As automakers search for weight reduction in all parts of the vehicle, Matthew Beecham spoke to Mark Findlay, managing director at UK engineering consultancy, Drive System Design (DSD) about how weight savings might be made in the transmission and driveline.
Just how important could the weight savings be?
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The next breakthrough in CO2 emissions reduction is likely to come from more aggressive weight reduction in major powertrain components. Every ten percent reduction in vehicle weight can cut fuel consumption by about 5-7 percent and, as all the easy gains, the so-called ‘low hanging fruit’ – have now been taken, manufacturers are searching more widely.
Typically the transmission is the third heaviest system on the vehicle, after the body structure and engine but, if taken to include the driveline and axles, it approaches the number one position. While extensive research has been undertaken into aluminium intensive body construction and ultra high strength steels, transmissions and axles are largely still manufactured from ‘conventional’ materials.
For example, despite considerable progress in manufacturing process refinement and component design optimisation, the transmission industry still relies largely on aluminium and iron castings for the majority of structural casings and covers. This reflects the considerable body of existing knowledge and historical investment in these techniques. However, with the potential to save between 20 and 30 percent in component weight, pressure is growing to switch to lighter materials.
Where does the process start?
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By GlobalDataTo identify potential alternative materials for a transmission casing, the primary functions of such a component must be considered. It has to resist internal loads created by the transmission of torque from input shaft to output shaft(s); transmit beam loads resulting from powertrain mounting requirements; contribute to powertrain bending stiffness; contain all lubricants and transmit internally generated heat out through the casing walls. With an anticipated vehicle life of between 10 and 20 years, satisfactory material longevity must also be considered in order to avoid any deterioration in mechanical properties.
Alternative materials processed by fabrication, composite construction, injection moulding and forging, present weight saving opportunities but also raise a number of design challenges in order to satisfy requirements for cost, strength, durability, lubrication, cooling and NVH.
Significant progress has been made in recent years in the development of new structural materials. These materials, which include: advanced ceramics; polymers; metals; and hybrid composite materials derived from these open up new opportunities for incorporation into transmissions.
Components, such as the gears and shafts, are highly stressed and are already near the limit of the steel properties that these parts are commonly made from. This is driving the development of new steel formulations. Therefore, it is more likely that plastics will first be used in less stressed components, such as the transmission casing.
What are the biggest obstacles to be overcome?
In common with many new materials and techniques, the reliable mass production of components at a competitive cost remains an obstacle long after the technical requirements have been satisfied. Experience gained from composite applications for vehicle bodies on the one hand, and small powertrain components on the other, has not yet led to the kind of production-feasible light weight composites that can replace heavy, structural castings.
We have been able to identify preferred material options for these challenging applications, such as metal or polymer matrices, based on manufacturing costs and the required component properties. We have also been able to broaden awareness of their potential.
Effective implementation of these innovative construction methods and materials also requires a very different design approach to traditional castings. DSD has a clear understanding of the fundamental material characteristics and failure modes, backed by well established and proven analytical techniques to predict NVH behaviour, structural stiffness and system performance reliably. Two further challenges remain, adoption in volume by the supply chain and better understanding of directional material properties. DSD is collaborating with one of the world’s leading plastic developers to understand the latter and is conducting research projects with OEMs and Tier 1s to stimulate the supply chain.
Which materials or technologies have emerged as the front-runners?
The structural materials are classified as ‘ceramics’, ‘polymers’ or ‘metals’ and two or more of these materials can be combined to form a composite that has properties superior to those of its constituents. Composites generally consist of fibrous or particulate reinforcements held together by a common matrix. Continuous fibre reinforcement enhances the structural properties of the composite far more than particles. However, fibre reinforced composites are also more expensive and difficult to produce for a component as complex as a transmission housing.
In the short term, we believe the solution requiring least disruption to existing automotive supply chains would be based on metal matrix composites (MMCs) that use filaments, whiskers or particles of high strength materials to enhance the properties of the base matrix in critical areas. Such materials are already in commercial production and we expect that selective reinforcement of a conventional casting by the use of MMC inserts will enable the use of lighter ‘thin wall’ designs with additional strength provided only where necessary. Research is being carried out to develop casting technologies that will enable the production of thin wall designs without the risk of porosity.
A more ambitious solution is possible in the medium term, using polymer matrix composites (PMCs). Lightweight polymers are already popular for non-structural covers, often incorporating metal inserts where fasteners generate local clamping forces. To handle the high structural loads found in transmission and axle casings, we propose the inclusion of larger metal inserts into the mould, forming a metallic skeleton to achieve the required strength in specific areas. By injecting the polymer around the metallic inserts, a hybrid structure is created that could be significantly lighter than a traditional design, without incurring additional costs.
Both these solutions avoid the labour intensive manufacturing processes associated with carbon composites because the time for resin application can be shortened through increasing injection pressure. Carbon composite materials are likely to be confined to niche vehicles in the medium term because it would require the development of new automation techniques to achieve cost-effective cycle times for the accurate lay-up of the laminated layers.
Do plastic casings offer other advantages or disadvantages?
The remainder of this interview is available on just-auto’s QUBE Global light vehicle transmissions and clutches market- forecasts to 2030
