DUAL CLUTCH TRANSMISSION M.ARUNA D.MADHUSREE

06741A0304 06741A0324

aruna _mech19@yahoo.com +91-9490982320 DEPARTMENT OF MECHANICAL ENGINEERING INTEL ENGINEERING COLLEGE

ANANTAPUR.ABSTRACT

In the development of automobiles transmission system occupy a very important position. To get the efficient transmission system with smooth running, the present day automobile industries are using a new technology for power transmission, which came in to light recently.

This paper mainly focuses on the transmission system of an automobile and dealt with the modifications made to existing transmission system for reducing the jerky motion and to increase the efficiency of the vehicle by using the new technology called Dual Clutch Transmission (DCT)

This paper elucidates the concept of DCT and the mechanism behind the DCT. The disadvantages in the automatic transmission, manual transmission and a clear cut of the application and advantage of DCT are discussed here. The paper emphasizes on the tremendous impacts that DCT will have in the near future.

INTRODUCTION

A few years ago no self-respecting driver would get excited about what type of gearbox was fitted to his car. But now things have changed. Due to the abrupt changes in the automobile technology the technically developing countries like India are mainly focused on the new technologies like Dual clutch transmissions. For generations of drivers, the automobile transmission has been defined by two choices: automatic or manual. Each offers its own set of advantages: smoother and easier operation for the automatic versus better economy and more driver control for the manual. For both of these the main basic component is the transmission system. The transmission system consists of the clutch, differential, and couplings. Out of these clutch plays a major role to engage and disengage the gears. It is a friction coupling. The main principle involved in the operation of a clutch is friction. When two surfaces are brought in contact and pressed they are united due to the friction between them. If now one is revolved the other also revolves. When properly operated it prevents the jerky motion of the vehicle and avoids the undue strain and the remaining parts of the power transmission system.

PRE-REQUISITES OF A CLUTCH

The clutch should be in a position that it should transmit maximum torque of the engine.

The clutch should be able to dissipate large amount of heat that is generated during the clutch operation due to friction.

It should be dynamically balanced. This is particularly required in the case of high speed engine.

The clutch should have suitable mechanism in order to reduce noise and to damp vibrations during the power transmission.

The clutch should be as small as possible.

The weight of the driven member of the clutch must be low, so that it will continue to rotate for any length of time after the clutch has been disengaged.

COMPARISION OF EXPERIMENTAL RESULTS OF MANUAL AND AUTOMATIC CLUTCHES

MANUAL AUTOMATIC

Acceleration 0-60 mph (0-100 km/h) sec

8.9(9.4)10.4(10.8)

Top speed mph (km/h) 130(210)127(205)

Performance (urban)

mpg (l/100km)

22.2(12.7)19.8(14.3)

Performance (extra urban) (l/100km)

39.6(7.1)37.9(7.4)

Carbon dioxide emissions gm/kg

219239

Preferences for one transmission type or the other vary, not only among drivers but also among regions. U.S. consumer preference has swung heavily over the years toward the automatic transmission; while as many as 90 percent of European drivers prefer the manual. It is in these markets that a third transmission type - a hybrid of the other two - offers the potential for future dominance.

In order to have both the advantages of the manual and automated technologies new technologies like Dual clutch transmissions (DCT) (Automated manual transmissions) have been invented. Automotive products showed the first modern idea for a dual clutch mechanism more than 20 years ago and called it the "hot shift gear box". The idea is to share the ratios in the gearbox between two clutches rather than one as normal. So odd ratios are controlled by one clutch and evens by the other. Imagine that the vehicle is accelerating in second gear. The gear box computer has already selected third, so when the gear is changed, one clutch opens and the other closes--the gear selection has already been done.

AUTOMATED MANUAL TRANSMISSIONS (AMT)

Automated manual transmissions are more efficient compared to full-blown autos, mainly because they don't demand a constant stream of energy-gobbling hydraulic pressure to work. But there are Mechanical limitations and Automated manual transmissions have been criticized for both shifting too slowly and 'nodding head' effect they impart during gear changes. In reality they are fine for small, fuel-efficient city cars but for all but the most exotic sports applications--their days are numbered.

Software and actuators are also developed that can control a variety of power trains, including electric, hydraulic, and pneumatic systems as well as conventional internal combustion engines and transmissions. A full-authority vehicle controller that includes an AMT and clutch system with software that automatically shifts gears when engine and road conditions warrant. The shifting is transparent to the driver, much as one would experience with an automatic transmission.

Because the system's hardware is based on a manual transmission, however, AMT-equipped vehicles achieve improved acceleration, fuel economy and emissions in comparison to those equipped with conventional automatic transmissions. The design eliminates the automatic's torque converter, transmission fluid pump and multiple clutches, reducing transmission weight by approximately one-third. The AMT is also less expensive to build than the automatic transmission that dominates the American market.

The configurations for an AMT are quite varied. Some have clutch pedals and some have shift levers, while some have neither, and others have combinations of these. However, even with all these possible derivatives there are only three basic AMT types:

Single-sided clutch (SSCT)

Double-sided clutch (DSCT)

Dual clutch transmission. (DCT)

SINGLE SIDED CLUTCH TRANSMISSION (SSCT)

The SSCT uses a current production manual transmission but incorporates actuators to automatically operate the clutch and gear shifting function. This type of AMT is by far the most cost-effective because it is able to use a conventional transmission without modifications to the existing gears and shafts. The power interruption is shorter than in a conventional manual transmission because the AMT engages/disengages the clutch and moves the shift lever faster than a human can, and also pre-stages the synchronizer in advance of the upcoming shift. Moreover, it precisely adjusts the engine speed - again, faster than a human can - to achieve near-synchronous shifting.

However, in situations where the power interrupt interval must be reduced even further by double-sided clutch transmission (DSCT)

DOUBLE SIDED CLUTCH TRANSMISSION (DSCT)

The shift quality that is derived from a double-sided clutch is superior to that of a single-sided clutch because the total clutch travel time is dramatically reduced. In demanding applications where even better shift quality is required, the continuous power transfer characteristic of a dual clutch transmission (DCT) configuration is required. Figure 1 shows the mechanism of DSCT

Figure.1

DUAL CLUTCH TRANSMISSION (DCT)

The other name for DCT is "DOUBLE SHIFT GEAR BOX". The main idea is to share the gearbox between two clutches. The DCT substitutes double clutches for the current production single-sided clutch to allow power from the engine to be transferred in parallel through two power paths. The housing containing both clutches would be connected through a concentric shaft system to two sets of gears, one set being the odd gears (1st, 3rd, 5th)

Figure.2

and the other the even gears (2nd, 4th, 6th). Synchronizers mounted on the input shaft would handle similar loads, allowing the output shaft to remain similar in construction to today's conventional two-shaft manual transmission.

Figure.3

In DCT, the torque flow of the wheels is not interrupted during the gear changes and the gear selection is ingenious. In all the modern gearboxes, gear teeth are cut at an angle to reduce noice and this creates the axial thrust along the centre line of the gears, trying to force them to slide away from each other.

According to Antonov principle, this force was balanced by centrifugal force in the rotating components to open and close clutches and transmits the drive through different sets of ratios. Figure.3 shows the working of the Antonovs principle. Lightweight is the major feature of all the new transmissions, despite the extra gear ratios.MECHANISM OF DUAL CLUTCH TRANSMISSION

COMPARISION BETWEEN THREE AMTS:

SSCTDSCTDCT

PerformanceIt shows poor performance among the remaining transmissions due to power interruption.It is the modified SSCT. Here the power interruption is decreased by a large extentIn this regard, the DCT rates highest among the three systems because it maintains continuous power during the shift.

Cost/weight/packagingThe cost, weight is same as the DSCT The DSCT is not so expensive, because the it does not require more maintenance; the system is not so complicated.The DCT is more expensive than the DSCT because it is more complex, has a greater number of parts and requires a longer transmission housing. The same factors also make the DCT dramatically heavier than the DSCT.

Vehicle fuel consumptionFuel consumption is more due to power interruption.Fuel consumption is decreased and some losses exist during the transmission of the gears.The DCT's lack of power interrupt also contributes to improved fuel consumption relative to the DSCT, but there may be some additional energy loss within the DCT transmission during the clutching event as one clutch disengages and the other engages. Because these clutches are modulated to provide a smooth transition, there would have to be some high-torque slippage of the clutches, resulting in some energy loss.

ADVANTAGES OF DCT

It is lightening fast and most importantly for smoothness and efficiency.

There is no interruption of torque from the engine to the wheels.

Ratios are wider spread than a 5-speed optimizating both acceleration and fuel consumption.

It is also 15mm shorter and 14kg lighter than a 5-speed equivalent and has a new type of torque converter, which disconnects completely at idle, rather than creeping, to further reduce consumption.

DISADVANTAGES

Due to the heavy multiple friction/separator plates, longer clutch plate housings, longer input shafting, an additional piston and additional fluid required for clutch actuation and cooling all contribute to the weight penalty. The DCT contains two clutch assemblies, each comprising a large number of friction plates and separator plates. Consequently, the clutch housings and shafts and their integral splines are longer and more difficult to fabricate. Both systems utilize a pump and control valve to actuate the clutches. However, the amount of fluid required to operate and cool the DCT is dramatically greater.

FUTURE SCOPE AND CONCLUSIONS Now the Mercedes-Benz is attempting to go one better with the first seven speed automatic passenger car transmissions, the 7G-Tronic reduces the fuel consumption by about 5% and also improving acceleration marginally. It will be employed in new SL-500.

By 2008, several other customers beyond VW also will use the dual-clutch technology. Volkswagen AG has been BorgWarner's development partner for DCT, which appears on VW, Audi, Skoda and SEAT-brand vehicles. VW refers to the transmission as DSG (direct-shift gearbox).

REFERENCES

AUTOCAR INDIA (magazines).

Summer 2004 Technology Today---SwRI publications.

Automobile technology---Dr.N.K.Giri.