Reducing vehicle weight is considered a straightforward way to improve fuel efficiency. Sridhar Lakshminarayanan provides an overview of today`s materials and their relative merits and demerits in light weighting.
With environmental protection agencies across the world raising concerns over the world where our future generations would be living in, stringent norms on Green House Gas (GHG) emissions have come into effect. As road vehicles are one of the major sources of GHG emissions, it has become a business imperative for automotive industry from the environmental and social responsibility perspectives to produce vehicles with lower GHG emissions. Designing more fuel efficient vehicles, alternative fuel vehicles, Hybrids and Electric vehicles (xEVs) have been thought of as the means to curb the GHG emissions, as less amount of fuel (fossil) burnt to travel the same distance amounts to less amount of noxious emissions.
Several industrialised nations have given car and truck makers specific and time-bound emission targets. For example, the US Environmental Protection Agency`s Corporate Average Fuel Efficiency (CAFE) target is 95 gm of CO2/km (fleet average) by 2025 which translates to a fleet average of 54 mpg, 95 gm of CO2/km for cars in the EU by 2020 and so on. Such emission regulations are expected to come into effect in India from 2017 and the target is predicted to be around 117 gm of CO2/km by 2025.
Apart from these statutory regulations, the need for fuel-efficient vehicles is complemented by the consumers` growing preference towards such vehicles in the wake of the spiralling fuel prices. Emerging technologies such as xEVs are still several years away from being viable options (both from economical and engineering standpoint). Reducing vehicle weight is considered a relatively straightforward way to improve the fuel efficiency of road vehicles (Figure 2) to ensure steady progress towards the target by 2025. Statistics-based studies have shown that a 10 per cent reduction in vehicle mass brings about a 6-8 per cent improvement in fuel efficiency.
The path to construction of lighter vehicles isn`t as short and simple as it appears. As occupant safety is an immutable statutory requirement in the road transportation industry, the quest for building lighter vehicles cannot undermine crashworthiness compulsions. To remain competitive in the market, automakers are also under immense pressure to include various safety (active and passive) and comfort systems which add a lot of weight to the vehicle. Considering all these factors, it is abundantly clear that `automotive light weighting is a heavy weight challenge`. Neither direct replacement of existing materials with lighter ones nor mere optimisation of existing designs will overcome this challenge. It is becoming increasingly obvious that only a properly engineered equilibrium between Geometry, Gauge and Grade will help the automotive community design and produce lighter vehicles, with no compromise in performance. This is what the industry today refers to as `3G optimisation`.
Opportunities for weight reduction
In an average passenger vehicle, powertrain, body and chassis/suspension amount to about 75 per cent of the total vehicle weight while these three systems in an average light truck add up to 98-99 per cent of the total weight. No weight reduction is too little in this context and any breakthrough achievements in powertrain, body, and chassis/suspension systems will bring measurable reductions in the overall vehicle weight.
Secondary benefits: Any considerable reduction in vehicle weight will also bring with it secondary benefits by way of mass de-compounding. Reduced vehicle weight further enables the potential to resize/optimise the powertrain, body and suspension systems, resulting in secondary mass reduction which is referred as mass de-compounding. Reduced vehicle weight also reduces wear on braking components and tyres extending their life, thus bringing in tertiary benefits in the form of reduced maintenance costs.
Materials: No single material is the clear winner in this race, as each material brings with it a penalty of some form or the other. Experts have opined that the light weighting brainteaser is all about the intelligent mix of materials. An overview of today`s materials and their relative merits and demerits is discussed in brief here.
Life cycle, drive cycle, end cost, recyclability and impact on environment are the major parameters that govern the choice of materials.
Aluminium: Due to its unparallelled combination of strength and low density, aluminium is the most sought-after material in the weight-conscious transportation industry (road and air). It has replaced steel in quite a few structurally intense applications in road vehicles. Higher strength-to-weight ratio and nearly complete recyclability are the two major factors that work in favour of aluminium as `the` lightweight material. The recent success of the Ford F-150 truck (2015) with an all-aluminium body has triggered much discussion in the industry. As high strength alloys (5 series, 6 series and 7 series) are becoming increasingly popular in the industry, aluminium is likely to play a pivotal role in this race.
On the flip side, complexity in joining methods, energy-intense manufacturing processes (relatively more emissions before the vehicle hits the road), loss of strength at elevated temperatures, and higher cost prevent it from becoming the unanimous choice. Aluminium has to improve its basic stiffness and consume less energy for manufacturing to enhance its share in a vehicle`s material composition.
High Strength Steels (HSS): Steel, which is considered the mother of all metals in the industry, is continuously rediscovering itself to stay in the race amidst all this talk about light weighting. It is believed that there are about 180 grades of steel in use today. Intrinsically higher strength, numerous grades for various specific applications, high strength at minimum cost differential, formability, abundance, matured manufacturing and joining technology make steel the primary choice. Steel giant Arcelor Mittal claims that VW replaced aluminium with HSS to reduce weight and cost in the 2013 version of Golf.
On the other hand, corrosion issues and low strength-to-weight ratio make steel lose some of its share to competition. Pohang University in South Korea has developed a new lightweight steel which is 13 per cent lighter than conventional steel and whose strength-to-weight ratio is comparable to that of titanium alloys at one-tenth the cost of titanium alloys. This indicates a lot of promise for steel as a serious contender.
Magnesium: Magnesium being lighter than aluminium and possessing reasonable basic strength is slowly finding its way into the auto industry. Due to its ductility, it is used in energy absorbing applications. Its peculiar feature of producing thin walled castings (2.4 mm thick) and low density stand in its favour. However, poor availability, cost, immature joining methods and corrosion are the challenges for magnesium.
Metal Matrix Composites (MMC): The possibility of combining various material systems (metal-ceramic-non-metal) gives the opportunity for unlimited variation in mechanical properties at lower densities. Thus, MMCs are a good future choice when it comes to light weighting. MMCs are being tested as cylinder liners for aluminium engine blocks - if this concept becomes successful, significant weight reductions are a distinct possibility in powertrain applications. Bonding issues, cost, and commercial availability are some of the factors that need improvement.
Carbon Fibre Composites (CFC): CFCs with superior strength-to-weight ratio and durable performance make them the most fancied material in the industry. However, CFC is not today`s mainstream technology. Extremely high costs and bonding issues make CFCs a choice that is still some years away. Manufacturing technology and infrastructure have to be enhanced for mass production. >
Plastics and other composites: Ease of producing complex 3D parts, low densities, and low costs make plastics a preferred choice over metals for moderate structural applications. However, their brittleness and vulnerability to heat limit their applications.
Each structure in the vehicle has a definite structural duty and the onus is on the structural engineer to identify the right material with optimum strength and its distribution. Traditionally, we have been retaining a lot of unwanted material in most structures on account of manufacturing constraints. For example, the rack and pinion steering gear housing as shown in Figure 4 is an aluminium casting. FE simulation of its driving and road loads suggests that stresses are concentrated only around the mounting foot. A lot of materials are retained along the insignificant tubular region to comply with minimum thickness requirements of the casting process. A properly optimised sheet metal structure with higher grade of aluminium could bring down the weight significantly.
Experts in the industry have suggested that, i) classification of structures based on their structural loading intensity, ii) viewing structures through the prism of vehicle level objectives, iii) including the effects of neighbouring systems while designing components, and iv) integration of parts for multi-functionality will help designers build structures with minimum weight without compromising on their functionality.
As we move away from the traditional material space, the challenge becomes more intense in the form of cost and joining methods. There are some joining solutions in the form of tailor welded blanks and structural adhesives which enable us to join different materials, grades, and geometries to build lighter and stronger parts.
Role of CAE
OEMs are changing the way they have been using Computer Aided Engineering (CAE) in the recent past. Virtual simulation is being introduced in the early stages of design to validate ideas revolving around novel structures and combination of materials. With new forms of materials such as metal matrix composites, aluminium foams, and composite polymers coming into play, CAE departments are being tasked with steep challenges in terms of improving the accuracy of representing these new material formulations. As a lot of research work is happening in joining methods, CAE teams have to work in collaboration with manufacturing faculties to develop methodologies for accurate representation of these joints and the impact of these joints on the strength of parts (welding reduces the strength of parts around weld areas). Currently, joining methodologies employed in the virtual world are simplified and come with a lot of assumptions.
Challenges and the road ahead
To win this light weighting contest, the industry has to strive hard to build lighter and stronger structures with minimal cost differential. This requires cross-disciplinary education in design, manufacturing, materials, and other sciences. CAE methodologies have to be evolved in close collaboration with material and manufacturing streams.
Scenario in India
With small cars being a major part of the Indian passenger car market, cost for the buyers and profit margins for the OEMs are two major factors that influence the business. As consumers are conscious of fuel economy, OEMs are under enormous pressure to offer cheaper cars with better fuel efficiency.
The Indian Government is cognizant of occupant safety and environmental balance, and is mulling the idea of enforcing safety and emission norms soon. The Indian CAFE standards are expected to be at an average of 113 gm of CO2/km by 2022. Amidst all this, it has become a necessity for Indian OEMs also to follow the global trend of light weighting. A survey report from Global Automotive Lightweight Materials (GALM) indicates a slight shift of buyers from A segment to B segment with A segment continuing to have the major share, which is a fillip to OEMs when it comes to investing in R&D for future programmes. However, the lack of local availability of the trending lightweight materials is a major obstacle.
Inspired by global trends in light weighting, Tata Motors is building its next-gen Sumo on a new platform, Ropter. It is employing a new `body integrated frame` technology with the body joined to chassis frame, as the name suggests. It is expected to bring the combined benefits of conventional ladder and monocoque structures with comparatively less weight.
Major passenger car maker, Suzuki, also announced its intent towards light weighting strongly with its recently released premium hatchback Baleno. The frame is made of basic shapes without bends and precisely engineered layouts for engine, suspension and fuel tank avoiding additional reinforcements. This has resulted in a lot of weight savings. Additional savings were achieved by integrating neighbouring systems (suspension and body) and considering the vehicle level objectives instead of system level objectives. As a result, this new vehicle was able to reduce about 100 kg in comparison to its predecessor, the Swift.
The industry has geared up to this challenge and a lot of innovation is taking place in every aspect of automotive engineering to reduce every gram of mass. The holistic objective of reduction in carbon footprint can be achieved in a smarter fashion by integrating the benefits of light weighting with powertrain improvements.
In the current context, light weighting is just about right weighting.
The author is Vice President & Global Delivery Head - CAE/Simulation, Satyam-Venture Engineering Services Pvt Ltd