (w FL, w FR); fuel supply control means (in 35, 36) for controlling the amount of fuel supplied to the engine (31) of the vehicle (1), characterized in that said fuel supply control means (in 35, 36) includes engine output reducing means (in 35) for reducing output from said engine (31) in response to said excessive slip state, by leaning means (35b, 35d) for leaning an air-fuel ratio of an air-fuel mixture supplied to the engine (31), and fuel cut-off means (35b, 35d) for cutting off fuel supply for a respective time interval, said respective time interval being a specified multiple value (N₀, N₁) of the time period between the beginning (TDC-signal) of successive intake strokes of the engine. Driving wheel slip control system according to claim 1, wherein said excessive slip detecting means (in 35) outputs a slip value corresponding to the amount of excessive slip of said motor vehicle (1), and said fuel cut-off means (35b, 35d) determines said multiple value in response to said slip value. Driving wheel slip control system according to claim 2, wherein said engine output reducing means (in 35) further includes means for causing said fuel supply means (in 35) to repeat alternate fuel cut and fuel leaning, when said excessive slip state is detected by said excessive slip detecting means (in 35). Driving wheel slip control system according to claim 3, wherein said fuel supply means are provided for stopping fuel supply for a first predetermined time interval. Driving wheel slip control system according to claim 2 or 3, wherein said engine output reducing means (in 35) comprises (a) first leaning means for leaning the air-fuel ratio of an air-fuel mixture supplied to the engine, when said slip value exceeds a first predetermined slip value (STB), (b) fuel cut-off means (35b, 35d) for cutting off fuel supply for a first predetermind time interval subsequent to the operation of said first leaning means said predetermined time interval being a specified multiple value (N₀) of the time period between the beginning (TDC-signal) of successive intake strokes of the engine, said multiple value being determined in response to said slip value, when said slip value exceeds a second predetermined slip value (FCM), wherein said second predetermined slip value (FCM) is greater than said first predetermined slip value (STB). Driving wheel slip control system according to claim 5, wherein said engine output reducing means (in 35) further includes second leaning means for leaning the air-fuel ratio of an air-fuel mixture supplied for a second predetermined time interval, when said slip value exceeds said second predetermined slip value (FCM). Driving wheel slip control system according to claim 6, wherein said first and second leaning means increase the degree of leaning of the air-fuel ratio as said slip value increases. Driving wheel slip control system according to claim 1, wherein said fuel cut-off means (35b, 35d) includes alternating means for causing said fuel supply control means (in 35, 36) to repeat alternate fuel cut and fuel supply in a manner such that fuel supply is cut off for a first predetermined time interval and fuel supply is then effected for a second predetermined time interval, when said motor vehicle (1) has a slip value (λ) exceeding a predetermined slip value (FCM, STB). Driving wheel slip control system according to claim 8, wherein said fuel supply control means (in 35, 36) supplies fuel to said engine (16) in an amount smaller than the amount of fuel required for an operating condition of the engine (16) assumed when the slip value (λ) of said motor vehicle (1) exceeds said predetermined slip value (FCM, STB) during said second time interval. Driving wheel slip control system according to claim 9, wherein said amount of fuel supplied to said engine (16) for said second predetermined time interval is set by a fuel decremental value determined in dependence upon an operating condition of said engine (16) assumed when the slip value (λ) of said motor vehicle (1) exceeds said predetermined slip value (FCM, STB). Driving wheel slip control system according to claim 10, wherein said fuel decremental value is dependent upon at least one of the rotational speed (Ne) of said engine (16) and pressure in an intake passage (32) of said engine (16). Driving wheel slip control system according to one of claims 8 to 11, wherein the ratio (N₀:N₁) between said first predetermined time interval and said second predetermined time interval is set to a value dependent upon an operating condition of said engine (16) assumed when the slip value (λ) of said motor vehicle (1) exceeds said predetermined slip value (FCM, STB). Driving wheel slip control system according to claim 12, wherein the ratio (N₀:N₁) between said first predetermined time interval and said second predetermined time interval is set to a value dependent upon at least one of the rotational speed (Ne) of said engine (16) and pressure in an intake manifold (32) of said engine (16). Driving wheel slip control system according to claim 8, wherein the ratio (N₀:N₁) between said first predetermined time interval and said second predetermined time interval is set to a value dependent upon a slip state of said at least one driving wheel (11, 12) assumed when the slip value (λ) exceeds said predetermined slip value (FCM, STB). Driving wheel slip control system according to claim 14, wherein the ratio (N₀:N₁) between said first pre-determined time interval and said second predetermined time interval is set to a value dependent upon at least one of the slip value (λ) and a variation (λ) of the slip value. Driving wheel slip control system according to claim 8, wherein said fuel supply control means repeats alternate fuel cut and fuel supply when the slip value (λ) exceeds said predetermined slip value (FCM, STB) after said slip value has a magnitude which is smaller than said predetermined slip value (STB, FCM), for less than a predetermined period of time (T.TRC). Driving wheel slip control system according to claim 5, wherein said engine output reducing means (in 35) comprises ignition timing retarding means for retarding ignition timing of said engine (16) which is determined by at least one operating parameter of said engine (16) when the slip value (λ) exceeds said second predetermined slip value (STB², FCM¹). Driving wheel slip control system according to claim 1, wherein said fuel cut-off means (in 35) includes first control means for repeating alternate fuel cut and fuel supply in a manner such that fuel cut is effected for a first predetermined time interval and then fuel supply is effected for a second predetermined time interval, when said motor vehicle (1) is in a first slip state which is during effecting slip control, and second control means for effecting continuous fuel cut for a third predetermined time interval longer than said first predetermined time interval, when said motor vehicle (1) is in a second slip state which follows a stable less slip condition. Driving wheel slip control system according to claim 1, wherein said fuel cut-off system includes first control means for repeating alternate fuel cut and fuel supply in a manner such that fuel cut is effected for a first predetermined time interval and then fuel supply is effected for a second predetermined time interval subsequent to said first time interval, when said motor vehicle (1) is in a first slip state which is during effecting slip control, and second control means for effecting continuous fuel cut so long as said motor vehicle (1) is in a second slip state which follows a stable less slip condition. Driving wheel slip control system according to claim 18 or 19 wherein said first slip state is a state to which said motor vehicle (1) shifts after staying for less than a predetermined period of time (T.TRC) in a less slip state, and said second slip state is a state to which said motor vehicle (1) shifts after staying for at least said predetermined period of time (T.TRC) in said less slip state. Driving wheel slip control system according to claim 8, wherein said fuel supply control means (in 35, 36) includes further engine output reducing means (in 35) associated with said excessive slip detecting means (in 35), and operable when the slip value (λ) returns to a value below said predetermined slip value (STB), for reducing output from said engine (16) to a value smaller than a value required for an operating condition of said engine (16) assumed immediately after the slip value (λ) has returned to a value below said predetermined slip value (STB). Driving wheel slip control system according to claim 21, wherein said further engine output reducing means (in 35) comprises mixture leaning means which leans a mixture supplied to said engine (16). Driving wheel slip control system according to claim 21, wherein said further engine output reducing means (in 35) comprises ignition timing retarding means for retarding ignition timing in said engine (16). Driving wheel slip control system according to any one of claims 21 to 23, wherein said further engine output reducing means (in 35) operates when the rotational speed (Ne) of said engine (16) is higher than a predetermined value immediately after the slip value (λ) returns to a value below said predetermined slip value (STB). Driving wheel slip control system according to any one of claims 21 to 23, wherein said further engine output reducing means (in 35) operates for a third predetermined time interval after said slip value (λ) returns to a value below said predetermined slip value (STB). Driving wheel slip control system according to any one of the preceding claims, wherein said fuel supply control means (in 35) further includes air fuel ratio correcting means (K WOT , T ACC , K O2 ) for correcting an air-fuel ratio of a mixture supplied to said engine (16) in response to engine operating parameters, and inhibiting means for inhibiting operation of said air-fuel ratio correcting means when said excessive slip state is detected. Driving wheel slip control system according to any one of the preceding claims wherein said fuel supply control means (in 35, 36) includes engine operating parameter detecting means (34, 38, 39, 40, 43) for detecting at least one engine operating parameter (PBA, Ne), and said engine output reducing means (in 35) reduces output from said engine (16) in a manner such that the reduction ratio depends upon said at least one engine operating parameter (PBA, Ne) so that the reduction ratio changes in proportion to said output from said engine (16).

Inventors: NARISAWA, NOBUYUKI, OOONO, TETSUYA
Applicants: HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: B60K28
IPC codes: B60K28