NUNEATON, UK - While most materials being used to lighten cars are the invention of new technology, some are not. Researchers have found they can use the natural fibers of flax to replace heavier glass fibers in composite panels.
The principal weight-saving materials are more exotic, such as metal foam that can be used in exterior panels and body structure elements, said Rudolf Stauber, BMW's top engineer for materials. He was addressing a seminar at the opening of a new, $1.6-million materials engineering laboratory at the UK's Motor Industry Research Association.
'For the cars of the future our main concerns are lightweight design, increase in safety, environmental sustainability, and economical manufacturing, use and recyclability,' Stauber said.
Metal foam is the latest new material for possible use in exterior and car body applications, Stauber said.
With a porosity of at least 50 percent, the foam weighs much less than solid steel. This could provide a high energy absorption capacity and lead to greater passive safety, Stauber said.
The car body structure offers many potential sites for metal foams, he said, particularly those suffering high deformation under crash conditions: doors, bumpers, rocker panels, longitudinal carriers, bulkheads and pillars.
Plastics and plastic composites continue to be the preferred materials for interior applications, Stauber said.
However, to improve recyclability, it is desirable to keep within one class of plastics. Separation processes are difficult and costly.
He said the BMW 5 series instrument panel was made from three different types of polyurethane, so it can be recycled at one time.
It uses a polyurethane hard foam substrate reinforced with a glass mat, a semi-hard polyurethane foam layer and a polyurethane skin sprayed on top.
Ventilation ducting under the fascia was also in the same material.
Environmental issues are important for trim panels where safety is less of an issue, Stauber said. Natural materials such as flax, jute and sisal are being researched.
Besides cutting weight in half, natural fiber composites offer advantages over glass fiber such as good sound insulation, better recycling and less wear of manufacturing tools. Cost is an issue.
In the engine compartment, BMW aims to reduce weight and cut fuel consumption by using smaller pistons.
A carbon-reinforced magnesium piston formed by squeeze casting is 30 percent lighter than an aluminum piston.
A magnesium piston would require a compatible cylinder liner. Stauber said two options are being explored: knitted carbon with ceramic particles and an aluminum alloy; and a porous structure of hard particles infiltrated with an aluminum alloy.
Valves made from traditional steel will no longer provide the levels of engine control needed for reduced fuel consumption and noise, Stauber said.
Valves of silicon nitride and titanium aluminide would be lighter, yet resistant to corrosion and heat. They would allow engine temperatures up to 1000 degrees Celsius, compared with the maximum 800 degrees tolerated by steel valves.
Aluminum can cut weight in the chassis. The aluminum rear axle introduced two years ago on the BMW 5 series weighs 40 percent less than a comparable steel axle.
Brake drums in a lightweight metal matrix composite offer advantages over carbon-based brakes. Although they are acceptable in race cars, he said, they are too expensive for series production.
He said the combination of new techniques and new materials should contribute to building a 'more efficient, safe, comfortable and environmentally friendly car.'