JP5885970B2 - Vehicle drive device - Google Patents

Description

  The present invention relates to a vehicle drive device, and more particularly to a vehicle drive device with improved durability of a friction clutch.

  A clutch device that interrupts transmission of torque is used in the middle of a vehicle power train, generally between an engine and a transmission. As a typical clutch device, there is a friction clutch device in which a driven plate having a friction material (referred to as a facing or lining) on a friction surface is frictionally slid and joined to a driving plate. The friction clutch device is subdivided into a wet type that supplies a cooling liquid such as oil to the friction surface and a dry type that relies on cooling by air. Further, the operation method includes a method driven by a driver depressing a clutch pedal and an automatic method driven by an actuator. In a dry friction clutch device, if the clutch disengagement operation is repeated, the friction material wears, affecting the operating characteristics and long-term durability performance each time.

  A clutch device has been proposed that incorporates means for automatically compensating for or reducing the influence of such friction material wear. For example, the hybrid vehicle drive device disclosed in Patent Literature 1 includes clutch control means for controlling clutch engagement / disengagement based on a stroke value that is an operation amount of the clutch actuator. The clutch control means is characterized in that the stroke value when the clutch is completely engaged is used as a control reference point, the reference point is measured in real time during each engagement operation, and the reference point is corrected. Thereby, it is said that the control accuracy of the engagement / release of the friction clutch is stably ensured regardless of the degree of progress of wear of the clutch facing.

  Patent Document 2 discloses a driving force transmission device that transmits a driving force between both rotating members by friction engagement, and an amorphous structure is provided on one of the sliding surfaces that frictionally engage. The diamond-like carbon thin film is provided. Thereby, it is said that abrasion hardly occurs and durability is improved. The material of the friction material is not limited to Patent Document 2, and various other proposals have been made.

JP 2010-143365 A JP 2006-250364 A

  By the way, although the technique of patent document 1 has an effect which stabilizes the operating characteristic of a friction clutch each time, the wear amount of a friction material cannot be reduced and durability performance is not improved. Moreover, although the technique of patent document 2 can reduce an abrasion amount by selecting the material of the friction material of a friction clutch, the fall of the durable performance by abrasion does not occur at all. In general, the amount of wear and the amount of heat generated by the friction material are greatly affected by the amount of work when the friction clutch is engaged. That is, if the work amount obtained by integrating the product of the clutch torque and the rotational speed difference transmitted during the joining operation over the joining operation time is large, the wear amount and the heat generation amount of the friction material are increased. As is well known, the amount of work of the clutch is large due to the joining operation at the time of vehicle start, and the amount of work is particularly large at the time of sudden start when the accelerator pedal is greatly depressed. In this case, a large amount of heat energy is generated until the synchronous rotation of the driving side member and the driven side member of the friction clutch is achieved and the lockup state is reached, which may affect the durability performance. If it is high, the risk increases.

  The present invention has been made in view of the above-mentioned problems of the background art, and restricts the work of the friction clutch as necessary when starting the vehicle to suppress the generation of heat energy, thereby improving the durability performance of the friction clutch. It is another object of the present invention to provide an improved vehicle drive device.

The vehicle drive device of the present invention includes an engine, a drive-side member that is rotationally connected to an output shaft of the engine, a driven-side member that is rotationally connected to a drive wheel and frictionally connects and separates from the drive-side member, A friction clutch that has an actuator for driving frictional engagement and separation between the side member and the driven side member, and that interrupts transmission of torque from the driving side member to the driven side member, and an operation amount of an accelerator pedal A target value of the output speed of the output shaft of the engine is set based on the state of the vehicle including the engine, the engine is controlled so that the actual output speed matches the target value of the output speed, and And a controller for controlling the actuator of the friction clutch, further comprising clutch temperature detection means for actually measuring or estimating the temperature of the friction clutch, Parts are have a start-time control means for setting a target value of the output speed on the basis of the operation amount and the friction clutch of the accelerator pedal temperature when the vehicle starts, the vehicle start control means, said friction clutch The target value of the output speed of the output shaft of the engine is set so that the temperature of the engine falls within a preset upper limit allowable temperature that does not affect the durability performance of the friction clutch. The friction clutch within a lock-up time between the target value of the output speed of the output shaft of the engine and the drive side member and the driven side member of the friction clutch reaching synchronous rotation after the accelerator pedal is operated. And the relationship between the amount of work and the temperature increase of the friction clutch, and the temperature increase to the temperature of the friction clutch when the vehicle starts. The to fit to the even upper limit allowable temperature addition, sets the output speed of the target value of the output shaft of said engine based on said two relationships.

  Further, it is preferable that the start time control means sets the target value of the output speed of the output shaft of the engine to be lower as the temperature of the friction clutch at the start of the vehicle is higher.

  Further, the engine may have a throttle valve for adjusting an intake air amount, and the control unit may adjust the opening degree of the throttle valve to control the actual output rotational speed.

The vehicle drive device of the present invention further includes clutch temperature detection means for actually measuring or estimating the temperature of the friction clutch, in addition to the engine, the friction clutch, and the control unit, and the control unit includes an operation amount of an accelerator pedal when starting the vehicle, There is a start time control means for setting a target value of the output speed of the output shaft of the engine (hereinafter abbreviated as engine output speed) based on the temperature of the friction clutch. That is, the temperature of the friction clutch, which has not been considered before, is taken into account when controlling the engine output speed when the vehicle starts. Therefore, when the amount of operation of the accelerator pedal is large and the temperature of the friction clutch is high when the vehicle starts, the target value of the engine output speed can be set low to limit the work of the friction clutch, and the generation of thermal energy can be suppressed. . Also, when the accelerator pedal operation amount is small or the friction clutch temperature is sufficiently low even when the accelerator pedal operation amount is large, the engine output corresponding to the accelerator pedal operation amount is not limited without limiting the work amount of the friction clutch. A target value for the number of revolutions is set, and at this time, a starting operation can be performed that meets the driver's request.
In addition, the target value of the engine output speed is set so that the temperature of the friction clutch falls within the preset upper limit allowable temperature that does not affect the durability of the friction clutch. Can be kept in. In addition, grasp the relationship between the target value of engine output speed when the vehicle starts and the amount of work done by the friction clutch, and the relationship between the amount of work and the temperature rise of the friction clutch, and the upper limit of the friction clutch temperature. Since the target value of the engine output rotational speed is set so as to be within the allowable temperature, the amount of work and the temperature increase performed by the friction clutch can be obtained with high accuracy. Therefore, the temperature of the friction clutch can be maintained with high accuracy within the upper limit allowable temperature, and the durability performance is improved more reliably than before.

  Further, in the aspect in which the target value of the engine output rotational speed is set lower as the temperature of the friction clutch is higher, the heat energy generated in the friction clutch can be appropriately adjusted when the vehicle starts, so that the temperature of the friction clutch does not increase excessively. , Durability performance is improved than before.

  Further, in the aspect in which the actual output speed is controlled by adjusting the opening of the throttle valve of the engine, the configuration of the vehicle drive device itself does not need to be changed, and the durability of the friction clutch can be improved only by a simple control change. Performance is improved than before.

It is a figure showing typically the vehicle drive device of the embodiment of the present invention. It is a figure which illustrates typically the function of the starting control means. It is a flowchart explaining the starting control flow of the control part containing a starting time control means. It is a figure explaining the starting operation | movement in the R point of FIG. 2 which is an example when the temperature of a friction clutch is low. It is a figure explaining the starting operation | movement in the P point of FIG. 2 which is an example when the temperature of a friction clutch is high.

  An embodiment for carrying out the present invention will be described with reference to FIGS. FIG. 1 is a diagram schematically illustrating a vehicle drive device 1 according to an embodiment of the present invention. Broken arrows in the figure indicate the flow of information and control. The vehicle drive device 1 according to the embodiment includes an engine 2, a friction clutch 6, a control unit 7, and the like.

  The engine 2 is mounted on a vehicle and disposed in an unillustrated engine room, and an engine 2 having a general method and structure can be used. The engine 2 has an output shaft 3, a throttle valve 4, and an intake air temperature sensor 5. The output shaft 3 rotates integrally with a crankshaft that is driven to rotate by a piston and outputs torque. The throttle valve 4 is disposed in the course of taking air in the engine room into the engine, and its opening degree is variably controlled by the control unit 7. When the throttle valve 4 is opened widely and the intake air amount increases, the amount of the air-fuel mixture containing fuel increases, and the output rotation speed and output torque of the output shaft 3 increase. The intake air temperature sensor 5 is a sensor that measures the temperature of air taken into the engine. In the present embodiment, the intake air temperature sensor 5 also serves as clutch temperature detection means for estimating the temperature of the friction clutch 6.

  The friction clutch 6 includes a driving side member 61, a driven side member 62, and an actuator 63, and interrupts transmission of torque from the driving side member 61 to the driven side member 62. The drive side member 61 is rotationally connected to the output shaft 3 of the engine 2. The driven member 62 is rotationally connected to the input shaft 81 of the transmission 8, and is further rotationally connected to the left and right drive wheels via a transmission output shaft and a differential device (not shown). A friction material (facing) is provided on a surface of the driven side member 62 facing the driving side member 61 so as to be frictionally connected and separated from the driving side member 61. The actuator 63 is a part that drives frictional engagement and separation between the driving side member 61 and the driven side member 62, and can be configured using a servo motor, a hydraulic operation mechanism, or the like.

  Here, the friction clutch 6 is disposed in the engine room together with the engine 2. Therefore, since the temperature of the friction clutch 6 substantially matches the air temperature in the engine room, the intake air temperature sensor 5 can also serve as the clutch temperature detecting means. In particular, when starting after the vehicle has been stopped for a certain period of time, the temperature in the engine room is made uniform, and therefore the temperature of the friction clutch 6 is accurately determined using the intake air temperature of the intake air temperature sensor 5. Can be estimated. Note that the temperature of the friction clutch 6 may be estimated by correcting the intake air temperature of the intake air temperature sensor 5 as necessary. Further, a dedicated temperature sensor may be provided in the vicinity of the friction clutch 6 so that the temperature of the friction clutch 6 is measured.

  The control unit 7 is an electronic control unit (ECU) that incorporates a microcomputer and operates by software. The control unit 7 acquires a detection signal from the intake air temperature sensor 5 of the engine 2 to estimate the temperature of the friction clutch 6, and acquires a detection signal of the operation amount of the accelerator pedal from the accelerator sensor 9. Then, the control unit 7 sets a target value of the output speed (engine output speed) of the output shaft 3 of the engine 2 based on the state of the vehicle including the operation amount of the accelerator pedal, and sets the actual value to this target value. The opening degree of the throttle valve 4 is variably controlled so that the output rotational speeds coincide. As an index indicating the state of the vehicle, in addition to the operation amount of the accelerator pedal, the vehicle speed, the speed selected by the transmission 8, the operation amount of the brake pedal, the steering operation amount of the steering wheel, and the like are referred to as appropriate. The control unit 7 controls the actuator 63 of the friction clutch 6 to control the joining operation and the cutting operation.

  Further, the control unit 7 has a start time control means 71 that sets a target value of the engine output speed based on the operation amount of the accelerator pedal and the temperature of the friction clutch when the vehicle starts. The starting time control means 71 is realized by software of the control unit 7. FIG. 2 is a diagram schematically illustrating the function of the starting control means 71. The horizontal axis in FIG. 2 is the accelerator opening degree Ac that indicates the operation amount of the accelerator pedal as a ratio, and the vertical axis is the target value Ne of the engine output speed. Also, the three curves C1 to C3 in the figure illustrate the setting curves C1 to C3 of the target value Ne using the temperature T of the friction clutch 6 as a parameter.

  As illustrated in FIG. 2, the target value Ne of the engine output speed is set based on the accelerator opening Ac on the horizontal axis and the temperature T of the friction clutch as a parameter. Qualitatively, the target value Ne of the engine output speed is set to be higher as the accelerator opening Ac at the time of starting the vehicle is larger, and the target value Ne of the engine output speed is set to be higher as the temperature T of the friction clutch at the time of starting the vehicle is higher. Set low. The starting time control means 71 holds a map in the form of a list of target values Ne using the accelerator opening Ac and the temperature T of the friction clutch 6 as parameters, and sets the target value Ne using this map.

  More specifically, when the accelerator opening degree Ac exceeds the accelerator opening degree Ac1 corresponding to the play size when the temperature T1 of the friction clutch is high, the target value Ne of the engine output speed of the setting curve C1 is steep from the idle speed Nei. Begin to increase. As the accelerator opening degree Ac increases, the target value Ne tends to increase, becomes almost saturated to the target value Ne1 at the accelerator opening degree Ac2, and does not change significantly thereafter, and becomes the target value Ne1 at the accelerator opening degree Ac3 (point P in the figure). ). Setting so as to be saturated at the target value Ne1 means that the work amount of the friction clutch 6 is limited. The purpose of this control is to keep the temperature T of the friction clutch 6 within an upper limit allowable temperature Tmax that does not affect the durability performance, as will be described in detail later.

  When the temperature T2 of the friction clutch is medium and the accelerator opening degree Ac1 is exceeded, the target value Ne of the engine output speed of the setting curve C2 starts to increase from the idle speed Nei more steeply than the setting curve C1. It is located above the curve C1. As the accelerator opening degree Ac increases, the target value Ne tends to increase, but gradually increases to the accelerator opening degree Ac3 and reaches the target value Ne2 (Ne2> Ne1) (point Q in the figure).

  When the temperature T3 of the friction clutch is low and the accelerator opening degree Ac1 is exceeded, the target value Ne of the engine output speed of the setting curve C3 starts to increase from the idle speed Nei more steeply than the setting curves C1 and C2, and is set. Located above the curves C1 and C2. As the accelerator opening degree Ac increases, the target value Ne tends to increase, but gradually increases to the accelerator opening degree Ac3 and reaches the target value Ne3 (Ne3> Ne2) (point R in the figure). This target value Ne3 is a value commensurate with the driver's request corresponding to the operation amount of the accelerator pedal, and is a value set in the prior art.

  That is, in the prior art, the target value Ne of the output rotation speed is treated as a function of the accelerator opening degree Ac without considering the temperature T of the friction clutch, and only the setting curve C3 is used exclusively. On the other hand, in this embodiment, when the temperature T1 and T2 of the friction clutch at the start of the vehicle is high to medium, the setting curves C1 and C2 are used, and the target value Ne1 of the engine output speed at the accelerator opening Ac3, Ne2 is set lower than the conventional target value Ne3.

  Next, a procedure for setting the target value Ne of the engine output speed will be described. FIG. 3 is a flowchart illustrating a start control flow of the control unit 7 including the start time control means 71. In step S1 in the figure, when the engine 2 is started and the first speed gear stage is selected by the transmission 8, preparation for starting is completed. Next, the start time control means 71 estimates the temperature T of the friction clutch 6 in step S2, and acquires the accelerator opening Ac indicating the operation amount of the accelerator pedal in step S3. Next, at step 4, the target value Ne of the engine output speed is set using the aforementioned map.

  Next, the actual start operation is controlled. In step S5, the control unit 7 controls the opening degree of the throttle valve 4 of the engine 2 to increase the actual output rotational speed. In parallel with this, the control unit 7 jointly controls the actuator 63 of the friction clutch 6 to increase the transmitted clutch torque Tc. As a result, the rotational speed difference ΔN between the driving side member 61 and the driven side member 62 of the friction clutch 6 gradually decreases, and the rotational speed difference ΔN disappears in step S6 to enter a lock-up state. Thereafter, the routine shifts to normal shift control.

  Next, the starting operation and action of the vehicle drive device 1 of the embodiment will be described. FIG. 4 is a diagram for explaining the starting operation at the point R in FIG. 2 as an example when the temperature T3 of the friction clutch 6 is low. FIG. 5 is a diagram for explaining the starting operation at the point P in FIG. 2, which is an example when the temperature T1 of the friction clutch 6 is high. 4 and 5, the horizontal axis indicates the common time t, the upper graph indicates the clutch torque Tc transmitted by the friction clutch 6, and the lower graph indicates the actual output of the output shaft 3 of the engine 2. The rotational speed Nout (that is, the rotational speed of the drive side member 61 of the friction clutch 6) and the input rotational speed Nin of the input shaft 81 of the transmission 8 (that is, the rotational speed of the driven side member 62 of the friction clutch 6) are shown.

  In FIG. 4 when the temperature T3 of the friction clutch 6 is low, when the start operation is started at time t0, the output rotational speed Nout of the engine 2 is set to the target value from the idle rotational speed Nei as shown in the lower graph. It increases sharply toward Ne3. On the other hand, the input rotational speed Nin of the transmission 8 gradually increases from 0 due to the frictional sliding action of the friction clutch 6. Then, as time elapses, the rotational speed difference ΔN (Nout−Nin) between the output rotational speed Nout and the input rotational speed Nin decreases (shown by hatching for convenience). Eventually, at time t3, the rotational speed difference ΔN becomes a lock-up state of 0, and both the output rotational speed Nout and the input rotational speed Nin reach the target value Ne3, and the start control ends.

  On the other hand, as shown in the upper graph, the clutch torque Tc (shown by hatching for convenience) transmitted by the friction clutch 6 gradually increases from 0 after time t0, and the clutch torque at time t3. Tc3 is reached. After the time t3, normal shift control is performed, the output rotation speed Nout and the input rotation speed Nin increase in synchronization, the clutch torque Tc further increases from Tc3, and the vehicle is accelerated.

  Here, the friction clutch 6 performs work corresponding to the product of the clutch torque Tc and the rotational speed difference ΔN. Further, when this work is integrated over the joint operation time tj3 from time t0 to time t3, the work amount performed by the friction clutch 6 can be obtained. This work amount is as large as before, the wear amount and the heat generation amount of the friction material of the friction clutch 6 are increased, and the temperature rise ΔT3 is also increased. However, since the initial temperature T3 of the friction clutch 6 at time t0 is low, even if a large temperature increase ΔT3 is added to the initial temperature T3, it remains within the upper limit allowable temperature Tmax. The upper limit allowable temperature Tmax is an upper limit temperature that does not affect the durability performance of the friction clutch 6, and can be set in advance in consideration of the thermal characteristics of the constituent members.

  Further, in FIG. 5 when the temperature T1 of the friction clutch 6 is high, when the start operation is started at time t0, the output rotational speed Nout of the engine 2 is determined from the idle rotational speed Nei as shown in the lower graph. It increases sharply toward the target value Ne1. On the other hand, the input rotational speed Nin of the transmission 8 gradually increases from 0 due to the frictional sliding action of the friction clutch 6. Then, as time elapses, the rotational speed difference ΔN (Nout−Nin) between the output rotational speed Nout and the input rotational speed Nin decreases (shown by hatching for convenience). Here, the target value Ne1 of the engine output speed is set lower than the target value Ne3 at the low temperature T3. Therefore, the lockup state is reached at time t1 when the joint operation time tj1 shorter than the joint operation time tj3 at the low temperature T3 has elapsed, and both the output rotation speed Nout and the input rotation speed Nin reach the target value Ne1.

  On the other hand, as shown in the upper graph, the clutch torque Tc (shown by hatching for convenience) of the friction clutch 6 gradually increases from 0 after time t0, and is lower than that at low temperature T3 at time t1. A small clutch torque Tc1 is reached. After the time t1, the output rotation speed Nout and the input rotation speed Nin increase in synchronization and reach the target value Ne3 of the low temperature T3 at the time t4. Further, after time t1, the clutch torque Tc is further increased from Tc1, and the vehicle is accelerated.

  Here, the work performed by the friction clutch 6 at the high temperature T1 and the amount of work to be performed can be obtained in the same manner as at the low temperature T3. The work and the work amount at the high temperature T1 are limited to be smaller than those at the low temperature of FIG. 4 and the prior art, the wear amount and the heat generation amount of the friction material of the friction clutch 6 are reduced, and the temperature increase ΔT1 is also reduced. However, since the initial temperature T1 of the friction clutch 6 at time t1 is high, when the small temperature increase ΔT1 is added to the initial temperature T1, the temperature approaches the upper limit allowable temperature Tmax. That is, if the target value Ne1 of the engine output speed is set higher than this, the durability performance of the friction clutch 6 may be affected.

  For the purpose of avoiding this possibility, the target value Ne of the engine output speed is set to be saturated to the target value Ne1 after the accelerator opening Ac2 of the setting curve C1 of FIG.

  Further, as described above, there is a specific causal relationship between the target value Ne of the engine output speed at the time of starting the vehicle and the amount of work performed by the friction clutch 4. Furthermore, there is a specific causal relationship between the amount of work performed by the friction clutch 4 and the temperature rise. The two causal relationships can be grasped by experiments, sample surveys, simulations, theoretical analysis, and the like. Although the description will be mixed, the above-described map for setting the output rotation speed Ne of the engine 2 is created in consideration of the two causal relationships grasped.

  According to the vehicle drive device 1 of the embodiment described above, the starting control means 71 sets the target value Ne of the engine output rotational speed to be lower as the initial temperature T1 of the friction clutch 6 is higher, so that the friction clutch 4 The temperature is set within the upper limit allowable temperature Tmax that does not affect the durability performance. Therefore, the temperature of the friction clutch 6 can be maintained with high accuracy within the upper limit allowable temperature Tmax, and the durability performance is improved more reliably than before.

  Further, the starting control means 71 limits the amount of work of the friction clutch 6 when the accelerator pedal operation amount is small when the vehicle starts or when the temperature of the friction clutch 6 is sufficiently low even if the accelerator pedal operation amount is large. First, a target value of the engine output speed corresponding to the amount of operation of the accelerator pedal is set, and at this time, a start operation that meets the driver's request can be performed.

  Furthermore, since the actual output speed Nout is controlled by adjusting the opening degree of the throttle valve 4 of the engine 2, the configuration of the vehicle drive device 1 does not need to be changed from the conventional one, and the friction can be achieved only by a simple control change. The durability performance of the clutch 6 is improved as compared with the prior art.

  In addition, you may comprise so that the function of the control part 7 and the start time control means 71 of this invention may be fulfill | performed by a coordinated control with a some electronic control apparatus (ECU). Further, since more clutch torque Tc is required when starting on a slope than when starting on a flat ground, the setting curves C1 to C3 may be corrected as appropriate. In addition, the present invention can be variously applied and modified.

1: Vehicle drive device 2: Engine 3: Output shaft 4: Throttle valve 5: Intake air temperature sensor (also serves as clutch temperature detection means)
6: Friction clutch
61: Drive side member 62: Driven side member 63: Actuator 7: Control unit 71: Control unit at start 8: Transmission 81: Input shaft 9: Accelerator sensor Ac, Ac1, Ac2, Ac3: Accelerator opening T, T1, T2, T3: Friction clutch temperature Ne, Ne1, Ne3: Target value of engine output speed Nei: Idle speed C1, C2, C3: Setting curve Nout: Actual output speed of engine Nin: Input speed of transmission Number ΔN: Speed difference (= Nout−Nin)
tj1, tj3: Joint operation time

Claims (3)

Engine,
A drive-side member that is rotationally connected to the output shaft of the engine; a driven-side member that is rotationally connected to a drive wheel and frictionally engages and separates from the drive-side member; and a friction-joint between the drive-side member and the driven-side member. A friction clutch that includes an actuator that drives engagement and separation, and that interrupts transmission of torque from the driving side member to the driven side member;
A target value for the output speed of the output shaft of the engine is set based on the state of the vehicle including the amount of operation of the accelerator pedal, and the engine is set so that the actual output speed matches the target value for the output speed. A vehicle drive apparatus comprising: a control unit that controls and controls the actuator of the friction clutch;
Clutch temperature detecting means for actually measuring or estimating the temperature of the friction clutch;
The control unit may have a starting time control means for setting a target value of the output speed based on the temperature of the operation amount and the friction clutch of the accelerator pedal when the vehicle starts,
The starting time control means includes:
A target value of the output speed of the output shaft of the engine is set so that the temperature of the friction clutch falls within a preset upper limit allowable temperature that does not affect the durability performance of the friction clutch;
The target value of the output rotation speed of the output shaft of the engine when the vehicle starts, and the lock-up time from when the accelerator pedal is operated until the drive side member and the driven side member of the friction clutch reach synchronous rotation The relationship between the amount of work done by the friction clutch, and the relationship between the amount of work and the temperature increase of the friction clutch,
A target value for the output speed of the output shaft of the engine is set based on the two relations so that the temperature rise is within the upper limit allowable temperature even if the temperature increase is added to the temperature of the friction clutch when the vehicle starts. A vehicle drive device.   2. The vehicle drive device according to claim 1, wherein the start time control unit sets a target value of an output rotation speed of the output shaft of the engine to be lower as a temperature of the friction clutch at the time of vehicle start is higher. 3. The vehicle drive device according to claim 1 , wherein the engine includes a throttle valve that adjusts an intake air amount, and the control unit adjusts an opening degree of the throttle valve to control the actual output rotational speed.

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