GB2279111A

GB2279111A - Vehicle wheel slip traction control

Info

Publication number: GB2279111A
Application number: GB9411057A
Authority: GB
Inventors: Bruno Valente; John William Robertson
Original assignee: Delco Electronics LLC; Motors Liquidation Co
Current assignee: Delco Electronics LLC; Motors Liquidation Co
Priority date: 1993-06-18
Filing date: 1994-06-02
Publication date: 1994-12-21
Anticipated expiration: 2014-06-02
Also published as: GB2279111B; GB9411057D0; GB9312640D0

Abstract

The controller compares a measure of target wheel slip with actual wheel slip and determines therefrom the amount of engine torque reduction required to reduce the difference to a predetermined level. A retardation value for retarding the occurrence of ignition in one or more engine cylinders is calculated on the basis of the required engine torque reduction and is limited to a predetermined value. Processing means determines if the retardation value can provide the required engine torque reduction and, if this is not the case, commands the selective cutting off of fuel to one or more engine cylinders. Additional spark retardation is provided if fuel cut off is not sufficient. The predetermined ignition retardation may be dependant on engine exhaust gas or convertor temperature. The ignition timing may be readjusted with delay following fuel cut off.

Description

TRACTION CONTROLLER The present invention relates to a method and apparatus for controlling vehicle wheel slip.

Traction control systems are provided in vehicles to limit slip of the driven wheels to maintain vehicle stability and to maximise vehicle acceleration. Although these systems are generally described as controlling traction, in practice they control the torque delivered to the driven wheels in order to optimise tractive and lateral tyre forces for the given tyre/road surface conditions.

In a typical traction control system, torque is controlled by adjustment of one or a combination of the calculated spark advance, the calculated fuel requirement, the throttle valve and, in an automatic transmission, the engaged gear and brake application.

The most basic systems use existing anti-lock braking system (BS) and powertrain control hardware which allows for the application of traction control for very little additional cost. Where such an arrangement is not possible or where performance is important, a combination of the above adjustments is used, which increases system complexity and cost.

The present invention seeks to provide an improved method of and apparatus for controlling the traction of a vehicle.

According to an aspect of the present invention, there is provided a method of controlling vehicle wheel slip comprising the steps of obtaining a measure of target wheel slip; obtaining a measure of actual wheel slip; determining the difference between the actual and target wheel slips and determining therefrom the amount of engine torque reduction required to reduce the difference to a predetermined level; determining on the basis of the required engine torque reduction a retardation value for retarding the occurrence of ignition in one or more engine cylinders relative to a calculated ignition time; limiting the retardation value to a predetermined value if the retardation value exceeds a threshold level; determining whether the retardation value can provide the required engine torque reduction; and, if the retardation value can not provide the required engine torque reduction, selectively cutting off fuel to one or more engine cylinders.

According to another aspect of the present invention, there is provided a traction controller for controlling vehicle wheel slip comprising target slip determining means for obtaining a measure of target wheel slip; sensing means for obtaining a measure of actual wheel slip; comparing means for deriving the difference between the actual and target wheel slips and for determining therefrom the amount of engine torque reduction required to reduce the difference to a predetermined level; ignition control means for determining on the basis of the required engine torque reduction a retardation value for retarding the occurrence of ignition in one or more engine cylinders relative to a calculated ignition time,.the ignition control means being adapted to limit the retardation value to a predetermined value if the retardation value exceeds a threshold level; processing means for determining whether the retardation value can provide the required engine torque reduction; and fuel control means for selectively cutting off fuel to one or more engine cylinders when the processing means determines that the retardation value can not provide the required engine torque reduction.

With controlled spark retardation and intermittent fuel starvation to individual cylinders, it is possible to obtain a desired reduction in engine torque while maintaining the temperatures of the engine exhaust gas and catalytic converter within acceptable limits. Moreover, lower engine exhaust gas and catalytic converter temperatures can be achieved by limiting the amount of spark retardation before cutting off fuel to the cylinders. Additional spark retardation can be allowed once the cutting off of fuel causes a reduction in the engine exhaust gas and catalytic converter temperatures.

In the preferred embodiment, the occurrence of ignition, when retarded, is retarded in all of the engine cylinders. The retardation is typically related to crankshaft angle and is applied as a change in the calculated ignition angle equivalent to a calculated ignition time.

For che avoidance of any doubt, the term traction controller is used to denote the elements which provide the functions described herein and may be included in a dedicated unit, or in any combination of units, including a controller of an anti-lock braking system, a transmission control module, torque control module and engine control module.

Driver comfort can be increased during traction control by retarding spark as much as possible before cutting off fuel to any of the cylinders.

In a preferred embodiment, traction can be controlled through engine control intervention only.

Preferably, the threshold level of the retardation value is equivalent to a predetermined ignition time and/or to an engine exhaust gas or catalytic converter threshold temperature. In this manner, it can be ensured that the temperature of the catalytic converter does not exceed critical limits.

Advantageously, the amount of cylinder fuel cut off is progressively adjusted on the basis of the difference between the torque reduction provided by the retardation value and the required engine torque reduction. It can thus be ensured that the minimum necessary amount of fuel is cut off.

In an embodiment, the amount of cylinder fuel cut off is limited, thereby ensuring that operation of the engine does not become excessively uncomfortable to the driver and passengers.

Preferably, the amount of fuel cut off is limited to a value at which only two engine cylinders are operational.

In a preferred embodiment, additional retardation of ignition timing is provided when the rate of cylinder fuel cut off has been limited.

Since the cutting off of fuel causes some engine cylinders to pass cool air to the engine exhaust, engine exhaust temperature entering the catalytic converter drops during cylinder fuel cut off, enabling ignition to be retarded further.

In an alternative embodiment, additional retardation of ignition timing is provided between each fuel cut off step. In the case where fuel is progressively cut off, this step provides additional spark retardation at each step, which can smooth engine operation during the fuel cut off stage and which can provide enhanced torque reduction.

Advantageously, the additional retardation provided is limited, for example, to a value equivalent to an engine exhaust gas or catalytic converter threshold temperature.

Preferably, the implementation of additional retardation in ignition timing is delayed following an adjustment in the amount of fuel cut off. Such a delay can compensate for the different delays in torque reduction experienced with spark retardation and fuel cut off.

In an embodiment, it is determined whether cylinder fuel cut off has been limited; whether the required torque reduction exceeds a preset reduction; and, if the required torque reduction exceeds the preset reduction, fuel delivery to all of the engine cylinders is disabled. This can provide a very fast reduction in engine torque in extreme conditions.

Preferably, fuel delivery is re-enabled when the engine speed has dropped to a predetermined speed or the required torque reduction drops below the preset reduction.

In the preferred embodiment, the target wheel slip and te actual eel slip are determined on the basis of the difference between the speeds of driven wheels and undriven wheels of the vehicle.

The required torque reduction may be determined as a function of the engaged transmission gear and of the difference between the target wheel slip and the actual wheel slip.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an embodiment of traction controller linked to other system controllers; Figure 2 is a schematic diagram of elements of the traction controller of Figure 1 for use in determining the amount by which engine torque is to be reduced in order to bring wheel slip to within acceptable limits; Figure 3 is a generalised flow chart of an embodiment of routine for reducing engine torque to maintain wheel traction; Figure 4 is a graph comparing brake torque with engine exhaust and catalytic converter temperatures; Figure 5 is a flow chart of an embodiment of routine for activating and deactivating traction control; Figure 6 is a flow chart of an embodiment of routine for retarding the occurrence of ignition; Figure 7 is a flow chart of an embodiment of routine for cutting off fuel to the engine cylinders; and Figure 8 is a series of graphs showing additional spark retardation during fuel shut off in a second embodiment of routine.

Referring to Figure 1, an embodiment of traction controller 10 is shown coupled to a plurality of other controllers for receiving information such as wheel speed, engine speed, engine load, brake pedal position and, optionally, gear status and torque control information; and for outputting information on the amount of torque reduction required to maintain or return wheel slip to within acceptable limits.

In this embodiment, the traction controller 10 is a separate component, although in some embodiments it could be combined with an electronic control module in a single unit.

The traction controller 10 receives wheel speed information from the vehicle anti-lock braking system (ABS) controller 12 through lines 14 to 20, each of which lines is associated with a respective vehicle wheel (not shown). This wheel speed information could also be obtained directly from wheel speed sensors.

Engine speed (RPM) and engine load, determined on the basis of throttle position (TPS), are also fed to the traction controller 10 via lines 22 and 24 respectively. This data is typically obtained from the vehicle engine control module (ECM) 26, although could equally be obtained directly from any suitable engine speed sensor and throttle position sensor.

Where the vehicle is equipped with an automatic transmission controlled by a transmission control module 28, the traction controller 10 may also obtain, on line 30, information as to which gear is engaged and, on line 32, information as to the amount of torque reduction commanded by the transmission control module 28 during, for example, a gear change.

Input line 34 provides information as to the status of the brake switch, that is as to whether the vehicle brakes are being applied; while line 36 is connected to a driver actuable switch for enabling or disabling the traction controller 10.

Lines 14 to 20 provide the traction contr.oller 10 with information representative of the wheel speeds, the information from the rear (undriven) wheels being used to calculate vehicle speed and vehicle acceleration.

The outputs of traction controller 10 include a torque reduction command line 38 for outputting a required torque reduction value for use by the engine control module 26 in reducing engine torque, in a manner described below.

An output line 40 provides a signal to a light or other indicator for giving the driver information as to the operating state of the traction controller 10, for example whether it is disabled, enabled or active. A serial data line 42 provides a programming input and also allows the traction controller 10 to be tested by a service engineer.

In this embodiment, the traction controller 10 uses the information it receives to monitor the difference between the speeds of the driven and the undriven wheels of the vehicle. If the difference is within a predetermined limit, the traction controller 10 determines that the amount of wheel slip is acceptable. On the other hand, if the difference in the speeds between the driven and the undriven wheels is greater than the predetermined limit, the traction controller 10 determines that there is an unacceptable amount of wheel slip and that engine torque must be reduced to reduce wheel slip and regain traction The operation of traction controller 10 is described in greater detail below with reference to Figures 2 and 3. Referring to Figure 2, there is shown a schematic diagram of the basic elements of traction controller 10 used in obtaining a measure of wheel slip and of the amount of torque reduction required to maintain or return wheel slip to an acceptable level.

From look-up table 50, the traction controller 10 determines a target velocity difference (6v arget), representative of the target difference between the speeds of the driven and undriven wheels, on the basis of the measured vehicle speed and calculated vehicle acceleration from the rear (undriven) wheels. Concurrently, the traction controller also obtains at 52 a measure of the actual difference (#v actual) in the velocities of the driven and undriven wheels, on the basis of the individual wheel speeds provided on input lines 14 to 20 from the ABS controller 12. Typically, the ABS controller 12 provides individual wheel speed signals which are converted by the traction controller 10 into units of vehicle speed and then used to determine the actual speed difference (Ovactual).

By means of comparator 54, the traction controller 10 calculates the difference between the actual and target velocity differences and produces a velocity error signal 6ver ro . Then, the error signal EverrOr is filtered through a conventional Proportional Integral Derivative filter 56 to produce an uncompensated torque reduction value TEPID representative of the torque reduction required to maintain or return wheel slip to within acceptable limits.

In this embodiment, the uncortipensated torque reduction value is obtained on the basis of the following equation: 2ID = KpRop(8Verror) + KINT(##verrordt) + + KDER(#verror/dt), (1) in which the term KPROP(#verror ) is the error signal Sverrcr multiplied by a proportional gain factor KPROP; ; KINT KINT(##verrordt) is the integral over time of the error signal AverrOr multiplied by an integral gain factor K and KDER(#v aVerror/dt) is the derivative over time of the error signal #v error multiplied by a derivative gain factor K DER Since the proportional and derivative terms in equation (1) are directly dependent upon the instantaneous wheel speeds, they generally give an adequate response time. However, since the integral term is dependent upon the history of the velocity error signal dv w in order to achieve a fast transient response following a sudden change in the error signal, a higher integral gain factor KIWT is required than during moderate changes. The proportional term generally compensates for any delay in transient response caused by the integral term.

The engaged gear is identified at 58 and the torque reduction value T is compensated to produce a torque reduction signal Trend for transfer to the engine control module 26. Compensation is provided on the basis of the following equation: T red = [Tt-D]KGEAR (2) in which K EAR is representative of the active transmission gear ratio and T D is obtained from equation (1; above. In this embodiment, the torque reduction signal Tred is in the form of a pulse-width modulated signal.

Following the compensation for the active gear ratio, the torque reduction signal T red is sent to the engine control module 26 for use in reducing the engine torque by spark retardation and/or discrete cylinder fuel cut off.

Referring now to the generalised flow chart of Figure 3, which shows an embodiment of routine carried out by the engine control module 26 for reducing engine torque.

At step 70, the engine control module 26 uses the received torque reduction signal Trend to calculate the amount of engine torque to be allowed under the given wheel slip conditions on the basis of the equation: %T1 = (Tz - T,,,)/T,, (3) in which %T is the percentage of available engine torque to be allowed; Td is the maximum available engine torque, determined on the basis of the engine speed (RPM) and engine load, which in this example is measured from the manifold absolute pressure (MAP); Tdl is set as the value of Td on activation of traction control; and T red is the torque reduction value obtained from the traction controller 10.

Equation (3) above depicts the relationship between the torque requirement at the driven wheels and the engine torque output, or maximum available torque. When traction control is first activated, the value (T - Trend) is the torque required by the driven wheels for the current tyre/road surface friction conditions. If there is a change in road surface friction cr if the desired torque reduction occurs, the value of T red will be altered as a result of the calculations carried out by the traction controller 10. If the engine load changes, the value of T will act to maintain a substantially constant engine output torque. Thus, in such circumstances, the value of %T1 will change to alter the amount of torque reduction provided by the electronic control module 26.

In this embodiment, the traction controller 10 also carries out a calculation based on equation (3) above to determine the point at which it can command the beginning/ceasing of traction control.

In this example, the traction controller 10 uses throttle position (TPS) as a measure of engine load when obtaining the value of Td.

Torque is first reduced by the electronic control module 26 by retarding the occurrence of ignition from the calculated spark advance, by means of the spark control unit 29. Although such retardation reduces engine torque effectively, it leads to an increase in the engine exhaust gas and catalytic converter temperatures. In order to prevent these temperatures from reaching a level which can adversely affect the operation of the catalytic converter, the engine control system limits spark retardation to no more than 110 after top dead centre (ATDC) or to a value at which the exhaust gas or catalytic converter temperature reaches a desired limit, for example 9000C.

The limited allowable torque value ignited is then converted at step 72 into a spark retardation value for use by the spark control means in reducing the engine output torque. In this embodiment, the amount of spark retardation is obtained as a function of the limited allowable torque value %T --ted and of the manifold absolute pressure (MAP).

If it is necessary to limit spark retardation in order to prevent excessive heating of the catalytic converter, it is likely that the required torque reduction Trend will not be achieved by the allowed spark retardation. The processor 27 of the electronic control module 26 determines this shortfall in engine torque reduction and accounts for this, at step 74, by providing additional torque reduction by means of selective cutting off of fuel to individual cylinders of the engine.

Step 74 of Figure 3 begins by determining whether further torque reduction is required on the basis of the following equation: %T2 = (Ti - T )/(Td * %Tl,liited) 1 (4) in which %T2 is the percentage of available torque after spark retardation. As will be apparent, this equation depicts the same relationship between tyre/road surface friction and engine torque as equation (3) above. The numerator, which represents the torque required for the measured tyre/road surface friction, is unchanged; while the denominator represents the available engine torque after spark retardation.

It will be evident from equations (3) and (4) that if spark retardation alone is sufficient, the value of %T2 obtained from equation (4) will be 100%. On the other hand, if spark retardation alone is insufficient, the value of %T2 will be less than 100%, indicating that engine torque must be reduced further.

If equation (4) reveals that further torque reduction is required, fuel is selectively cut off at step 74, at a frequency calculated on the basis of the available torque for the current engine conditions, in a manner similar to the calculation for spark retardation.

Fuel is selectively cut off, by means of the fuel control unit 31, by omitting one fuel delivery event every predetermined number of scheduled fuel delivery events. In a typical sequential fuel injection scheme, this results in random cylinders being starved of fuel in an intermittent manner.

The frequency of fuel cut off is determined on the basis of the following equation: N = %T2/(1 - %T2), (5) in which T is the percentage of available engine torque obtained from equation (4) above and N is the number of consecutive events in which fuel is injected between each non fuelling event. In practice, the value of N calculated from equation (5) is the result of the calculation rounded to the nearest integer value. For example, if equation (5) produces a value between 2.50 to 3.49, it will be rounded to a value of N = 3.

In addition to cutting off fuel, step 74 also causes the electronic control module 26 to default to open loop fuel control in which a high air to fuel ratio, in this example of 14.6, is used to prevent the catalytic converter from overheating.

As the required torque reduction increases, the frequency at which fuel is cut off is increased up to a set limit of N=1 in equation (5) above, which in a four cylinder engine is equivalent to the engine operating on two cylinders alone.

In severe conditions, when fuel cut off is not sufficient to provide the required reduction in engine torque, the electronic control module 26 retards spark further at step 76 of Figure 3. This is possible since two of the engine cylinders (in the case of a four cylinder engine) will be passing cool air to the engine exhaust, thereby cooling the catalytic converter. The amount of additional spark retardation is provided on the basis of a gradually increasing retardation up to a temperature limit, based on engine speed (RPM) and manifold absolute pressure (MAP), or until the value of N in equation (5) increases. Should the value of N increase, the amount of additional spark retardation is gradually reduced until the value of N again reaches its lower limit or the amount of additional spark retardation reaches zero.

In extreme conditions, such as on polished ice, the electronic control module 26 adopts a maximum torque reduction measure, at step 78, which makes use of a conventional fuel intervention engine speed limiting algorithm. This algorithm is enabled if, after a delay period, the required engine torque reduction Tie exceeds a predetermined value based on the engine speed (RPM) and manifold absolute pressure (MAP). The algorithm then uses the current engine speed (RPM) as a target speed and ceases fuel delivery to all cylinders. Once the engine speed falls below the established target speed or the required torque reduction falls below the predetermined value, normal fuelling is resumed. The delay period is provided to allow the additional spark advance of step 76 to ramp up to its limit and to allow for a gradual increase in engine and vehicle speeds as this algorithm is repeatedly activated and deactivated.

As will be apparent from the above, steps 70 to 78 are repeated in cyclical manner in order to provide up to date values of required torque reduction, spark retardation and fuel cut off.

Therefore, fuel is only cut off when the limited spark retardation is insufficient to provide the required reduction in engine torque; while additional spark retardation is only provided when fuel cut off is insufficient.

The effects produced by the routine of Figure 3 can be seen in the graph of Figure 4, which compares brake (shaft) torque with exhaust and catalytic converter temperatures over the entire torque reduction range provided with the routine of Figure 3.

In the example of Figure 4, engine torque is initially reduced by retarding spark up to a point at which the catalytic converter temperature reaches a predefined temperature limit, for example 900"C.

At this point, sequential fuel cut off is enabled, which at first has relatively little effect due to the large default value chosen for N. When the value of N reaches 1, in other words when only two cylinders are receiving fuel, the system provides additional spark retardation until either the 110 ATDC spark limit is reached or the exhaust gas or catalytic converter temperature limit is reached.

An example of routines for carrying out steps 72 to 78 of Figure 3 are shown in Figures 5 to 7.

Referring to Figure 5, there is shown a flow chart of an embodiment of routine for activating and deactivating traction control. The routine starts at step 100 at which it is determined whether the throttle position TPS is greater than a predetermined threshold position KTCSTPS. If the throttle position does not exceed the threshold position, the routine passes through step 102, to set a traction control flag TCSACT to zero to deactivate traction control, before exiting.

On the other hand, if the throttle position TPS is greater than the threshold position KTCSTPS, the routine passes to step 104 to determine if the coolant temperature is greater than a preset reference temperature KTCSCOOL. If this is not the case, the routine is exited through step 102, deactivating traction control.

If the coolant temperature is above the reference temperature KTCSCOOL, the routine passes to step 106 to determine if traction control is already active. If this is not the case, the routine passes to step 108 to determine whether the required torque reduction TRED is above a minimum level KTREDMIN. If the required torque reduction is not above this level, the routine is left on the basis that wheel slip is within acceptable limits.

On the other hand, if the required torque reduction TRED is greater than the minimum, the routine passes to step 110 to determine whether the change in required torque reduction (TRED - TREDOLD) during successive passes through the routine is greater than a predetermined minimum change KDELTRED.

If this is the case, the routine passes to step 112 to set the traction control flag TCSACT to one to activate traction control. If the change in required torque reduction is less than the predetermined change, the routine passes to step 114 to set the current required torque reduction TRED as the old required torque reduction TREDOLD in preparation for a subsequent pass through the routine.

Returning to step 106, if it is determined that the traction control flag has been set to one, in other words that traction control has already been actived, the routine passes to step 116 to determine whether the required torque reduction TRED is less than or equal to an exit value KTCSEXIT and if this is not the case, the routine is exited on the basis that torque reduction is required and that traction control is appropriate. On the other hand, if the determination at step 116 is positive, the routine passes to step 118 to calculate the new value Tdi on the basis of the equation: Td, =tea1 + (Td - Tdl)*KTCSXFLT, (6) in which KTCSXFLT is a constant determined by experiment.

At step 120, the routine determines whether the newly calculated value of Tdl is equal to the value Td and, if this is the case, the routine determines that engine torque is at the required level and deactivates traction control by setting the traction control flag TCSACT to zero at step 122. On the other hand, if the newly calculated value of Tdi is not equal to the value T , the routine determines that traction control is still required and is exited immediately.

When traction control is activated, the electronic control module 26 performs the routines of Figure with Figure 3.

From step 202 or 204, the routine passes to step 206 to determine if the maximum frequency of fuel cut off has been reached, that is if N = 1 in this embodiment. If the engine control module 26 has not activated fuel cut off, then the value of N will be at its default value of 255 (in this embodiment) and the routine will pass to step 208 to reduce the additional spark retardation by a predetermined amount on the basis of the equation: XSPKRTD = XSPKRTD * KXSPKRT2/256, (7) in which XSPKRTD is the amount of additional spark retardation and KXSPKRT2 is a predetermined constant. If there has been no additional spark retardation, the value of XSPKRTD will be zero and step 208 will produce no change.

The routine then passes to step 210 to calculate the amount of spark retardation on the basis of the following equation: TCS~SPARK RETARD = FTCSPKRT(MAP,%T1) + XSPKRTD, (8) in which FTCSPKRT(MAP,%T1) is a first level of spark retardation determined as a function of engine load, in this example the manifold absolute pressure (MAP), and the value of %T1 obtained at step 202 or 204; and XSPKRTD is the additional spark retardation obtained at step 208, 212 or 214.

Returning to step 206, if the maximum frequency of fuel cut off has been reached, that is N=1, the routine passes to step 212 to determine if the set amount of additional spark retardation has reached or exceeded a predetermined maximum value FXSPKRTA obtained as a function of engine speed (RPM) and engine load, in this case manifold absolute pressure (MAP). If the predetermined value has been reached or exceeded, the routine passes directly to step 210, bypassing step 214. On the other hand, if the maximum value of additional spark retardation has not been reached, the routine passes to step 214 to increase the additional spark retardation by a predetermined amount on the basis of the following equation: XSPKRTD = FXSPKRTA(RPM,MAP) * (KXSPKRT1/256) + XSPKRTD, (9) in which FXSPKRTA(RPM,MAP) is a proportional value determined as a function of engine speed and engine load, in this case manifold absolute pressure; and KXSPKRT1 is a preset constant.

From step 214 the routine passes to step 210 to determine the amount by which ignition is to be retarded.

Referring now to Figure 7, there is shown a flow chart of an embodiment of routine for selectively cutting off fuel to the engine cylinders and fqr cutting fuel to all the cylinders when torque reduction required to maintain desired wheel slip has exceeded a predetermined level.

This routine first carries out, at step 300, the calculation of equation (4) above to find the percentage of torque reduction %T. after spark retardation and then proceeds to step 306 at which the routine carries out the calculation of equation (5) above to find the frequency of fuel cut off, that is the value N. In alternative embodiments, step 306 could find the value of N by any other suitable method or algorithm.

At step 308 the air to fuel ratio is set at 14.6 and closed loop fuel control is disabled to prevent the catalytic converter from overheating due to an excess of fuel. Step 308 is bypassed if fuel shutoff is not desired. The routine then passes to step 310 to determine if the value of N determined at step 306 is equal to 1, that is if the frequency of fuel cut off has reached the preset maximum value.

If this is not the case, implying that the frequency can still be increased, the routine passes to step 312 to reset the timer TFSODLY used to measure the time delay for the maximum torque reduction mode described above and then to step 314 to set a flag TFSO false, this flag being used to disable fuel delivery to all of the engine cylinders during the maximum torque reduction mode.

Returning to step 310, if the value of N = 1, implying that the frequency of fuel cut off can not be increased further, the routine passes to step 316 to determine whether the required reduction in engine torque can be achieved by maximum spark retardation and maximum fuel cut off, using the following equation: TRED > FTREDMAX * KTREDMUL/256, (10) in which FTREDMAX is the amount of torque reduction which can be provided with initial spark retardation, when the engine is running on two cylinders and the additional spark retardation obtained from equation (7); and KTREDMUL is a "fudge factor" or smoothing factor of known form.

If the required torque reduction can be achieved, the routine is left via steps 312 and 314.

On the other hand, if the required torque reduction can not be achieved, the routine passes to step 318 to determine whether the count in the delay timer TFSODLY has reached the predetermined delay time KTFSODLY and, if this is the case, the routine passes to step 320 to set the flag TFSO to true so as to cut off fuel to all of the engine cylinders.

On the other hand, if the predetermined delay has not elapsed, the routine passes to step 322 to set the new reference engine speed TFSORPM to the current engine speed and then to step 324 to increment the count in the delay timer TFSODLY by one. The reference engine speed is used to disable step 320 when the engine speed drops below the reference engine speed after step 320 has been enabled.

In an alternative embodiment, the steps of cutting of fuel to the cylinders and of retarding spark further on cutting off fuel are carried out together.

When it is determined at step 74 that further torque reduction is required as a result of the limitation of the amount of spark retardation, this embodiment passes to an intermittent fuel cut off mode. This intermittent fuel cut off mode has similarities with the fuel cut off steps of the embodiment shown in Figures 5 to 7 and continues until it reaches a predetermined maximum fuel cut off (for example when N = 1). In this embodiment, fuel cut off is determined on the basis of the following equation: InvN = (1-%T2)/%T2, (11) which can be considered the inverse of equation (5) above and representative of the number of consecutive cylinders not to be fuelled (InvN) before fuel is again delivered to one cylinder. In practice, the value InvN calculated from equation (11) is rounded to the nearest integer value from the actual value (InvNact) obtained. For example, if the actual value (InvNact) calculated is between 2.50 to 3.49, the value of InvN will be 3. The actual fuel cut off value InvNact and the rounded value InvN are stored in memory for future use.

It has been found that the InvN factor can under some conditions effectively replace the total fuel cut off mode of step 78 of the first described embodiment. Better torque reduction control can be realised since fuel reduction is calculated from the absolute torque reduction. In practice, both equations (5) and (11) are utilised in this embodiment, in dependence upon the value of InvN, as will become apparent below.

Additional spark retardation is combined with the above fuel cut off step so as (i) to reduce torque for all conditions of the fuel cut off mode; (ii) to determine the amount of additional spark retardation required on the basis of the rounding off error-between the rounded fuel cut off value InvN and the actual fuel cut off term InvNact calculated by equation (11) (or the equivalent values calculated from equation (5)); and (iii) to compensate for the delay between a missed fuelling event and an actual reduction in nominal torque.

The additional spark retardation is calculated from one of the following equations: XSPKRTD = FXSPKRTD*(N - Nact + 0.5)*NMUL (12) or XSPKRTD = FXSPKRTD*(InvNact - InvN + 0.5)*InvNMUL (13) in which XSPKRTD is the amount of additional spark retardation to be produced, FXSPKRTD is the maximum additional spark retardation allowable, NMUL is a multiplier value based on the value of N and InvNMUL is a multiplier value based on the value InvN. The maximum additional spark retardation FXSPKRTD is a predetermined value based upon the amount of catalytic converter cooling which will occur as a result of the fuel cut off and is dependent also upon the engine load (for example manifold absolute pressure MAP and engine speed RPM). The multiplier value NMUL is based on the value N and, in connection with equation (12), determines the proportion of maximum additional spark advance which should be used on the basis of the difference between the rounded fuel cut off value N and the actual value Nact. In this embodiment, the multiplier value NMUL varies between zero and one and is in the form of a table accessible on the basis of the rounded fuel cut off value N. The multiplier value InvNMUL is similar to the multiplier term NMUL and is associated with the inverse fuel cut off term InvN.

Equation (12) is used when InvN = < 1, while equation (13) is used when InvN > 1.

As will be apparent from the above equations (12) and (13), as the actual fuel cut off values Nact and InvNact change without a corresponding change in the rounded values N and InvN, the amount of additional spark retardation will change to give a corresponding change in torque reduction. However, when the rounded fuel cut off value N or InvN changes, the amount of additional spark retardation is reset to zero.

The effects of equations (12) and (13) can be seen in Figure 8, in which the lower graph shows the rounded fuel cut off values N and InvN against the actual values Nact and InvNact calculated from equations (5) and (11). The upper graph shows the additional spark retardation XSPKRTD added between changes in fuel cut off and calculated from equation (12) or (13). As can be seen, the additional spark retardation varies from zero when a change in fuel cut off has just occurred to the maximum spark retardation FXSPKRTD *NMVL or InvNMVL for the measured engine operating conditions.

A commanded change in spark retardation will result in a delay in the change in engine torque by a maximum of approximately 1800 of crankshaft rotation. However, as a result of the time required to inject, ingest, compress and finally ignite a normal amount of fuel in one cylinder, a missed fuelling event will not result in a change in torque for approximately 3600-7200 of crankshaft rotation.

In order to compensate for the difference in these delays, following a change in the fuel cut off values N and InvN, a delay is effected before implementation of additional spark retardation, as can be seen in Figure 8. The length of the delay is calibratable and based upon a number of reference periods after an injection of fuel is missed, each reference period in this example being 720" divided by the number of cylinders in the engine.

This step of delaying the resetting the value of XSPKRTD to zero may also be used with the first described embodiment (for example just before step 76 of the routine of Figure 3).

This second embodiment effectively converts the independent torque reduction steps of the first described embodiment into a combination of interactive steps which can provide a smoother change in torque and therefore improved perceived vehicle smoothness.

Although the above-described embodiments are based on a vehicle having two driven and two non-driven wheels, it will be immediately apparent to the skilled person that these embodiments could be modified for, for example, a four wheel drive vehicle. Moreover, it will also be apparent that the above-described embodiments could be adapted for use with an engine having any number of cylinders.

The disclosures in British patent application no 9312640.7, from which this application claims priority, and in the abstract accompanying this application are incorporated herein by reference.

Claims

Claims:

1. A method of controlling vehicle wheel slip comprising the steps of obtaining a measure of target wheel slip; obtaining a measure of actual wheel slip; determining the difference between the actual and target wheel slips and determining therefrom the amount of engine torque reduction required to reduce the difference to a predetermined level; determining on the basis of the required engine torque reduction a retardation value for retarding the occurrence of ignition in one or more engine cylinders relative to a calculated ignition time; limiting the retardation value to a predetermined value if the retardation value exceeds a threshold level; determining whether the retardation value can provide the required engine torque reduction; and, if the retardation value can not provide the required engine torque reduction, selectively cutting off fuel to the engine cylinders.

2. A method according to claim 1, wherein the threshold level of the retardation value is equivalent to a predetermined ignition time and/or to an engine exhaust gas or catalytic converter threshold temperature.

3. A method according to claim 1 or 2, comprising the step of progressively adjusting the amount of cylinder fuel cut off on the basis of the difference between the torque reduction provided by the retardation value and the required engine torque reduction.

4. A method according to claim 3, comprising the step of limiting the amount of cylinder fuel cut off.

5. A method according to claim 4, comprising the step of limiting the amount of fuel cut off to a value at which only two engine cylinders are operational.

6. A method according to claim 4 or 5, comprising the step of providing additional retardation of ignition timing when the rate of cylinder fuel cut off has been limited.

7. A method according to claim 4 or 5, comprising the step of providing additional retardation of ignition timing between each fuel cut off step.

8. A method according to claim 7, comprising the steps of resetting ignition timing at each adjustment in the amount of cylinder fuel cut off and delaying the step of resetting ignition timing for a delay period following an adjustment in the amount of cylinder fuel cut off.

9. A method according to any one of claims 6 to 8, comprising the step of limiting the additional retardation of ignition timing.

10. A method according to claim 9, wherein the additional retardation is limited to a value equivalent to a engine exhaust gas or catalytic converter threshold temperature.

11. A method according to any one of claims 4 to 10, comprising the steps of determining whether cylinder fuel cut off has been limited; determining whether the required torque reduction exceeds a preset reduction; and, if the required torque reduction exceeds the preset reduction, disabling fuel delivery to all of the engine cylinders.

12. A method according to claim 11, comprising the step of re-enabling fuel delivery when the engine speed has dropped to a predetermined speed or the required torque reduction drops below the preset reduction.

13. A method according to any preceding claim, wherein the target wheel slip and the actual wheel slip are determined on the basis of the difference between the speeds of driven wheels and undriven wheels of the vehicle.

14. A method according to any preceding claim, wherein the required torque reduction is determined as a function of the engaged transmission gear and of the difference between the target wheel slip and the actual wheel slip.

15. A traction controller for controlling vehicle wheel slip comprising target slip determining means for obtaining a measure of target wheel slip; sensing means for obtaining a measure of actual wheel slip; comparing means for deriving the difference between the actual and target wheel slips and for determining therefrom the amount of engine torque reduction required to reduce the difference to a predetermined level; ignition control means for determining on the basis of the required engine torque reduction a retardation value for retarding the occurrence of ignition in one or more engine cylinders relative to a calculated ignition time, the ignition control means being operative to limit the retardation value to a predetermined value if the retardation value exceeds a threshold level; processing means for determining whether the retardation value can provide the required engine torque reduction; and fuel control means for selectively cutting off fuel to the engine cylinders when the processing means determines that the retardation value can not provide the required engine torque reduction.

16. A traction controller according to claim 15, wherein the ignition control means is operative to set the threshold level of the retardation value to a level equivalent to a predetermined ignition time and/or to an engine exhaust gas or catalytic converter threshold temperature.

17. A traction controller according to claim 15 or 16, wherein the fuel control means is operative to adjust progressively the amount of cylinder fuel cut off on the basis of the difference between the torque reduction provided by the retardation value and the required engine torque reduction.

18. A traction controller according to claim 17, wherein the fuel control means is operative to limit the amount of cylinder fuel cut off.

19. A traction controller according to claim 18, wherein the fuel control means is operative to limit the amount of fuel cut off to a value at which only two engine cylinders are operational.

20. A traction controller according to claim 18 or 19, wherein the ignition control means is operative to provide additional retardation of ignition timing when the rate of cylinder fuel cut off has been limited.

21. A traction controller according to claim. 18 or 19, wherein the ignition control means is operative to provide additional retardation of ignition timing between each fuel cut off step.

22. A traction controller according to claim 21, wherein the ignition control means is operative to reset ignition timing at each adjustment in the amount of cylinder fuel cut off and to delay the step of resetting the ignition timing for a delay period following an adjustment in the amount of cylinder fuel cut off.

23. A traction controller according to any one of claims 20 to 22, wherein the ignition control means is operative to limit the additional retardation of ignition timing.

24. A traction controller according to claim 23, wherein the ignition control means is operative to limit the additional retardation to a value equivalent to a engine exhaust gas or catalytic converter threshold temperature.

25. A traction controller according to any one of claims 18 to 24, wherein the processing means is adapted to determine whether cylinder fuel cut off has been limited, to determine whether the required torque reduction exceeds a preset reduction and, if the required torque reduction exceeds the preset reduction, to command the ceasing of fuel delivery to all of the engine cylinders.

26. A traction controller according to claim 25, wherein the processing means is adapted to command the re-enabling of fuel delivery when the engine speed has dropped to a predetermined speed or the required torque reduction drops below the preset reduction.

27. A traction controller according to any one of claims 15 to 26, wherein the target slip determining means and the sensing means are operative to obtain a measure of target wheel slip and actual wheel slip on the basis of the difference between the speeds of driven wheels and undriven wheels of the vehicle.

28. A traction controller according to any one of claims 15 to 27, wherein the comparing means is adapted to determine the required torque reduction as a function of the engaged transmission gear and of the difference between the target wheel slip and the actual wheel slip.

29. A method of controlling vehicle wheel slip substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

30. A traction controller for controlling vehicle wheel slip substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.