Outside of motorsport, regular users of advanced carbonfibre-reinforced plastic (CRFP) structures include McLaren, BMW and Lamborghini. The Italian firm built a new factory in 2010 to make the Aventador monocoque in-house, developing its own process called ‘RTM-Lambo’. RTM – for resin transfer moulding – usually involves pressing the dry, resin-free lay-up together in a two-part mould and injecting the resin. RTM-Lambo is a lower-cost method of making high-quality body components. The top half of the Aventador tub is made and part cured using the ‘prepreg’ method. Prepreg is carbonfibre mat pre-impregnated with resin. It comes in rolls and is stored in a refrigerator, like ready-made puff-pastry, to stop it going off. Prepreg is the best way to make components with a shiny Class A finish, but it is expensive, so the bottom half of the tub is made using RTM-Lambo, like RTM but with a small, precise amount of resin added first to allow pre-forming of the basic shape. Finally, top and bottom components are pressed together in one large mould, impregnated and cured to complete the monocoque. Past master McLaren handed the world’s largest order for CRFP production car monocoques to Salzburg firm Carbo Tech when McLaren Automotive was launched in 2010. Unlike Lamborghini’s approach, McLaren’s Monocells were made using straightforward RTM in steel moulds weighing 35 tonnes. The objective was not to speed up the process but to squeeze the ultimate performance from the chassis. Even so, each tub could be made 10 times faster than that of the McLaren F1 supercar. Now McLaren has it’s own resource, the £50m Innovation Centre in Sheffield, to produce Monocells and Monocages in-house, the first of which was delivered to the McLaren Production Centre in March. The barrier to carbonfibre in the mainstream has been cost, partly because of the raw material itself but mainly because manual handling of the carbonfibre still plays a big part in the manufacture of complex structures. Production and process costs account for 33 to 75% of the overall cost of components. The BMW-Series ‘Carbon Core’ is a hybrid design with steel components embedded in the mould along with carbon, but despite rumours that the subsequent 5 Series bodyshell would follow suit, it didn’t, sticking with steel, aluminium and magnesium. The closest yet to a mainstream CRFP bodyshell has been that of the BMW i3 but it, and its construction, may not necessarily continue after the current model. A breakthrough in cost cutting may be on the horizon, though. Voith Composites developed an automated system called the Voith Roving Applicator to make the CRFP rear panel and parcel shelf for the Audi A8, calling it “a breakthrough technology in the composites world”. For now, though, aluminium remains a favourite for mass-produced cars and is more sustainable, taking 95% less energy to recycle it than to make the stuff in the first place. Forging ahead Developed by Lamborghini with golf club manufacturers Callaway Golf and hailed as a material of the future, Forged Composite consists of short carbon fibres mixed with resin, injection moulded and cured in just two or three minutes. It was used to make the Sesto Elemento concept car a few years ago and some small production parts today, but while 30% cheaper than RTM-Lambo, it has yet to achieve the breakthrough in car manufacturing once hinted
Origin: Under the skin: How carbonfibre is trickling down to the mainstream
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Under the skin: why hybrid makers love a biscuit tin
If last week’s column was anything to go by, EV motors are far more interesting than they look. Once people get their heads around them, they may hold the same fascination that engines have enjoyed since they were invented. The electric machines (the correct name for motor-generators) in EVs today take various forms electrically, but the one thing they have in common is that they are all radial flux machines. Flux is the scientific name for the direction taken by the magnetic fields but, in simple terms, it means they are cylindrical in shape with a rotor spinning inside a cylindrical stator. There is, however, another type of electric machine emerging, and it’s one that some car makers are looking at intently, especially for integrating with a combustion engine to hybridise it, or tacking onto a small engine to make a range-extender generator. It’s the axial flux ‘biscuit tin’ motor. If biscuit tin sounds derogatory, it isn’t (and come on, who doesn’t like a Hobnob?). It’s the simplest way to describe the shape: short in length and large in diameter. This is useful because it means that in some applications they can use space more efficiently than a radial flux motor, such as when sandwiched between an engine and gearbox on a transverse engine. They have other important advantages, such as the ability to generate more torque than a radial flux equivalent. Whereas the rotor (which rotates) of a conventional radial flux machine is relatively small in diameter because it is housed inside the stator (which remains static), the rotor and stator of the axial flux machine are like two large dinner plates facing one another. So both plates’ magnets, the fields of which interact to generate torque and spin the rotor, are set much further away from the motor’s driveshaft. Because of that, the force they produce has more leverage on the main shaft. That means more torque, or the same torque for less power consumption. A good way to visualise that is to think of the steering wheel on a car. Imagine taking the wheel off and grasping the steering column with your hand. It would be pretty difficult to turn. Moving the effort you’re applying further away from the column – by using a wheel – gives you more leverage, or turning force. The same thing happens with an axial flux motor. More companies are developing the technology, and one of those in the forefront is the Oxford-based firm Yasa. Its P400 electric machine can be sandwiched between an engine and transmission in a ‘P2’ hybrid configuration or be used standalone. At just 80mm thick, this sliver of a machine weighs only 24kg, develops peak power of 215bhp (160kW) and 273lb ft peak torque. Yasa also has a complete electric drive unit (EDU) concept comprising a motor, controller and two-speed powershift automatic transmission. The motor itself produces peaks of 402bhp and 368lb ft and yet it weighs in at just 85kg. That compares pretty favourably with the average four-cylinder engine, which weighs around 150kg without the transmission. The hub of the matter It’s only a matter of time before electric machines move to the wheels, freeing up space and possibly changing the way cars look. The axial flux design lends itself perfectly to this. US firm Protean Electric has yet another design, its ‘inside out’ permanent magnet motor, with the stator on the inside and rotor on the
Origin: Under the skin: why hybrid makers love a biscuit tin
Under the skin: the latest CVT gearbox technology
Toyota’s umpteenth incarnation of the Corolla is now on sale, mainly in Hybrid form. It’s billed as having an ‘e-CVT’, which at first had our news antennae all a-quiver. In fact, e-CVT is simply another marketing moniker for essentially the same hybrid driveline concept Toyota came up with in the 1990s for the first Prius and has stuck with ever since. Originally called the Toyota Hybrid System (THS), it then also became Hybrid Synergy Drive (HSD), giving a nod to the fact that it was also used by Lexus and even sold to a couple of other car makers. Swapping cogs, gear changing, shifting: whatever your favourite expression, gearboxes and cars go together like sticky toffee pudding and custard – unless it’s a CVT. Some drivers loathe the way a CVT’s soaring engine revs are disconnected from the car’s rate of acceleration – known as the ‘rubber band effect’. The CVT was made famous by DAF when it launched the first production version, the Variomatic, in 1958. Instead of a complex box of cogs, it consisted of two pulleys of continuously variable diameter, connected by a belt. To give the lowest ratio (like first in a manual), the engine-driven pulley is at its smallest diameter and the second pulley, driving the wheels, at its largest. As speed increases the engine-driven pulley gets bigger and the drive pulley smaller, increasing the ratio – so the car speeds-up. Controlled not by a computer but by a vacuum, it continuously and automatically adjusts for hills and harder acceleration or cruising. The design has been used by many manufacturers over the years, including Audi, Ford and Fiat. CVTs are not all alike, though. Although Toyota offers a CVT in the new Corolla (but not in the UK), its hybrid drive e-CVT is nothing like the original Variomatic and there’s no belt. Instead, it consists of two electric motor-generators (MG1 and MG2) connected to a planetary gearbox. The whole caboodle has the engine at one end and the driven wheels at the other. Planetary gear sets exist aplenty inside conventional automatics. The compact package consists of a sun, planets and an enclosing ring gear and resemble a desk toy of the solar system. There are only a few components, but making the drive take different routes through the mini solar system allows the two motor-generators to perform different roles. MG1 can start the engine and at other times act as a generator to charge the hybrid battery. MG2 can act as a drive motor on its own or with the engine and also a generator to perform a regenerative braking role. MG1 can also apply small amounts of torque to the gear set to control the balance between the engine and electric drive from MG2, and there are many more combinations. The system allows electric-only drive by decoupling the engine (without the need for a clutch), and it’s small and compact. So not all CVTs are what they seem. This latest one is clever and mega-efficient, and it’s not surprising the basic idea has endured for more than 20 years. Reverse engineering Bosch’s electronically controlled version of the original CVT remains mechanically simple. Despite CVTs being scorned by some, Dutch rallycross star Jan de Rooy dominated with his DAF 55 and 555s in the 1970s. DAFs were banished to their own category in the annual Dutch backwards racing championship (yes, really, it used to be a thing) because CVTs enabled them to drive as fast in reverse as
Origin: Under the skin: the latest CVT gearbox technology
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