中英文文献翻译—运动型7速双离合器变速器系统

附录

附录A外文文献原文

7-Speed Dual Clutch Transmission System for Sporty Application ABSTRACT:With its 7-speed dual clutch transmission, ZF has introduced an innovative transmission for sporty applications. The close ratios combined with extremely spontaneous drive behavior makes it an ideal transmission for sporty applications. This article describes the compact gear set with lubrication by injection for improving the level of efficiency and increasing the engine-speed-strength, the dual clutch unit as well as the hydraulic control unit, which is based on the pre-control principle, are also described in detail. The hy-draulic control principle provides the option of a hydraulic cruise mode in the event of an electronics failure. In addition to the transmission design, functional features that also highlight the sporty character of the transmission are described in detail.

Key words: Automatic transmission; Dual clutch; Vehicle connection; Efficiency

1 Introduction

When it comes to the field of automatic transmissions, dual clutch systems currently represent the benchmark in terms of spontaneity and sportiness. In this type of transmission, which is based on a countershaft transmission, these advantages are combined with a very direct "vehicle connection", high rpm performance, and excellent transmission efficiency.

The 7-speed dual clutch transmission for the standard driveline presented here is designed for a torque capacity of up to 520 Nm and rotational speeds of up to 9250 rpms. In order to be able to achieve these performance data in the existing installation space, a concept was developed in which an oil chamber as well as lubrication by injection are used. Before introducing the transmission′s several unique features in more detail below, an overview of the basic transmission design will be presented, Fig. 1.The engine torque is introduced to the dual clutch via a torsion damper (not shown in Fig. 1). The multidisk clutches in the dual clutch are radially nested in one another and transfer the torque to both input shafts in the countershaft transmission gear set. In this case, due to the installation space, the countershaft is not located under the main shaft, but is tilted laterally. This becomes possible because the concept is based on lubrication by injection with a dry sump.

On the one hand, lubrication by injection improves heat removal, on the other, there are no noticeable losses due to the gears splashing in the oil pan. The oil is supplied to the transmission via an internal gear pump which is driven by a spur gear train behind the dual clutch. With the help of a spur gear train, the drive unit has the advantage that, via different gear ratio phases and depending on the intended use, the flow rate and the max. speed of the pump can be adapted. An additional advantage is that based on theresulting I proved installation space, an optimal ratio between the pump width and the pump diameter can be achieved for the pump′s level of efficiency. The hydraulic control unit is arranged under the gear set. The hydraulic unit supplies the clutch, based on need, with pressure and cooling oil as well as shift actuators. The latter are arranged laterally to the gear set and work with double-acting cylinders. The sensor for detecting the position of the gearshifts is attached directly onto the four gearshifts. The transmission has an external control unit.

Fig.1Overview dual clutch transmission (DCT)

2 Seven speeds with sophisticated stepping-a concept for extrme sporti- ness

The gear set concept of the dual clutch transmission introduced here was developed in house taking into consideration the following requirements:

High power density

High speed endurance strength up to 9250 rpm Variability and modular design

Representation of transmission-ratio spreads of about 4.7 and 6.8 with 7 speeds

Use of existing synergies for manual transmissions

After extensive systematic development of the gear set in which many thousands of variants were produced and compared, the gear set concept that is illustrated in Fig. 2 is the final variant and the ideal concept for achieving the goals specified.

The gear set selected is based on the constant drive concept and consists of two concentric drive shafts each of which are driven by one of the two multidisk clutches in the

Fig.2Gear set scheme of 7D variant

dual clutch, two countershafts also concentric to one another, a main shaft and an output shaft. The gear ratios are engaged by the four synchronizer units A/B, C/D, E/F, and G/H, which are arranged on the main shaft and on the hollow countershaft and these are connected to the loose wheels or the adjacent shafts. An important feature in the gear set is the connectability of both countershafts through the C/D synchronizer unit. In the D shift position, the gear ratios selected in this way can be doubly used which reduces construction costs compared to conventional dual clutch gear sets. Similarly, this feature is used in first gear because then the vehicle is started up using the more powerful K1 clutch. Because of this dual use of the last gear level in the transmission for the first and second gear, the desired ratio step 1-2 is achieved through the transmission ratios of both constant drive phases.

The use of the K1 clutch for starting up in first gear results inevitably in the direct gear also being assigned to the odd subsection. In this case, the fifth and seventh gears can be selected as a direct drive. With this feature, it was possible to develop a modular gear set which, on just a few changes,contains two different transmission gear ratio variants with fundamentally different characters.

For the first version, with an overall spread of about 4 . 7 , the seventh gear is selected as a direct gear (called the 7D variant). Fig. 2 shows the relevant gear set diagram with the performance flows in all speeds. Due to its sophisticated gear steps, this transmission is highly suitable for very sporty vehicles that need only a "little" transmission stepping due

to the high rotating engine. Optimal tractive power can be provided at any time during

vehicle operation.

The second version is based on the 7D variant, however, fifth gear was selected as the direct drive. When maintaining the torque multiplication ratio and in adapting the transmission ratio of several lower gear levels, you get the 5D variant with a considerably higher transmission-ratio spread for vehicles with increased comfort demands and simultaneously reduced consumption.

Fig. 3 illustrates the design of the 7D variant. The main similarity with existing manual transmissions for standard transmissions is noticeable. Due to the compact gear set design, the sufficient shaft dimensioning and the favorable arrangement in proximity of the bearing of the high transmitting ratios, central bearing glasses were not necessary despite the proportionally large bearing clearance.Overall, only two housing bearing levels are necessary where the front level is located behind both constant gears. In addition, a very compact and inexpensive transmission design could be implemented based on the bearing concept selected, especially in the area of the hollow shaft.

Fig.3Sectional Drawing of 7D variant

3 The dual clutch

The central module of this highly topical transmission concept is the wet dual clutch. With a broad spectrum of technical features, it implements the functional provisions of the transmission control unit and thus distinguishes the special character of this transmission concept.

Very fast delay times, low inertia and good, comfortable friction value progressions facilitate, very sporty handling with highly dynamic gear shifting and comfortable cruising at a high level of efficiency. The dual clutch placed directly on the transmission input accepts the engine torque from thtorsion damper and feeds it to one of the two subsections, depending on the situation.

Safety considerations have led to a "normall open" design.

The radial arrangement of the multidisk pack age represents the best combination of performanc and installation space need, Fig. 4.

Fig.4Dual clutch

Careful lining and oil selection as well as intensive enhancement of this tribological system are the requirements for comfort and performance of this clutch throughout its service life.

Through intense testing and detailed calculations, it was possible to achieve a very high therma loading capacity. As part of the process, the lining type, dimensioning, and grooving as well as equal distribution of thermal load and oil flow in the multidisk package are decisive design features.

Low torque drag even with low temperatures as well as high speed endurance strength support comfort and a high level of sportiness, but are also important safety requirements.

Rotating, centrifugal force-compensating clutch cylinders with hysteresis optimized gaskets make the clutches easy to control. Integrated plate springs reliably accept rapid piston resetting even at high speeds.

In the case of an open clutch, only transmission input shafts with very low additional mass inertia are used. This supports rapid synchronizing sequences and a long service life of the synchronizer units.

4 The hydraulic control unit

In the present dual clutch transmission, the hydraulic control unit fulfills the following tasks:

Actuating the dual clutch

Shifting the gearshifts, i. e. engaging/synchronizing the gear

Cooling the dual clutch

Gear lubrication

Emergency stop function in case of complete failure of transmission electronics

Several features in the hydraulic control unit as well as criteria for the selection of the control concept are going to be described in more detail below.

4.1 Performance

The use of the dual clutch transmission in sporty vehicles demands high performance from the hydraulic control unit, especially with regard to the first two tasks because the timely "handling" of these tasks come into play in gear shifting and gear shifting times.

That is why particular value is placed on the selection of the right control unit concept as part of the system design. During the decision process, the choice was made, in principle, between two concepts, Fig. 5.

Fig.5Control concept direct control / precontrol

Precontrol of the valves

Direct control of the valves (so-called cartridge valves)

In case of direct control, the valve that is used for pressure control, e.g. a clutch, is directly connected to the power-generating proportional solenoids and provides the main pressure to the corresponding clutch pressure.

The precontrol uses the pressure that is supplied by a pressure controller, for example, to actuate an additional valve that supplies the clutch pressure from the main pressure.

To assess the performance of both concepts, a larger number of compared measurements were performed with different systems, of which two systems shall be considered here:

ZF hydraulic control unit with precontrol for DCT standard drive

Comparative hydraulic control unit with direct control

A reference clutch was used as the clutch to engage. Criteria for assessing the performance were (see also Fig. 6):

Fig.6Delay, increase/rise, and fall times. Red curve: Power /Electric current. Green curve: Clutch

pressure

Delay time, 1 to 4

Time of step response until clutch inflation pressure, 1 to 2

Time of the step response up to 90% of the main pressure 1 to 3

Time of pressure drop (emptying times), 5 to 6

Fig. 6 shows, as an example, the times for a transmission oil temperature of + 20°C to be reached. One notices that the direct control first in dicates a lower delay time (14.3 ms) compared to the precontrol (30.1 ms), see also time of brand 1to 4.

For increase to clutch inflation pressure or to 90% of the main pressure shows, however, the advantage of the precontrolled system (see also summarizing tab 1).

Emptying times, also present a disadvantage for direct control. Trans-mission oil temperature of -20°C also show comparable results for step responses and fall times.

All of the tests support the statement that direct control has an advantageous effect with small oil volumes. However, if large oil volumes have to be transported, precontrol valves are to be preferred due to larger opening cross-sections.

4.2 Operational safety

Operational safety is determined essentially due to the soiling tendency because the so-called silting can lead to the valves getting jammed. Provocation tests with transmission-specific environmental conditions (dirty oil) demonstrated the influences of soiling on the characteristic curves. Technical, trouble-free characteristic curve progressions could be illustrated only with a high dither amplitude in valve actuation, which leads, in turn, to increased valve wear-and-tear due to the micro movements that it causes. The increased tendency toward soiling can result needing a fine filter.

4.3 Costs

In addition to the delay time comparison as well as assessing the operational safety, the costs were relevant for a final evaluation. The compari son with regard to the hydraulic and electro-mag netic components shows that a precontrol system has cost benefits compared to a direct control system. Added to this are the higher flows with the actuation of direct control valves, which, in turn, result in a more expensive TCU. Furthermore, in opting for precontrol, ZF is able to "pool" together pressure controllers in large quantities because these, too, are used in the automatic ZF planetary gear set.

4.4 Emergency stop function

In case there is a complete outage in the transmission electronics, a hydraulic emergency stop function is actuated in the transmission. The clutch that is pressurized with a larger amount of pressure in the event of a system outage will continue to be pressurized. This condition is maintained until an adjustable engine speed threshold is achieved, then the clutch opens in order to prevent the engine from being choked. It is not possible to re-start this system.

5 Sporty functions

For function developers, the dual clutch transmission offers the opportunity to combine the comfort of a stepped automatic transmission with the dynamics and sportiness of a countershaft transmission. Connected, therefore, are typical " catalog values," such as time from zero to 100 kilometers per hour or the time from 80 to 120 kilometers per hour with correspondingly fast kick-down shifting, but also subjective acceleration sensitivity during a shifting sequence where the purist among the manual transmission drivers still wants to feel that jolt of acceleration.

One function especially designed for the dual clutch transmission in sports cars is the "race start"function. The race start is a function used to achieve optimal acceleration from a

standstill, i.e. in the shortest time from 0 to 100 km/h. The sequence progresses as follows: The engine is brought to a suitably high rpm with the clutch engaged in first gear. The driver simultaneously actuates the brakes with the lef foot so that the clutch can already be lightly engaged and the gas pedal (full throttle) in order to bring the vehicle up to the target speed. By simultaneously pressing and holding an operating element, such as the selector lever or a push button on the steering wheel, the race start intention is conveyed to the system, the engine speed adjusted and the start up prevented until the driver releases the brake. During the race start, the clutch is closed under the control of the wheel slip with which the optimal acceleration is achieved and by exploiting the dynamic engine torque (inertia torque). The entire procedure progresses automatically once the driver releases so that even an inexperienced driver

can achieve the best possible drive performance figures. Obviously, the driver can cancel the procedure by removing his/her foot from the gas pedal or touching the brakes. Also, the system recognizes when the street conditions do not permit a race start, such as wet roads, for example. Due to the optimal start-up and a shifting sequence into second gear free of traction interruption (see also sports shifting), the race start function enables the acceleration time of 0 to 100 km/h to be improved by an average of 0.2 sec compared to a car with a manual transmission. At the same time, this functionality helps avoid improper use and resulting clutch overload.

The top chart in Fig. 7 illustrates the engine and transmission input shaft speed, the lower chart shows the vehicle′s longitudinal acceleration. Starting with a cranking speed of 6,800 rpm, the clutch begins to close, which leads to an engine pressure up to about 4,000 rpm. The dynamic engine torque used to achieve this results in an acceleration of 0.7-0.9 g. In the process, noticeable vibrations in the transmission input shaft speed signal develop due to the wheel slip regulation. After about 1.2 sec, the vehicle is accelerated only by the engine torque with approx. 0.5 g. It must be mentioned here that this test was performed using a vehicle with very high traction. In most cases, a starting speed of only up to about 4,000 rpm is reasonable.

A further function developed for the dual clutch transmission is so-called sports shifting. This is described in more detail below.

In general, a gear-shift change by the driver is only perceived acoustically by the change in the engine speed. The transition from the acceleration level of the original gear to

Fig.7Measurement of a race star

the new gear should be made smoothly and continuously. This also corresponds to the standard shifting sequences in auto-matic and dual clutch transmissions. However, many drivers of sporty cars wish that they had the option of both distinctive comfort shifting sequences as well as sporty shifting sequences, which, besides the haptic response (acceleration jolt), also have an acceleration advantage as a result. To this end, the dynamic engine torque can also be used again. The requirement for this is the torque capacity of the dual clutch which has to be able to transmit this torque increase. As the possible torque increase depends on the gradients of the engine speed, this can be used particularly effectively in shifting gears with a large speed difference with the target gear (large ratio spread/ratio step), which is why the gear changes 1-2, 2-3, and 3-4 are offered. In the process, sports shifting from the frst to second gear can serve as a supplement to the ace start for improving the acceleration time from to 100 km/h. As the use of the dynamic torque is pure application topic, we distinguish, as a rule,between three shifting systems. Fig. 8 illustrates he stylized differences and features between the hifting systems, Fig. 9 shows an original measurement from a prototype vehicle.

The top chart shows the respective engine and ransmission speed, the bottom chart shows the orques from both clutches. The bottom line in the hart represents the clutch from the target gear that is used to achieve the torque increase during engine sp eed adjustment

and thereby acceleration gains.