GB2140514A - Automatic transmissions - Google Patents

Abstract

A creep preventing device for vehicles equipped with an automatic transmission has a starting oil passage (111) connected to an oil pressure source (R) and provided at its intermediate portion with a throttle (75); an engine braking oil passage (126) connected to the oil pressure source and a communicating means interposed between the starting oil passage (111) and the engine braking passage (126), and the working oil passage (29), the communicating means operating in response to the operation of a manual valve (Vm) so as to bring the starting oil passage to communication with the working oil passage at the time of starting and to bring the engine braking oil passage to communication with the working oil passage at the time of engine braking. This ensures that should the creep-preventing valve (Vc), which partially depressurises the first-speed (starting) clutch (C1) when it is energized, stick open, engine braking will still be available with the manual valve (Vm) in position I. The creep-preventing valve is solenoid-operated under electronic controls and may be hydraulically closed. <IMAGE>

Description

SPECIFICATION Automatic transmissions This invention relates to a creep preventing device for vehicles equipped with an automatic transmission.

With a speed change lever being set in the drive position (forward-movement position) during a stop, vehicles equipped with an automatic transmission present the so-called creep phenomenon that the vehicle tends to move forward against the intention of the driver due to the dragging torque of a torque converter.

Such a creep force is going to apply a brake force to the engine during the idle operation. To hold the number of revolutions of the engine during the idle operation at a reference value, therefore, it is required to increase the opening degree of a throttle valve during the idle operation to a certain extent for compensating the brake force. This increases fuel consumption as a matter of course and was one of primary causes to deteriorate the fuel cost of vehicles equipped with an automatic transmission.

In view of the above, it has been previously proposed that the idle operation state of a vehicle is detected and the pressure exerting on a starting clutch is bypassed to an oil tank to interrupt power transmission between the engine and driving wheels, so that the load upon the engine becomes the same level as that in vehicles equipped with a manual transmission. Such an improvement, however, is accompanied by the following problem in the case of the automatic transmission in which the gear train, e.g. low speed-gear train, used during idle operation is used also for engine braking, although there is no problem when the whole system functions safely. Namely, assuming here that the system is held in the creep preventive state for any reason, the effect of the engine brake in the above-mentioned gear train is entirely nullified.In the vehicle in which the shift position is arranged to make the engine brake effective in this gear train, therefore, the vehicle can be braked only by the foot brake which directly brakes the wheels. This is quite inconvenient from the view point of safety, and it is necessary to provide a suitable countermeasure to ensure the safety through elimination of such a kind of trouble. Examples of such troubles are a failure in the sensor system to judge the present state of vehicle erroneously as being in the idle operation state even though the vehicle is cruising, and a failure in the actuator to erroneously hold the transmission in the creep preventive state despite the sensor system and the judging system are safe.Particularly, the oil pressure change-over valve which is the final actuator tends to suffer from unsmoothed movement due to foreign matters in the oil so that, in some cases, it cannot be reset to the inoperative position because of small resetting force of the reset spring.

This problem is not encountered by vehicles in which there is a shift position at which the engine brake is put into effect with the low-speed gear train. However, in the vehicles in which the engine braking is effected with the middle speed gear train, the gear ratio of the medium speed gear train cannot be decreased unlimitedly because a too small gear ratio impairs the effect of the engine brake. Consequently, in such vehicles, there is a restriction to the reduction of the fuel consumption.

According, an object of the invention is to provide a creep preventing device for vehicles equipped with automatic transmission in which the creep with the gear train for starting is prevented while permitting an engine braking with this gear train, improved to ensure the engine braking effect even in case of a failure in the creep preventing mechanism while it is in the operating condition.

To this end, according to the invention, there is provided a creep preventing device for vehicles equipped with an automatic transmission. The device has a starting oil passage connected to an oil pressure source and provided at its intermediate portion with a throttle; an engine braking oil passage connected to the oil pressure source in parallel to the engine braking passage; and a communicating means interposed between the starting oil passage and the engine braking passage, and the working oil passage, the communicating means being adapted to operate in response to the change-over operation of the manual change-over means so as to bring the starting oil passage to the working oil passage at the time of starting and to bring the engine braking oil passage to the working oil passage at the time of engine braking.

According to a first featurte of the invention, even in the case of a failure in the creep preventing valve while the same is in the operative position, the working oil of high pressure is supplied to the working oil passage from the engine braking oil passage bypassing the restriction thereby to bring the frictional engaging elements into engaging condition. It is, therefore, possible to put the engine brake into effect without fail even in case of the failure in the creep preventing valve.

According to a second feature of the invention, the creep preventing valve has an oil chamber in which oil pressure acts to operate the creep preventing valve in the closing direction, and means are provided in connection with a manual change-over means so as to bring the oil chamber into communication with an oil pressure source when the manual change-over means are operated at the time of the engine braking.Thus, since the oil pressure from the oil pressure source is introduced to the creep preventing valve at the time of the engine braking to move this valve to the closing position, the spool valve is forcibly moved to put the engine brake into effect without fail, even in the event of a failure in the electric system for operating the creep preventing valve upon detect of the idle state of the vehicle or even in the event of a failure of the operation of the creep preventing valve due to foreign matters contained by the oil.

The above and other objects, features and advantages of the invention will become clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

Figure l is a schematic view of an automatic automotive transmission to which the invention is applied, the transmission having three stages of forward movement and one stage of backward movement; Figure 2 is a hydraulic circuit diagram of the transmission shown in Figure 1; Figure 3 is an illustration showing the connection of oil passages when a manual valve is in the position for engine braking; Figure 4 is a hydraulic control circuit diagram of another embodiment of the invention; and Figure 5 is an illustration showing the connection of oil passages when a manual valve is shifted to a position for holding the first speed.

Referring first to Figure 1, there is shown a schematic view of an automatic transmission for an automobile with three stages of forward movement and one stage of backward movement, the output of an engine E is transmitted, via a torque converterT, an auxiliary transmission M and a differential device Df in said order, from a crank shaft 1 to driving wheels W, W' to drive the latter. The torque converter T comprises a pump vane wheel 2 connected to the crank shaft 1, a turbine vane wheel 3 connected to an input shaft 5 of the auxiliary transmission M, and a stator vane wheel 4 connected through a one-way clutch 7 to a stator shaft 4a relatively rotatably supported on the input shaft 5.The torque transmitted from the crank shaft 1 to the pump vane wheel 2 is transmitted to the turbine vane wheel 3 fluid-dynamically, and when amplifying action of torque is effected during that period, the stator vane wheel 4 bears the reaction thereof as known per se.

A pump driving gear 8 for driving an oil pressure pump P shown in Figure 2 is provided on the right end of the pump vane wheel 2, and a stator arm 4b for controlling a regulator valve Vr shown in Figure 2 is fixediy mounted on the right end of the stator shaft 4a.

Between the input and output shafts sir 6 parallel to each other of the auxiliary transmission M, there are provided in parallel a low-speed stage gear train G1, a middle-speed stage gear train G2, a high-speed stage gear train G3 and a reverse gear train Gr. The low-speed stage gear train G1 comprises a driving gear 17 connected to the input shaft 5 through a low-speed stage clutch C1 serving as a frictionally engaging element for starting and engine brake, and a driven gear 18 fixedly mounted on the output shaft 6 and meshed with the gear 17. The middle-speed stage gear train G2 comprises a driving gear 19 connected to the input shaft5 through a middle-speed stage clutch C2 and a driven gear 20 connected to the output shaft 6 through a switching clutch C and meshed with the gear 19.The high-speed stage gear train G3 comprises a driving gear 21 fixedly mounted on the input shaft 5 and a driven gear 22 connected to the output shaft 6 through a high-speed clutch C3. The reverse gear train Gr comprises a driving gear 23 formed integrally with the driving gear 19 of the middle-speed stage geartrain G2, a driven gear 24 connected to the output shaft 6 through the switching clutch Cs and an idle gear 25 meshed with both the gears 23,24. The switching clutch Cs is disposed between the driven gears 20 and 24, and the driven gears 20 and 24 can be selectively connected to the output shaft 6 by shifting a selector sleeve 26 of the clutch Cs to a forward-movement position on the lefthand orto a backward-movement position on the righthand in the Figure.

If only the low-speed stage clutch C1 is engaged when the selector sleeve 26 is retained at the forward movement position as shown, the driving gear 17 is connected to the input shaft 5 to establish the low-speed stage gear train G1,through which the torque is transmitted from the input shaft 5 to the output shaft 6. Next, when the middle-speed stage clutch C2 is engaged while the low-speed stage clutch C1 is disengaged, the driving gear 19 is connected to the input shaft 5 to establish the middle-speed stage gear train G2, through which the torque is transmitted from the input shaft 5 to the output shaft 6.When the high-speed stage clutch C3 is engaged while the low-speed stage clutch C1 and the middle-speed stage clutch C2 are disengaged, the driven gear 22 is connected to the output shaft 6 to establish the high-speed stage gear train G3, through which the torque is transmitted from input shaft 5 to the output shaft 6. Next, when the selector sleeve 26 is switched to the reverse position on the righthand and only the middle-speed stage clutch C2 is engaged, the driving gear 23 and driven gear 24 are connected to the input shaft 5 and output shaft 6, respectively, to establish the reverse gear train Gr, through which gear train Gr, the torque is transmitted from the input shaft 5 to the output shaft 6.

The torque transmitted to the output shaft 6 is transmitted from an output gear 27 provided on the end of said shaft 6 to a large-diameter gear 28 of the differential device Df.

Referring now to Figure 2, a hydraulic pump P sucks the oil from an oil tank R and feeds the oil under pressure into a working oil passage 29. This pressurized oil is regulated in pressure to a predetermined level by a regulator valve Vr, and thereafter fed to a manual valve Vm. This oil pressure is termed as the line pressure Pt.

The regulator valve VF has a pressure regulating spring 30 and a spring receiving cylinder 31 for carrying an outer end thereof, and this spring receiving cylinder 31 can be moved to left and right to adjust the set load of the pressure regulating spring 30. The stator arm 4b comes into contact with the outer surface of the spring receiving cylinder 31 so as to apply the reaction force acting on the stator vane wheel 4, that is, the stator reaction force thereto. A stator spring 32 for carrying the stator reaction force is connected to the spring receiving cylinder 31. Accordingly, if the stator reaction force increases, the stator spring 32 is compressed, as a consequence of which the spring receiving cylinder 31 is moved leftwards to increase the set load of the pressure regulating spring 30 with the result that the oil pressure P4 in the working oil passage 29 is increased.

A part of pressure oil regulated by the regulator valve Vr is introduced into the torque converter T via an inlet oil passage 34 having a throttle 33 to pressurize the interior of the converter T to prevent a cavitation, said internal pressure being determined by the magnitude of the throttle 33, the strength of a spring 37 of a check valve 36 provided on an outlet oil passage 35 of the torque converter T, and the like.

The oil having passed through the check valve 36 is returned to the oil tank R via an oil cooler 56. The surplus portion of pressurized oil discharged from the oil pressure pump P is introduced to a lubricating oil passage 38 from the regulator valve Vr and fed to various lubricating portions. A pressure regulating valve 39 is connected to the lubricating oil passage 38 in order to secure the oil pressure required to the minimum.

When the manual valve Vm is in the neutral position as shown, the pressurized oil fed to the valve Vm is not fed to any of said clutches C1, C2, C3 nor to various other hydraulically operating portions. As a result, the three clutches C1, C2, C3 are all disengaged and hence the torque of the engine E is not transmitted to the wheels W, W'.

When the manual valve Vm is moved leftwards by one step from the illustrated position and shifted to the drive position D, the operating oil passage 29 from the hydraulic pump P is communicated with oil passages 43, 118 and an oil passage 111 for starting is communicated with an operating oil passage 41a leading to an oil pressure cylinder 40a of the low-speed stage clutch C1 and provided with a unidirectional throttle 75. On the other hand, an oil passage 112 is disconnected from an operating oil passage 41 b leading to an oil pressure cylinder 40b of the middle-speed stage clutch C2 and an oil passage 1 13a is disconnected from an exhaust port 114. An oil passage 115 remains communicated with an exhaust port 116.The working oil passage 43 is communicated with a spring chamber 42 of an oil pressure servo-motor Sm for shifting the selector sleeve 26, and therefore, a piston 44 of the servo-motor Sm remains at a position moved leftwards as shown to hold the selector sleeve 26 at the forward-movement position as shown in Figure 1 through a shift fork 45. Thus, the reverse gear train Gr is placed in an inoperative state. An inlet oil passage 46 in communication with an input port of means for generating oil pressure proportional to vehicle speed, that is, a governor valve Vg is branched from the operating oil passage 29, and a first signal oil passage 47 extends from an output port of the valve Vg.

The governor valve Vg, which is known per se, is rotated about its own rotational axis 49 by a gear 48 meshed with the large-diameter gear 28 of the differential device Df. Thus, the rotational speed thereof is proportional to the vehicle speed, and therefore, the governor valve Vg can release the oil pressure proportional to the vehicle speed, that is, a governor pressure Pg, to the first signal oil passage 47 by the action of the centrifugal force acting on a weight 51 of a spool valve body 50.

An oil passage 53 is branched from the working oil passage 43 and connected to a first throttle valve Vt1 through a modulator valve 54. The modulator valve 54 is a pressure reducing valve which is biased toward the closing side by a spring force and urged toward the opening side by the modulator pressure at an output port 54a, and it determines an upper limit value of the inlet pressure of the first throttle valve Vtl.The first throttle valve Vt1, which is well known, comprises a spool valve body 55, a control spring 58 for biasing the valve body 55 leftwards, a return spring 57 for biasing the valve body 55 rightwards, a control spring 59 for carrying the outer end of the control spring 58, a control cam 60 rotated in association with an increase in opening degree of the throttle valve of the engine E to move the control piston 59 leftwards, an adjusting bolt 51 capable of adjusting the set load of the return spring 57 and the like. When the control piston 59 is moved leftwards, the displacement thereof causes the spool valve body 55 to be pushed leftwards through the control spring 58.With this leftward movement, the oil pressure released to a second signal oil passage 52 acts on a left shoulder 55a of the spool valve body 55 so as to push back the spool valve body 55 rightwards, and therefore, after all, the first throttle valve Vt1 can release the oil pressure proportional to the opening degree of the throttle valve of the engine E, that is, a throttle pressure Pt, to the second signal oil passage 52.

Incidentally, counterclockwise movement of the control cam 60 throttles the degree of communication between an oil passage 117 and the oil tank R continuously.

The first and second signal oil passages 47, 52 are connected to pilot oil pressure chambers 62a, 62b; 63a, 63b, respectively, on both ends of a low-middle speed shift valve V1 and a middle-high speed shift valve V2.

Thereby, spool valve bodies 64, 65 of these shift valves V1, V2 receive at both end surfaces thereof said governor pressure Pg and throttle pressure Pt and are operated as follows: That is, the spool valve body 64 of the low-middle speed shift valve V1 initially remains at a position moved rightwards as shown by the force of a spring 66, and therefore, an oil passage 118 is communicated with the operating oil passage 41 a through the oil passage 111,so that the low-speed stage clutch C1 is pressurized and engaged.Next, when the vehicle speed increases to increase the governor pressure Pg and the force for moving the spool valve body 64 leftwards by the governor pressure Pg overcomes the force for moving the valve body 64 rightwards by the throttle pressure Pt and spring 66, a click ball 68 moving together with the valve body 64 in a click motion mechanism 67 provided on the right end of the valve body 64 gets over a fixed locating projection 69 and the valve body 64 is rapidly switched to a leftwardly moved position. Thereby, the oil passage 111 is communicated with a drain oil passage 119 and the oil passage 118 is communicated with an oil passage 70. On the other hand, the oil passage 70 is disconnected from the drain passage 120. In this state, if the middle-high speed shift valve V2 is at the position as shown, the oil passage 70 is communicated with an oil passage 113 provided with a unidirectional throttle 121 and further with the operating oil passage 41 b through the manual valve Vm. Thus, the operating oil is supplied to the oil pressure cylinder 40b so as to pressurize and engage the middle-speed stage clutch C2. As a result, the middle-speed stage gear train G2 is established.

When the vehicle speed further increases, the similar operation occurs also in the middle-high speed shift valve V2, whereby the spool valve body 65 of the valve V2 is moved leftwards due to the increasing governor pressure Pg to communicate the oil passage 113 with a drain oil passage 122 and the oil passage 70 with the working oil passage 41 c leading to the oil pressure cylinder 40c of the high-speed stage clutch C3, and on the other hand, the operating oil passage 41 c is disconnected from a drain oil passage 123. Thus, the middle-speed stage clutch C2 is released from its engaged state, and instead the high-speed stage clutch C3 is pressurized and engaged to establish the high-speed stage gear train G3.

To alleviate the shock at the time of speed change, accumulators 72,73,74 are connected in parallel to the clutches C1, C2, C3 hydraulically. A 1-2 orifice control valve 124 is provided in the drain oil passage 119, and a 2-3 orifice control valve 125 is provided in the drain oil passage 122, respectively. The control pressure Pc from the second throttle valve Vt2 is guided to back pressure chambers 77, 78,79 of those accumulators 72, 73,74 through an oil passage 106.The second throttle valve Vt2 is interposed between an oil passage 105 branched from the oil passage 53 and the oil passage 106, and it comprises a spool valve body 107, a control spring 108 for biasing the valve body leftwards, a control piston 109 for carrying the outer end of the control spring 108, and a control cam 110 rotated in association with an increase in opening degree of the throttle valve of the engine E to move the control piston 109 leftwards. When the control piston 109 is moved leftwards, the displacement thereof causes the spool valve body 107 to be pushed leftwards through the control spring 108.With this leftward movement, the oil pressure released to the oil passage 106 acts on a left shoulder 1 07a of the spool valve body 107 so as to push back the spool valve body 107 rightwards. In this way, the second throttle valve Vt2 can release the control pressure Pc proportional to the opening degree of the throttle valve of the engine E to the back pressure chambers 77 -79 of the accumulators 72-74 through the oil passage 106.

Incidentally, counterclockwise movement of the control cam 110 throttles the degree of communication between the drain oil passage 120 and the oil tank R continuously.

When the vehicle speed lowers, the valve body 65 of the middle-high speed shift valve V2 is first moved rightwards to shift the change gear ratio from the high speed stage to the middle-speed stage. When the vehicle speed is further lowered, the valve body 64 of the low-middle speed shift valve V1 is moved rightwards to give the change gear ratio of the low-speed stage. It is noted to be of importance that the operating oil fed to the low-speed stage clutch C1 passes the unidirectional throttle 75 in such change gear ratio of the low-speed stage with the shift lever being at the drive position D.

The halfway part of the operating oil passage 41 a is connected to the accumulator 72, and an oil passage 84 directly leading to the oil tank R is connected to the halfway part of the operating oil passage 41a. A creep preventing valve Vc of pilot type is disposed halfway of the oil passage 84.

The creep preventing valve Vc has a spool valve body 85 which is housed in a cylindrical valve chamber 86 formed in the intermediate part of the passage 84 so as to divide the oil passage 84 into the upstream portion 84a and the downstream portion 84b, whereby oil chambers 87, 88 are defined in the upper and lower parts of the valve body 85, respectively. A return spring 89 for biasing the spool valve body 85 downwards is housed in the upper oil chamber 87. The spool valve body 85 comprises a pair of upper and lower lands 90, 91, an annular groove 92 located therebetween, and orifices 93,94 causing the groove 92 to be communicated with the upper oil chamber 87 and the lower oil chamber 88, respectively.The annular groove 92 is always communicated with the upstream portion 84a, and the upper land 90 opens and closes a port 95 of the downstream portion 84b opened to the valve chamber 86 upon the vertical movement thereof.

The port 95 is preferably formed to have a circular section or the like such that the effective opening area is gradually increased as the upper land 90 moves upwards.

An end wall member 97 having an orifice 96 is disposed on the upper end of the valve chamber 86, and a solenoid valve 99 including a pilot needle valve 98 adapted to open and close the orifice 96 is disposed above the member 97. The needle valve 98 is arranged to close the orifice 96 by the resilient force of a spring 100 and open the orifice 96 when a solenoid 101 is excited. In an opened state, the orifice 96 is communicated with a bypass oil passage 102 branched from the downstream portion 84b of the oil passage 84.

When the solenoid 101 is excited and the orifice 96 is opened, the upper oil chamber 87 is communicated with the oil tank R through the orifice 96 and the bypass oil passage 102, whereby the oil pressure in the upper oil chamber 87 is lowered. Accordingly, the spool valve body 85 moves upwards by the force acting on the second pressure receiving surface 85b exposed to the lower oil chamber 88, thus opening the oil passage 84. Since the pressure in the operating oil passage 41 a, that is, the low-speed stage clutch C1 at this time is determined by the resilient force of the return spring 89 and the area of the second pressure receiving surface 85b of the spool valve body 85, occurrence of the creep phenomenon can be prevented by setting said pressure lower than the engaging pressure of the low-speed stage clutch C1.

Incidentally, when the valve body 64 of the low-middle speed shift valve V1 or the valve body 65 of the middle-high speed shift valve V2 is moved leftwards to establish the change gear ratio of the middle-speed stage or the high-speed stage, the oil pressure in the operating oil passage 41 a becomes zero with the result that the spool valve body 85 will not move upwards and the creep preventing valve Vc will not be operated.

The solenoid 101 of the solenoid valve 99 is grounded through a transistor 140, to the base of which is applied the output from an AND gate 141 through a resistor 146. To the input terminals of the AND gate 141 there are connected a shift position sensor 142 adapted to produce a high level signal when the shift lever has been shifted to the drive position D, an engine speed sensor 143 which detects the speed of revolution of the engine E and outputs a high level signal when the detected speed is lower than a reference value, a braking sensor 144 which outputs a high level signal when the brake pedal is trod on, and a throttle sensor 145 adapted produce a high level signal when the accelerator pedal is in the idle position.

Thus, when the vehicle is stopped at a street intersection with the foot placed on the brake pedal, the outputs ofthe all sensors 142 to 145 assume high levels to excite the solenoid 101, whereby the creep preventing valve Vc is opened and occurrence of the creep phenomenon is prevented.

In the above process of creep prevention, the throttle 75 disposed halfway of the oil passage for starting plays an essential role. Now assuming that a diameter of the throttle 75 is d, a diameter of the downstream portion 84b of the oil passage 84 in the creep preventing valve Vc is Di, a discharge pressure of the hydraulic pump P is Pe, and an oil pressure in the working oil passage 41a isp, the following equation is obtained.

Substitution of Pe = 8.5 kg/cm2, p = 1.0 kg/cm2, and d=1.7mm into the above equation results in Di= 2.8 mm. In practice, other factors should be considered, and therefore, Di is set in a range of 5-6 mm +. In any case, Di has not a so large value. To attain good responsivity at the time of starting, the creep preventing valve Vc is disposed as near as possible to the low-speed stage clutch C1.

Referring to Figure 3, when the manual valve Vm is shifted to a shift position other than the drive position D, e.g., an engine brake position, namely, a first speed retaining position I, the working oil passage 41a is disconnected from the starting oil passage 111 in accordance with the movement of the valve body 80 but is brought into communication with the oil passage 43 through the engine braking oil passage 126 which runs in parallel to the starting oil passage 111, and further with the working oil passage 29 leading to the hydraulic pump P. Consequently, the hydraulic pump P and the hydraulic cylinder 40a of the low-speed stage clutch C1 are directly connected to each other without the intery of the undirectional throttle 75 in the starting oil passage 111.Consequently, the low-speed stage clutch C1 is engaged to establish the low-speed stage gear train G1 thereby to put the engine brake into effect.

As an alternative, when the valve Vm is shifted to a middle-speed stage retaining position II or reverse position Re, only the middle-speed stage clutch C2 is pressurized and engaged to establish the middle-speed stage gear train G2 or reverse gear train Gr. In particular, when the manual valve Vm is shifted to the reverse position Re, the piston 44 of the servo-motor Sm receives the pressurized oil on the left end face thereof and the spring chamber 42 is communicated with the oil tank R, whereby the piston 44 is moved rightwards to establish the reverse gear train Grays previously notes. Besides, a shift position Pk in the manual valve Vm designates a parking position.

Next, the operation of this emnodimentwill be explained.

The operation of this embodiment is as follows. Assume here that the manual valve Vm is shifted to the drive position D to decelerate and stop the vehicle. As a result, the valve bodies 65,66 of the middle-high and low-middle shift valves V2,V1 are moved to the right to bring the oil passage 118 into communication with the starting passage 111 so that oil pressure of about 1 Kg/cm2 is supplied to the working oil passage 41 a due to the throttling effect of the throttle 75, thereby to engage the low-speed stage clutch Cl. When the vehicle stops at a street intersection, the creep preventing valve Vc is opened so that the oil pressure in the hydraulic cylinder 40a comes down below the engaging pressure so that the low-speed stage clutch C1 is disengaged, thereby to prevent the occurrence of the creep phenomenon.

According to the invention, the engine braking can be put into effect even if the creep preventing valve Vc is prevented from moving from the open position due to a short-circuiting in the transistor 140 or due to a small resetting force of the reset spring 89 particularly when the resistance to the movement of the creep preventing valves is increased by foreign matters contained in the oil. In order to put the engine brake into effect, the manual valve Vm is shifted to the engine braking position, i.e. to the first-speed retaining position I. As a result, as explained before in connection with Figure 3, the starting oil passage 111 is disconnected from the working oil passage 41 a while the engine braking oil passage 126 leading to the hydraulic pump P is communicated with the working oil passage 41 a.Consequently, high pump discharge pressure of about 8.5 Kg/cm2 is directly lead to the working oil passage 41 a. Therefore, even if the creep preventing valve Vc is keep opened, the oil pressure sufficient for engaging the low-speed stage clutch C1 is introduced into the hydraulic cylinder 40a. Consequently, the low-speed stage gear train G1 is established to put the engine brake into effect in spite that the creep preventing valve Vc is kept opened.

Considering that the aforementioned discharge pressure pe = 8.5 Kg/cm2 is sufficient for transmitting the maximum torque of the engine E and that the engine brake is the internal friction torque necessary for idling the engine E, it will be understood that the engaging pressure of the low-speed stage clutch C1 need not be so high, although it depends on the gear ratio of the low-speed stage gear train G1.

Figures 4 and 5 show another embodiment of the invention in which the same reference numerals are used to denote the same parts or members as those of the preceding embodiment. In this embodiment, the working oil passage 41 a is provided with a undirectional throttle 75'. In addition, the creep preventing valve Vc' is so constructed as to have an upper oil chamber 87 connected to an oil passage 127 through a check valve 128 which permits the oil to flow only towards the upper oil chamber 87. The oil passage 127 can be brought into communication with the working oil passage 29 or with the oil passage 116 leading to the oil tank R, through the manual valve Vm.Namely, when the manual valve Vm is in the neutral position N as shown in Figure 4, and when the same is in the drive position D shifted one stage leftwards from the neutral position N, the oil passage 127 is communicated with the oil passage 116 through a groove 82 formed in the outer peripheral surface of the spool 80 of the manual valve Vm. However, when the manual valve Vm is shifted to the first-speed retaining position I as shown in Figure 5,the oil passage 127 is allowed to communicate, through the groove 81, with the working oil passage 29 leading to the hydraulic pump P.

Between the orifice 93 and the upper oil chamber 87, disposed is a check valve 129 which is adapted to prevent the backward flow of the pressurized oil from the upper oil chamber 87 to the lower oil chamber 88.

This check valve 129 is retained by the end of the reset spring 89 at a predetermined position. The operation of this embodiment is as follows.

It is now assumed that the vehicle is stopped, for example, at a street intersection with the foot placed on the brake pedal. In this case, since all sensors 142 to 144 produce high level signals, the output of the AND gate 141 assumes a high level, and therefore, the transistor 140 is conducted to excite the solenoid 101.

Accordingly, the pilot needle valve 98 is lifted up and the upper oil chamber 87 is communicated with the oil tank R through the orifice 96, bypass oil passage 102, oil passage 127, manual valve Vm and the oil passage 116. Consequently, the pressure in the upper oil chamber 87 is decreased to allow the spool valve body 85 to move upwardly thereby to open the oil passage 84. As a result, the oil pressure in the working oil passage 41 a is decreased below the engaging pressure of the low-speed stage clutch C1 thereby to prevent occurrence of the creep phenomenon.

During running of the vehicle, the pilot needle valve 98 has been lowered to close the orifice 96 so that the valve body 85 is moved downwardly by the force of the reset spring 89 thereby to close the oil passage 84, whereby the creep preventing function is dismissed. Assume here that the pilot needle valve 98 is undesirably kept in the raised position by a continuous excitation of the solenoid 101 due to an internal short-circuiting of the transistor 140. In this case, the oil passage 84 is kept opened so that the oil pressure supplied to the low-speed stage clutch C1 is decreased. Consequently, the engine brake cannot be put into effect unless the device of the invention is incorporated. This problem, however, can be overcome by the present invention as will be understood from the following explanation.According to the invention, as the manual valve Vm is shifted to the first speed retaining position I as shown in FigureS to effect the engine braking, the oil passage 127 is brought into communication with the working oil passage 29 so that the oil pressure from the hydraulic pump P comes to act in the upper oil chamber 87. Therefore, a downward force is applied on the valve body 85 by the hydraulic pressure acting on the upper pressure receiving surface 85a of the valve body 85, so that the valve member 85 is moved downwardly to press and engage the low-speed stage clutch C1 to attain sufficient engine braking effect.

Assume here also that the resistance to the movement of the valve body 85 is increased due to foreign matters contained in the oil so that the valve body 85 cannot be moved to the closing position solely by the force of the reset spring 98. In this case also, the valve body is pressed by the strong oil pressue so that the oil passage 84 can be closed.

In this embodiment, the manual valve Vm can take a second-speed retaining position II between the drive position D and the first-speed retaining position I. The oil passage 127 is allowed to communicate with the working oil passage 29 also when the manual valve Vm takes the second-speed retaining position 11. With this arrangement, in the event a failure, it is possible to move the valve body 85to the closing position when the manual valve Vm selects the second-speed gear train before the engine brake is put into effect with the first-speed gear. This is quite advantageous from the view point of safety.When the manual valve Vm takes the second-speed retaining position II, the oil pressure in the working oil passage 41 a is theoretically zero, so that the downward force acting on the valve body 85 is further increased. When the manual valve Vm is moved from the drive position D to the first-speed retaining position I, the manual valve Vm necessarly passes the second-speed retaining position II to ensure a margin of time.

In the embodiments described hereinbefore, the high pressure is supplied to the creep preventing valve Vc through the manual valve Vm. This arrangement, however, is not exclusive and it is possible to use a separate change-over valve operatively connected to the manual valve Vm or another interface such as electricity. The communication between the oil passage 127 and the oil tank R when the manual valve Vm takes the drive position D may be accomplished by another change-over valve.

Claims (4)

1. In a creep preventing device for vehicles equipped with an automatic transmission comprising: a fluid type torque converter; an auxiliary transmission connected to said torque converter and having a frictionally engaging element for establishing a gear train for starting and engine braking; a manual change-over means for manual change-over of said frictionally engaging element between the position for starting and the position for engine braking; and a creep preventing valve disposed in an oil passage branching from a working oil passage for supplying said frictionally engaging element with the working oil and leading to an oil tank, said creep preventing valve being adapted to open said passage during idle operation of said vehicle; an improvement which comprises: a starting oil passage connected to an oil pressure source and provided at its intermediate portion with a throttle; and engine braking oil passage connected to said oil pressure source in parallel to said engine braking passage; and a communicating means interposed between said starting oil passage and said engine braking passage, and said working oil passage, said communicating means being adapted to operate in response to the change-over operation of said manual change-over means so as to bring said starting oil passage to said working oil passage at the time of starting and to bring said engine braking oil passage to said working oil passage at the time of engine braking.

2. In a creep preventing device for vehicles equipped with an automatic transmission comprising: a fluid type torque converter; an auxiliary transmission connected to said torque converter and having a frictionally engaging element for establishing a gear train for starting and engine braking; a manual change-over means for manual change-over of said frictionally engaging element between the position for starting and the position for engine braking; and a creep preventing valve disposed in an oil passage branching from a working oil passage for supplying said frictionally engaging element with the working oil and leading to an oil tank, said creep preventing valve being adapted to open said passage during idle operation of said vehicle; an improvement which comprises that said creep preventing valve is provided with an oil chamber in which oil pressure acts to urge said creep preventing valve in the closing direction, and that means are provided in connection with said manual change-over means for bringing said oil chamber to the oil pressure source in response to the change-over operation of said manual change-over means at the time of engine braking.

3. A creep preventing device according to claim 2, wherein said manual change-over means has a third position between the position for starting and the position for engine braking, said third position being for establishing a gear train other than that for the engine braking, and wherein said means for bringing said oil chamber to said oil pressure source is arranged to permit said oil chamber to be communicated wit said oil pressure source also in response to the shifting of said manual change-over means to said third position.

4. A creep preventing device for vehicles equipped with automatic transmissions, substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 4 and 5 of the accompanying drawings.