Transmission considerations: beyond the manual gearbox.

As far as the majority of European drivers are concerned, the only transmission worth considering is the manual, which accounts for four out of every five gearboxes found in new cars sold in the region. There are two key factors in this dominance: (1) the extra cost in purchase price of automatic transmissions; (2) the in-built prejudice against automatics. The typical European driver still believes that not having a shift stick dilutes control of the car. "In Europe, if a customer can obtain an air conditioning system or light-alloy sports wheels for the same extra cost as an automatic transmission, the majority choose the other options," said Dr--Ing Gerhard Wagner, group vice president, ZF in his plenary speech at the Innovative Automotive Transmissions conference organized by the Car Training Institute in Berlin at the beginning of December 2005. "However, it should be noted that the extra cost charged for an automatic transmission is not in any way related to their manufacturing costs. Instead, the whole issue is governed by the pricing policy of the vehicle manufacturers which still determines that some items of equipment are cost options."

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Wagner continued, "The figures quoted in sales brochures for fuel economy, emission levels and 0-100 km/h acceleration times are the next obstacle to overcome when selling a vehicle equipped with an automatic transmission. If these figures are significantly worse than those of a vehicle with a manual transmission, European customers tend as a general rule to favor the manual transmission. As a consequence, the requirement for low fuel consumption and good performance figures form an essential component in any automatic transmission development work."

Things, though, could be about to change in Europe as the transmission has now come into the front line in the carmakers' quest to meet emissions legislation. "The primary requirements and customer benefits underpinning the further development of automatic transmissions," said Wagner, "include a reduction of fuel consumption, a reduction of pollution, vehicle performance improvement, increasing torque capacity, greater comfort, sportiness, noise reduction, resource conservation, and competitiveness."

Over the last 10 to 15 years, various automatic transmission technologies, including step automatic transmission, AMT (automated manual transmission), CVT (continuously variable transmission), and DCT (dual clutch transmission), have been developed. The jury is now out as to which of these will prove the most practical or, more likely, the one most accepted by the public. What has been noticeable in Europe over the last years is that the transmission has started to appear in automotive advertising as a feature for differentiating between vehicles. Examples of this include the Audi Multitronic, the ZF Mytronic and the Mercedes-Benz 7G Tronic. However, it is the dual clutch, otherwise known as the "twin clutch," transmission, with its extremely sporty image and ability to satisfy the comfort requirements drivers look for in an automatic, that may be set to make an impact on the transmission market. It also has the added bonus of being praised by the enthusiast press. This transmission is based on a countershaft design that essentially comprises two inter-related manual transmissions with two input shafts, one for the odd numbered and one for even numbered gears which share a single output shaft. Powershifts are undertaken using the dual clutch which can switch engine power between the two transmission sectors under load with no interruption to traction.

Developed for use in front-transverse driven vehicles based on the Golf platform, Volkswagen, working closely with BorgWarner, was the first carmaker to bring it to volume production with the DSG250 ("dual shift gearbox," in its parlance) in 2003. It is now offered on vehicles as diverse as the VW Golf, the VW Touran, a small minivan, and the 3.2 liter Audi TT. The price premium for the VW DSG can be compared with that for a 6-speed multi-ratio transmission. Comparing like for like under test conditions, a DSG version of the VW Golf R32 shows a 3 to 10% improvement over a manual version in 0-100 km/h acceleration times.

By the end of '05, the DSG transmission accounted for 90,000 units for the German group which, while a relatively small number, is just the start. Viewed as a future trendsetter in specialist circles, it is under intense development by almost the entire European automotive industry. The work is focusing on mid-range and high-torque transmissions with wet clutches and hydraulic actuation and on low torque transmissions with dry clutches and electro-mechanical actuation. Development activities also cover virtually all driveline configurations, although it will mainly be used by vehicles with front-transverse drive. Other drive configurations are due to follow later.

However, this does not mean to say that the dual clutch transmission will elbow out other alternative automatics, just that it sees it in pole position. While the demand for improved fuel economy in Japan has generated a trend towards CVTs for front-wheel driven vehicles, particularly in the B and C segments, there is no such enthusiasm for this type of transmission in Europe. This is despite various carmakers, including Audi with its Multitronic, Ford with the C-Max, and Mercedes-Benz with the latest generation A-Class, promoting it over the last few years. In front-transverse applications, when compared to the familiar automatic 4-speed multi-ratio transmissions traditionally used in this sector, CVT units achieve fuel economy benefits of up to 10% and a 5% to 9% improvement compared with the 5-speed automatic. When compared with a modern 6-speed multi-ratio transmission, though, there is hardly any difference in fuel economy figures although the CVT is slightly faster when it comes to acceleration. While the CVT offers the highest standard of operating comfort of any transmission system, there is one thing that militates against its success in Europe, and that is its "rubber band" image that remains firmly fixed in the public mind. Although very few actually drove such a transmission when it was available in the 1970s, such was the withering criticism of the DAF Variomatic that it left an indelible mark on the public psyche. No matter how good modern CVTs are with the incorporation of open-loop and feedback systems, nothing will shift public opinion.

Another transmission that has also been slightly stigmatized in Europe is the automated manual transmission (AMT). "Ride comfort is a major criterion for any automatic transmission, but automated transmission systems differ substantially in terms of the level of shift comfort and spontaneity they afford when changing gears," said Wagner. "In the case of automated manual transmissions, this has given rise to customers going so far as to refuse to accept this kind of transmission system in certain vehicle segments, as a consequence of which vehicle manufacturers have stopped offering this option in those areas. It is therefore clear just how important the criteria of shift comfort and spontaneity are as key customer preferences in any development work on automatic transmission systems."

Conversely, the image of the new automatic 6- and 7-speed transmissions has helped them to gain very rapid market penetration in Europe. Wagner noted that following the launch of the new-generation ZF 6-speed automatic transmissions in 2001, the 5-speed unit had been more or less phased out by 2005. When compared to their 5-speed counterparts, the modern 6- and 7-speed automatic transmissions have been able to deliver very significant benefits to customers, he said. They are able to achieve fuel economy improvements in excess of 5%, while further measures being undertaken by ZF inside the automatic 6-speed transmissions scheduled for launch next year will further reduce fuel consumption by between 3 and 6%. Figures published for the 7-speed Mercedes-Benz transmission have recorded a 6-11% better fuel consumption than the earlier 5-speed transmission in the European driving cycle. They are also capable of cutting the acceleration time from 0-100 km/h by 4%.

These new transmissions are being employed primarily on rear-wheel-drive vehicles in the mid-range, luxury class and SUV sectors in Europe. More recently, though, automatic 6-speed transmissions have also made their appearance in front transverse applications. While the advent of the dual clutch transmission has raised the bar for the developers of the multi-ratio automatic transmission ZF claims that its newest 6-speed transmission, following an upgrade to the electronic control unit, matches the response times of a dual clutch transmission. "In the past, multi-ratio automatic transmissions based on planetary design principles were developed very much with comfort as the key priority," said Wagner. "Developers consequently paid relatively little attention to the notion of sports performance in gear change operations. However, the introduction of the 6-speed transmission has seen an improvement in the responsiveness, which is becoming more the focus of development."

There is not one transmission system that ideally satisfies all the applicable requirements, said Wagner. Selecting a transmission depends greatly on the vehicle parameters while different vehicle driveline configurations also require different solutions. There is also the factor of customer acceptance in different markets. "In the long term, the cheapest system will win over the largest section of the market," said Wagner. "When looking at costs, though, there is a key point that should not be forgotten and that there is not a one size fits all solution. Different systems can each constitute a commercial optimum for their respective and different driveline configurations."

By William Kimberley, European Correspondent, [e-mail]