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Automotive Design


4 Continuously variable transmissions (CVTs) have been used in mass- produced vehicles since the 1950s, but the technology has enjoyed a steady increase in popularity over the past 10 years. Paul Stevens reports on some of the latest developments and applications, including pressure-charging systems and hybrid power trains.


CVT gathers momentum and expands its horizons


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ontinuously variable transmission (CVT) technology still accounts for only a small proportion of all transmissions installed in cars and commercial vehicles, but CVTs are


steadily growing in popularity, largely due to the fuel savings that can be achieved. Already used for a number of cars and commercial vehicles, CVTs are now being investigated by manufacturers of diverse vehicle types, from trucks to micro-cars, as well as for indirect applications. Rather than offering a set of fixed gear ratios,


CVTs can change steplessly through an infinite range of ratios. One advantage of a CVT is that it enables the engine to run at a wider range of speeds for a given road speed; for example, the engine speed might be set to deliver peak efficiency or maximum power (which is usually higher than the speed at which peak efficiency is achieved). CVT - and its close relative the infinitely


variable transmission (IVT) - is not new to the industry: DAF introduced the first CVT-equipped vehicle in 1958; more recently, Nissan boasted about selling one million cars with CVT in 2007, while Audi (with some A4 models), BMW (in some Minis) and Ford are also major users. But while the technology has been used in


various cars - including a number of sports cars - it


has potential applications beyond being simply a gearbox replacement. In the UK, a consortium of automotive companies has developed a prototype Flywheel Hybrid System for Premium Vehicles (FHSPV), which adds up to 60kW of recovered energy to the engine’s output. The consortium, which is currently testing prototypes, anticipates that future models will reduce fuel consumption by 20 per cent. Compared with conventional hybrid systems,


flywheel hybrids reduce the number of energy conversions, thereby helping to improve the overall efficiency of the regenerative braking system. Rather than converting kinetic energy into electricity for storage in a battery, a small CVT, connected to the car’s rear differential, transfers the energy directly into a compact, high-speed flywheel. When the driver reapplies the accelerator, the CVT smoothly transfers the energy back to the wheels. This project is part-funded by the Technology


Strategy Board, the UK’s national innovation agency. Industrial partners include Jaguar Land Rover, Flybrid Systems, Ford, engineering consultancies Prodrive and Ricardo, and transmission experts Torotrak and Xtrac. Based on a flywheel developed by Flybrid


Systems, the FHSPV spins at speeds of up to 60 000rpm so that it achieves a high energy density,


Fig. 1. A CVT module would form a critical part of Volvo’s flywheel-based kinetic energy recovery system.


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