Article
Articles, Issue 42 - Autumn 2011

Vehicle Light-weighting and Sustainability

The British automotive industry is a large and critical sector within the UK economy.  It accounts for 820,000 jobs, exports finished goods worth £8.9bn annually and adds over £10 billion to the UK economy each year. However, the UK automotive industry is currently facing great challenges; road transport released 132 million tonnes of CO2 in 2008, accounting for 19% of the total UK annual CO2 emission and its global competitiveness is also threatened by the emerging new economic powers, such as China and India. Additionally, the UK government is committed to reducing CO2 by 80% in all sectors by 2050 and the EU requires 95% recovery and reuse of end of live vehicles (ELVs) by 2015.

A solution to these challenges comes from the development and manufacture of low carbon vehicles (LCVs), and this is clearly presented in the vision of the UK automotive industry set by the New Automotive Innovation and Growth Team (NAIGT).

Strategic importance of vehicle light-weighting

Whilst hybrid or electric powertrain systems provide opportunities for lower tailpipe emissions and improvements in fuel economy, the key system components typically increase vehicle mass by 100-40 kg for C-class vehicles with hybrid systems, as demonstrated by the new 2012 Honda Civic sedan hybrid models which will add 100 kg (9.4%) to kerb weight over the non hybrid models.

Fig 1. Comparison of mass for 2009-1020 hybrid vehicles versus similar non-hybrid vehicles in the US

From US based figures , an average of 9.0% increase in mass has been seen for hybrid vehicles over their non-hybrid equivalent models (Figure 1). These statistics included compact cars, large saloon (sedans), crossovers, full-size sport utility vehicles, luxury vehicles, and full-sized pick-up trucks. At the upper end of the spectrum in terms of vehicle size, the BMW ActiveHybrid X6 weighs 260 kg (11.0%) more than its non-hybrid counterpart.

The scene is different for electric vehicles;  a C-class vehicle typically shows a mass increase of 500 kg or more subject to the vehicle’s range. In addition to the increases in weight from the powertrain systems there is also a continuing trend in overall vehicle mass increase, owing largely to product enhancements in passenger comfort, infotainment and safety performance requirements.

These additional mass increases offset some of the true potential benefits that these newer powertrain systems offer — consequently there is a major opportunity to achieve considerably greater vehicle performance through weight reduction by the deployment of lightweight materials and innovative lightweight vehicle architectures.

A review of the energy usage in the vehicle system provides an understanding of the benefits of the alternative powertrain systems and weight reduction.

Fig 2. Energy budget for B-class vehicle with 1.4 litre gasoline engine

Figure 2 shows the energy budget for a B Class vehicle operating on the NEDC with a 1.4 litre gasoline engine. For this drive cycle, about one third of the energy produced as work is imparted as kinetic energy of the vehicle and about one third is used to overcome rolling resistance, both of which are related to the vehicle mass.

Fig 3. Energy budget for B-class battery electric vehicle

Figure 3 shows that the much higher tank-to-wheel efficiency of a battery electric vehicle (BEV) means that, potentially, for the same proportion of drive cycle energy losses, there is a greater proportional benefit to the total vehicle energy requirement in reducing the mass of the BEV than for the conventional vehicle.

Vehicle light-weighting is an effective approach to improve fuel economy and reduce CO2 emissions. CO2 emissions per km are directly related to vehicle curb weight . Studies have shown that for every 10% reduction in vehicle weight a  3.5% improvement in fuel efficiency can be gained (depending on drive cycle) . In terms of greenhouse effect, this means that every 100 kg weight reduction results in CO2 reduction up to 3.5 g of CO2 per kilometre driven . In addition to these primary benefits, vehicle light-weighting reduces the power required for acceleration and braking, which provides the opportunity to employ smaller engines, transmissions and braking systems. These savings have been termed secondary weight reduction and would allow further reductions in CO2 of up to 8.5 g/km.

Approaches to vehicle mass reduction

A critical requirement for achieving future generation low emission vehicles with enhanced fuel economy may be addressed directly through optimisation of vehicle mass.

A wide range of opportunities are available to achieve weight reduction within the vehicle key systems including body-in-white (BIW); closures; interior trim and equipment; powertrain and chassis.  To achieve suitable weight reduction a number of different strategies may deployed from methods such as material substitution through to structural optimisation, and increased structural and functional integration. In opting to use lightweight materials many improvements in mass reduction will typically be accompanied by increases in manufacturing or bill of material (BOM) costs and therefore careful consideration in the deployment of weight reduction strategies is essential where material substitution alone may be inefficient.

One of the major systems of the vehicle is the BIW which represents about one-quarter of the overall vehicle mass and is the core structure of the vehicle. The BIW is so fundamental to the vehicle that sometimes it is the only portion of the vehicle that is researched, designed and analysed in mass reduction technology studies . Over many years there has been a fundamental material shift from wood, cast iron and steel to high-strength steel (HSS), advanced high-strength steel (AHSS), aluminium (Al), magnesium (Mg) and polymer matrix composites (PMCs).

Between 1995 and 2007, the use of aluminium in vehicle structures increased by 23%, PMCs by 25% and magnesium by 127%.

Further vehicle mass reduction can be achieved by mass-optimised design technology. Mass optimisation from a whole vehicle perspective opens up the possibility for much larger vehicle mass reduction. For example, secondary mass reduction is possible since reducing the mass of one vehicle part can lead to further reductions elsewhere due to reduced requirements of the powertrain, suspension and body structure to support and propel the various systems.

New and more holistic approaches that include integrated vehicle system design, secondary mass effects, multi-materials concepts and new manufacturing processes are expected to contribute to vehicle mass optimisation for much greater potential mass reduction.

As reviewed by Lutsey , there have been 26 major research and design  programmes worldwide on vehicle mass reduction.

Compared to a steel structure, the HSS intensive body structure by the Auto Steel Partnership achieved 20-30% mass reduction, the aluminium intensive body structures of the Jaguar XJ, Audi A8 and Audi A2 achieved 30-40% mass reduction  and a multi-material body structure featuring more a mix of 37% aluminium , 30% magnesium  and 21% PMCs by the Lotus High Development Programme achieved 38% mass reduction.

It is clear that although a single material approach can achieve substantial mass reduction the greatest potential comes from an integrated multi-material approach that exploits the mass and functional properties of Al, Mg, PMCs and AHSS.

Despite the greater use of the higher cost advanced materials, mass-optimised vehicle designs could have a moderate or even slightly negative cost impact on new vehicle  if a holistic vehicle design approach is applied.

The Lotus High Development Programme demonstrated a 32% whole vehicle mass reduction could be achieved with a 2% reduction in cost, whilst the VW Super Light Car achieved a 35% body mass reduction for a cost of <€8 for every kilogram of mass reduction. The combination of a multi-material concept and a mass-optimised whole vehicle design approach can achieve significant mass reduction with a minimal or moderate cost impact on vehicle structure and it is most likely that the future materials for LCVs are an optimised combination of Al, Mg, PMCs and AHSS.

The drive towards sustainability

Closed-loop recycling of advanced automotive materials, however, has been missing from nearly all the LCV programmes worldwide, which have concentrated on the reduction of CO2 emission during the use phase of vehicles produced from primary advanced materials. 

The production energy of all primary automotive materials is always much greater than that of their secondary (recycled) counterparts . For instance, production of 1 kg aluminium from the primary route uses 45 kWh electricity and releases 12 kg CO2, whilst 1 kg of recycled aluminium uses only 5% of that energy and generates 95% less CO2.

Detailed life cycle analysis (LCA) has shown that a primary Al intensive car can only achieve an energy saving after more than 20,000 km driven compared with its steel counterpart, while a secondary Al intensive car will save energy from the very beginning of vehicle life.

If all the automotive materials can be effectively recycled in a closed-loop through advanced materials development and novel manufacturing technologies, the energy savings and cost reduction for the vehicle structure will be considerably more significant.

Summary

The vision of automotive manufacturers is that future low carbon vehicles (LCVs) are achieved by a combination of multi-material concepts with mass-optimised design approaches through the deployment of advanced low carbon input materials, efficient low carbon manufacturing processes and closed-loop recycling of ELVs.

Advanced materials will include Al, Mg and PMCs, which are all supplied from a recycled source. A holistic and systematic mass-optimised design approach will be used throughout the vehicle (including chassis, trim, etc.) not only for mass reduction and optimised performance during vehicle life but also facilitating reuse, remanufacture and closed-loop recycling at the end of vehicle life.

Novel manufacturing processes will be used to reduce materials waste and energy consumption, shorten manufacturing steps and facilitate parts integration and ELV recycling. Fully closed-loop ELV recycling will be facilitated by new materials development, novel design approaches, advanced manufacturing processes and efficient disassembly technologies, all of which will be effectively guided by a full life cycle analysis.

Author: Jason Rowe

About lotusproactive

Lotus proActive is an e-magazine published quarterly by Lotus Engineering, covering engineering articles, industry news and articles from within Group Lotus (Cars, Engineering, Originals and Racing).

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