Car plant

5 min read

The use of natural products in automotive assembly takes root at the British Motor Show. Stuart Nathan reports

Low-carbon motoring usually conjures ideas of energy-efficient engines, alternative fuels or even the varieties of electric cars whose development has been closely followed by the media (especially The Engineer).

But although the powertrain forms a large part of the car's carbon footprint, it is not the whole story. The embodied energy in the vehicle — the energy, and associated carbon, that goes into making the car and its component materials —is also a small, but significant, proportion.

The estimates of embodied energy vary. Honda says building a car emits 810kg of carbon dioxide; the UK Society of Motor Manufacturers and Traders says it is 600kg. A recent study by Volkswagen estimated that almost a third of a car's carbon footprint comes from its manufacture. These figures do not include the energy or carbon needed to make the steel or the electricity used in the factory. However, this is dwarfed by the emissions from running the car.

Mark Ellis of the Nissan Technology Centre Europe, Cranfield, says his company has tried to estimate where the energy is used across a car's lifetime.

For a Micra doing 200,000km during its lifetime, manufacture of the car and spares, and servicing accounts for about 9 per cent of the total energy profile. Raw materials production accounts for 6 per cent, and the remaining 85 per cent comes from the road operation. 'The area we can most influence ourselves is reducing the energy use in manufacture and materials selection, so we're focusing on that quite strongly,' he said.

Nissan is not alone. Finding new, lower-energy materials is a target of research projects across industry and academia, and some striking results have now started to appear.

At the Warwick Manufacturing Group, Ben Wood recently led a project to produce an 'eco-racing car', made as far as possible from natural materials. Starting with a steel tubular spaceframe, his team added side body panels made from a composite of hemp matting impregnated with a resin based on rapeseed oil. The Cornwall-based boatbuilder Ecocats provided the main body panels, made from a resin based on cashew nut shell oil.

The tyres contained a cornstarch derivative rather than an oil-based filler, while the brakepads contained jute rather than aramid pulp. The oils and lubricants were based on plant oil esters, and were 95 per cent biodegradable after 28 days.

Fuelled with a blend of 15 per cent unleaded petrol with 85 per cent bioethanol, the Eco One car has a higher power-to-weight ratio than a Ferrari Enzo, can do 0-60mph in four seconds, and has a maximum speed of about 150mph.

Wood's goal was to prove that cars built from, and fuelled with, plant-based products need not be slow and lumbering. 'My goal is to make a race car that's 95 per cent biodegradable or recyclable,' he said.

While the Eco One is a research project, sacrificing looks for performance, its ideas have been taken up in a slicker way by Lotus. Now on show at the British International Motor Show's green motoring pavilion is what looks like a standard model of the Elise — until you look closer.

Instead of the usual leather-clad interior the seats, door panels and gear stick are swathed in tweedy-looking wool and the carpets are made from sisal, a variety of hemp long used to make durable matting.

Other materials are also derived from nature. The body panels, like those of Eco One, are a composite reinforced by hemp, rather than glass fibre. 'That was a great way of reducing the carbon footprint of the car, because the hemp is grown locally, which cuts down on transportation and, of course, hemp fixes CO2 as it grows,' said Lee Preston, environmental manager at Group Lotus. 'And it's also lighter than glass fibre.'

That helped with the goal of creating a car lighter than a standard Elise, even though it incorporated more technology.

The Eco-Elise has solar panels on the roof to help power electrics and the composite in the body panels is reinforced by hemp

The resin of the composite was a standard polyurethane, although plant-derived resins were tested. 'The problem with the natural resins was that you had to heat them to a much higher temperature to cure them, and that would have increased our gas usage. We have an ethos of reducing energy use on-site, so we went away from that option,' said Preston.

The goal of reducing weight was challenging but central to the reduction of overall emissions; the lighter the car, the less fuel it will use. 'The Eco-Elise is specifically designed to already be a lightweight car; it's optimised for track performance,' said Preston.

It incorporates solar panels in its roof to generate some of the power for its electrics, and these add to the weight. However, with lighter hemp composite panels, specially designed wheels that are a total of 15.8kg lighter than standard Elise wheels, and even a lightened stereo system, the Eco-Elise is 32kg lighter than a standard Elise S.

But the biggest energy savings come from the paint. Lotus uses a hybrid paint system, with solvent-based primers and clearcoats sandwiching a water-based colour coat. 'But we wanted to go fully waterbased, which has never been done in this country before,' said Preston.

Working with paint supplier DuPont, the company developed an entirely water-based system that can be sprayed by hand on to the hemp composite panels and can be cured at a cooler temperature than the hybrid system. 'We achieved a 10-20 per cent reduction in gas usage,' said Preston. 'That's likely to be the biggest thing we've learned from this project to be carried over into production, although we're also very encouraged by the results we had from using hemp.'

Preston sees the incorporation of solar panels as a major achievement. 'Just try to find a flat panel, in a car as curvy as an Elise, where you can incorporate it!' The panels sit in the doubly-curved roof panels of the car and generate 14W, enough to power a small speaker.

But incorporation is possible, and more panels in different parts of the vehicle — especially on larger models — could provide a significant proportion of car's electricity.

Mass producers are also keen on lighter, lower-energy materials, said Nissan's Mark Ellis. 'We're looking at a lot of recycled materials and biopolymers and recycled is probably our biggest opportunity at the moment: each Qashqai has about 40kg of recycled materials.'

The largest contributor to weight reduction is likely to be replacing glass, particularly in the upper parts of the vehicle, with lighter polycarbonate. 'I would expect a lot of polymer-type applications to replace steel, and lightweight metals like aluminium, along with potentially more composites from the performance side of the market,' said Ellis.

'These are technologies that have already been tried and tested in our niche models; for example, our GTR Skyline model has extensive use of carbon fibre, and a lot of high-strength, high-performance aluminium grades, which we're using as a trial for the more mass-production models.'

The carbon fibre could also be superceded by natural materials, Ellis said. NTCE has tested hemp, as well as other natural fibres and polymers.

Paint technology is trickier in mass production, however. 'If we were to lay down a brand-new factory, or scrap and rebuild a facility, we'd probably go to water-borne paints as the technology develops. But our Sunderland and Barcelona plants are very constrained by the existing layout and what's possible within these facilities.'

Also, waterborne grades need higher temperatures. 'This is something we encounter very frequently,' he said.

'For the paint, we'd gain in losing the solvents, but we'd lose in the higher gas bill for the oven. It's a delicate balancing act.'