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SPECIAL ON TYRES FOR ELECTRIC VEHICLES
As accumulator limitations of weight and autonomy are gradually resolved, the dream of an all-electric car is coming closer to realization. And tyres are also called on to make a contribution with increasingly less rolling resistance
Pietro Paolo Marziali
When man realized that the sources of fossil fuels were not infinite, it marked the start of a race to find and exploit alternative energies that would lighten up the dark horizon of petroleum derivates. All possible alternatives were explored: hybrid, methane gas, biomass, etc.. And, lastly, electricity. For a long time, cars driven by electric engines were at the bottom of the list of candidates for industrial production. There are two reasons: weight and limited battery autonomy. But here, too, the researchers worked, and continue to work intensely on finding something that will replace the classic zinc-lead combination. A definitive answer has still to be found, but the new lithium-ion accumulators are a considerable step ahead compared to the solutions of only ten years ago.
And so at the various car shows manufacturers continue to shine the spotlight on their creations in this field. The purpose is to show that they have the technology that will lead to the production of a car that does not need petrol. The first step was the hybrid vehicle. Basically, it is a classic internal combustion engine coupled in parallel to an electric engine driven by onboard batteries. This second unit comes into operation when supplementary power is needed or where internal combustion engines are prohibited. Now that we have hybrid know-how, the next step is to make a totally electric vehicle. This task is decidedly more arduous, because satisfactory autonomy can only be achieved by installing battery packs (the classic lead types) that are so heavy that they compromise vehicle performance. The companies that make accumulators are working hard on these problems and they have come up with lithium-ion batteries that are much lighter and more reliable than others. And they can be used to make the prototypes that will almost completely meet drivers’ expectations. In the majority of cases they are the “concept cars” exhibited like manufacturers’ visiting cards at various shows. Only in some cases (fewer than the fingers of one hand), these vehicles have set off for dealership showrooms and have begun a real commercial life. In the United States, Honda produced a batch of one thousand Fit EVs for leasing to the same number of ecologically-spirited users. The model, it’s small, can travel 123 miles on one lithium-ion battery charge, but for longer distances it has an auxiliary 92 kWh petrol engine. In Europe, Renault made the Fluence Z.E., a mid-size car that can be recharged from a normal electrical outlet. Even though the new accumulators are beginning to give reasonable mileage, the designers are concentrating on everything that will save energy during driving and thereby increase vehicle autonomy. Among the components that are most involved in the search for economy are tyres which, as usual, need characteristics that are conflicting. The first essential quality is minimum rolling resistance. Smoothness is achieved with minimum contact areas, very little deformation during rotation and minimum heating, all characteristics that absorb energy. On the other hand, total diameters are fairly wide so that each segment of the tyre contacts the ground less frequently. And the syping on the tyre segment should not be too long because it heats up during deformation. Continuing in this spirit, the tyres for electric cars must be “well” inflated. The task of the technicians is to reconcile these requests so that the needs of electric car manufacturers are already met at the first stage.
Many manufacturers have decided to invest their resources in research programmes and have shown the results to the public. The commercially available tyres include the Goodyear Efficient Grip, that began as a summer tyres but with some adaptations it has become a prototype for electric cars. According to Goodyear, the advantages are lower consumption, very low noise, good braking performance on the wet, combined with RunOnFlat technology. The characteristics that make the tyre particularly suitable for electric cars are narrowness combined with large diameter rims, which contribute, together with a special internal construction, to considerably reduced rolling resistance. The designers have also taken into consideration the high torque an electric engine generates even at low revs; this required a special tread pattern and a new compound to eliminate any irregular wear. Renault was convinced by all these elements and it has decided to equip the Fluence Z.E. with Efficient Grip (the size chosen is the 205/55 R16).
At the 2010 Geneva Motor Show, Bridgestone presented a model called Ecopia EP150. The standard Ecopia, with the improvement of certain characteristics that are typically important for electric cars, has added the EP150 suffix to its name. This tyre has reduced rolling resistance, more efficient consumption and lower CO2 emissions. Japanese technicians achieved this by combining the development of the Nano Pro-Tech-Tm technology with an optimized tread profile. The new tread design now has connected blocks and thin ribs that improve contact pressure and grip on wet roads. Thanks to these modifications, rolling resistance has improved by 15% and wet braking performance has also benefitted. This version of Ecopia was used as the reference tyre during the last stages of the development of the Nissan Leaf, also a 100% electric car, but smaller.
Continental has dedicated to electric cars the Conti.eContact, which the manufacturer in Hannover made to equip the Twizy. Part car and part four-wheeled scooter with fairings, the vehicle has a 15 kW engine that can reach 80 km/h. The Twizy’s wheel arches house the 125/80 R13 in front coupled with the 145/80 R13 behind. But this is all that physically exists of the BluEco programme, the future family of German tyres specifically for electric vehicles and whose logo already appears on the sidewalls of the Twizy’s Conti.eContact. The rest of the programme is still cloaked in industrial secret restrictions. Information from the development centre at Continental is scant, succinct and basically similar to the salient points of competing programmes, so much so that not even a photo of the Conti.eContact has been distributed. The focal point of the development of these tyres is rolling resistance (as always, to limit energy absorption) and noise rather than performance at high speeds. But without compromising safety, however. Conti is also in favour of developing products with wide diameters – like 20” – even though they were forced keep to Renault’s requirements for the Twizy.
Michelin presented its first Michelin Tire for the Electric Car (abbreviated to EV) at the Geneva Motor Show in 2009. Even though it is for electric cars, it still gives good grip and mileage. It is lighter and has lower rolling resistance; Michelin states that it has been reduced by 15% and is very silent, which is important in a tyre mainly for city use. The wide diameter of the EV saves current because a certain distance is covered with fewer revs. This is confirmation that the design of electric cars and tyres should be in parallel so that there is sufficient space in the wheel arch for the new tyre sizes. Michelin has developed its EV with fairly wide diameters (the preference is 21 inches) and a narrow tread. The low profile integrated with a wide diameter reduces rolling resistance and lowers energy consumption to a minimum.
Returning briefly to the Far East, Yokohama Japan has a new product, Blue Earth, a technologically-advanced “green” tyre. It does not target electric vehicles specifically, but it can be easily adapted to their needs. The technicians have been able to balance wet and dry performance, comfort, respect for the environment and, above all, rolling resistance with the consequent energy saving. Blue Earth owes its performance to fundamental components like the Nano Blend Compound, which combines performance that at one time was considered incompatible with saving energy, durability and grip. Another technological device that distinguishes Blue Earth is the very thin and light Advanced Inner Liner that reduces the natural loss of pressure by 36% and also has a positive effect on CO2 emissions. Blue Earth is made from 80% renewable materials, such as oil from citrus rind and natural rubber; combined with 36% lower aerodynamic resistance, it is a very “thrifty’ tyre. As we said, the product is not exclusively for electric vehicles but is suitable also for small and mid-size hybrids, so it will have a normal commercial life. The first sizes available also on the Italian market are 185/65 R15, 195/65 R15 and 215/45 R17.
Pirelli has presented the Cinturato P1 in a range with four variants (P1, P2, P6 and P7), ideally to reduce impact on the environment during production and throughout its life. Of these variants, the one of most interest is the Cinturato P1, a special version of the basic model specifically for electric cars. The tread pattern of the P1 takes us back to the first half of the last century and the historical Isotta Fraschini, but it has strong ecological and technological content. The Cinturato P1 Special Edition is the result of the technological partnership between Pirelli and Audi for the electric version of the A8. The joint project began fairly recently – January last year. Working in close collaboration, Audi Special Project Development and Pirelli Research and Development in Milan have studied every detail to make a narrow, lightweight tyre that will give good stability and meet all the needs of a heavy electric car like the R8-E-Tron, even at the prototype stage. Pirelli created the Cinturato P1 for this special application by adapting not only the compound but also the tread pattern so that the vehicle will be as quiet as possible while maintaining grip when cornering and braking on the wet. The Cinturato P1 “Special edition” uses a compound made from new materials and an innovative casing structure to achieve the goals mentioned above. The Cinturato P1 saves fuel, lasts longer, reduces CO2 emissions and noise level, always with special attention to safety, driving pleasure and good mileage. It is precisely to meet these aims that it has been made in two experimental sizes: 125/60 R21 and 145/50 R21 for equipping not only the Audi R8-E-Tron but also vehicles by other manufacturers who want to take advantage of the experienced gained by the Italian company on this project.
Working along the same lines is Vredestein, well-known for its collaboration with Giugiaro for product styling. The confirmation of this relationship is given by Quatrac Lite, the most recent product from the partnership between the two companies which made its appearance at the Geneva Motor Show; it was mounted on the latest “concept car”, the Proton Emas 3 by Giugiaro Design. The Quatrac Lite began life as a tyre for hybrid or electric vehicles and it combines energy efficiency, safety and driving comfort. Thanks to its Full Polymer Compound, aerodynamic sidewalls and “flexing points”, the Quatrac Lite has particularly low rolling resistance. Water dispersion is by three longitudinal grooves and numerous lateral grooves located in the shoulders, combined with a large number of sipes that give good grip even on rough ground.
The tread pattern reduces noise, a characteristic that will be harmonized this year by an EU regulation that also includes rolling resistance and grip on the wet. Quatrac Lite promises good energy saving with low rolling resistance to make this a genuinely ecological tyre. In addition to the symbol required by law, blades of grass are drawn at the top of the sidewall, another demonstration that behind the designers is the creative pen of Giugiaro.