A crashworthy diet

Car makers are running out of ways of shedding weight off their models to meet new EU emissions targets. Drivers just want too many advanced features. David Fowler reports.

Car manufacturers are losing the race to make their vehicles lighter in time to meet new EU emissions targets as they reach the limits of available weight savings on body structures. While major car companies and their suppliers attempt to shed every last kilogram from their bodyshells, other factors – not least consumer demand for bigger vehicles laden with state-of-the-art technologies – are pulling them in the opposite direction.

In 2008 new European rules for fleet average economy come into operation. Under the agreement between the EU and Acea, the European car makers’ association, carbon dioxide emissions averaged across each manufacturer’s total new car sales must be no more than 140g/km, and the industry is worried it won’t make it.

Some factors are working in the manufacturers’ favour. One is the increasing popularity of diesel, another is the launch of smaller cars. DaimlerChrysler’s Smart and A-Class and BMW’s Mini are helping bring down the average of these makers of mainly big executive saloons. But other trends are going in the opposite direction. Heavier 4×4 vehicles are becoming more popular. In addition, customer demand for equipment such as power steering and air-conditioning adds both to weight and fuel consumption.

The industry has started to face up in the last few months to the fact that it will be very difficult to achieve the targets by 2008,’ said Jon King, director of Corus Automotive, a supplier of metals to the car industry. ‘The rate of improvement has slowed.’

It may even be going into reverse. The UK average CO2 emissions for 2001 was 168g/km. In 2002 there was a marginal increase to 169g/km.

One of the main ways to improve fuel economy is to reduce weight. Every 10kg in the car is equivalent to 0.05 litres of fuel per 100km and the obvious place to start is the bodyshell, the car’s single biggest component. Until the mid-1970s the weight of bodyshells and complete cars declined steadily as designs became more efficient. Since then gains have been offset by higher levels of equipment and the need to meet increasingly high crashworthiness standards.

In a presentation at the recent SSAB Swedish Steel Prize seminar, ‘s advanced body engineering manager Kaj Fredin stunned his audience with a graph apparently showing the company’s intention to cut body weight by as much as 70 per cent over the next few years. Such a reduction would be nothing short of revolutionary, but it is unclear how it could be achieved using available materials and technologies.

The Ultra Light Steel Auto Body (ULSAB) series of projects demonstrated the weight reductions that could be achieved by using high-strength steel and other technological advances (see sidebar).

Even in the late-1990s high-strength steel was still little used by the car industry, but Volvo began to feature it in the 740 model of 1984 where the proportion was about 15 per cent by weight. By the time of the S40 and the XC90, Volvo’s latest 4×4, thatproportion had risen to just over 50 per cent (see sidebar below). Despite this the body structure of the new S40 weighed 311kg compared with the 266kg of the outgoing 1996 model.

Fredin says that for Volvo, with its reputation for safety to maintain, the question of weight reduction is particularly difficult. And for any car manufacturer such reductions have to be cost-effective. ‘There is a direct conflict between crash protection and reducing weight,’ he said. ‘There is also a conflict between reduced weight and cost.’Using high-strength steel has been a cost-effective way for Volvo to maintain weight at a more or less constant level. But today we are at quite a high percentage of high-strength steel and it will be difficult to include more cost-effectively. To take a large step in reducing weight we need to do something else.’

There are a range of materials and techniques at the disposal of body engineers, said Fredin. Alternatives include lighter but more expensive metals such as aluminium, magnesium and even titanium (see sidebar below). Aluminium was once seen as a potential rival for steel, but it is now generally accepted that the two can exist together if used appropriately. Corus’s King said that when it comes to use in the primary body structure of volume cars, aluminium is at a disadvantage. This is because car makers’ installed asset bases have a heavy investment in steel presses, assembly facilities and paint shops. However, there is more scope for aluminium’s use in bolt-on parts such as suspension components as well as body closures and skin panels.

Neil Ridley, director of Arup Automotive – the consultant that recently worked with Mayflower to design the aluminium body structure of the Ford GT – said other disadvantages are that its higher cost is compounded by price volatility, and the issue of its durability. Aluminium will always eventually fail due to fatigue if subjected to enough stress cycles, whereas at low, but repeated stresses, steel does not.

This is not to say that the material does not have considerable promise. BMW’s new 5-Series is widely praised as an example of using aluminium appropriately. The entire structure forward of the dashboard is aluminium, benefiting both overall weight and contributing to a 50-50 front/rear weight distribution, which helps handling.

Ridley said: ‘If you’re going to use aluminium you don’t have to go for all-aluminium, just the appropriate material for the appropriate place. The 5-Series is the right way to go.’

Magnesium is also beginning to gain a foothold in the automotive market in some quite tightly defined applications, notably cross-car beams such as the instrument pack beam behind the dashboard.

Magnesium generally has to be cast, which limits it to components with open cross-sections – favourable for casting – and places where it won’t normally be seen. The cross-car beam has to carry the weight of components mounted in the central console such as the air-conditioning unit, and in a crash it takes loads from the steering column and airbags. In a side impact it has the important function of bracing the passenger safety cell. A fairly large, yet lightweight section in magnesium is well suited to these tasks. Seat frames and inner door structures are other possible applications.

Stainless steel is another material with good potential, according to Fredin. Its benefits include high strength, good formability and durability. Cost is a disadvantage but this is coming down. Because of its formability, Fredin expects it to take over certain applications where extra high-strength steels, such as press-hardened boron steels, are now used.

Sandwich materials can be used where noise or vibration is a problem. These typically consist of two sheets of steel of 0.2-0.5mm thick with a layer of rubber or elastomeric sound-deadening material in between.

For strength and welding the sandwich can be treated as a conventional steel sheet, but saves the weight of separate sound deadening. Structural foams, typically urethanes or epoxies, can replace the additional steel reinforcement that is normally needed in components such as B-pillars (the post that forms the rear of the front door frame).

Continuous joining techniques – bonding or seam laser-welding instead of spot welding – also have great potential for improving crash performance, stiffness and durability. ‘Continuous welding might cost more,’ said Ridley, ‘but the improved stiffness allows material to be taken out.’

It is clear, therefore, that a formidable amount of effort is going into research to get body weight down. Will it be enough? Using all these techniques, how much weight could be saved from a car body in, for example, the next four years? Fredin declined to confirm the spectacular target suggested in his Swedish presentation. Instead he put the figure at a more modest, but still significant, 25 per cent.

Corus’s King came up with a similar figure: ‘If you could put the most up-to-date steel technology into a car body, the saving would vary between 10 and 25 per cent. In practice it would be almost impossible to achieve in a single model cycle for cost reasons.’King also pointed out one clear drawback: ‘The body is only about 25 per cent of the total vehicle mass, so you have to take quite a lot out to get any benefit.’ Overall, King finds it hard to see where the quantum leap will come from. ‘I don’t think anyone can see a revolution coming.’

Car designers, then, will have to look to a range of alternative methods to meet economy targets. But few of these are at the point where they could make a sizeable, immediate impact.

Further evolutionary improvements in economy will continue, in both petrol and diesel engines, through advances in direct injection technology.

‘Engine downsizing’, a current buzz-phrase among engine specialists, involves using a smaller, turbocharged power plant to do the work of a bigger one. In theory, this is one of the best ways to achieve both an economy gain and a weight saving, but the problem is a lack of power at low revs before the turbo boost comes in.

Hybrid cars, combining a petrol or diesel engine with electric power, offer the best hope of a step change in economy. But widespread acceptance, in Europe at least, still looks some years away.

There is one final area with the potential for huge gains, maybe big enough to achieve the weight savings needed to meet the emissions targets. This is ‘by-wire’ technology, the replacement of mechanical systems with electronics.

Brake-by-wire would eliminate brake fluid and its associated reservoirs and hydraulic pipework. Steer-by-wire would not only abolish the steering column, steering gear and its associated weight, but could also allow the whole interior and dashboard of the car to be rethought. If hard objects could be moved sufficiently far from the occupants, there would be less need for airbags.

In Europe the car industry is waiting impatiently for regulators to approve by-wire technology as safe before beginning its roll-out into the mass market.

Who are these regulators? The EU colleagues of the very bureaucrats who are setting the emissions targets.

Sidebar: Volvo S40: enhanced crashworthiness

The front body structure of the S40 is divided into several zones, each with a different task in the process of deformation through which crash forces are absorbed. To give each zone the relevant properties, four different grades of steel, from conventional mild steel to ultra high strength were used. The outer zones are responsible for most of the deformation; the closer the collision forces get to the passenger compartment, the less the materials used deform.

The front bumper incorporates a rigid UHSS (boron steel) cross member. The straight sections of the side members are ductile high-strength steel, optimised for high energy absorption. This is where most of the deformation is in a collision.

Further back, the member just before the A-post is designed to act as a barrier for the cabin space, and to help prevent the front wheel penetrating the interior. This section is extremely rigid and is made of extra high-strength steel. A rigid cross member connects the A-posts and lower side members, helping to maintain the cabin space in a bad crash. The patented technique allowed Volvo to achieve the same safety performance as the larger S80 saloon with a 200mm shorter structure, said advanced body engineering manager Kaj Fredin.

For the XC90’s front crash structure Volvo improved its crash performance by designing it as a space-frame of members in direct compression or tension rather than relying on their strength as beams in bending.

Sidebar: Titanium: the low-density, high-strength option

Titanium is suitable for exhausts and suspension springs, according to Roger Thomas, European development manager of Timet Automotive. Motorsport has proved the metal’s suitability for connecting rods, valves and valve springs, where its low density and fatigue strength are advantages.

It has been used for the exhaust of the Z06 Corvette since 2001 and for suspension springs on the Ferrari 360 Modena Challenge Stradale and VW Lupo 1.4FSI. In springs not only is weight saved but also, because of the metal’s lower shear modulus, the spring can be shorter. As the density of titanium is only about half that of steel, a spring would weigh 60-70 per cent less.

This helps with packaging constraints, and in a suspension system optimised around a titanium spring more weight can be saved in the associated components than in the spring.

Sidebar: Ultra light steel bodies

The Ultra Light Steel Auto Body project was set up by a consortium of steelmakers in the late-1990s, at least in part as a riposte to a perceived threat from aluminium. Audi had just brought out the all-aluminium A8 and Ford had produced numerous prototypes of a volume model in its Aluminium Intensive Vehicle project.

The main features of the first ULSAB report in 1998 were:<br>1. more use of high-strength steel<br>.2. tailor-welded blanks, in which the strength of a component is tailored to the loads it has to carry by welding two or more pieces of steel sheet of differing strengths or thicknesses together before the pressing process<br>.3. reducing part count<br>.4. hydroforming (a part is formed by forcing a tube into the shape of a mould by hydraulic pressure from inside)<br>.

‘Against a conventional body structure, then using little high-strength steel, ULSAB identified a potential for mass saving of up to 25 per cent,’ said Corus Automotive director Jon King.

Since then the industry has made considerable use of higher-strength steels and tailored blanks, but it was impossible to realise all the potential immediately, said King. ‘How much vehicle manufacturers could achieve varied from one model programme to another, depending on their existing asset bases.’

For example, to make a component out of high-strength steel a heavier press might be needed, but the investment cannot always be justified immediately. ‘But over two or three new models that percentage was probably achievable.’

ULSAB went on to look at closures (doors, bonnet, boot) and other components such as the suspension. This culminated in ULSAB AVC (advanced vehicle concepts) in 2002 which took a more radical approach with wider use of advanced high-strength steels, tailored blanks and hydroforming. The emphasis, though, was not so much on weight as crashworthiness. King said that recent successes of companies such as Renault in achieving five-star results in the Euro NCAP crash tests are generally attributable to use of high-strength steel.

Corus has shifted focus to helping individual car makers to implement the new technology. The first high-strength steels were harder to press, but more advanced steels and high-strength low-alloy steels have better formability, said King. Another innovation, suited to body panels, is bake-hardened steels, which gain strength in the paint-baking process.

Hydroforming is now possible with high-strength steels, giving it the potential to replace members made up of more than one part such as B-pillars with their reinforcement into a single component.

Corus and Arup are working to improve their understanding and ability to model the new materials. The use of aluminium for entire car bodies has remained limited to low-volume models such as the A8 and the latest Jaguar XJ. With Audi’s small hatchback A2 looking like the high water mark in volume terms, and given that a successor is not planned, talk of rivalry between steel and aluminium is replaced by a consensus that both have strengths.

Ridley said: Steel will continue to be big, but aluminium has a very rosy future. With car makers producing more niche models and production runs decreasing, there’ll be more scope for other materials.’