Drive device for a motor vehicle

Abstract

Drive device for a motor vehicle with a start / stop system (15) comprising a control unit (25) which, upon application of the brake pedal by the driver, triggers an engine stop (Sstop) as soon as a stopping process (ΔtAH) occurs and the actual brake pressure (pist) generated by the brake pedal application exceeds a brake pressure threshold (BDS), wherein the control unit (25) triggers a restart of the engine (Sstart) upon application of the brake pedal by the driver as soon as the actual brake pressure (pist) generated by the brake pedal application falls below the brake pressure threshold (BSD) and is at 0, i.e., the brake pedal is fully released, wherein during the stopping process (ΔtAH) until the vehicle comes to a standstill (tstand), the control unit (25) sets the brake pressure threshold (BDS) to a lower limit (UG).so that even with slight brake pedal actuation, the corresponding actual brake pressure (pist) exceeds the brake pressure threshold (BDS) and the control unit (25) triggers the engine stop, wherein the control unit (25) maintains the brake pressure threshold (BDS) unchanged at the lower limit (UG) from the point (tstand) of the vehicle coming to a standstill, and this over a service safety time interval (ΔtG), wherein the control unit (25) sets the brake pressure threshold (BDS) to an upper limit (OG) after the service safety time interval (ΔtG) has elapsed, wherein for a renewed engine start in the service safety time interval (ΔtG), the actual brake pressure (pist) is to be reduced by a pressure difference (Δp) to below the brake pressure threshold (BDS) by corresponding release of the brake pedal by the driver, characterized in that the control unit (25) has an evaluation unit (27) which determines,whether, during the operational safety time interval (ΔtG), there is an increased brake pedal load with a correspondingly increased actual brake pressure (pist) compared to the stopping process (ΔtAH) or the point at which the vehicle comes to a standstill (tstand), and that if there is no increased actual brake pressure (pist), the evaluation unit (27) leaves the brake pressure threshold (BDS) unchanged at the lower limit (UG) during the operational safety time interval (ΔtG), and that if there is an increased actual brake pressure (pist), the evaluation unit (27) raises the brake pressure threshold (BDS) from the lower limit (UG) to a safety value (SW) during the operational safety time interval (ΔtG).

Description

The invention relates to a drive device for a motor vehicle with a start / stop automatic system according to the preamble of claim 1. In a standard start / stop system, a control unit triggers an engine stop as soon as it detects a vehicle stop and the actual brake pressure generated by the driver's brake pedal exceeds a brake pressure threshold. Conversely, the control unit triggers a restart of the engine as soon as the actual brake pressure generated by the driver's brake pedal falls below the brake pressure threshold, and in particular reaches zero, meaning the brake pedal is fully released. The start / stop system sets the brake pressure threshold to a lower limit or minimum value during the braking process until the vehicle comes to a complete stop. This means that even with slight brake pedal application, the corresponding actual brake pressure exceeds the brake pressure threshold, triggering the control unit to shut off the engine. During the stopping process described above (i.e., after the engine has already stopped), the driver intuitively releases the brake pedal almost completely until the vehicle comes to a standstill, in order to achieve a smooth stop without any pitching motion. Against this background, the lower limit of the brake pressure threshold during the stopping process is set so low that even with almost complete release of the brake pedal, the corresponding actual brake pressure still exceeds the threshold. This ensures that only when the brake pedal is completely released will the brake pressure drop below the threshold, thus resulting in the engine restarting during the stopping process. Otherwise, even with minimal pressure on the brake pedal, the corresponding actual brake pressure exceeds the threshold (in order to maintain the engine stop). With regard to the operational safety of the start / stop system, it is known from the prior art that, from the moment the vehicle comes to a standstill, the control unit maintains the brake pressure threshold unchanged at the lower limit (i.e., a minimum value of, for example, 0.1 bar) for a safety time interval, which may be, for example, 3 seconds. After this safety time interval has elapsed, the control unit sets the brake pressure threshold to an upper limit (for example, 3 bar). If, due to insufficient brake pedal depressurization by the driver, the corresponding actual brake pressure falls below the brake pressure threshold now set at the upper limit, the control unit triggers a restart of the engine. For restarting the engine during the operational safety interval, the following applies according to current technology: The actual brake pressure must be reduced by releasing the brake pedal by a pressure difference to below the brake pressure threshold (set to the minimum value). In start / stop operation on an incline, this leads to the following problem: During the stopping process (i.e., after the engine has already stopped), the driver will intuitively release the brake pedal almost completely as the vehicle comes to a standstill in order to cause the vehicle to pitch forward. Subsequently, however, the driver applies increased pressure to the brake pedal, resulting in a correspondingly higher actual brake pedal pressure, to prevent the vehicle from unintentionally rolling backward.During the operational safety interval, the engine restarts when the driver releases the brake pedal, reducing the actual brake pressure by a pressure differential to below the brake pressure threshold (set to the minimum value). In this case, the driver's release of the brake pedal during the operational safety interval inevitably leads to the vehicle rolling backward. This is perceived as disruptive by the driver and may only be stopped by a roll detection system separate from the start / stop system. DE 10 2015 220 807 A1 discloses a method for controlling a motor vehicle powertrain. DE 10 2015 203 453 A1 discloses a method for assisting a vehicle in starting on an incline with a hill start assist system. DE 10 2015 206 658 A1 discloses an adaptive start-stop system for a motor vehicle with an automatic transmission, taking obstacles into account. DE 199 27 975 A1 discloses a device for automatic engine speed control of a motor vehicle in order to enable a smooth transition of the vehicle from a brake release state to a vehicle drive state. DE 101 28 155 A1 relates to a braking system for vehicles in which several systems of individual and separate brake fluid lines are provided and each line is equipped with at least one brake pressure holding device which, after the brake pedal is released, continuously maintains the brake fluid pressure in a wheel cylinder arranged in the brake fluid line and which releases the held brake pressure depending on an increase in the starting driving force of the vehicle, wherein the release of the held brake fluid pressure takes place at each brake pressure holding device with a time delay. DE 10 2014 205 176 A1 discloses a method for operating a motor vehicle with a start / stop function. This includes stopping the vehicle's internal combustion engine using the start / stop function, detecting a road inclination at the vehicle's position, determining a threshold value for an operating parameter of the vehicle's service brake from the road inclination, monitoring the operating parameter, and requesting a restart of the internal combustion engine using the start / stop function as soon as the threshold value is exceeded by the operating parameter. DE 10 2007 010 488 A1 relates to a method for starting and / or stopping the engine of a motor vehicle, in which the engine is started at least when, with the motor vehicle stationary, a brake actuation device is released from an active brake position or is released at least to such an extent that a first threshold brake position is reached or fallen below, whereby a brake force representing the active brake position is maintained with a predetermined time delay. The object of the invention is to provide a drive device for a motor vehicle with a start / stop automatic system that enables more comfortable operation compared to the prior art. The problem is solved by the features of claim 1. Preferred embodiments of the invention are disclosed in the dependent claims. According to the characterizing part of claim 1, the control unit of the start / stop system comprises an evaluation unit. This unit determines whether, during the operational safety interval, there is increased brake pedal load with a correspondingly higher actual brake pressure compared to the stopping process or the vehicle coming to a standstill. If no increased actual brake pressure is present, the evaluation unit leaves the brake pressure threshold unchanged at the lower limit value during the operational safety interval. Conversely, if an increased actual brake pressure is present, the evaluation unit raises the brake pressure threshold during the operational safety interval from the lower limit value (i.e., the minimum value) to a safety limit value. Raising the brake pressure threshold from the lower limit to the safety value reduces the pressure difference by which the actual braking value must be reduced so that it falls below the brake pressure threshold for a restart of the engine. The safety value to which the brake pressure threshold is raised from the lower limit (i.e., minimum value) is calculated so that the brake pressure reduction corresponding to the release of the brake pedal does not lead to any unintended vehicle roll. This means that even during the aforementioned brake pressure reduction, sufficient braking power is still provided to prevent the vehicle from rolling. In one design variant, the safety value can correspond approximately to the upper limit of the brake pressure threshold. Furthermore, the safety value is always lower than the actual brake pressure during the operational safety time interval. The invention is based on the fact that, during the stopping process (i.e., after the engine has already stopped), the driver intuitively releases the brake pedal almost completely as soon as the vehicle comes to a standstill in order to achieve a smooth stop without any pitching of the vehicle. The lower limit during the stopping process and during the operational safety interval is preferably set so low that even with almost complete release of the brake pedal, the corresponding actual brake pressure still exceeds the brake pressure threshold, thus preventing an unintentional engine start (for example, due to a further reduction in brake pedal pressure). In this configuration, it is therefore ensured that the brake pressure threshold is only undershot, and thus a restart of the engine during the operational safety interval occurs, when the brake pedal is actually completely released. An embodiment of the invention is described below with reference to the accompanying figures. Figure 1 shows a block diagram of a drive train for a two-track vehicle; Figure 2 shows a simplified block diagram relating to program modules of the start / stop system according to a comparative example not covered by the invention; Figures 3 and 4 are brake pressure-time diagrams illustrating a start / stop operation according to the prior art; Figure 5 shows a view corresponding to Figure 2 of a start / stop system according to the invention; and Figure 6 shows a diagram corresponding to Figure 4, explaining the start / stop operation of the vehicle according to the invention. Figure 1 shows the drivetrain of a two-track vehicle. This drivetrain comprises an internal combustion engine 1, a starting element 3, and a transmission 5, which drives the vehicle wheels of one axle via an axle differential 7. An engine control unit 9 of the internal combustion engine 1, a control unit 11 of the starting element 3, and a transmission control unit 13 of the transmission 5 are connected to a start / stop system 15. Based on a multitude of driving parameters, this system generates an engine stop signal (Sstop) or an engine start signal (Sstart), which can be used to control the drivetrain accordingly. In Fig. 1, the start / stop automatic system 15 is shown in signal connection with a brake pedal sensor 17, which detects a hydraulic actual brake pressure in the hydraulic brake lines and transmits this to the start / stop automatic system 15, and with a vehicle speed sensor 19, which transmits an actual vehicle speed. The invention specifically relates to a start-stop driving operation in which the engine restarts during a subsequently described operational safety time interval ΔtG. For a simpler understanding of the invention, a conventional start-stop driving operation is first described with reference to Figures 2, 3, and 4: Figure 2 shows a simplified, incomplete block diagram indicating a number of program modules required in the start / stop system 15 for signal processing during the operational safety time interval ΔtG. Figure 2 shows a detection module 19 that determines whether a stop process ΔtAH (i.e., vehicle speed is in the range of 0 to 25 km / h) is occurring or not. Furthermore, the detection module 19 determines whether the operational safety time interval ΔtG is present or not. The detection module 19 is connected to a converter module 21, in which a brake pressure threshold BDS, described later, is set to either an upper limit OG or a lower limit UG. The converter module 21 is in turn connected to a comparator module 23, which determines whether the actual brake pressure pist is greater than the brake pressure threshold BDS. Both the detection module 19 and the comparator module 23 are connected to an input side of a control unit 25, in which either an engine stop signal Sstop or an engine start signal Sstart is generated. The converter module 21 sets the brake pressure threshold BDS to the lower limit UG during a stopping process ΔtAH until the vehicle comes to a standstill t and during the subsequent operational safety time interval ΔtG.After the service safety time interval ΔtG has expired, the converter module 21 sets the brake pressure threshold BDS to an upper limit (for example, 3 bar). In Fig. 3, at time tstopp, a starting process ΔtAH was determined by the investigation module 19, which lasts until the vehicle comes to a standstill. Furthermore, at time tstopp, the actual brake pressure pist is above the brake pressure threshold BSD, which is set to the lower limit UG. The brake pressure threshold (BDS) is set to the lower limit (UG) and the upper limit (OG) for the following reasons: During the stopping process (ΔtAH) (i.e., after the engine has already stopped), the driver intuitively releases the brake pedal almost completely as soon as the vehicle comes to a standstill (tstand), in order to achieve a smooth stop without any vehicle pitching. The lower limit (UG) during the stopping process (ΔtAH) and during the operational safety time interval (ΔtG) is set so low (for example, 0.1 bar) that even with the almost complete, intuitive release of the brake pedal, the corresponding actual brake pressure (pistn) still exceeds the brake pressure threshold (BDS) (set to its minimum value).This ensures that only when the brake pedal is completely released does the brake pressure threshold BDS fall below the limit and thus the engine restart during the operational safety time interval ΔtG, but an unintentional restart due to excessive brake pedal release is prevented. In Fig. 3, at a time tstart within the operational safety time interval ΔtG, the driver fully releases the brake pedal. This causes the brake pressure threshold BDS, set to the lower limit UG, to fall below the actual brake pressure pist, thus triggering a restart of the engine. Figure 4 shows another start-stop driving operation in which the driver intends to restart the engine during the operational safety time interval ΔtG, specifically at time tstart. In contrast to Figure 3, in Figure 4, after the vehicle comes to a standstill, the driver applies increased pressure to the brake pedal, resulting in a correspondingly higher actual brake pressure pist. This increased pressure on the brake pedal after the vehicle comes to a standstill prevents unintentional rolling during the operational safety time interval ΔtG, such as rolling backward on a slope. If, in this scenario, the driver intends to restart the engine during the operational safety time interval ΔtG, the increased actual brake pressure pist must be reduced by a corresponding release of the brake pedal by the driver by a pressure difference Δp to below the brake pressure threshold BDS in order to trigger the engine start. However, such a reduction of the actual brake pressure pist by the pressure difference Δp inevitably leads to a backward rolling motion of the vehicle, which is perceived as disruptive by the driver. The following describes a start-stop system 15 with reference to Figures 5 and 6, which prevents the uncomfortable rollback of the vehicle: Accordingly, the start-stop system 15 shown in Figure 5 has an evaluation unit 27 in addition to the program modules already explained with reference to Figure 2. The evaluation unit 27 determines whether, during the operational safety time interval ΔtG, there is increased driver-side brake pedal pressure with a correspondingly increased actual brake pressure pist compared to the stopping process ΔtAH or the point at which the vehicle comes to a standstill. If there is no increased actual brake pressure pist, the evaluation unit 27 leaves the brake pressure threshold BDS unchanged at the lower limit value UG (minimum value at, for example, 0.1 bar) during the operational safety time interval ΔtGun.In contrast, if an increased actual brake pressure is present, the evaluation unit 27 raises the brake pressure threshold BDS during the operational safety time interval ΔtG from the lower limit value UG to a safety value SW. In Fig. 6, the safety value SW is set just below the increased actual brake pressure pist. This reduces the pressure differential Δp by which the actual brake pressure pist must be reduced to below the brake pressure threshold BDS in order to initiate a restart of the engine. The safety value SW is set so high that no unintended vehicle roll occurs during the brake pressure reduction via the pressure differential Δp, which correlates with the release of the brake pedal.

Claims

Drive device for a motor vehicle with a start / stop system (15) comprising a control unit (25) which, upon application of the brake pedal by the driver, triggers an engine stop (Sstop) as soon as a stopping process (ΔtAH) occurs and the actual brake pressure (pist) generated by the brake pedal application exceeds a brake pressure threshold (BDS), wherein the control unit (25) triggers a restart of the engine (Sstart) upon application of the brake pedal by the driver as soon as the actual brake pressure (pist) generated by the brake pedal application falls below the brake pressure threshold (BSD) and is at 0, i.e., the brake pedal is fully released, wherein during the stopping process (ΔtAH) until the vehicle comes to a standstill (tstand), the control unit (25) sets the brake pressure threshold (BDS) to a lower limit (UG).so that even with slight brake pedal actuation, the corresponding actual brake pressure (pist) exceeds the brake pressure threshold (BDS) and the control unit (25) triggers the engine stop, wherein the control unit (25) maintains the brake pressure threshold (BDS) unchanged at the lower limit (UG) from the point (tstand) of the vehicle coming to a standstill, and this over a service safety time interval (ΔtG), wherein the control unit (25) sets the brake pressure threshold (BDS) to an upper limit (OG) after the service safety time interval (ΔtG) has elapsed, wherein for a renewed engine start in the service safety time interval (ΔtG), the actual brake pressure (pist) is to be reduced by a pressure difference (Δp) to below the brake pressure threshold (BDS) by corresponding release of the brake pedal by the driver, characterized in that the control unit (25) has an evaluation unit (27) which determines,whether, during the operational safety time interval (ΔtG), there is an increased brake pedal load with a correspondingly increased actual brake pressure (pist) compared to the stopping process (ΔtAH) or the point at which the vehicle comes to a standstill (tstand), and that if there is no increased actual brake pressure (pist), the evaluation unit (27) leaves the brake pressure threshold (BDS) unchanged at the lower limit (UG) during the operational safety time interval (ΔtG), and that if there is an increased actual brake pressure (pist), the evaluation unit (27) raises the brake pressure threshold (BDS) from the lower limit (UG) to a safety value (SW) during the operational safety time interval (ΔtG). Drive device according to claim 1, characterized in that, after the vehicle has come to a standstill, the driver prevents an unintentional rolling movement of the vehicle, in particular a rolling backward on a road slope, by applying increased pressure to the brake pedal. Drive device according to claim 1 or 2, characterized in that by raising the brake pressure threshold (BDS) from the lower limit (UG) to the safety value (SW) the pressure difference (Δp) is reduced by which the actual brake pressure (pist) must be reduced so that the brake pressure threshold (BDS) is not undercut for a new engine start. Drive device according to claim 1, 2 or 3, characterized in that the safety value (SW) to which the brake pressure threshold (BDS) is set is dimensioned such that during the brake pressure reduction correlated with the release of the brake pedal via the pressure difference (Δp) there is no unintentional vehicle rolling movement, i.e. the vehicle remains safely stationary until the engine is restarted. Drive device according to one of the preceding claims, characterized in that the safety value (SW) corresponds to the upper limit (OG), and / or that the safety value (SW) is smaller than the current actual brake pressure (pist) during the operational safety time interval (ΔtG). Drive device according to one of the preceding claims, characterized in that the driver intuitively releases the brake pedal almost completely during the stopping process (ΔtAH) upon the vehicle coming to a standstill (tstand) in order to achieve a comfortable stop without a vehicle pitching motion, and in particular that the lower limit (UG) during the stopping process (ΔtAH) and during the operational safety time interval (ΔtG) is dimensioned so small that even with almost complete release of the brake pedal, the corresponding actual brake pressure (pist) still exceeds the brake pressure threshold (BDS), thereby ensuring that the brake pressure threshold (BDS) is only undershot and thus the engine restarts during the operational safety interval (ΔtG) when the brake pedal is actually completely released.

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