TORGUE CONTROL METHOD OF A ROAD VEHICLE

Abstract

A control method of the torque of a road vehicle having a powertrain system provided with an engine and a driveline which transmits the torque generated by the engine to the road surface; the method contemplates the steps of: determining a target torque; modeling the powertrain system as a single physical component which presents a characteristic mechanical inertia and a characteristic torsional elasticity; determining a current load torque of the vehicle; determining a target torsion of the powertrain system according to the target torque and the current load torque; determining a current torsion of the powertrain system and a current torsion speed of the powertrain system; determining a requested torque on the basis of the energy balance according to the target torsion, the current torsion, the current torsion speed, and the current load torque; and using the requested torque on the basis of the energy balance to control the torque generation of the engine.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Application No. 07425459.0 filed on Jul. 25, 2007.

TECHNICAL FIELD

The present invention relates to a torque control method of a road vehicle.

BACKGROUND

A road vehicle comprises a powertrain system, which is provided with an engine which generates a torque, and with a driveline which transmits the torque generated by the engine to the road surface; e.g. in a front-wheel drive car provided with an internal combustion engine, the driveline comprises a clutch, a gearbox, a differential, two axles shafts and two wheels each presenting a metallic rim supporting a rubber tire.

A powertrain system of a road vehicle presents a characteristic mechanical inertia and a characteristic torsional elasticity, i.e. to transmit the torque generated by the engine all the components of the powertrain system must undergo a torsional deformation or a torsion which may be more or less accentuated according to the mechanical features of each component. When a sudden variation in the torque transmission occurs (e.g. when the clutch is rapidly released with the gear engaged and the drive wheels have a good grip on the road surface), such a variation of the torque determines a corresponding variation in the longitudinal acceleration of the vehicle. Due to the torsional elasticity present in the powertrain system, a step variation of the torque triggers oscillations in the longitudinal vehicle acceleration about a final value which are progressively damped. Such oscillations in the longitudinal acceleration of the vehicle are damped rather rapidly (in the order of a few seconds); however, if not effectively counterbalanced, they are clearly perceived by the vehicle occupants and thus they are annoying.

In order to eliminate the oscillations in the longitudinal acceleration of the vehicle triggered by a variation of the torque generated by the engine, it has been suggested to implement a maximum damping algorithm in the torque control strategy. The use of a maximum damping algorithm is effective, because it is capable of either completely or nearly completely eliminating the oscillations in the longitudinal acceleration of the vehicle, but not very efficient, because it determines a considerable deceleration of the load increase of the transmission system, thus compromising the vehicle dynamic reactivity, in a manner perceivable by the driver.

SUMMARY

It is the object of the present invention to provide a control method of the torque of a road vehicle which is free from the above-described drawbacks and which is concurrently easy and cost-effective to implement.

According to the present invention, there is provided a control system of the torque of a vehicle as claimed in the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings which illustrate a non-limitative embodiment thereof, in which:

FIG. 1 is a diagrammatic view of a powertrain system of a rear-wheel drive car which implements the torque control method object of the present invention;

FIG. 2 is a block chart which shows the operation of a control unit of the powertrain system in FIG. 1; and

FIG. 3 is a further block chart which shows the operation of the control unit of the powertrain system in FIG. 1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In FIG. 1, numeral 1 indicates as a whole a car provided with two front wheels (not shown) and two rear drive wheels 2 which receive the torque from a powertrain system 3.

The powertrain system 3 comprises an internal combustion engine, which is arranged in front position and is provided with a drive shaft 5 which rotates at an angular speed ω_m, and a driveline 6 which transmits the torque generated by the internal combustion engine 4 to the road surface through the rear drive wheels 2. The driveline 6 comprises a mechanical gearbox 7 provided with a primary shaft 8 which rotates at an angular speed ω₁ and is connectable to the drive shaft 5 and with a secondary shaft 9 which rotates at an angular speed ω₂ and is connected to a propeller shaft 10 which transmits the motion to the rear drive wheels 2. The propeller shaft 10 ends with a differential 11, from which a pair of axle shafts 12 depart, each of which is integral with a rear drive wheel 2. Between the drive shaft 5 and the primary shaft 8 of the gearbox 7 there is interposed a servo-controlled plate clutch 13 to connect and disconnect the drive shaft 5 from the primary shaft 8 of the gearbox 7.

The powertrain system 3 further comprises an electronic control unit 14 (normally named ECU and diagrammatically shown), which controls the powertrain system 3 and drives the internal combustion engine 4 to adjust the torque generation.

As shown in FIG. 2, the electronic control unit 14 implements a control strategy of the torque generated by the internal combustion engine 4 which contemplates determining in a known manner a torque T_e-rqst requested by the driver essentially according to the position of an accelerator pedal (alternatively, the torque T_e-rqst may be requested by an automatic cruise control which replaces the driver). The requested torque T_e-rqst is processed by a signal processing algorithm, which is implemented in a calculation block 15 (normally identified by the acronym ESDD—Energy Shaping Driveline Damping) and according to the requested torque T_e-rqst determines the set-point torque T_e-sp which represents the instantaneous torque which must be generated by the internal combustion engine 4; the set-point torque T_e-sp is directly supplied to a controller 16, which adjusts the internal combustion engine 4 to make the internal combustion engine 4 itself generate the set-point torque T_e-sp.

The internal combustion engine 4 instantaneously delivers an effective torque T_e at a number N of revolutions to the drive shaft 5; the effective torque T_e generated by the internal combustion engine 4 is thus transmitted to the road surface by means of the driveline 6. The effective torque T_e is estimated by means of a estimator 17 (known) and the number N of revolutions is measured by a specific sensor 18 (typically a phonic wheel); thus, the estimated/measured values of the effective torque T_e and the number N of revolutions is transmitted as feedback to the calculation block 15 in which the signal processing algorithm which determines the set-point torque T_e-sp is implemented.

As shown in FIG. 3, the signal processing algorithm, which determines the set-point torque T_e-sp and is implemented in the calculation block 15, contemplates processing the requested torque T_e-rqst in a limiting block 19, in which the requested torque T_e-rqst is limited within a range comprising a minimum torque T_min (i.e. the minimum torque deliverable by the internal combustion engine 4) and a maximum torque T_max (i.e. the maximum torque deliverable by the internal combustion engine 4). The minimum torque T_min and the maximum torque T_max values take into account the physical limits of the internal combustion engine 4 and may be variable according to the conditions of the internal combustion engine 4.

The output of the limiting block 19 is supplied to a filtering block 20, according to which a filtering of the low-pass type is performed to eliminate both the possible high-frequency noise and excessively rapid torque change requests not compatible with the physical response limits of the internal combustion engine 4 (typically from an automatic cruise control which replaces the driver). The filtering features depend on the type of transient (acceleration or deceleration of the car 1) and on a certain number of signals representing the state of the car (such as, for example, the transmission ratio engaged by the gearbox 7 and the number N of revolutions of the internal combustion engine 4).

The filtering block 20 provides the requested and filtered torque T_fil, which is processed by a maximum block 21 which outputs the maximum value between the requested and filtered torque T_fil and an idling torque T_idle which is supplied by an idle speed adjuster and represents the minimum torque value which must be delivered by the internal combustion engine 4 to maintain the required idling ratio and thus to avoid the undesired stopping or an undesired irregular operation of the internal combustion engine 4 itself. The output of the maximum block 21, i.e. the maximum between the requested and filtered torque T_fil and the idling torque T_idle, represents the target torque T_e-targ which represent the torque target which must be reached and maintained in stationary conditions (i.e. after exhausting the transient of oscillatory nature determined by the torsional elasticity of the powertrain system 3).

The target torque T_e-targ is processed by a calculation block 22, which according to the target torque T_e-targ determines the target torsion Δθ_targ of the powertrain system 3. In other words, the powertrain system 3 presents a characteristic mechanical inertia and a characteristic torsional elasticity, thus in order to transmit the torque generated by the internal combustion engine 4 all the components of the powertrain system 3 which must undergo a torsional deformation or torsion. In the algorithm implemented in the calculation block 22, the powertrain system 3 is modeled as a single physical component which presents a characteristic mechanical inertia and a characteristic torsional elasticity and which in order to transmit a certain torque must undergo a corresponding torsion; consequently, the calculation block 22 is capable of estimating what the target torsion Δθ_targ of the model of the powertrain system 3 should be in order to be able to transmit the target torque T_e-targ.

The target torsion Δθ_targ is calculated by means of the following equation:

$\Delta {\vartheta }_{\mathrm{targ}}=f_{\mathrm{Tw}}^{-1}\left(\frac{J_v}{J_e+J_v}·T_{e\text{-}\mathrm{targ}}+\frac{J_e}{J_e+J_v}·T_r\right)$

Δθ_targ target torsion of the powertrain system 3 [rad];ƒ_Tw(Δθ) function which renders the elastic reaction torque of the powertrain system 3 [Nm];J_e moment of inertia of the internal combustion engine 4 transmitted to the drive shaft 5 [kg m²];J_v moment of inertia of the car 1 transmitted to the drive shaft 5 [kg m²];T_e-targ target torque [Nm];T_r current load torque of the car 1 transmitted to the drive shaft 5 [Nm].

In other words, the powertrain system 3 is modeled as a single physical components which presents a characteristic mechanical inertia and a characteristic torsional elasticity and which in order to transmit a certain torque must undergo a corresponding torsion; thus by exploiting such a model of the powertrain system 3 there is determined the function ƒ_Tw(Δθ) which renders the elastic reaction of the powertrain system 3 according to the torsion Δθ of the model of the powertrain system 3. Finally, the inverse of the function ƒ_Tw(Δθ) is determined in order to determine the target torsion Δθ_targ according to the target torque T_e-targ.

The current load torque T_r of the car 1 is estimated by an observer 23, which receives as input the effective torque T_e and the number N of revolutions and outputs, in addition to the current load torque T_r of the car 1, also the current torsion Δθ of the powertrain system 3 and the current torsion speed Δθ′ of the powertrain system 3 (i.e. the first derivative in time of the current torsion Δθ).

The target torsion Δθ_targ of the powertrain system 3 is supplied to a further calculation block 24, which according to the target torsion Δθ_targ is capable of determining the requested torque T_e-EC on the basis of the energy balance. In other words, the algorithm implemented in the calculation block 24 is based on the energy balance which allows to obtain the following equation:

$T_{e,\mathrm{EC}}=\frac{J_e+J_v}{J_v}·\left[\frac{E\left(\Delta {\vartheta }_{\mathrm{targ}},0\right)-E\left(\Delta \vartheta ,\Delta {\vartheta }^{\prime }\right)}{\Delta {\vartheta }_{\mathrm{targ}}-\Delta \vartheta }-\frac{J_e}{J_e+J_v}·T_r+b·\Delta {\vartheta }^{\prime }\right]$

T_e-EC requested torque on the basis of the energy balance [Nm];J_e moment of inertia of the internal combustion engine 4 transmitted to the drive shaft 5 [kg m²];J_v moment of inertia of the car 1 transmitted to the drive shaft 5 [kg m²];E(Δθ, Δθ′) function which renders the total mechanical energy of the powertrain system 3 [J];Δθ_targ target torsion of the powertrain system 3 [rad];Δθ current torsion of the powertrain system 3 transmitted to the drive shaft 5 [rad];Δθ′ current torsion speed of the powertrain system 3 transmitted to the drive shaft 5 [rad/s];b damping coefficient of the powertrain system 3 [Nm/s];T_r current load torque of the car 1 referred to the drive shaft 5 [−m].

The function E(Δθ,Δθ′) which renders the total mechanical energy of the powertrain system 3 (i.e. the sum of the inertia energy and of the elastic torsional energy of the powertrain system 3) is based on the following equation:

$E\left(\Delta \vartheta ,\Delta \stackrel{.}{\vartheta }\right)\equiv \frac{1}{2}·\frac{J_eJ_v}{J_e+J_v}·\Delta {\vartheta }^{\mathrm{\prime 2}}+V\left(\Delta \vartheta \right)$

E(Δθ, Δθ′) total mechanical energy of the powertrain system 3 [J];J_e moment of inertia of the internal combustion engine 4 transmitted to the drive shaft 5 [kg m²];J_v moment of inertia of the car 1 transmitted to the drive shaft 5 [kg m²];Δθ current torsion of the powertrain system 3 transmitted to the drive shaft 5 [rad];Δθ′ current torsion speed of the powertrain system 3 transmitted to the drive shaft 5 [rad/s];V(Δθ) function which provides the elastic torsional energy of the powertrain system 3 [J].

The calculation block 24 determines the requested torque T_e-EC on the basis of the above-described energy balance outside singularity zone I indicated by the following expression:

(Δθ,Δθ′)εI,I≡{(Δθ,Δθ′):|Δθ_targ−Δθ|<Θ_switch,|Δθ′|<Ψ_switch}

Δθ current torsion of the powertrain system 3 transmitted to the drive shaft 5 [rad];Δθ′ current torsion speed of the powertrain system 3 transmitted to the drive shaft 5 [rad/s];Δθ_targ target torsion of the powertrain system 3 [rad];Θ_switch first threshold value;Ψ_switch second threshold value.

In other words, calculation block 24 determines the requested torque T_e-EC on the basis of the energy balance described above only when the difference between the target torsion Δθ_targ is significantly different from the current torsion Δθ and the current torsion speed Δθ′ is significantly different from zero.

When the calculation block 24 is no longer capable of determining the requested torque T_e-EC on the basis of the above-described energy balance, a calculation block 25 determines the requested torque T_e-CL on the basis of the maximum damping, for example by using the following equation:

T _e-CL =T _e-targ −G _i·Δθ′

T_e-CL requested torque on the basis of the maximum damping [Nm];T_e-targ target torque [Nm];G_i damping constant;Δθ′ current torsion speed of the powertrain system 3 transmitted to the drive shaft 5 [rad/s].

The value of the damping constant G_i may be variable according to the drive axle and is proportional to the desired damping degree.

The requested torque T_e-EC on the basis of the energy balance and the requested torque T_e-CL on the basis of the maximum damping are supplied to coordinator 26, which also receives the target torque T_e-targ and has the function of selecting which inputted torque to be use as set-point torque T_e-sp which is directly supplied to the controller 16 (as previously described). During the first instants of a transient triggered by a change of the requested torque T_e-rqst, the requested torque T_e-EC on the basis of the energy balance is used as set-point torque T_e-sp, while when the transient triggered by a change of the requested torque T_e-rqst is close to being exhausted (i.e. the current torsion Δθ is close to the target torsion Δθ_targ and the current torsion speed Δθ′ is slow), the requested torque T_e-CL on the basis of the maximum damping is used as set-point torque T_e-sp; finally, when the transient triggered by a change of the requested torque T_e-rqst is exhausted (i.e. the current torsion Δθ is equal to the target torsion Δθ_targ and the current torsion speed Δθ′ is null), the target torque T_e-targ is used as set-point torque T_e-sp. It is worth noting that that coordinator 26 also receives the current torsion Δθ and the current torsion speed Δθ′ in order to determine when to use the requested torque T_e-EC on the basis of the energy balance and when to use the target torque T_e-targ.

The above-described control strategy of the torque generated by the internal combustion engine 4 in case of transient triggered by a change of requested torque T_e-rqst presents many advantages because it is simple and cost-effective to implement in a modern car 1 (i.e. does not require the installation of additional components with respect to those already normally present) and is above all concurrently effective (i.e. capable of completely eliminating the longitudinal acceleration oscillations of the car 1) and efficient (i.e. does not affect the dynamic performance of the car 1 because it is capable of minimising the transient time). Simulations have demonstrated that the above-described control strategy of the torque generated by the internal combustion engine 4 in the case of transient triggered by a change of the requested torque T_e-rqst is capable of halving the transient time with respect to a traditional control strategy based only on a maximum damping algorithm.

Claims

1. A control method of the torque of a road vehicle comprising a powertrain system provided with an engine presenting a drive shaft and a driveline which transmits the torque generated by the engine to the road surface through the drive wheels; the method comprises the steps of: determining a target torque (Te-targ); andcontrolling the generation of torque by the engine on the basis of the target torque (Te-targ);the method is characterized in that it comprises the further steps of:modeling the powertrain system as a single physical component which presents a characteristic mechanical inertia and a characteristic torsional elasticity and which must undergo a corresponding torsion to transmit a certain drive torque;determining a current load torque (Tr) of the vehicle;determining a target torsion (Δθtarg) of the powertrain system according to the target torque (Te-targ) and the current load torque (Tr);determining a current torsion (Δθ) of the powertrain system and a current torsion speed (Δθ′) of the powertrain system;determining a requested torque (Te-EC) on the basis of the energy balance according to the target torsion (Δθtarg), the current torsion (Δθ), the current torsion speed (Δθ′), and the current load torque (Tr); andusing the requested torque (Te-EC) on the basis of the energy balance to control the torque generation of the engine.

2. A method according to claim 1, wherein the target torsion (Δθtarg) is calculated by means of the following equation:

3. A method according to claim 1, wherein the requested torque (Te-EC) on the basis of the energy balance is calculated by means of the following equation:

4. A method according to claim 3, wherein the function E(Δθ,Δθ′) which renders the total mechanical energy of the powertrain system is based on the following equation:

5. A method according to claim 3, wherein the requested torque (Te-EC) on the basis of the energy balance is determined only when the difference between the target torsion (Δθtarg) is significantly different from the current torsion (Δθ) and the current torsion speed (Δθ′) is significantly different from zero and thus outside the singularity zone (I) indicated by the following expression: (Δθ,Δθ′)εI,I≡{(Δθ,Δθ′):|Δθtarg−Δθ|<Θswitch,|Δθ′|<Ψswitch}Δθ current torsion of the powertrain system transmitted to the drive shaft [rad];Δθ′ current torsion speed of the powertrain system transmitted to the drive shaft [rad/s];Δθtarg target torsion of the powertrain system [rad];Θswitch first threshold value;Ψswitch second threshold value.

6. A method according to claim 5, and comprising the further steps of: determining, within the singularity zone (I), a requested torque (Te-CL) on the basis of the maximum damping; andusing the requested torque (Te-CL) on the basis of the maximum damping to control the generation of engine torque (4).

7. A method according to claim 6, wherein the requested torque (Te-CL) on the basis of the maximum damping is calculated by means of the following equation: Te-CL=Te-targ−Gi·Δθ′Te-CL requested torque on the basis of the maximum damping [Nm];Te-targ target torque [Nm];Gi damping constant;Δθ′ current torsion speed of the powertrain system transmitted to the drive shaft [rad/s].

8. A method according to claim 6, and comprising the further steps of: using the requested torque (Te-EC) on the basis of the energy balance to control the torque generation of the engine during the first instants of a transient triggered during a change of the target torque (Te-targ);using the requested torque (Te-EC) on the basis of the maximum damping to control the torque generation of the engine when the transient triggered by a change of the target torque (Te-targ) is close to being exhausted; andusing the target torque (Te-targ) for controlling the torque generation of the engine when the transient triggered by a change of the target torque (Te-targ) is exhausted.

9. A method according to claim 1 and comprising the further step of determining the current torsion (Δθ), the current torsion speed (Δθ′), and the current load torque (Tr) by means of an observer (23) which receives as input an effective torque (Te) generated by the engine and a number (N) of revolutions of the engine.

10. A method according to claim 1 and comprising the further steps of: determining a torque (Te-rqst) requested by a driver or by an automatic cruise control;limiting the requested torque (Te-rqst) within a range, which comprises a minimum torque (Tmin) and maximum torque (Tmax) and takes the physical limits of the engine into account;filtering the limited requested torque (Te-rqst) by means of a low-pass type filter to obtain a requested and filtered torque (Tfil); anddetermining the target torque (Te-targ) as the maximum between the requested and filtered torque (Tfil) and an idling torque (Tidle).