Dual clutch transmission control method, dual clutch transmission, and vehicle mounted therewith

A method of a dual clutch transmission, a dual clutch transmission, and a vehicle equipped with the same which can reduce the load on a clutch on a start gear side to reduce the wear thereof and therefore make the clutch replacement interval longer. There are a first input shaft configured to be connected to a first clutch and a second input shaft configured to be connected to a second clutch. A set of odd-numbered gears and a set of even-numbered gears are arranged respectively across the first input shaft and second input shaft and an output shaft. When a vehicle starts, an absorbed energy Eabs by the second clutch is calculated while a start gear is partially connected to the second input shaft. The first clutch is partially connected (half clutch state) to the first input shaft, to which a support gear is synchronously engaged, when the absorbed energy exceeds a set value which is a predetermined threshold, the support gear having a gear ratio greater than that of the start gear by one speed or higher.

This application claims the benefit under U.S.C. Section 371, of PCT International Application No. PCT/JP2012/066705, filed Jun. 29, 2012 and Japanese Application No. 2011-148159 filed Jul. 4, 2011, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of controlling a dual clutch transmission, a dual clutch transmission, and a vehicle equipped with the same which reduce the load on both clutches and reduce the wear thereof to thereby improve the durability thereof, the dual clutch transmission including at least two input shafts and two clutches to smoothen gear shift operations.

BACKGROUND ART

Heretofore, a dual clutch transmission (hereinafter, referred to as the DCT) including two clutch systems has been developed for improvement in the gear shift time of an automated manual transmission (hereinafter, referred to as the AMT). The DCT generally includes a clutch for each of an even-numbered gear set and an odd-numbered gear set and shifts by switching the clutches. For this reason, a gear shift operation of an odd-numbered gear (or an even-numbered gear) can be performed while an even-numbered gear (or an odd-numbered gear) is being used. This DCT allows a quick gear shift with no gear shift time lag. Moreover, since the DCT transmits power by clutches, it has a simple structure and the power loss is small, or the transmission efficiency is good, which leads to less fuel consumption.

Here, the conventional DCT will be described with reference toFIGS. 7 and 8. As shown inFIG. 7, a DCT1X includes a first input shaft11, a second input shaft12, a first clutch C1, a second clutch C2, a countershaft13, gears G1to G6, a gear GR, coupling sleeves S1to S3, and a coupling sleeve SR.

The power of an engine (internal combustion engine) is received from a crankshaft2through the first clutch C1or the second clutch C2, and that power is transmitted to an output shaft3after its speed is changed at one of the gears.

The second input shaft12is formed in a hollow shape, and the first input shaft11is coaxially inserted in the second input shaft12. The gears G1, G3, G5, and GR are arranged on the first input shaft11, and the gears G2, G4, and G6are arranged on the second input shaft. The power can be transmitted by connecting the first clutch C1to the first input shaft11or the second clutch C2to the second input shaft, and synchronously engaging one of the coupling sleeves S1to SR provided on the countershaft13to one of the gears G1to GR.

The clutch C1includes a flywheel C1a, a clutch cover C1b, a release bearing C1c, a diaphragm spring C1d, a pressure plate C1e, and a clutch disk C1fformed of a lining, a torsion damper, a thrust, and the like. The clutch C2has a similar configuration as well.

As shown inFIG. 8, the DCT1X described above further includes an electronic control unit (“ECU”)20, a clutch operation mechanism21which operates the clutch C1or the clutch C2, and a synchronous engagement mechanism22which operates the coupling sleeves S1to SR. Hydraulic pistons or the like can be used for the clutch operation mechanism21and the synchronous engagement mechanism22.

Next, the operation of this DCT1X during start will be described. This DCT1X uses the gear G1as a start gear DG1. When the vehicle stops travelling and the engine stops, the ECU20disconnects the first clutch C1and the second clutch C2and synchronously engages the coupling sleeve S1to the start gear DG1. When the vehicle starts, the ECU20connects the first clutch C1to the first input shaft11. Circular arrows inFIG. 8illustrate the transmission of power during this state.

Then, the ECU20synchronously engages the coupling sleeve S2to the gear G2so that smooth acceleration will be performed. In this way, in the case of a shift from the start gear DG1to the gear G2, the first clutch C1is disconnected (hereinafter, expressed as being fully disconnected), and the second clutch C2is connected to the second input shaft12(hereinafter, expressed as being fully connected). Since the connection can be switched back and forth as described above, gear shift operations can be done smoothly.

Here, as described above, the DCT normally uses a predetermined gear such as the first gear or the second gear for start. Thus, the clutch to be used for start is either the one for the odd-numbered gears or the one for the even-numbered gears. Such a clutch is subjected to high load when brought into a connected state during start and wears accordingly. Thus, one of the clutches, the one for the odd-number gears or the one for the even-numbered gears, wears faster.

A clutch of a sufficiently large volume may be used to prevent this clutch wear. It is, however, difficult to secure a sufficiently large volume in the case of a DCT with two clutches housed in a small space. Meanwhile, there are devices employing a method that involves switching the start gear based on the worn states of the clutches, a start condition, etc. (see Patent Document 1 and Patent Document 2, for example). These devices can make the wear of the clutches even by selecting the appropriate start gear based on the worn states of the clutches. This, however, leads to a problem of changing the feel during start, which impairs the driving comfort of the vehicle.

PRIOR ART DOCUMENTS

Patent Document 1: Japanese patent application Kokai publication No. 2006-132562

Patent Document 2: Japanese patent application Kokai publication No. 2008-309325

SUMMARY OF THE INVENTION

To solve the above problem, the inventor has invented a method of controlling a dual clutch transmission that involves bringing both clutches into a half clutch state at the moment of start so as to reduce the load on the clutch on the start gear side and thereby reduce the wear thereof. This method is a method that sets multiple torque paths by connecting also the clutch other than the clutch linked to the start gear so as to increase the clutch volume.

By using this method, the clutch load is distributed. Thus, the durability of the clutch on the start gear side can be expected to improve. However, since the clutch other than the clutch on the start gear side is used in a half clutch state with a relatively large rotational speed difference, its wear may possibly increase. For this reason, it cannot be simply said that starting by using both clutches is always favorable.

The present invention has been made in view of the above problem, and an object thereof is to provide a method of controlling a dual clutch transmission, a dual clutch transmission, and a vehicle equipped with the same which can reduce the load on one of clutches to suppress the occurrence of wear of only the one clutch and therefore make the clutch replacement interval longer, and which can also reduce increase in the wear of the other clutch that occurs due to use of both clutches, without requiring any additional component and also without changing the feel during start.

A method of controlling a dual clutch transmission for achieving the above-described object is a method of controlling a dual clutch transmission a method of controlling a dual clutch transmission which includes at least a first input shaft configured to be connected to a first clutch and a second input shaft configured to be connected to a second clutch, and in which a set of odd-numbered gears and a set of even-numbered gears are arranged every one step for alternation respectively between the first the second input shafts and an output shaft, and in a case of starting transmission of power from a power source to the output shaft, the transmission of the power is started by synchronously engaging a start gear being one of the gears for start to the second input shaft and connecting the second clutch to the second input shaft, characterized in that the method comprises: calculating an absorbed energy by the second clutch while partially connecting the second clutch to the second input shaft, to which the start gear is synchronously engaged, after starting the transmission of the power from the power source to the output shaft; and partially connecting the first clutch to the first input shaft, to which a support gear is synchronously engaged, when the absorbed energy exceeds a predetermined threshold, the support gear having a gear ratio smaller than that of the start gear by one speed or higher or having a gear ratio greater than that of the start gear by one speed or higher.

By using both clutches at the moment when the vehicle starts, the wear of the clutch on the start gear side can be reduced. Thus, the clutch replacement interval can be made longer. On the other hand, since the clutch on the side other than the start gear side is used in a half clutch state with a relative large rotation speed difference, its wear may possibly increase.

According to this method, the absorbed energy by the clutch on the start gear side is calculated, and that absorbed energy is compared with a set value which is the predetermined threshold. The clutch on the support gear side is used when the absorbed energy exceeds the predetermined threshold. Thus, the wear of the clutch on the support gear side that occurs due to the use of both clutches can be reduced.

The energy absorbed by the clutch is calculated by the following formulae 1 and 2 with the torque transmitted from the power source (internal combustion engine) and the value of the difference between the rotational speed inputted to the clutch on the start gear side and the rotational speed outputted from the clutch on the start gear side. Here, the rotational speed inputted to the clutch on the start gear side is Nin (rpm), the rotational speed outputted from the clutch on the start gear side is Nout, the torque transmitted from the engine is T (Nm), the absorbed energy is Eabs (J), and the power loss is L (W).

It takes time before the clutch is fully connected when it is used in a half clutch state for a relative long period of time during start, particularly when the vehicle is on a hill or heavily load. As a result, the energy absorbed by the clutch increases. In this case, the other clutch is brought into a half clutch state so that part of the engine torque can be transmitted to the other clutch. In this way, the wear of both clutches can be reduced, and therefore the durability of both clutches can be improved.

On the other hand, in the cases other than the above-described case, only the clutch on the start gear side is used for starting, so that the wear of the clutch on the support gear side can be reduced. In these cases, the load on the clutch on the start gear side is small, and there is no need to use both clutches.

Moreover, the above-described method of controlling a dual clutch transmission further comprises synchronously engaging the start gear and the support gear to the first input shaft and the second input shaft, respectively, when the transmission of the power from the power source to the output shaft stops. According to this method, the above operations and effects can be achieved by simply switching the clutches after start.

A dual clutch transmission for achieving the above-described object is a dual clutch transmission which includes at least a first input shaft configured to be connected to a first clutch and a second input shaft configured to be connected to a second clutch, and in which a set of odd-numbered gears and a set of even-numbered gears are arranged every one step for alternation respectively between the first and the second input shafts and an output shaft, and in a case of starting transmission of power from a power source to the output shaft, the transmission of the power is started by synchronously engaging a start gear being one of the gears for start to the second input shaft and connecting the second clutch to the second input shaft, characterized in that the dual clutch transmission comprises a support gear and a control device, the support gear having a gear ratio greater than that of the start gear by one speed or higher, or smaller than that of the start gear by one speed or higher, and the control device includes a control of calculating an absorbed energy by the second clutch while partially connecting the second clutch to the second input shaft, to which the start gear is synchronously engaged, after starting the transmission of the power from the power source to the output shaft, and a control of partially connecting the first clutch to the first input shaft, to which the support gear is synchronously engaged, when the absorbed energy exceeds a predetermined threshold.

According to these configurations, the above-described effects can be achieved without adding any component to a conventional dual clutch transmission. Thus, the cost can be reduced.

Moreover, the above-described dual clutch transmission further comprises an input-rotational-speed sensor configured to detect a rotational speed inputted to the second clutch, and an output-rotational-speed sensor configured to detect a rotational speed outputted from the second clutch, and the control device further includes a control of calculating torque transmitted from the power source, and a control of calculating the absorbed energy based on the torque and the value of a difference between the rotational speed inputted to the second clutch and the rotational speed outputted from the second clutch.

According to this configuration, the absorbed energy can be calculated by calculating the torque transmitted from the engine (power source) and assigning that torque and the input rotational speed detected by the input-rotational-speed sensor and the output rotational speed detected by the output-rotational-speed sensor into the formulae 1 and 2 mentioned above. In this way, whether to start by using both clutches or to start by using only one of the clutches can be determined.

In addition, in the above-described dual clutch transmission, the control device further includes a control of synchronously engaging the start gear and the support gear to the first input shaft and the second input shaft, respectively, when the transmission of the power from the power source to the output shaft stops. According to this configuration, the above-described operations and effects can be achieved by simply switching the clutches.

A vehicle for achieving the above-described object is equipped with the above-described dual clutch transmission. According to this configuration, the wear of the clutches can be made even, and the feel during start does not change. Thus, a vehicle with good driving comfort can be provided.

According to the present invention, it is possible to reduce the load on one of clutches to suppress the occurrence of wear of only the one clutch and therefore make the clutch replacement interval longer, and also to reduce increase in the wear of the other clutch that occurs due to use of both clutches, without requiring any additional component and also without changing the feel during start.

DESCRIPTION OF THE EMBODIMENTS

Hereinbelow, methods of controlling a dual clutch transmission, dual clutch transmissions, and vehicles equipped with the same of first and second embodiments according to the present invention will be described with reference to the drawings. Note that the same components and operations as those of the conventional dual clutch transmission (DCT)1X shown inFIGS. 7 and 8will be denoted by the same reference numerals, and description thereof will be omitted. Moreover, while the first and second embodiments according to the present invention will be illustrated by using a six-speed DCT, the number of gears is not limited and the DCTs of the present invention may be an eight-speed DCT, for example.

As shown inFIG. 1, a dual clutch transmission (hereinafter, referred to as the DCT)1of the first embodiment according to the present invention, includes a first input shaft11, a second input shaft12, a first clutch C1, a second clutch C2, a countershaft13, gears G1to G6, a gear GR, coupling sleeves S1to S3, a coupling sleeve SR, an ECU (control device)20, a clutch operation mechanism21, and a synchronous engagement mechanism22, and uses a configuration similar to that of the conventional DCT X1shown inFIG. 8. However, as shown inFIG. 1, the configuration is different from that of the conventional DCT X1in that: the gear G2and the gear G3serve as a start gear DG2, and a support gear SG3, respectively; the clutch operation mechanism21is configured to be capable of simultaneously operating both clutches C1and C2; and a second-clutch input-rotational-speed sensor23and a second-clutch output-rotational-speed sensor24are added.

This DCT1is not limited to the above configuration as long as it is an automated manual transmission, and no limitation is imposed on the arrangement of both input shafts, the numbers of the input shafts and the clutches mounted, the number of the gears, and so on. For example, it is possible to employ a configuration in which both input shafts are disposed not coaxially but in parallel with each other with the countershaft disposed between the input shafts. Moreover, the present invention may be applied to a triple clutch transmission including three clutches. Thus, as long as the DCT is a conventional one, no additional component needs to be particularly added. Accordingly, the cost can be reduced.

In the above configuration, the gear G2is the start gear DG2, and the gear G3which synchronously engages with the first input shaft11at a gear ratio higher than the start gear DG2by one speed is the support gear SG3. This start gear may be set to any gear as long as it is the gear G1(first speed) or higher. For example, when the gear G1is the start gear, the gear G2is set as the support gear.

The ECU20is configured to control the whole power plant including the transmission through electric circuits. The ECU20also controls the engine and is a microcontroller which performs total electrical control. In the case of an automatic transmission vehicle, the ECU20stores therein optimal control values for all the possible traveling states, and controls the mechanisms by causing sensors to detect the current state and selecting the optimal values from the stored data based input signals from the sensors.

This ECU20controls the connection of the first clutch C1to the first input shaft11and the connection of the second clutch C2to the second input shaft12independently and simultaneously. Moreover, the ECU20can also control each of the first clutch C1and the second clutch C2to bring them into a half clutch (partially connected) state. The half clutch state refers to a state where the clutch is not fully engaged. In this state, the drive power from the engine can be adjusted and transmitted to power transmission systems such as the transmission, transfer case, and differential gear. Thus, the drive power can be transmitted to the wheel(s) even during a low-speed traveling state where the traveling speed of the vehicle does not match the engine speed or during a stopped state.

In addition, the ECU20also performs control which brings the gears G1to GR into synchronous engagement with the first input shaft11and the second input shaft12through the coupling sleeves S1to SR. This control can bring the odd-numbered gear G1, G3, or G5into synchronous engagement while the even-numbered gear G2, G4, or G6is being used, for example, so as to achieve a smooth gear shift operation.

Further, the ECU20calculates an engine torque T. Furthermore, the ECU20calculates an absorbed energy Eabs by using the aforementioned formulae with the engine torque T and pieces of information detected by the second-clutch input-rotational-speed sensor23and the second-clutch output-rotational-speed sensor24to be described later. The ECU20determines whether or not the absorbed energy Eabs thus calculated is greater than a set value Elim which is a predetermined threshold. These calculation methods use the formula 1 and the formula 2 below installed in the ECU20as programs and calculate necessary values automatically.

The clutch operation mechanism21only needs to be capable of operating the clutches C1and C2to connect them to the first input shaft11and the second input shaft12, respectively, and operating the clutches C1and C2simultaneously. The clutch operation mechanism21is formed of a hydraulic piston, an electromagnetic actuator, and the like, for example. The synchronous engagement mechanism22includes shift forks which swing the coupling sleeves S1to SR, and only needs to be capable of operating these shift forks. The synchronous engagement mechanism22is formed of a hydraulic piston, an electromagnetic actuator, and the like, for example. The clutch operation mechanism21and the synchronous engagement mechanism22are not limited to the configurations described above; the clutch operation mechanism21only needs to be capable of operating the clutches C1and C2, and the synchronous engagement mechanism22only needs to be capable of operating the coupling sleeves.

The second-clutch input-rotational-speed sensor23is a sensor capable of detecting an input rotational speed Nin of the second clutch C2, and the second-clutch output-rotational-speed sensor24is a sensor capable of detecting an output rotational speed Nout of the second clutch C2. The input rotational speed Nin is the rotational speed of the crankshaft2, and an existing crank angle sensor can be used. Moreover, the output rotational speed Nout is the rotational speed of the second input shaft12which is lower than the input rotational speed Nin due to the presence of the second clutch C2, and an existing speed sensor or the like, can be used. Considering the gear ratio of the support gear G3, this second-clutch output-rotational-speed sensor24can be provided to the output shaft3instead of being provided to the second input shaft12.

Next, the operation of the DCT1will be described with reference toFIG. 2. As shown in Part (a) ofFIG. 2, at the time of stopping the vehicle with its shift level (not shown) set at a D range, the ECU20synchronously engages the start gear DG2and the support gear SG3to the second input shaft12and the first input shaft11, respectively. The D range refers to a drive range which is a mode used during normal travel and allowing a completely automatic gear shift function such that the vehicle can travel basically through accelerator and brake pedal operations only from start to high-speed travel to stop. This operation before the vehicle starts is not limited to the stopping of the vehicle with the shift level set at the D range as described above, as long as the start gear DG2and the support gear SG3are synchronously engaged.

Then, as shown in Part (b) ofFIG. 2, when the vehicle starts, the second clutch C2is gradually connected to the second input shaft12from a disconnected state of the clutch and set to a half clutch (partially connected) state. Meanwhile, the ECU20calculates the engine torque T, and the second-clutch input-rotational-speed sensor23and the second-clutch output-rotational-speed sensor24calculate the input rotational speed Nin and the output rotational speed Nout, respectively. The ECU20assigns these into the aforementioned formulae 1 and 2 to calculate the absorbed energy Eabs by the second clutch C2. The ECU20determines whether or not the absorbed energy Eabs thus calculated is greater than the set value Elim which is a predetermined value.

If the absorbed energy Eabs is greater than the set value Elim, the ECU20partially connects the first clutch to the first input shaft as shown in Part (c) ofFIG. 2. In this way, both clutches C1and C2transmit the torque at the moment of the start. Thus, in addition to the second clutch C2on the start gear DG2side, the first clutch C1on the support gear SG3side takes part of the torque transmission.

Then, the input rotational speed Nin and the output rotational speed Nout detected by the second-clutch input-rotational-speed sensor23and the second-clutch output-rotational-speed sensor24are sent to the ECU20, and the ECU20calculates a rotational speed difference ΔN therebetween (Nin−Nout). When this rotational speed difference ΔN falls below a set value Nlim which is a predetermined value, the first clutch C1on the support gear SG3side is disconnected from the first input shaft11(hereinafter, expressed as being fully disconnected) as shown in Part (d) ofFIG. 2. After the first clutch C1is fully disconnected, the second clutch C2on the start gear DG2side is fully connected to the second input shaft12(hereinafter, expressed as being fully connected).

Since this operation uses both clutches C1and C2during start, the wear of the second clutch C2on the start gear DG2side can be reduced. Accordingly, the replacement interval for both clutches C1and C2can be made longer.

Moreover, since whether or not to use the first clutch C1is determined by determining whether or not the absorbed energy Eabs by the second clutch C2, which shifts to a half clutch state earlier than the other, is greater than the set value Elim, it is possible to suppress increase in the wear of the first clutch C1which occurs due to the use of both clutches C1and C2.

Further, in the case of a gear shift from the start gear DG2to accelerate the speed, the acceleration can be done smoothly by simply switching the clutches C1and C2since the support gear SG3remains synchronously engaged with the first input shaft11.

Next, a method of controlling the DCT1will be described with reference toFIG. 3. First, the ECU20performs step S1of determining whether or not the vehicle is stopped in the D-range state. If determining that the vehicle is stopped in the D-range state, the ECU20then performs step S2of synchronously engaging the start gear DG2and the support gear SG1to the second input shaft12or the first input shaft11. In step S2, the ECU20synchronously engages the start gear DG2and the support gear SG3by operating the synchronous engagement mechanism22to swing the coupling sleeve S2and the coupling sleeve S3.

Then, the ECU20performs step S3of resetting the value of the absorbed energy Eabs by the second clutch C2; the absorbed energy Eabs used in the last start is reset. Then, the ECU20performs step S4of determining whether or not an operation to start the vehicle is performed. If determining that an operation to start the vehicle is performed, the ECU20then performs step S5of connecting the second clutch C2to the second input shaft12in a half clutch state. In step S5, the second clutch C2which has been fully disconnected from the second input shaft12is gradually connected to the second input shaft12until it reaches a half clutch state. At this point, the first clutch C1is fully disconnected, the second clutch C2is in a half clutch state or is shifting from a fully disconnected state to a half clutch state, the start gear DG2is synchronously engaged, and the support gear SG3is synchronously engaged.

Then, the ECU20performs step S6of determining whether or not the second clutch C2is fully connected. The ECU20proceeds to the next step since the second clutch C2is in a half clutch state or is shifting from a fully disconnected state to a half clutch state. Then, the ECU20performs step S7of calculating the rotational speed difference ΔN between the input rotational speed Nin and the output rotational speed Nout of the second clutch C2. Then, the ECU20performs step S8of determining whether or not the rotational speed difference ΔN is smaller than the set value Nlim which is a predetermined threshold.

If this rotational speed difference ΔN is smaller than the set value Nlim, the second clutch12stays in a half clutch state only for a short period of time and gets fully connected to the second input shaft12quickly. Accordingly, the wear of the second clutch C2is relatively small. Thus, in this case, the ECU20proceeds to step S12to be described later. Here, the set value Nlim is set preferably to such a value that “set value Nlim=rotational speed difference ΔN>0.”

Then, if determining that the rotational speed difference ΔN is the set value Nlim or more, the ECU20performs step S9of updating the calculation of the absorbed energy Eabs by the second clutch C2. If the rotational speed difference ΔN is the set value Nlim or more, the absorbed energy Eabs by the second clutch C2is large. Thus, it may take time before the second clutch C2gets fully connected in some cases. In such cases, the wear of the second clutch C2is severe. In step S9, the ECU20calculates the absorbed energy Eabs by the second clutch C2by using the aforementioned calculation methods. The ECU20then performs step S10of determining whether or not the absorbed energy Eabs exceeds the set value Elim. If the absorbed energy is equal to or smaller than the set value Elim, the ECU20returns to step S6.

Then, if the absorbed energy Eabs exceeds the set value Elim, the ECU20performs step S11of bringing the first clutch C1on the support gear SG3side into a half clutch state. The absorbed energy Eabs exceeding the set value Elim refers, for example, to when the vehicle starts on a hill or when the vehicle is heavily loaded. In such a situation, the second clutch C2stays in a half clutch state for a relatively long period of time. That is, it takes time before the second clutch C2gets fully connected, and the wear thereof is severe. For this reason, in this situation, both clutches C1and C2are used to start the vehicle. In this way, the wear of the second clutch C2can be reduced.

Once completing step S11, the ECU20then turns to step S6. At this point, the first clutch is in a half clutch state, the second clutch is in a half clutch state, the start gear DG2is synchronously engaged, and the support gear SG3is synchronously engaged. From this state, the rotational speed difference ΔN of the second clutch. C2starts to decrease gradually. Then, the ECU20again calculates the rotational speed difference ΔN in step S7and determines in step S8whether or not the rotational speed difference ΔN is smaller than the set value Nlim which is a predetermined threshold.

If determining that the rotational speed difference ΔN is smaller than the set value Nlim, the ECU20then performs step S12of determining whether or not the first clutch C1is fully disconnected. Since the first clutch C1is in a half clutch state, the ECU20then performs step S13of fully disconnecting the first clutch C1Once completing step S13, the ECU20returns to step S6. At this point, the first clutch is fully disconnected, the second clutch is in a half clutch state, the start gear DG2is synchronously engaged, and the support gear SG3is synchronously engaged.

Through steps S6to S12, determining this time that the first clutch C1is fully disconnected, the ECU20performs the next step S14of fully connecting the second clutch. Then, the ECU20returns to step S6and ends this control method since the second clutch is fully connected. In the final state, the first clutch is fully disconnected, the second clutch is fully connected, the start gear DG2is synchronously engaged, and the support gear SG3is synchronously engaged. In the case of a gear shift after this from the start gear DG2to the support gear SG3to accelerate the speed, the acceleration can be done smoothly by simply switching the first clutch C1and the second clutch C2.

The absorbed energy Eabs does not become large in cases other than when the half clutch state continues for a relatively long period of time, such as when the vehicle starts on a hill or is heavily loaded. Thus, the ECU20returns to step S6from step S10. Since the first clutch C1is fully disconnected, the ECU20performs the step S14and ends the method. In the case where it does not take time before the second clutch C2gets fully connected, starting the vehicle by using both clutches C1and C2causes the first clutch to be used in a half clutch state with a relatively large rotational speed difference and thereby increase the wear thereof. For this reason, in the case where it does not take time before the second clutch C2gets fully connected as described above, the first clutch C1is not used and only the second clutch C2is used for start.

According to this method, in the case where the second clutch C2on the start gear DG2side is used in a half clutch state for a long period of time and it takes time before the second clutch C2gets fully connected, thereby increasing the wear thereof, both clutches C1and C2are used to reduce the load on the second clutch C2on the start gear DG2side. In this way, the wear of the second clutch C2can be reduced. Accordingly, the replacement interval for the second clutch C2can be made longer. On the other hand, in the case where the second clutch C2is fully connected quickly and the wear thereof is relatively small, the first clutch C1is not used. In this way, unnecessary increase in the wear of the first clutch can be suppressed. In addition, the above-described operation and effect can be achieved as long as both clutches C1and C2can be operated independently and simultaneously. Thus, no additional component is needed for a conventional DCT. Accordingly, the cost can be reduced. Further, since the start gear DG2is always the same gear each time start is performed, the wear of both clutches C1and C2can be reduced without changing the feel during start.

Next, how each part operates in the above control method will be described with reference toFIG. 4. It is assumed that: time t0is time at which a start operation is performed; time t1is time at which the rotational speed difference ΔN is greater than the set value Nlim and the absorbed energy Eabs exceeds the set value Elim; and time t2is time at which the rotational speed difference ΔN falls below the set value Nlim.

At the time t0, a start operation is performed. In response to determining that start operation, the second clutch C2is brought into a half clutch state. The input rotational speed Nin of the start gear DG2becomes constant after a short period of time, while the output rotational speed Nout keeps increasing gradually, thereby decreasing the rotational speed difference ΔN gradually from a value greater than the set value Nlim. Meanwhile, the absorbed energy Eabs keeps increasing gradually. At the time t1at which the rotational speed difference ΔN is greater than the set value Nlim and the absorbed energy Eabs exceeds the set value Elim, the first clutch C1on the support gear SG3side is brought into a half clutch state. At the time t2at which the rotational speed difference ΔN falls below the set value Nlim, the support gear SG3is fully disconnected, and the second clutch C2on the start gear DG2side starts to be fully connected.

As can be seen from the above operations, by applying the control method of the present invention to a conventional DCT, it is possible to reduce the load on both clutches C1and C2and therefore reduce the wear thereof.

Next, a dual clutch transmission of the second embodiment according to the present invention will be described with reference toFIG. 5. As shown inFIG. 5, a support gear SG1and an acceleration gear AG3are provided instead of the aforementioned support gear inFIG. 1. Moreover, a first-clutch output-rotational-speed sensor25is added. This support gear SG1is a gear G1which synchronously engages with the first input shaft11at a gear ratio lower than the start gear DG2by one speed. This start gear may be set to any gear as long as it is the gear G2(second speed) or higher. Moreover, the support gear only needs to have a gear ratio lower than the start gear by one speed or higher and synchronously engage with the input shaft other than that for the start gear. For example, when the gear G3is the start gear, the gear G2and the gear G4are set as the support gear and the acceleration gear, respectively.

Next, a method of controlling the dual clutch transmission1of the second embodiment according to the present invention will be described with reference toFIG. 6. Steps S21to S32in this control method are generally the same as steps S1to S12in the control method of the first embodiment described earlier. However, steps S27and S28are different in that the input rotational speed and the output rotational speed of the first clutch C1are detected, because the support gear SG1has a lower gear ratio than the start gear DG2. At this point, the first clutch is in a half clutch state, the second clutch is in a half clutch state, the start gear DG2is synchronously engaged, and the support gear SG1is synchronously engaged.

If determining in step S32that the first clutch C1is not fully disconnected, the ECU20then performs step S33of determining whether or not the synchronous engagement of the support gear SG1is released. Since the synchronous engagement of the support gear SG1is not released, the ECU20performs next step S34of releasing the synchronous engagement of the support gear SG1. Once completing step S34, the ECU20returns to step S26. At this point, the first clutch C1is in a half clutch state, the second clutch C2is in a half clutch state, the start gear DG2is synchronously engaged, and the support gear SG1is released from its synchronous engagement.

Then, through steps S26to S33, determining that the synchronous engagement of the support gear SG1is released, the ECU20then performs step S35of fully disconnecting the first clutch C1. Then, the ECU20performs step S36of synchronously engaging the acceleration gear AG3. Once completing step36, the ECU20returns to step S26. At this point, the first clutch C1is fully disconnected, the second clutch C2is in a half clutch state, the start gear DG2is synchronously engaged, the support gear SG1is released from its synchronous engagement, and the acceleration gear AG2is synchronously engaged.

Then, the ECU20determines in step S32that the first clutch C1is fully disconnected, and performs step S37of fully connecting the second clutch C2. Then, the ECU20returns to step S26and determines in step S26that the second clutch C2is fully connected, and ends this control method. According to this method, the same operation and effect as those described above can be achieved.

A vehicle equipped with the above-described DCT1can make the wear of both clutches C1and C2even and therefore make the replacement interval for both clutches C1and C2longer than conventional cases. Moreover, the above-described operation and effect can be achieved without changing the feel during start, and therefore a vehicle with good driving comfort can be provided.

The methods of controlling a dual clutch transmission of the present invention can reduce the load on the clutch on the start gear side and thus reduce the wear thereof and therefore make the clutch replacement interval longer, without requiring any additional component and also without changing the feel during start. Moreover, the methods of controlling a dual clutch transmission of the present invention can reduce the wear of the clutch on the support gear side since the absorbed energy by the clutch on the start gear side is calculated to determine whether or not to use both clutches. In addition, the methods of controlling a dual clutch transmission of the present invention can make a gear shift operation after start smooth. Accordingly, the methods of controlling a dual clutch transmission of the present invention can be utilized in large-sized vehicles such as trucks equipped with a dual clutch transmission to achieve low fuel consumption via smooth gear shift operations.