METHOD FOR OPERATING A VEHICLE COMBINATION

A method is for operating a vehicle combination having a tractor and at least one trailer. A brake control unit of a tractor brake system monitors the braking behavior of the tractor or the vehicle combination, and if necessary the brake pressure at each of the wheel brakes of the tractor, and initiates the deceleration of the trailer. To improve the driving safety of the vehicle in the event of limitation of the functionality of components of the vehicle combination, at least one vehicle component of the tractor and/or of the trailer, the vehicle component being relevant for driving safety, is monitored with regard to functionality, and in the event of detection of a limitation of the functionality of a component, the brake control unit of the tractor brake system decelerates the vehicle combination using a trailer brake system of the trailer.

TECHNICAL FIELD

The disclosure relates to a method for operating a vehicle combination having a tractor vehicle and having at least one trailer vehicle having an anti-lock brake system.

BACKGROUND

The tractor vehicle and the at least one trailer vehicle of the vehicle combination have pneumatically actuatable wheel brakes at each wheel. The wheel brakes of such service brake systems are adjustable in terms of their braking action by way of the brake pressure that is set. The tractor vehicle brake system has an electronic brake control unit that monitors the braking behavior of the tractor vehicle or of the vehicle combination as a whole. The brake control unit is configured to adjust the brake pressure at each of the wheel brakes of the tractor vehicle and to adjust a trailer brake pressure with which a trailer brake system of a relevant trailer vehicle is controlled in open-loop or closed-loop fashion in terms of its braking action. Here, the brake control unit varies the brake pressure and/or the trailer brake pressure as required, irrespective of the inputs made by a driver of the vehicle combination. In so doing, the brake control unit implements an anti-lock brake system (ABS) for the tractor vehicle, and adjusts the brake pressure if there is a locking tendency at particular wheels. Specifically, only a braking force corresponding to the friction coefficient of the roadway can be utilized during any braking operation. If the braking force that is input exceeds the maximum braking force that is transmissible at one or more wheels, the wheels begin to lock, which can cause the vehicle to become unstable. An ABS system permanently monitors the rotational speed of each wheel on the basis of measurement signals from rotational speed sensors, and from these ascertains the respective brake slip values. This may be performed for example by comparing the wheel speed that is ascertained from the wheel rotational speed with a vehicle reference speed that is ascertained from all of the wheel speeds of the vehicle. If a locking tendency of the wheel is identified from the brake slip value thus ascertained, that is, if a slip limit has been reached or overshot, the brake control unit performs control by adjusting the brake pressure. Here, in a first step, the brake pressure is lowered in order to subsequently adjust the brake pressure of the relevant wheel along the slip limit by closed-loop control. The braking moment is increased again until such time as a braking moment that corresponds to the friction coefficient of the roadway is reached.

US 2018/0354478 and US 2019/0315324 disclose a method for adjusting brake pressures at pneumatically actuatable wheel brakes of a vehicle combination, wherein a tractor vehicle brake system has a brake control unit, and a trailer brake system has a separate anti-lock brake system with its own brake electronics unit.

In the known method, the brake control unit of the tractor vehicle brake system communicates with the brake electronics unit of the trailer brake system and receives information relating to the brake slip value of each axle unit of the trailer vehicle. If it is apparent from the evaluation of the brake slip value that there is a demand for braking at the trailer vehicle, the brake control unit of the tractor vehicle outputs a corresponding trailer brake pressure that is provided to the trailer vehicle at a trailer control valve of the tractor vehicle.

US 2021/0370898 discloses a further method in which the brake control unit of the tractor vehicle brake system communicates with a brake electronics unit of the trailer brake system in order to receive brake slip information. If the driver actuates the brake pedal and thus the service brake valve, the brake control unit of the tractor vehicle brake system detects the setpoint deceleration determined by the driver and also the present actual deceleration of the vehicle combination. The actual deceleration of the vehicle combination is adjusted to the setpoint deceleration through coordinated actuation of the service brake systems, that is, of the tractor vehicle brake system and of the trailer vehicle brake system.

Electrical, electronic or mechanical faults at a wheel, at a brake or at other vehicle components or systems that are relevant for the driving safety of the vehicle may occur during travel of the vehicle combination. If the driver does not notice the fault and/or the associated limitations in functionality of the vehicle component in question, unchanged onward travel of the vehicle combination could pose a considerable risk to the driving safety of the vehicle combination and/or of other road users.

SUMMARY

It is an object of the present disclosure to improve the driving safety of the vehicle in the event of limitation of the functionality of vehicle components of the vehicle combination.

According to the disclosure, the above object is, for example, achieved by a method for operating a vehicle combination having a tractor vehicle and having at least one trailer vehicle, wherein a brake control unit of a tractor vehicle brake system monitors a braking behavior of the tractor vehicle or of the vehicle combination, and, if necessary, initiates a respective brake pressure at each of a plurality of wheel brakes as well as a deceleration of the trailer vehicle. The method includes: monitoring at least one vehicle component of at least one of the tractor vehicle and the trailer vehicle with regard to a functionality thereof, wherein the at least one vehicle component is relevant for driving safety; and, decelerating the vehicle combination via the brake control using a trailer brake system of the trailer vehicle in an event of a detection of a limitation of the functionality of a vehicle component.

According to the disclosure, at least one vehicle component of the tractor vehicle and/or of the trailer vehicle, the vehicle component being determined as being relevant for the driving safety of the vehicle and thus also of the vehicle combination, is monitored with regard to its functionality, and in the event of detection of a limitation of the functionality of a vehicle component, the brake control unit of the tractor vehicle brake system decelerates the vehicle combination using a trailer brake system4of the trailer vehicle6. If a limitation of the functionality of a vehicle component occurs which is relevant for the driving safety of the vehicle, the brake control unit of the tractor vehicle initiates a deceleration of the trailer vehicle, in order to reduce the speed of the vehicle combination, irrespective of the driving behavior of the driver. The braking of the trailer vehicle (and not of the tractor vehicle situated ahead of it in the vehicle combination) has the advantage that the vehicle combination is straightened during the deceleration maneuver. This also rules out a situation in which, in the event of a limitation of the functionality of a vehicle component of the tractor vehicle, the driving safety of the vehicle is possibly yet further adversely affected by an initiation of a braking operation for decelerating the vehicle by activation both of the brake system of the tractor vehicle and of the at least one trailer vehicle.

The disclosure relates both to vehicle combinations with trailer vehicles that are towed using drawbars and to semitrailer trucks in which a tractor vehicle tows the semitrailer.

The vehicle combination is advantageously decelerated by virtue of the brake control unit of the tractor vehicle brake system adjusting a trailer brake pressure with which the trailer brake system of the trailer vehicle is controlled in open-loop or closed-loop fashion in terms of its braking action.

In the case of a vehicle combination with an electronic brake system (EBS), the brake control unit of the tractor vehicle commands a brake electronics unit of a trailer vehicle, by way of a message with continuously updated message content, to brake the trailer vehicle in accordance with the message.

In an embodiment, following detection of a functional limitation of a vehicle component, closed-loop control of a maximum speed ascertained or specified for this situation is performed by the brake control unit through adjustment of the trailer brake pressure. In other words, the adjustable trailer brake pressure is the manipulated variable in a closed control loop, the controlled variable of which is the driving speed of the vehicle combination. Through influencing of the trailer brake pressure, the driving speed of the vehicle combination is influenced such that the driving speed is approximated to the ascertained or specified maximum speed, that is, the maximum speed that is reduced in relation to the present driving speed is adopted.

The value of the maximum speed that is set by closed-loop control by way of the trailer brake pressure is adapted to the quality of the affected vehicle component and/or to the degree of impairment of the functionality. Here, the vehicle speed may be reduced to a standstill, or to a maximum speed that may be continuously updated. Not only specifications relating to typically occurring faults but also the present driving demands of the driver, specifically an acceleration or braking demand of the driver, may be taken into consideration in an ascertainment of the maximum speed.

The adjustment of the driving speed to a maximum speed by closed-loop control can advantageously be activated by the driver, such that the driver can make the decision to at least temporarily continue the journey at a reduced driving speed. A corresponding actuating device for activating the adjustment to the maximum speed by closed-loop control is arranged in the area of the driver's seat. The adjustment of the driving speed to a maximum speed by closed-loop control can advantageously be activated by actuation of the accelerator pedal, such that an existing device can be used for activating the closed-loop control. For example, if the driver actuates the accelerator pedal as far as a stop, the vehicle combination is not brought to a standstill but is adjusted by closed-loop control to a particular maximum speed, the magnitude of which is adapted to the detected functional limitation or to the detected fault and/or identified driving situation.

In an embodiment of the disclosure, the maximum speed can be increased by—possibly repeated—actuation of the accelerator pedal, such that higher driving speeds are available for the driver to choose in accordance with their assessment of the hazard situation. For example, if the driver actuates the accelerator pedal several times in close succession as far as the stop, which indicates the activation of the closed-loop control of the driving speed, the setpoint variable (maximum speed) is selected by the driver in accordance with this and is set by closed-loop control by way of the trailer brake pressure.

As soon as the closed-loop control of the trailer brake pressure has commenced and brake pressure has been actively provided at the trailer brake system, an information signal is advantageously output from the control unit to an engine control unit with the message that the engine should no longer generate drive torque.

It is advantageously the case that, for the detection of a functional limitation of a vehicle component, the rotational speeds of the wheels are monitored. No additional measuring devices are necessary for the required information if the component vehicles in question of the vehicle combination are equipped with anti-lock brake systems. For the detection of the functionality of each wheel, the measured values detected by the anti-lock brake system for the rotational speed of the wheel are taken into consideration. This embodiment of the disclosure is therefore particularly advantageous in the case of vehicle combinations with trailer vehicles that are equipped with a dedicated anti-lock brake system. Here, the anti-lock brake system of the trailer vehicle—in particular the brake electronics unit thereof—provides the wheel rotational speed information that is desired for the monitoring of the functionality of the vehicle components in accordance with the disclosure.

A particular functional impairment can be inferred from a conspicuous value of the rotational speed of a wheel, that is, a rotational speed that is conspicuous in relation to other wheels of the vehicle, in the presence of certain attendant circumstances. For example, during unbraked travel of the vehicle, the decrease of a wheel rotational speed points toward the impediment of the free rotation of the wheel, which is an indication of permanently grinding brake pads of the brake or of some other damage to brake components at the wheel. In particular in the case of permanent grinding of the brake pads, there is the risk that brake components can overheat, often very quickly, in a manner that damages the brake. As a result of the driving speed being reduced in accordance with the disclosure, by adjustment or increase of the trailer brake pressure and subsequent adjustment of the driving speed by closed-loop control to a maximum speed, the occurrence of damage that goes unnoticed by the driver is counteracted.

During braked travel of the vehicle, a wheel is conspicuous in terms of its measured rotational speed if the rotational speed indicates no brake slip or only very little brake slip of the wheel, that is, the wheel is not contributing, or is contributing only abnormally little, according to the setpoint deceleration, to the work being done to decelerate the vehicle. This behavior is an indication of a malfunction of a component of the wheel brake between the point at which pressure is fed into the brake cylinder and the brake friction surface/brake pad as brake components of the brake, for example a broken brake line. In particular if the conspicuous rotational speed occurs at one wheel (or both wheels) of the front axle, this constitutes a malfunction that is critical in terms of driving safety. Following detection of the malfunction, the brake control unit of the tractor vehicle brake system automatically reduces the driving speed of the vehicle combination, irrespective of the driving behavior of the driver, through corresponding adjustment of the trailer brake pressure to the maximum speed that is intended for this functional limitation.

If one wheel of the vehicle combination is permanently locked, or is locked several times in close succession, without the driver or an external brake system (driver assistance system) performing a braking operation, then the monitoring of the functionality can infer that a limitation is present owing to a mechanical fault. For example, in drum brakes, it is often then the case that the S-shaped cam that pushes the brake shoes apart and against the brake drum has turned too far owing to an incorrect readjustment of the air gap between brake drum and brake shoes.

A severe impediment of a wheel that is detected through monitoring of the wheel rotational speed may also indicate sudden bearing damage.

In a further embodiment, for the detection of a functional limitation of a vehicle component, the tire pressure of vehicle wheels is monitored. If a suddenly or quickly decreasing tire pressure is detected during the monitoring of the tire pressure, this is evaluated as a state which poses a risk to the driving safety of the vehicle combination, and a deceleration of the vehicle combination in accordance with the disclosure is initiated. This is advantageous in particular if the wheel affected by the detected pressure loss is a steered wheel, generally a front wheel. The braking of the vehicle combination by way of a trailer vehicle causes the vehicle combination to be straightened, and the tractor vehicle situated at the front remains steerable.

In a further embodiment, for the detection of a functional limitation of a vehicle component, the functionality of electrical connections between tractor vehicle and trailer vehicle is monitored. In the event of a fault or an interruption of the electrical connection, which often cannot be noticed by the driver, the trailer brake pressure is automatically increased and the trailer vehicle is decelerated, wherein the driver is also made aware, by the automatic deceleration, that a functional limitation has been detected.

It can advantageously be the case that, for the detection of a functional limitation of a vehicle component, the functionalities of axle suspension arrangements are monitored. Here, through evaluation of the measurement signal of an axle load sensor, damage to the spring elements, for example to a leaf spring, can be indirectly identified. In the case of an axle with air suspension, the functionality of a pneumatic support bellows, for example, is monitored. Through evaluation of the measurement signal from a pressure sensor of the support bellows, specified situations that are associated with a limitation of the functionality are identified, and the brake control unit of the tractor vehicle brake system thus decelerates the vehicle combination in accordance with the disclosure by correspondingly adjusting the trailer brake pressure.

In a further embodiment, for the detection of a functional limitation of a vehicle component, the functionality of the steering system of the tractor vehicle is monitored. According to the disclosure, if a functional limitation that is of significance for the driving safety of the vehicle is ascertained through evaluation of the measurement signals from the rotational speed sensors of the wheels of the tractor vehicle, in particular those of the front axle (steering axle) of the tractor vehicle, of the measurement signal from a steering wheel angle sensor (LWS), of the measurement signals from axle load sensors of the front axle and/or of a signal from a lane keeping assistance system (LDW, LDWS, AFIL, LKAS), the vehicle combination is decelerated by corresponding adjustment of the trailer brake pressure, determination of a maximum speed, and subsequent adjustment of the driving speed to the maximum speed by closed-loop control.

In a further embodiment, for the detection of a functional limitation of a vehicle component, the functionality of the wheel brakes (friction brakes) of the tractor vehicle is monitored. An evaluation of the measurement signals from the rotational speed sensors of the wheels of the tractor vehicle, and information relating to the mass of the tractor vehicle and of the trailer vehicle and/or relating to the setpoint deceleration determined by the driver, are taken into consideration in the monitoring for detecting damage to the wheel brakes of the tractor vehicle. The mass of the tractor vehicle can be ascertained for example via axle load sensors of the tractor vehicle, and the mass of the trailer vehicle can be ascertained indirectly by ascertainment of the mass of the vehicle combination and the mass of the tractor vehicle, wherein the mass of the tractor vehicle is subtracted from the ascertained mass of the vehicle combination. The mass of the vehicle combination is ascertained from data relating to the drive power of the vehicle (vehicle combination), the data being obtained from an engine control unit and being ascertained by the engine control unit during phases of positive acceleration of the vehicle, and simultaneously detected vehicle speed data. According to the disclosure, if it is ascertained that, during travel with high capacity utilization (loading) of the tractor vehicle and with the capacity of the trailer vehicle not being fully utilized, the braking action of the wheel brakes of the tractor vehicle, which decelerates the vehicle combination, exhibits a strongly progressive decrease depending on the setpoint deceleration specified by the driver, the vehicle combination is decelerated by corresponding adjustment of the trailer brake pressure, determination of a maximum speed, and subsequent adjustment of the driving speed to the maximum speed by closed-loop control. In this way, so-called brake fading is counteracted, in the case of which a functional impairment of the wheel brakes that is of significance for the driving safety of the vehicle is present immediately following an intense conversion of kinetic energy into thermal energy by the wheel brakes, for example following a relatively long period of downhill travel and/or several successive, relatively intense braking operations starting from relatively high driving speeds.

DETAILED DESCRIPTION

FIG.1shows an electrical and pneumatic diagram of the brake systems of a vehicle combination, specifically of the tractor vehicle brake system3of a tractor vehicle5of the vehicle combination7and of a trailer brake system4of a trailer vehicle6. In the diagram, electrical lines are illustrated using solid lines, and pneumatic lines are illustrated using dotted lines. In the embodiment shown, the tractor vehicle5comprises two axles8,9, with wheels10being arranged on both sides at each of the axles. The trailer vehicle6likewise has two trailer axles12,14, each having wheels10. To brake the wheels10, each wheel10is assigned a pneumatically actuatable wheel brake13, the wheel brakes being parts of the relevant service brake system of a vehicle of the vehicle combination7.

In the driver's cab of the tractor vehicle5, there are arranged an accelerator pedal17and a brake pedal15, the brake pedal being coupled to a service brake valve16. By actuating the brake pedal15, the driver of the tractor vehicle5can open pneumatic brake lines between pressure medium reservoirs19,20and the wheel brakes13. Via the accelerator pedal17, the driver adjusts the driving speed of the vehicle combination7.

In the embodiment shown, the wheel brakes13of the front axle8of the tractor vehicle5are assigned to a common first brake circuit21, whereas the wheel brakes13of the rear axle9can be actuated via a second brake circuit22. The first pressure medium reservoir19is in this case assigned to the first brake circuit21. The second brake circuit22of the rear axle9is supplied with pressure medium via a second pressure medium reservoir20. The second brake circuit22is constructed analogously to the first brake circuit21.

Both in the tractor vehicle brake system3and in the trailer brake system4, a pressure control valve27is connected upstream of each wheel brake13, each of which pressure control valves is electrically actuatable. To receive control signals28, the pressure control valves27of the tractor vehicle brake system3are connected to a brake control unit30. The pressure control valves27of the trailer brake system4are connected to a brake electronics unit31. The pressure control valves27are each a combination of at least two solenoid valves, specifically an inlet valve32and an outlet valve33. The inlet valve32serves in principle for increasing brake pressure or, by being closed, for maintaining the brake pressure P at a wheel brake13of a wheel10, whereas, in order to reduce the brake pressure P, the outlet valve33is opened and the brake cylinder, respectively connected on the wheel side, of the wheel brake13of a wheel10is ventilated. The inlet valve32and the outlet valve33of the pressure control valves27are preferably 2/2 directional valves.

The brake control unit30and the brake electronics unit31are configured and provided for influencing the brake pressure in the relevant component vehicle of the vehicle combination7as required. For this purpose, the rotational behavior of the respective wheels10is monitored. Each wheel10of the tractor vehicle5and each wheel10of the trailer vehicle6is assigned a rotational speed sensor34that generates measurement signals35providing information regarding the rotational behavior of the relevant wheel10. The rotational speed sensors34of the tractor vehicle brake system3are connected to the brake control unit30, and the rotational speed sensors34of the trailer brake system4are connected to the brake electronics unit31. The brake control unit30and the brake electronics unit31evaluate the measurement signals35from the respectively connected rotational speed sensors34.

The brake electronics unit31, the rotational speed sensors34and the pressure control valves27of the trailer brake system4are the main elements of an anti-lock brake system36of the trailer vehicle6. Irrespective of an anti-lock brake function (ABS) of the brake control unit30of the tractor vehicle5, the brake electronics unit31monitors the locking tendency of the individual wheels10of the trailer vehicle6by way of the rotational speed sensors34or by evaluating the measurement signals35from the rotational speed sensors. If a locking tendency of a wheel10is identified, the brake electronics unit31counteracts locking of the wheels10by actuating the one or more pressure control valves27and by adjusting the brake pressures prevailing at the wheel brakes along the slip limit by closed-loop control.

The tractor vehicle brake system3comprises a coupling head23to which the trailer brake system4of the trailer vehicle6can be coupled. Via the coupling head23, the tractor vehicle brake system3provides a pneumatic trailer brake pressure P-A for the trailer brake system4. The coupling head23is assigned a trailer control valve24that controls the connection between a third pressure medium reservoir25and the pneumatic coupling head23. The trailer brake system4has a trailer brake circuit26, in which the trailer brake pressure P-A prevails and can be connected through to all wheel brakes13of the trailer brake system4. The actuation of the trailer control valve24is performed by actuation of the service brake valve16by the driver using the brake pedal15, or is performed by the brake control unit30of the tractor vehicle brake system3irrespective of actuation of the service brake valve16by the driver. For this purpose, the tractor vehicle brake system3has a trailer pressure control valve38which is actuatable by the brake control unit30of the tractor vehicle brake system3and which is constructed analogously to the pressure control valves27connected upstream of the wheel brakes13. The inlet valve32and the outlet valve33of the trailer pressure control valve38are actuated by the brake control unit30of the tractor vehicle brake system3. For this purpose, the tractor vehicle brake system3has a fourth brake circuit45with a separate fourth pressure medium reservoir46. The fourth brake circuit45has a 3/2 directional valve47, a double check valve48and a trailer pressure control valve38, which is constructed analogously to the pressure control valves27of the wheel brakes13, and the inlet valve32and outlet valve33of which are actuated by the brake control unit30of the tractor vehicle brake system3by way of control signals28. The control signals28give rise to a control brake pressure P-AKV in the brake pressure line49to the trailer control valve24, which control brake pressure causes the trailer control valve24to provide a trailer brake pressure P-A to the trailer brake system4of the trailer vehicle6.

As will be discussed in more detail further below with reference toFIG.2, vehicle components of the tractor vehicle5and of the trailer vehicle6that are determined as being relevant for driving safety are monitored with regard to their functionality. If a vehicle component of the vehicle combination7is impaired in terms of its functionality, the brake control unit30decelerates the driving speed of the vehicle combination7by correspondingly adjusting the trailer brake pressure P-A, and for this purpose actuates the trailer pressure control valve38.

In the present embodiment, the functionality of wheels10, wheel brakes13, axle suspension arrangements39and electrical connections42between tractor vehicle5and trailer vehicle6are monitored. For this purpose, the measurement signals35from the wheel rotational speed sensors34of the wheels10are taken into consideration, and the rotational speed n of the respective wheels10is evaluated with regard to functional impairment of the wheels10themselves and/or of the respectively associated wheel brakes13. Furthermore, each of the wheels10is assigned a tire pressure sensor11, the measurement signal18of which, with information relating to the tire pressure R, is evaluated in order to monitor the tire pressure R. The functionality of axle suspension arrangements39is also monitored. For this purpose, the respectively monitored axle9is assigned a corresponding measuring device, the measurement signal40from which is evaluated. Here, the monitoring of functionality may be performed at selected wheels10or axles9, or at all wheels10or axles8,9, of the tractor vehicle5or of the vehicle combination7. Finally, in the embodiment shown, the functionality of the electrical connection42between the tractor vehicle5and the trailer vehicle6is monitored by evaluation of the measurement signal41of a measuring device43that is assigned to the electrical connection.

The brake control unit30of the tractor vehicle brake system3and the brake electronics unit31of the trailer brake system4communicate with one another via an electrical connection42between the vehicles (component vehicles) of the vehicle combination7. Monitoring information relating to the trailer vehicle6is made available in the tractor vehicle5via the connection42, the information being transmitted by way of an information signal29. The information signal29contains information relating to the trailer brake system4, including information relating for example to the rotational speeds of the wheels10of the trailer vehicle6or the slip of each of the wheels, as evaluated by the anti-lock brake system36of the trailer vehicle6. The information from the anti-lock brake system36of the trailer vehicle6is evaluated by the brake control unit30of the tractor vehicle5for braking force distribution purposes or for the purposes of being input into driver assistance systems. The information signal29from the trailer vehicle6is at the same time used for the purposes of monitoring the functionality of vehicle components of the trailer vehicle6in accordance with the disclosure.

The deceleration of the vehicle combination7in the event of detection1of an impairment of functionality of a vehicle component that is relevant for driving safety is illustrated inFIG.2. In the embodiment shown, the wheel rotational speeds n are evaluated by way of the measurement signals35from the rotational speed sensors34, the tire pressures R at the wheels10are evaluated by way of the measurement signals18from the respective tire pressure sensors11, the measurement signals40from the measuring devices at the axle suspension arrangements39are evaluated, giving information relating to the functionality f1of the axle suspension arrangement, and the measurement signal41from the measuring device43at the electrical connection42between tractor vehicle5and trailer vehicle6is evaluated for the purposes of detecting the functionality f2of the electrical connection42.

The functionality f3of the steering system of the tractor vehicle is also monitored. For this purpose, the detection1detects the measurement signal from a steering wheel angle sensor52.

Taking into consideration the measurement signals35from the rotational speed sensors34, the measurement signals40from the measuring devices at the axle suspension arrangements39, and data from an engine control unit50relating to the drive power of the engine, in order to ascertain vehicle masses, the functionality f4of the wheel brakes of the tractor vehicle is also monitored. For the detection of the functionality f4of the wheel brakes, the engine control unit50provides an information signal51to the brake control unit, which information signal communicates the present drive power of the engine and, with this variable, allows the mass of the tractor vehicle to be ascertained. A rapid diminishment of the braking action of the wheel brakes of the tractor vehicle, so-called fading, can thus be identified. In such a situation, if it is ascertained that a load of the tractor vehicle is high and that the trailer vehicle is not fully laden, the onset of critical driving situations can be prevented by decelerating the vehicle combination by way of the trailer vehicle.

If it is apparent from the detection1that the functionality of a vehicle component is impaired, a deceleration of the vehicle combination7is initiated by adjustment44of a corresponding trailer brake pressure P-A in accordance with the identified nature and severity of the functional impairment. At the same time as the adjustment of a trailer brake pressure P-A, an information signal is output from the brake control unit30to an engine control unit, the content of which information signal stipulates that, temporarily, for as long as the information signal is present, no power that drives the vehicle is to be generated by the drive engine (engine).

The adjustment of the trailer brake pressure P-A and the associated deceleration of the vehicle combination7by braking of the trailer vehicle6can be influenced by the driver of the vehicle combination7by actuation of the accelerator pedal17in a specified manner, for example by brief depression of the accelerator pedal17as far as a stop. In this case, a maximum speed vM can be specified for closed-loop control2and controlled in closed-loop fashion through the adjustment44of the trailer brake pressure P-A. For this purpose, the present driving speed v is taken into consideration, and if a driving speed v is higher than the maximum speed vM, a deceleration by way of the trailer brake pressure P-A is initiated. The adjustment of the driving speed v by closed-loop control2to a maximum speed vM can be activated by actuation of the accelerator pedal17. The value of the specified maximum speed vM is obtained from a characteristic map memory37in which advantageous empirical values for relevant situations of malfunctions, or functional limitations, of particular vehicle components are stored. The maximum speed vM is read out from the characteristic map memory37in accordance with the result of the detection1.

The magnitude of the maximum speed vM is adapted to the functional limitation ascertained during the detection1or to the detected fault or identified driving situation (high-speed freeway travel). The specification of the maximum speed vM is increased by repeated actuation of the accelerator pedal17, such that higher driving speeds are available for the driver to choose in accordance with their assessment of the hazard situation. Thus, if the driver actuates the accelerator pedal17several times in close succession, the setpoint variable (maximum speed) is selected by the driver in accordance with this and is set by closed-loop control by way of the trailer brake pressure P-A and the drive torque of the engine.

LIST OF REFERENCE DESIGNATIONS (PART OF THE DESCRIPTION)