Optimising transmission performance: the role of lubricants
By: Torsten Murr, Wednesday, August 19, 2009, AutomotiveWorld.com
Mounting public interest in cutting carbon emissions and obtaining best value from consumer spending is driving a flurry of innovations in passenger car technology. Research and Development (R&D) is promoting the commercial rollout of a range of technologies and manufacturing techniques designed to increase fuel efficiency and reduce carbon emissions from fossil fuel combustion. Vehicle parts are being machined to be lighter to reduce total vehicle weight, two speed engine ancillaries are in development (for example by Antonov) to vary gear ratios according to RPM, while tyre dimensions, weight and rolling resistance are also being continually optimised to deliver greater fuel efficiency and reduced emissions.
Many leading automotive manufacturers are focusing R&D efforts on transmission developments to realise greater fuel efficiency, while maintaining overall vehicle performance. Achieving this performance combination could significantly improve vehicle market appeal as it would limit the sacrifices, such as handling, acceleration and extra features, consumers may otherwise have to make when opting for greater fuel efficiency. This is leading to OEM investment in six, seven, and eight-speed, continuously variable, dual-clutch, auto shift manual and select shift manual, adaptive control and electronically-controlled transmission technologies. Audi, for example, predicts that 90% of its models could use dual-clutches in the future, while Ford is planning to introduce automatic six-speed transmissions on more than half of its North American vehicle range through 2009.
Although often overlooked, transmission fluids need to be developed in-line with these technological developments and shifts in production for the performance potential of the different transmissions to be fully realised. Use of a fluid that has not been specifically developed for a higher performance transmission means it is unlikely that intended fuel efficiency benefits can be achieved and can lead to component damage, resulting in costly vehicle recalls or corrective repairs under warranty agreements.
Use of a fluid that has not been specifically developed for a higher performance transmission means it is unlikely that intended fuel efficiency benefits can be achieved and can lead to component damage.
Development of high performance fluids
A high quality, designed-for-purpose fluid provides the correct level of transmission friction control and helps prevent gear slipping and accelerated component wear. In addition, a high performance fluid will offer high temperature and oxidative resistance, shear stability and seal compatibility. These characteristics can help to extend oil life and transmission life and performance, helping to yield greater power output and fuel efficiency.
To support automotive manufacturers in developing fuel efficient transmissions, fluid specialists, with Shell Lubricants in a leading role, have improved their ability to test both component durability and fluid suitability through investment in the development of a range of test equipment. The test equipment covers all areas of automotive manufacturing, ensuring that all transmissions can be tested. This helps fluid suppliers to continuously gauge lubricant performance and efficiency as well as the suitability of prototype fluids.
The Shell Lubricants driveline rig is a prime example of this process. The rig replicates actual driving conditions to test transmissions under a range of vehicle speeds and loads. Furthermore, it can simulate road corners and bends to ascertain the variation in revolutions. This enables accurate identification of the fluid requirements of a transmission during day-to-day operation across its average lifecycle.
This process is supported by screener tests such as the so-called SRV friction test or the four-ball testing, which evaluates the friction and wear characteristics of lubricants. This allows different levels of friction to be screened and simulated in relation to increases in torque and helps determine the required temperature stability of the lubricant.
The test methods also enable manufacturers to focus on delivering solutions for complex areas of vehicles, beyond just the engine. With a range of new high performance transmissions to cater for, a number of measurements have to be in place for appropriate lubricants to be developed.
Changing conditions
It is increasingly important for fluid suppliers to work with OEMs and components suppliers to truly understand the operational demands of new clutch designs and develop high performance fluids. Fluid requirements vary across the range of transmissions and there is no single fluid that can effectively perform in manual, dual, automatic and continuously variable transmissions. Each of the transmission types require different levels of lubricant viscosity depending on the speed they operate at, the level of metal-to-metal contact and the design of the gears and clutch plates. All of these factors affect the level of friction generated during operation, which needs to be accurately controlled to optimise torque and fuel efficiency.
The demands placed on fluids will intensify as OEMs aim to extend oil replacement intervals and use smaller volumes of oil in transmissions.
However, although there is no ‘one-size-fits-all’ lubricant solution, there are similarities between the operational demands of the different transmissions. The demands placed on fluids will intensify as OEMs aim to extend oil replacement intervals and use smaller volumes of oil in transmissions. Furthermore, the transmissions will generally operate at higher speeds and temperatures, exerting further pressure on fluid performance. These factors can lead to high levels of oil stress, reducing lubricant viscosity and oil life and compromising fluid performance and friction control, increasing the potential risk of accelerated component wear and poor transmission performance and power output.
The use of different materials such as plastics and elastomers in seals could present further challenges. These new materials are designed to maximise component durability and improve environmental compliance and transmission fluids need to be developed to maintain seal compatibility to avoid potential leaks and loss of pressure.
Partnerships
Partnership structures with OEM and auto supplier customers are becoming increasingly vital for specialist teams at the major lubricant suppliers. Shell Lubricants teams work increasingly closely with these customers, effectively becoming part of in-house transmission development teams in order to develop non-standard, non-commodity solutions for increasingly demanding specific applications. This has led to key transmission fluid developments, including that used for the six- and eight-speed transmissions developed and manufactured by ZF and now used in high performance and luxury cars. For example, for the development of the six-speed unit, Shell Lubricants played a pivotal role through the development of a bespoke fluid. Ensuring optimum performance from the car, the transmission is lighter, more compact and uses between 5-7% less fuel in normal driving conditions.
Developing and using bespoke high performance fluids will help OEMs to cost-effectively realise the performance potential of the new transmission technologies and meet market demand for fuel efficient and low-carbon vehicles.
Other partnerships have included the development of a bespoke solution for a leading, but un-named OEM which required an improved automatic transmission fluid to meet the demands of its new innovative five- and six-speed transmissions that required a unique lubricant. A low viscosity fluid to meet rigorous product specifications was subsequently developed, which is expected to be used in as many as two million transmissions a year.
The reality is that new transmission technologies are playing an increasingly instrumental role in improving passenger car fuel efficiency. Developing and using bespoke high performance fluids will help OEMs to cost-effectively realise the performance potential of the new transmission technologies and meet market demand for fuel efficient and low-carbon vehicles.
Torsten Murr is Transmission Fluid Technology Manager at Shell Lubricants, which collectively refers to Shell Group companies engaged in the lubricants business. Shell claims the position of global leader in branded lubricants and the largest marketer of lubricants, with 13% (in volume terms) of the global lubricants market.
Published on Wednesday, August 19, 2009
Free e-newsletters
Our free E-Newsletter service is an excellent way of staying up to date with news and features from around the global auto industry.
