1、外文资料 1 Transmissions Transmissions have to compromise on either ride comfort or efficiency, but a new approach to the dog engagement gearbox could improve both. With tightening emissions regulations, carmakers are not just confining their efforts to improving combustion and after-treatment. Many are
2、 finding that modern engines are so advanced that the benefits of some engine technologies are small compared to the huge development costs involved. Its important to look at the whole vehicle in order to improve emissions. As the second most expensive piece of kit in the car, the transmission is th
3、e logical next place to look. Of all transmission technologies, the manual gearbox is the most efficient； around 96percent of the energy that is put in comes out of the other end. But not everyone can drive one or wants to. Because you have to dip the clutch pedal, its less comfortable to drive in h
4、eavy traffic. It makes the driver tired and the torque interruptions head-nod effect on passengers can be wearing. The drivers clutch control and corresponding torque interruptions are also the manuals weak point. When accelerating up through the gearbox, each up-shift requires the driver to cut the
5、 torque momentarily by lifting the gas pedal and dipping the clutch. It may just take a second to complete the operation, but during this time the vehicle is losing speed and acceleration. At the opposite and of the spectrum is the traditional automatic. Its shift quality is good thanks to its torqu
6、e converter, but efficiency is relatively poor despite recent advances. Because of this ,a lot of the current research is trying to find an efficient alternative to the conventional automatic. The main technologies are continuously variable transmissions (CVTs)； dual clutch transmissions(DCTs) and a
7、utomated manual transmissions(AMTs).They all offer different benefits over the conventional planetary automatic. The CVT uses a belt chain or toroidal shaped dish drive to vary an infinite number of gear ratios. It has improved efficiency and cost when compared to conventional automatics.Its advanta
8、ge comes from its simplicitu. It consists of very few components； usually a rubber or metal-link belt； a hydraulically operated driving pulley, a mechanical torque-sensing driving pulley, microprocessors and some sensors. The transmissions works by varying the distance between the faces of the two m
9、ain pulleys.The pulleys have V-shaped grooves in which the connecting belt rides. One side of the pulley is fixed axially； the other side moves, actuated by hydraulics. When actuatec, the cylinder can increase or reduce the amount of space between the two sides of the pulley. This allows the belt to
10、 ride lower or higher along the walls of the pulley, depending on driving conditions. This changes the gear ratio. A torodial-type design works in a similar way but runs on discs and power-rollers. The stepless nature of its design is CVTs biggest draw for automotive engineers. Because of this, a CV
11、T can work to keep the engine in its optimum power range, thereby increasing efficiency and mileage. A CVT can convert every point on the 外文资料 2 engines operating curve to a corresponding point on its own operating curve. The transmission is most popular with Japanese carmakers and Japanese supplier
12、 JATCO is a major producer. But in the US and Europe driving styles are different. Uptake has been slow despite Audi and other manufacturers having offered CVT otions on their ranges. The DCT is, in effect, two manual gearboxes coupled together. Gear shifts are made by switching from one clutch on o
13、ne gearbox to another clutch on the other. The shift quality is equal to a conventional automatic, but slip, fluid drag and lydraulic losses in the system result in only slightly improved efficiency and acceleration over the conventional planetary automatic. Developing the control strategy is costly
14、 too. Recent advances in conventional automatic technology have weakened the argument to develop and set up production for CVT or DCT. says Bill Martin, managing director of transmission firm zeroshift. Some carmakers have cancelled DCT projects because of the cost. The cheapest way to build an auto
15、matic is with an AMT. AMTs use actuators to replace the clutch pedal and gear stick of a conventional manual. They keep the high efficiency and acceleration of a manual gearbox, but the shift quality on some models is lacking. Torque interruptions and the head-nod effect are the most common complain
16、t. so what is the alternative? There are always new ideas in transmissions, but Zeroshift says that its technology has efficiency benefits over a manual, delivering fuel economy improvements to city driving. Shift quality can also be equal to that of a refined automatic. Zeroshifts approach is an up
17、grade to the AMT. The synchromesh is replaced with an advanced dog enqaqement system. Dog engagement has been used for many years in motor sport to allow fast shifts. Conventional dog boxes are unsuitable for road use as the large spaces between the drive lugs or dogs create backlash, an uncomfortab
18、le shunt caused by the sudden change in torque direction. Zeroshifts technology solves this problem by adding a second set of drive dogs. It has also made each of the two sets of dogs only capable of transmitting torque in one or other opposing directions. By controlling the engagement and disengage
19、ment of the two sets you can shift into the new gear befor disengaging the previous gear, says Martin. The shift quality is smoother than a typical modern six-speed automatic luxury car. The shift is instant and the torque is not interrupted.This philosophy is used for both up and down shifts. In co
20、nventional AMT there is an emissions spike during a shift due to the need to back off and reintroduce throttle, this is eliminated by going seamless, says Martin. This also reduces fuel consumption. It is a relative newcomer to the transmission sector, but the firm says that it is already attracting
21、 the attention of major European and US carmakers. The big draw is as a low-cost alternative to DCT, says Martin. Because the manual gearbox architecture is largely maintained, production costs and complexity are not greater than for a conventional AMT. Development of the controls 外文资料 3 side is als
22、o considerably cheaper. Music to the ears of engineers trying to cut emissions and costs. Most of the carmakers have seen the system at least once, says Martion. Some signed us immediately. Some have said not yet. None have said no. That may be the clearest sign yet that when it comes to powertrain
23、developments, carmakers are starting to focus on the transmission. HOW ZWROSHIFT WORKS The hardware consists of two sets of bullets. mounted and actuated on two independent bullet rings. both sets of bullets run on the common hub, which is attached to the shaft with splines. Each bullet has a specia
24、l profile. On one side they have an angled face for engagement. These are diagonally opposed, allowing the bullet to have a drive function for one gear and an overrun function for the other gear. The engagement faces taper backwards slightly to ensure the bullet latches onto the engaged gear under l
25、oad. the opposite corners have a ramp, which pushes the bullet out of the previous gear once the new gear has been engaged. In neutral both bullet rings are positioned midway between the ratios. To select first gear, the bullets are moved into mesh with the engagement dogs.The bullets are actuated v
26、ia shift forks conected to the shift actuators. The driving bullets lock first gear to the output shaft and transfer torque from the gearwheel onto the output shaft. The first gear overrun bullets are also moved into gear to lock the wheel to the output shaft in the opposite direction. This transfrs
27、 torque from the gearwheel onto the output shaft when the throttle closes and the engine overruns. This eliminates the backlash youd expect from a dog engagement gearbox. To shift up with an open throttle, first gears overrun bullets are unloaded and move in to engage second gear. This is followed b
28、y the previous driving ring which becomes unloaded when second gear is taken up. If the bullet is stopped from engaging fully-dog-face to dog-face-the second gear wheel opens an engagement window due to the relative speed difference. With the bullet pushed against the engagement dog compliance betwe
29、en the fork and actuator allows the stored energy to fire the bullet into the window. The first gear overrun bullets have now become the second gear drive bullets. As second gear takes over, the load is removed from the first gear drive bullets. These bullets are now no longer held by their retention angie and can be either moved out of gear by actuators or pushed out of gear by contact with the ramp face of the bullet. The first gear drive bullets then move across into engagement with second gear. In second gear, the roles of the bullets are reversed. Audi Roadjet
