03-08-2012, 10:08 AM
ZEROSHIFT TRANSMISSION
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INTRODUCTION
The world is dominated by two forms of transmission, themanual transmission (MT) and the Automatic Transmission (AT). The MT has a single dry clutch operated by the driver whilst the AT uses wet clutches that are automatically controlled. There are pros and cons to both transmissions. The MT is the most efficient transmission available. This lends itself to providing good fuel economy for the vehicles it is employed in. The efficiency of the MT is dependent on the load applied. Over a representative drive cycle its efficiency can approach 97% . The MT is very easy to manufacture having very few parts. It is reliable and easy to maintain. Its main weakness is that it is less easy to drive than an AT especially in congested traffic as it requires the driver to operate the clutch for each gear shift. Gear shifts are sometimes not smooth which causes discomfort for passengers who will notice an interruption of engine torque to the wheels when the transmission is between gears. In severe situations the interruption of torque can cause dynamic instability of the vehicle resulting in loss of control. The AT has many advantages which include ease of driving and very smooth shift quality. The choice of gear is determined by a transmission control unit (TCU) and this will ensure that the best gear is chosen for the vehicle road speed depending on whether the driver requires fuel economy or performance. Drivers are willing to pay a premium for this type of transmission
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even though performance and fuel economy are inferior to the MT. A reduced performance and fuel economy is apparent because of the lower efficiency of the transmission. Not only are there parasitic losses to account for to operate the hydraulic pump, but large amounts of slip in the torque converter generates heat loss which warms the transmission fluid rather than provide torque to the wheels. In recent years a lock up clutch bypasses the torque converter for a large proportion of the journey which can improve the overall efficiency dramatically. Over a representative drive cycle the efficiency of an AT can be as high as 86% . If a transmission could have the benefits of both the MT and AT and the weaknesses of neither, this would introduce a third option to segment the market. Traditionally, the AMT has matched the MT for fuel economy but has suffered from inferior performance (compared to the MT) and bad shift quality. Zeroshift corrects these latter two issues allowing simultaneously good fuel economy, performance and shift quality. The combination of these has not previously been seen in any other transmission.
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ZEROSHIFT CONCEPT
The zeroshift gearbox has a hub assembly consisting of a pair of drive rings, selector frocks, hub and acradle with springs.Shifting is done by a gear change barrel.Barrel is controlled by the signals from TCU. TCU can be set in automatic mode and semi automatic mode. In automatic mode gear change takes place based on engine speed, load. In semi automatic mode traditional feel is retained by introduction of hand selector buttons. The Zeroshift concept is to remove the synchromesh components within a manual transmission and replace them with Zeroshift rings, Figure 1. Synchronisation is performed externally with the assistance of torque intervention performed by the engine control unit and an automated clutch (standard equipment in an AMT). The Zeroshift rings act as a pair and represent a dog engagement type transmission. The unacceptable backlash found in dog engagement transmissions is eliminated by splitting the dog into two halves. The first Zeroshift ring is engaged to take up drive whilst the SecondZeroshift ring is brought in afterwards to take up the backlash. Figure 1.Two Zeroshift rings between two gears.
When initiating a shift from the neutral position, one drivering is engaged with the dog faces on the gear and thesecond ring joins the first axially adjacent which providesdrive in the opposite direction to oppose engine
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overrunand, more importantly, eliminate perceivable backlash. This ring is unloaded and is the ring to make the next shift and as such requires less shift force i.e. much less than synchroniser force (in the order of 1/25th). Figure 2.Neutral position. It is easier to see the actions of these rings if we look at an exploded view, Figure 2. Each pair of rings has three pairs of “bullets”. Here we see just one pair of bullets for illustration purposes. On one side of the bullet is aretention angle to take up the drive whilst on the oppositeside is a ramp face to disengage drive. The unique feature of Zeroshift is that it is able to changegear in zero time, just like a digital switch. We can see inFigure 3 how by shifting the blue ring and its bullets witha shift fork, drive is taken up on the 1st gear (top ofdiagram). Backlash is taken up by shifting the red ring,Figure 4(1st gear engaged).
To perform a Zeroshift gearshift, whilst still driving in 1st gear on the blue ring, the unloaded red ring may be moved to the 2nd gear (bottom of diagram), Figure 5. As 2nd gear is rotating faster, it overruns 1st gear and takes up drive on the red ring allowing the engine torque to be handed over. At no time is the engine torque interrupted from being transferred to the vehicle’s wheels. The gearshift is complete when the original blue ring is no longer loaded on 1st gear and is able to be actuated over to 2nd gear to take up backlash, (Figure 6. 2nd gear engaged). Such a concept has wide application not only because ofimproved acceleration performance but also fuel economy and shift quality. The instantaneous shift is by definition a torque handoverfrom one gear to another. This creates a torque orenergy spike in the driveline which is managed bydissipation through a combination of engine and transmission management, clutch control and system compliance. The result is a continual delivery of torque and unaffected vehicle stability.
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BENAFITS
The main benefits of Zeroshift are: fuel economy performance ease of manufacture
FUEL ECONOMY
The fuel economy of a vehicle is a function of many components including its powertrain. The two main components of the powertrain that determine fuel economy are the engine and transmission. It has been stated by more than one company that to reduce CO2 it is twice as cost effective to develop the transmission rather than the engine. Having developed such technologies, the additional manufacturing cost of engine technology is more than twice that of transmission technology for the same benefit in fuel economy . It is therefore beneficial both in terms of development and manufacturing costs to first seek a fuel economy improvement via transmission technology. To date, the MT has provided better fuel economy than the AT . A technology that has demonstrated on occasions, even better fuel economy than the MT is the AMT. This fuel economy is not due to the in gear efficiency of the transmission as it is no more efficient than the MT but due to the gear shift schedule. On a drive cycle an AMT is able to shift to a taller gear earlier whilst the MT has to stick with a mandatory gear
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schedule. In real driving with the AMT in automated mode, an improvement in fuel economy over the MT can be achieved for the same reason. It is unlikely that the driver will always be in the best gear for all situations. At highway cruise the AMT will be no better than the MT. It may be slightly worse if parasitic losses are present to operate a hydraulic pump continuously. Zeroshift is an AMT which has no significant parasitic losses when in gear. A small amount of power is required during the gear shifting. There is no hydraulic actuation so there are no parasitic losses to operate a hydraulic pump. Both the gear actuation and clutch control can be electrically operated from the standard 12V vehicle electrical system.