Four-wheel drive control method for a multi-motor plug-in hybrid vehicle

Abstract

The present invention provides a four-wheel drive control method for a multi-motor plug-in hybrid vehicle, belonging to the technical field of automobiles. It solves the problem that the existing control cannot simultaneously meet the requirements of economy, stability and power performance. The four-wheel drive control method for the multi-motor plug-in hybrid vehicle, according to the state of charge, discharge capacity, accelerator pedal state, shift lever position, chassis stability control state and rear drive motor performance state of the high-voltage power battery, when the prerequisite conditions for activating the four-wheel drive mode are met, according to the driver operation information, vehicle state information and road surface state information monitored in real time, when it is determined that the vehicle belongs to an aggressive driving condition, a slope condition or a tendency to lose stability, the vehicle is controlled to enter the four-wheel drive mode, and according to the corresponding road surface state information, driver operation information and vehicle state information, the distribution ratio of the driving torque between the front and rear axles is adjusted. The present invention can balance the power performance, stability and economy of the vehicle in different scenarios.

Description

Technical Field

[0001] The present invention belongs to the technical field of automobiles and relates to a four-wheel drive control method for a multi-motor plug-in hybrid electric vehicle. Background Art

[0002] A plug-in hybrid electric vehicle (PHEV) is a new energy vehicle that lies between a pure electric vehicle and a fuel vehicle. It has the engine, transmission, drive system, fuel circuit, and fuel tank of a traditional vehicle, as well as the battery, motor, and control circuit of a pure electric vehicle. Moreover, the battery has a relatively large capacity and a charging interface. It combines the advantages of a pure electric vehicle (EV) and a hybrid electric vehicle (HEV), and can not only achieve pure electric and zero-emission driving, but also increase the vehicle's cruising range through the hybrid mode. In the vehicle model setting of the multi-motor plug-in hybrid electric vehicle involved in this solution, the front-wheel drive system consists of an engine + a 3DHT hybrid transmission (with P1 + P2 motors built-in), and the rear-wheel drive system consists of a P4 motor + a fixed-ratio reduction drive axle. Its four-wheel drive mode can be achieved through the cooperation of the engine and the P4 motor, or through the combination of the P2 motor and the P4 motor under pure electric driving conditions.

[0003] In recent years, with the development and progress of automotive technology, in addition to meeting economy requirements, users have an increasing demand for vehicle performance, including attributes related to driving pleasure such as power performance and vehicle stability. However, when the current multi-motor plug-in hybrid electric vehicle is running, the power control unit often only considers economy when selecting a power source, aiming for the lowest fuel consumption, while ignoring the vehicle's power performance, especially the vehicle's stability performance. Often, it is necessary for the chassis control unit to intervene in the power source and for traditional mechanical braking to intervene to ensure its stability performance. However, the intervention of braking inevitably sacrifices the vehicle's power performance and cornering sensitivity, greatly reducing the vehicle's handling pleasure and unable to fully utilize the hardware performance of the multi-motor plug-in hybrid electric vehicle. It can be seen that the existing power control unit cannot meet the requirements of economy, stability, and power performance simultaneously during control. Summary of the Invention

[0004] The purpose of the present invention is to address the above problems existing in the prior art and propose a four-wheel drive control method for a multi-motor plug-in hybrid electric vehicle. The technical problem to be solved is: how to balance the vehicle's power performance, stability, and economy in different scenarios.

[0005] The purpose of the present invention can be achieved by the following technical solutions: A four-wheel drive control method for a multi-motor plug-in hybrid electric vehicle, including the following steps:

[0006] Judge whether the prerequisite conditions for activating the four-wheel drive mode are met according to the state of charge, discharge capacity, accelerator pedal state, shift lever position, chassis stability control state, and rear drive motor performance state of the high-voltage power battery;

[0007] When the prerequisite conditions for activating the four-wheel drive mode are met, continuously monitor the driver operation information, vehicle state information, and road surface state information;

[0008] Judge whether the vehicle belongs to one of the severe driving conditions, uphill conditions, or unstable tendency conditions according to the driver operation information, vehicle state information, and road surface state information;

[0009] When the vehicle belongs to the severe driving conditions, uphill conditions, or unstable tendency conditions, control the vehicle to enter the four-wheel drive mode, and adjust the distribution ratio of the driving torque between the front and rear axles according to the corresponding road surface state information, driver operation information, and vehicle state information.

[0010] The working principle of this four-wheel drive control method is as follows: First, judge whether the vehicle meets the activation conditions of the four-wheel drive mode, that is, when the state of charge, discharge capacity, accelerator pedal state, shift lever position, chassis stability control state, and rear drive motor performance state of the high-voltage power battery all meet the preset conditions, it is judged that the vehicle meets the prerequisite conditions for activating the four-wheel drive mode. Setting the above several parameters as prerequisite conditions is mainly because to achieve four-wheel drive mode driving, the rear drive motor needs to be able to work normally, and the normal operation of the rear drive motor is related to the performance state of the rear drive motor, the state of charge and discharge capacity of the high-voltage power battery, and the accelerator pedal state and shift lever position determine whether the vehicle is in a driving state. In addition, this four-wheel drive control method is to solve the problem that the chassis control unit intervenes and affects the power performance. Therefore, the monitoring of the chassis stability control state ensures the necessity of entering the four-wheel drive mode. Therefore, only when the above several conditions are met, the vehicle can continue to judge whether to enter the four-wheel drive mode. When the prerequisite conditions for activating the four-wheel drive mode are met, the driver operation information, vehicle state information, and road surface state information are continuously monitored, and it is judged whether the vehicle belongs to the severe driving conditions, uphill conditions, or has an unstable tendency according to the monitored information. When it is judged that it belongs to one of these three situations, the vehicle is controlled to enter the four-wheel drive mode, and the distribution of the driving torque between the front and rear axles is dynamically adjusted according to the corresponding information monitored in real time, so that the vehicle ensures its stability without the intervention of the chassis control unit. Through this method, the in-depth integration of the vehicle's power performance, stability, and economy control goals is realized, so that different performance goals are taken into account in different scenarios, and the drawback that the vehicle must rely on the intervention of the chassis control unit for stability control and sacrifice the operation pleasure when the vehicle is in severe driving conditions, uphill conditions, or has an unstable tendency is avoided.

[0011] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, it is determined whether the prerequisite conditions for activating the four-wheel drive mode are met:

[0012] Whether the state of charge of the high-voltage power battery exceeds the state of charge threshold;

[0013] Whether the discharge capacity exceeds the discharge threshold;

[0014] Whether the throttle pedal state is in the depressed state;

[0015] Whether the shift lever position is in the D gear;

[0016] Whether the chassis stability control state is in the unactivated state;

[0017] Whether the performance state of the rear drive motor has no fault;

[0018] When all of the above six conditions are met, it is determined that the prerequisite conditions for activating the four-wheel drive mode are satisfied; conversely, when any one of the above six conditions is not met, it is determined that the prerequisite conditions for activating the four-wheel drive mode are not satisfied. Setting the prerequisite conditions for activating the four-wheel drive mode can ensure more reliable four-wheel drive control of the vehicle.

[0019] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the driver operation information includes the throttle pedal opening and the steering wheel steering angle; the vehicle state information includes the vehicle yaw rate, lateral acceleration, longitudinal acceleration, wheel speed, and vehicle speed; the road surface state information includes the road surface gradient and the road surface adhesion coefficient.

[0020] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operations for determining whether the vehicle belongs to an aggressive driving condition include:

[0021] Compare the throttle pedal opening with the throttle opening threshold;

[0022] When the throttle pedal opening is greater than the throttle opening threshold, it is determined that the vehicle belongs to an aggressive driving condition. The throttle pedal opening can intuitively reflect the driver's dynamic requirement for speed. When it is determined that the vehicle belongs to an aggressive driving condition, enter the four-wheel drive mode to avoid a single front-wheel drive or rear-wheel drive, where the vehicle cannot meet the driver's high dynamic response requirement for sudden acceleration, improve the vehicle's driving efficiency, and save energy consumption.

[0023] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operations for determining whether the vehicle belongs to a slope condition include:

[0024] Compare the road surface gradient with the gradient threshold;

[0025] When the road surface gradient is greater than the gradient threshold, it is determined that the vehicle belongs to a slope condition.

[0026] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operations for determining whether the vehicle has an instability tendency include:

[0027] Comparing the steering wheel angle with the steering wheel angle threshold;

[0028] Comparing the vehicle yaw rate with the yaw rate threshold;

[0029] Comparing the vehicle lateral acceleration with the lateral acceleration threshold;

[0030] Comparing the vehicle speed with the vehicle speed threshold;

[0031] When the steering wheel angle is greater than the steering wheel angle threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; or when the vehicle yaw rate is greater than the yaw rate threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; or when the vehicle lateral acceleration is greater than the lateral acceleration threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency. At high vehicle speeds and with a large steering wheel angle, in the original operation process, the vehicle must rely on the chassis control unit to intervene in stability control. In this method, it enters the four-wheel drive mode, and by reasonably distributing the driving forces of the front and rear axles, the stability and steering sensitivity are improved without increasing the hardware cost and reducing the requirements for chassis control.

[0032] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operations for determining whether the vehicle has an instability tendency further include:

[0033] Calculating the single-axle wheel slip ratio based on the wheel speed and the vehicle speed;

[0034] Comparing the single-axle wheel slip ratio with the wheel slip ratio threshold;

[0035] When the single-axle wheel slip ratio is greater than the wheel slip ratio threshold, it is determined that the vehicle has an instability tendency.

[0036] In this step, the wheel speed includes the left front wheel speed, the right front wheel speed, the left rear wheel speed, and the right rear wheel speed. As long as the single-axle wheel slip ratio calculated from any wheel speed and the vehicle speed is greater than the wheel slip ratio threshold, it can be determined that the vehicle has an instability tendency, and further operations are then carried out to make the vehicle balance stability and power performance.

[0037] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, it further includes:

[0038] Comparing the road surface adhesion coefficient with the road surface adhesion coefficient threshold;

[0039] When the road surface adhesion coefficient is less than the road surface adhesion coefficient threshold, it is determined that the vehicle has a possibility of instability.

[0040] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes:

[0041] When the vehicle is in an aggressive driving condition, the vehicle enters the four-wheel drive mode. At this time, the axle load transfer amount between the front and rear axles is calculated based on the longitudinal acceleration, the front axle load and the rear axle load are recalculated based on the axle load transfer amount, and the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the front axle load to the rear axle load. When in an aggressive driving condition, the load transfers from the front axle to the rear axle, that is, the front axle load is the original front axle load minus the axle load transfer amount, and the rear axle load is the original rear axle load plus the axle load transfer amount. The front axle driving torque and the rear axle driving torque are then distributed according to the ratio of the front axle load to the rear axle load to ensure vehicle stability and driving performance.

[0042] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes:

[0043] The operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes:

[0044] When the vehicle is in a slope condition, the vehicle enters the four-wheel drive mode. At this time, the front axle vertical load and the rear axle vertical load are calculated based on the road surface slope, and then the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the front axle vertical load to the rear axle vertical load. The slope condition includes uphill and downhill. When going uphill, it will cause the load to transfer from the front axle to the rear axle. At this time, the front axle vertical load is less than the rear axle vertical load, and the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the front axle vertical load to the rear axle vertical load, so that a larger proportion is distributed to the rear axle. When going downhill, it will cause the load to transfer from the rear axle to the front axle. At this time, the front axle vertical load is greater than the rear axle vertical load, and the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the front axle vertical load to the rear axle vertical load, so that a larger proportion is distributed to the front axle, ensuring that when the vehicle is driving in a slope condition, it can take into account the vehicle's power performance, stability and economy.

[0045] In the above four-wheel drive control method for a multi-motor plug-in hybrid vehicle, the operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes:

[0046] When the vehicle has a tendency to become unstable, the vehicle enters the four-wheel drive mode and compares the yaw rate with the target yaw rate:

[0047] When the yaw rate exceeds the target yaw rate, it is determined as oversteering, and the distribution ratio of the driving torque between the front and rear axles is dynamically adjusted according to the difference between the target yaw rate and the yaw rate, so as to adjust the distribution ratio of the front axle to be more than that of the rear axle;

[0048] When the yaw rate is less than the target yaw rate, it is determined as understeer. According to the difference between the target yaw rate and the yaw rate, the distribution ratio of the driving torque between the front and rear axles is dynamically adjusted to make the distribution ratio of the front axle less than that of the rear axle.

[0049] The target yaw rate takes the vehicle speed and the steering wheel steering angle as inputs and is calculated according to a seven-degree-of-freedom vehicle dynamics model. During the operation, the driving torques of the front and rear axles are dynamically adjusted in a PID closed-loop control manner to ensure that the vehicle takes into account power performance, stability, and economy.

[0050] In the above four-wheel drive control method of a multi-motor plug-in hybrid vehicle, the operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes:

[0051] When the vehicle has a tendency to lose stability, the vehicle enters the four-wheel drive mode. At this time, according to the difference between the target slip ratio and the single-axle wheel slip ratio, the driving torque of the slipping axle is transferred to the non-slipping axle to reduce the slip ratio of the slipping axle to the target slip ratio.

[0052] The target slip ratio is obtained by looking up a table based on the vehicle speed and the road surface adhesion coefficient.

[0053] The single-axle wheel slip ratio includes the left front wheel slip ratio, the right front wheel slip ratio, the left rear wheel slip ratio, and the right rear wheel slip ratio. The left front wheel slip ratio, the right front wheel slip ratio, the left rear wheel slip ratio, and the right rear wheel slip ratio are respectively calculated from the vehicle speed and the wheel speeds of their respective wheels. During the operation, the driving torque of the slipping axle is transferred to the non-slipping axle in a PID closed-loop control manner. If the slipping axle is the front axle, the driving torque of the front axle is transferred to the rear axle by a predetermined value until the slip ratio of the front axle is reduced to the target slip ratio, ensuring that the vehicle takes into account power performance, stability, and economy.

[0054] Compared with the prior art, the four-wheel drive control method of this multi-motor plug-in hybrid vehicle has the following advantages:

[0055] 1. This invention sets the conditions for entering the four-wheel drive mode, enabling the vehicle to control the driving force by a single front axle or rear axle in scenarios where four-wheel drive intervention is not required, improving the driving efficiency of the vehicle, saving energy consumption, and entering the four-wheel drive mode only when the conditions for entering the four-wheel drive mode are met, and dynamically adjusting the driving torques of the front and rear axles, achieving a deep integration of the vehicle's power performance, stability, and economy control goals, and making different control goals taken into account in different scenarios.

[0056] 2. The four-wheel drive control of this invention does not require the intervention of the chassis control unit, reducing the requirements for chassis control, and there is no increase in other hardware costs in this invention, reducing costs. Description of the Drawings

[0057] Figure 1 This is the control flow chart of the present invention. Detailed implementation manners

[0058] The following are specific embodiments of the present invention. In combination with the accompanying drawings, the technical solutions of the present invention will be further described, but the present invention is not limited to these embodiments.

[0059] As Figure 1 shown, during the driving process of the multi-motor plug-in hybrid vehicle, the four-wheel drive control method of the present invention monitors in real time the state of charge, discharge capacity, accelerator pedal state, shift lever position, chassis stability control state, and rear drive motor performance state of the high-voltage power battery, and determines whether the state of charge of the high-voltage power battery exceeds the state of charge threshold; whether the discharge capacity exceeds the discharge threshold; whether the accelerator pedal state is in the depressed state; whether the shift lever position is in the D gear; whether the chassis stability control state is in the unactivated state; whether the rear drive motor performance state has no fault; when all these six conditions are met, it is determined that the preconditions for activating the four-wheel drive mode are met; otherwise, when any one of the above six conditions is not met, it is determined that the preconditions for activating the four-wheel drive mode are not met, and the monitoring continues.

[0060] Among them, the state of charge threshold and the discharge threshold are preset. Also preset are the throttle opening threshold, slope threshold, steering wheel angle threshold, yaw rate threshold, lateral acceleration threshold, vehicle speed threshold, road surface adhesion coefficient threshold, and wheel slip rate threshold. Among them, the state of charge threshold is set to 20% of the total charge amount, the discharge threshold is set to 30 kW, the throttle opening threshold is set to 60% of the total throttle opening, the slope threshold is set to 20%, the steering wheel angle threshold is set to 30 degrees, the yaw rate threshold is set to 0.3 rad / s, the lateral acceleration threshold is set to 0.5 m / s2, the vehicle speed threshold is set to 50 km / h, the road surface adhesion coefficient threshold is set to 0.3, and the wheel slip rate threshold is set between 10% and 20%, such as 15%.

[0061] When the preconditions for activating the four-wheel drive mode are met, the driver operation information, vehicle state information, and road surface state information are monitored in real time. Among them, the driver operation information includes the throttle pedal opening and the steering wheel angle; the vehicle state information includes the vehicle yaw rate, lateral acceleration, wheel speed, vehicle speed, and longitudinal acceleration, and the wheel speed includes the left front wheel speed, right front wheel speed, left rear wheel speed, and right rear wheel speed; the road surface state information includes the road surface gradient and the road surface adhesion coefficient. The road surface adhesion coefficient is specifically composed of a pre-set value and a self-learning value. The pre-set value can be set to 0.7-0.8. When the driving torque remains unchanged and a single-wheel skidding condition occurs, the road surface adhesion coefficient decreases by the pre-set self-learning value. For example, if the self-learning value is 0.1, the current road surface adhesion coefficient of the vehicle can be obtained at this time; conversely, when increasing, the road surface adhesion coefficient increases by the pre-set self-learning value, and the vehicle is on a high-adhesion road surface. Among them, the range of the road surface adhesion coefficient is between 0.1 and 1.0.

[0062] According to the driver operation information, vehicle state information, and road surface state information, it is determined whether the vehicle belongs to one of the severe driving conditions, slope conditions, or instability tendency. The specific operation is as follows:

[0063] Compare the throttle pedal opening with the throttle opening threshold;

[0064] Compare the road surface gradient with the gradient threshold;

[0065] Compare the steering wheel angle with the steering wheel angle threshold;

[0066] Compare the vehicle yaw rate with the yaw rate threshold;

[0067] Compare the vehicle lateral acceleration with the lateral acceleration threshold;

[0068] Compare the vehicle speed with the vehicle speed threshold;

[0069] Compare the road surface adhesion coefficient with the road surface adhesion coefficient threshold;

[0070] Calculate the single-axle wheel slip ratio based on the wheel speed and vehicle speed, and compare the single-axle wheel slip ratio with the wheel slip ratio threshold;

[0071] When the throttle pedal opening is greater than the throttle opening threshold, it is determined that the vehicle is in an aggressive driving condition; when the road surface gradient is greater than the gradient threshold, it is determined that the vehicle is in a slope condition; when the single-axis wheel slip rate is greater than the wheel slip rate threshold, it is determined that the vehicle has an instability tendency; when the steering wheel steering angle is greater than the steering wheel angle threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; when the vehicle yaw rate is greater than the yaw rate threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; when the vehicle lateral acceleration is greater than the lateral acceleration threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; when the road surface adhesion coefficient is less than the road surface adhesion coefficient threshold, it is determined that the vehicle has a possibility of instability.

[0072] When the vehicle is in an aggressive driving condition, a slope condition, or has an instability tendency, control the vehicle to enter the four-wheel drive mode, and adjust the distribution ratio of the driving torque between the front and rear axles according to the corresponding road surface state information, driver operation information, and vehicle state information; the specific operations include:

[0073] When the vehicle is in an aggressive driving condition, that is, when the throttle pedal opening is greater than the throttle opening threshold, the vehicle enters the four-wheel drive mode. At this time, calculate the axle load transfer amount between the front and rear axles according to the longitudinal acceleration, recalculate the front axle load and the rear axle load based on the axle load transfer amount, and adjust the distribution ratio of the driving torque between the front and rear axles according to the ratio of the front axle load and the rear axle load. The axle load transfer amount is calculated according to the existing calculation formula, such as the front axle load F z1 Obtained through the following formula:

[0074]

[0075] The rear axle load F z2 Obtained through the following formula:

[0076]

[0077] Among them, G is the vehicle weight, m is the vehicle mass, ∑T i is the set of the inertial resistance couple moments acting on the front and rear wheels and the inertial resistance couple moment on the flywheel of the transverse engine, h g is the height of the vehicle's center of mass, f is the friction coefficient, r is the wheel radius, L is the vehicle wheelbase, a and b are the distances from the vehicle's center of mass to the front and rear axles, and u is the longitudinal vehicle speed.

[0078] When the vehicle is in a slope condition, that is, when the road surface slope is greater than the slope threshold, the vehicle enters the four-wheel drive mode. At this time, the vertical load of the front axle and the vertical load of the rear axle are calculated according to the road surface slope, and then the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the vertical load of the front axle and the vertical load of the rear axle. The slope condition includes uphill and downhill. When going uphill, it will cause the load to transfer from the front axle to the rear axle. At this time, the vertical load of the front axle is less than the vertical load of the rear axle, and the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the vertical load of the front axle and the vertical load of the rear axle, so that a larger proportion is distributed to the rear axle. When going downhill, it will cause the load to transfer from the rear axle to the front axle. At this time, the vertical load of the front axle is greater than the vertical load of the rear axle, and the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the vertical load of the front axle and the vertical load of the rear axle, so that a larger proportion is distributed to the front axle. The vertical load of the front axle and the vertical load of the rear axle are calculated by existing calculation formulas. For example, when going uphill, the vertical load F1 of the front axle is:

[0079]

[0080] The vertical load F2 of the rear axle is:

[0081] Where G is the vehicle weight, α is the road surface slope, ∑T i is the set of the inertial resistance couple moments acting on the front and rear wheels and the inertial resistance couple moment on the flywheel of the transverse engine, h g is the height of the vehicle's center of mass, f is the friction coefficient, r is the wheel radius, L is the vehicle wheelbase, and a and b are the distances from the vehicle's center of mass to the front and rear axles.

[0082] In this embodiment, it also includes a method for estimating the vehicle mass. When accelerating on a flat road surface with the vehicle acceleration stable and the steering wheel angle less than the steering wheel angle threshold, the vehicle mass is calculated according to Newton's second law through the total vehicle driving force and the longitudinal acceleration. When the vehicle acceleration is stable, the integral of the difference between the acceleration calculated by the vehicle model and the acceleration received by the chassis control unit is accumulated regularly into the total vehicle mass, so that when the vehicle starts next time, it can be controlled according to the pre-stored total vehicle mass to avoid the situation of being unable to control.

[0083] When the vehicle is in both the intense driving condition and the slope condition, the torque is distributed according to the slope condition.

[0084] When the vehicle has a tendency to become unstable, such as when the slip ratio of a single-axle wheel is greater than the wheel slip ratio threshold, the vehicle enters the four-wheel drive mode. At this time, according to the difference between the target slip ratio and the slip ratio of the single-axle wheel, the driving torque of the slipping axle is transferred to the non-slipping axle to reduce the slip ratio of the slipping axle to the target slip ratio. The operation of transferring the driving torque of the slipping axle to the non-slipping axle can be controlled according to the PID closed-loop control method, and the driving torque corresponding to the slipping wheel is redistributed to the wheels with good adhesion to ensure the stability of the vehicle driving. The target slip ratio is obtained by looking up a table based on the vehicle speed and the road surface adhesion coefficient. The table can pre-store the corresponding information of the vehicle speed, road surface adhesion coefficient and target slip ratio through experiments in advance. The single-axle wheel slip ratio includes the left front wheel slip ratio, the right front wheel slip ratio, the left rear wheel slip ratio and the right rear wheel slip ratio, and the left front wheel slip ratio, the right front wheel slip ratio, the left rear wheel slip ratio and the right rear wheel slip ratio are respectively calculated from the vehicle speed and the wheel speed of their respective wheels.

[0085] When the vehicle has a tendency to become unstable, such as when the steering wheel steering angle is greater than the steering wheel angle threshold and the vehicle speed is greater than the vehicle speed threshold, or when the vehicle yaw rate is greater than the yaw rate threshold and the vehicle speed is greater than the vehicle speed threshold, or when the vehicle lateral acceleration is greater than the lateral acceleration threshold and the vehicle speed is greater than the vehicle speed threshold, or when the road surface adhesion coefficient is less than the road surface adhesion coefficient threshold, the vehicle enters the four-wheel drive mode and, according to the difference between the target yaw rate and the actual yaw rate, adopts the yaw rate closed-loop correction method to dynamically adjust the torque distribution between the front and rear axles. The yaw rate closed-loop correction method can be PID closed-loop control. When the yaw rate exceeds the target yaw rate, it is determined as oversteering, and according to the difference between the target yaw rate and the yaw rate, the distribution ratio of the driving torque between the front and rear axles is dynamically adjusted to adjust the distribution ratio of the front axle to be more than that of the rear axle; when the yaw rate is less than the target yaw rate, it is determined as understeering, and according to the difference between the target yaw rate and the yaw rate, the distribution ratio of the driving torque between the front and rear axles is dynamically adjusted to adjust the distribution ratio of the front axle to be less than that of the rear axle; the target yaw rate is calculated based on the vehicle speed and the steering wheel steering angle as inputs according to the seven-degree-of-freedom vehicle dynamics model. The yaw rate is collected by the inertial unit sensor SAS.

[0086] In the three cases of aggressive driving conditions, hill road conditions and instability tendency, with the instability tendency as the highest priority, the vehicle preferentially dynamically adjusts the driving torque of the front and rear axles according to the information under the instability tendency. When the vehicle does not belong to the above three cases, the power control unit automatically selects to enter the front-wheel drive or rear-wheel drive mode to avoid the impact of long-term four-wheel drive activation on the vehicle fuel consumption and improve the economic index of the vehicle. Moreover, with the use of this method, the vehicle power performance is not reduced due to the improvement of stability, and the vehicle well achieves the balance of power performance, stability and economy in different scenarios.

[0087] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Those skilled in the art to which the present invention pertains may make various modifications or supplements to the described specific embodiments or use similar means for substitution, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A four-wheel drive control method for a multi-motor plug-in hybrid vehicle, characterized in that It includes the following steps: Based on the state of charge, discharge capacity, throttle pedal state, shift lever position, chassis stability control state, and rear drive motor performance state of the high-voltage power battery, determine whether the preconditions for activating the four-wheel drive mode are met. The preconditions for activating the four-wheel drive mode include: The state of charge of the high-voltage power battery exceeds the state of charge threshold; The discharge capacity exceeds the discharge threshold; The throttle pedal state is in the depressed state; The shift lever position is in the D gear; The chassis stability control state is in the unactivated state; The rear drive motor performance state has no fault; When all of the above six conditions are met, it is determined that the preconditions for activating the four-wheel drive mode are met; conversely, when any one of the above six conditions is not met, it is determined that the preconditions for activating the four-wheel drive mode are not met; When the preconditions for activating the four-wheel drive mode are met, continuously monitor the driver operation information, vehicle state information, and road surface state information; Based on the driver operation information, vehicle state information, and road surface state information, determine whether the vehicle belongs to one of the severe driving conditions, slope conditions, or instability tendency; When the vehicle belongs to the severe driving conditions, slope conditions, or instability tendency, control the vehicle to enter the four-wheel drive mode, and adjust the distribution ratio of the driving torque between the front and rear axles according to the corresponding road surface state information, driver operation information, and vehicle state information.

2. The four-wheel drive control method of the multi-motor plug-in hybrid vehicle according to claim 1, wherein, The driver operation information includes the throttle pedal opening and the steering wheel steering angle; the vehicle state information includes the yaw rate, lateral acceleration, longitudinal acceleration, wheel speed, and vehicle speed; the road surface state information includes the road surface slope and the road surface adhesion coefficient.

3. The four-wheel drive control method of the multi-motor plug-in hybrid vehicle according to claim 2, characterized in that, The operations for determining whether the vehicle belongs to the severe driving conditions or slope conditions include: Compare the throttle pedal opening with the throttle opening threshold; Compare the road surface slope with the slope threshold; When the throttle pedal opening is greater than the throttle opening threshold, it is determined that the vehicle belongs to the severe driving conditions; when the road surface slope is greater than the slope threshold, it is determined that the vehicle belongs to the slope conditions.

4. The four-wheel drive control method of the multi-motor plug-in hybrid vehicle according to claim 3, characterized in that, The operations for determining whether the vehicle has an instability tendency include: Compare the steering wheel steering angle with the steering wheel angle threshold; Compare the yaw rate with the yaw rate threshold; Compare the lateral acceleration with the lateral acceleration threshold; Compare the vehicle speed with the vehicle speed threshold; When the steering wheel steering angle is greater than the steering wheel angle threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; or when the yaw rate is greater than the yaw rate threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency; or when the lateral acceleration is greater than the lateral acceleration threshold and the vehicle speed is greater than the vehicle speed threshold, it is determined that the vehicle has an instability tendency.

5. The four-wheel drive control method for a multi-motor plug-in hybrid vehicle according to claim 4, wherein The operations for determining whether the vehicle has an instability tendency also include: Calculate the single-axle wheel slip ratio based on the wheel speed and the vehicle speed; Compare the single-axle wheel slip ratio with the wheel slip ratio threshold; When the single-axle wheel slip ratio is greater than the wheel slip ratio threshold, it is determined that the vehicle has an instability tendency.

6. The four-wheel drive control method of a multi-motor plug-in hybrid vehicle according to any one of claims 2-5, characterized in that, The operations for adjusting the distribution ratio of the driving torque between the front and rear axles include: When the vehicle is in an aggressive driving condition, the vehicle enters the four-wheel drive mode. At this time, the axle load transfer amount between the front and rear axles is calculated based on the longitudinal acceleration. The front axle load and the rear axle load are recalculated based on the axle load transfer amount, and the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the front axle load and the rear axle load.

7. The four-wheel drive control method for a multi-motor plug-in hybrid vehicle according to any one of claims 2-5, characterized in that, The operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes: When the vehicle is in a slope condition, the vehicle enters the four-wheel drive mode. At this time, the front axle vertical load and the rear axle vertical load are calculated based on the road surface slope, and then the distribution ratio of the driving torque between the front and rear axles is adjusted according to the ratio of the front axle vertical load and the rear axle vertical load.

8. The four-wheel drive control method of the multi-motor plug-in hybrid vehicle according to claim 4, wherein The operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes: When the vehicle has a tendency to become unstable, the vehicle enters the four-wheel drive mode, and the yaw rate is compared with the target yaw rate: When the yaw rate exceeds the target yaw rate, it is determined as oversteering. According to the difference between the target yaw rate and the yaw rate, the distribution ratio of the driving torque between the front and rear axles is dynamically adjusted to adjust the distribution ratio of the front axle to be more than that of the rear axle; When the yaw rate is less than the target yaw rate, it is determined as understeering. According to the difference between the target yaw rate and the yaw rate, the distribution ratio of the driving torque between the front and rear axles is dynamically adjusted to adjust the distribution ratio of the front axle to be less than that of the rear axle; The target yaw rate is calculated based on the vehicle speed and the steering wheel steering angle as inputs according to a seven-degree-of-freedom vehicle dynamics model.

9. The four-wheel drive control method for a multi-motor plug-in hybrid vehicle according to claim 5, characterized in that, The operation of adjusting the distribution ratio of the driving torque between the front and rear axles includes: When the vehicle has a tendency to become unstable, the vehicle enters the four-wheel drive mode. At this time, according to the difference between the target slip ratio and the single-axle wheel slip ratio, the driving torque of the slipping axle is transferred to the non-slipping axle to reduce the slip ratio of the slipping axle to the target slip ratio; The target slip ratio is obtained by looking up a table based on the vehicle speed and the road surface adhesion coefficient; The single-axle wheel slip ratio includes the left front wheel slip ratio, the right front wheel slip ratio, the left rear wheel slip ratio, and the right rear wheel slip ratio. The left front wheel slip ratio, the right front wheel slip ratio, the left rear wheel slip ratio, and the right rear wheel slip ratio are respectively calculated from the vehicle speed and the wheel speeds of their respective wheels.

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