Economist: More Efficient Tyres
Fuel-Saving Tyres: Hysterectomy
Silica extracted from rice husks makes for greener tyres.
Tyres are remarkable pieces of engineering. At high speed in slippery bends they provide only a few square centimetres of contact with the road, yet they help a driver steer safely around the corner. Once, they were made entirely of natural rubber. These days they also contain rubber-like polymers, layers of steel braiding and textile reinforcements, all of which improve performance and cut fuel consumption. Now Pirelli, an Italian tyremaker, is manufacturing fuel-saving tyres that are greener still by extracting one of their ingredients from rice husks.
One good way to save fuel is to reduce rolling resistance, which is caused, in part, by a vehicle’s weight repeatedly squashing its tyres. As the tyres bounce they convert kinetic energy into heat, thus wasting it. Hysteresis loss, as this is known, can be reduced by mixing a tyre’s rubber with powdered material that has strong chemical bonds in it.
In the past, the material of choice has been carbon black, a sooty substance made by the incomplete combustion of petroleum products. More recently, silica (silicon dioxide, which comes from sand) has come to the forefront. Silica is better because it lowers hysteresis loss without reducing a tyre’s grip. It thus cuts rolling resistance by around 30%, compared with its predecessor. That translates into a 5-7% reduction in fuel consumption. And silica also increases a tyre’s wet grip.
Brazil bounces back
Sand is cheap. But processing it into something suitable for use in tyres requires effort and money. So if nature were to provide grains of silica suitable for use without processing, that would be a bonus. And nature does. Grasses contain tiny pieces of silica, called phytoliths…whose job is to discourage herbivores, both vertebrate and insect. Pirelli’s engineers realised that these defensive weapons are the ideal size to add to tyres in order to control hysteresis loss, and that a ready supply of them is available in the husks left over from the milling of rice.
Rice husks were once waste. These days they have some value because they are used as fuel in small-scale electricity generators. But from Pirelli’s point of view, that is a good thing, because what the firm is interested in is the phytoliths left behind in the ash—and until now the ash itself really was waste. The firm has set up a factory in Meleiro, a town in a rice-growing area of southern Brazil, to extract phytoliths and put them in tyres. The ash comes from rice husks burned to help power the factory.
A tonne of rice produces around 200kg of husks and those, in turn, yield 40kg of silica. According to Daniele Lorenzetti, who is in charge of the project, by 2015 the factory will be providing nearly a third of the silica Pirelli needs for the 400,000 tonnes of tyres it makes in Brazil.
The technology could spread fast, especially in other rice-growing areas. For Brazilians that would have a delicious irony. The Amazon rainforest was the original home of rubber trees, but Brazil’s rubber industry was devastated when seeds smuggled to Asia were used to set up rival plantations. By taking an Asian crop and using it to make better tyres, they will be getting their own back.