A global effort is underway to develop the technology for a range of vehicles designed to go further and faster than conventional hybrids. Stuart Nathan reports

Hybrid vehicles are now a common sight on our roads and everyone is familiar with their claims of greater efficiency and fuel economy — hence lower carbon emissions — than conventional internal combustion engine vehicles.

But car manufacturers are developing a new range of vehicles which, they believe, could top even the hybrids' performance. In the UK, the Technology Strategy Board (TSB) is investing heavily in a range of projects to accelerate the development of these Range-Extended Electric Vehicles (REEVs).

REEVs, also known as plug-in hybrid electric vehicles (PHEVs) are, as the name suggests, essentially electric cars. Yet they are also a type of hybrid. The vehicle's drive system is fully electric —that is, an electric motor drives the car 100 per cent of the time. The motor is powered by banks of batteries that can be charged overnight in a domestic power supply. A four to six-hour charging period would provide sufficient power for a small, set number of miles, generally targeted at between 20 and 60.

After this power is exhausted, a small onboard generator switches on to recharge the batteries to extend the range of the vehicle. This generator could use a wide range of technologies. If it is a petrol or diesel engine, the car is essentially a hybrid, although unlike current models such as the Toyota Prius the vehicle is never driven by the internal combustion engine. The generator could also be a fuel cell.

The cars generally also include regenerative braking systems to conserve power and return charge to the batteries when the vehicle brakes, which also improves the range. A single load of fuel is generally targeted at taking the vehicle about 300 miles.

REEVs are potentially lower-emission vehicles than standard hybrids. In operation, the electric phase produces no emissions (although this does not take account of any carbon emissions associated with generating electricity used to charge the batteries). And because the electric motors drive the car all the time, the generator only has to keep the batteries charged — it does not have to produce the variable power load needed to drive a car over different terrains, for example.

REEVs, like many alternative automotive drive technologies, are not new. In the early days of the car, many different technologies were tried. Porsche, in fact, produced an electric hybrid as early as 1899. The first vehicle to use a technology similar to a REEV system was a concept car made by General Motors in 1969. And diesel-electric locomotives, which have been around for decades, use a system similar to a REEV, with a large onboard diesel generator providing power to heavy-duty electric motors. Cost consideration, and limitations in battery technology, have limited their commercial possibilities until now.

At the Detroit Motor Show, held at the beginning of the year, half a dozen REEV concepts were on display. Chevrolet's Volt, first launched a year previously, uses a chassis packed with lithium ion batteries which provide 16kWh of power, running a 120kW electric motor. The generator component is a one-litre 'flex-fuel' engine, capable of running on petrol or petrol-ethanol blends. Other types of generator, including fuel cells, could also be used.

Other REEVs on show at Detroit included GM's Cadillac and Saturn (the US equivalent of Vauxhall); Chrysler in particular turned heads with its Jeep Renegade, a small dune buggy-type vehicle, which is claimed to achieve 110mpg fuel economy by linking a small diesel engine to a lithium-ion battery pack.

The batteries have always been a sticking-point for electric cars, the major concerns being weight, speed of charging, power storage capability and safety — a large number of lead-acid batteries in a car would never be an ideal solution. Most current hybrids use nickel-hydrogen batteries. But the emerging lithium-ion (Li-ion) technology seems to provide a good platform for REEVs, being more compact and environmentally-friendly than nickel-hydrogen (lithium is produced from seawater rather than mining) and demand seems set to rise.

VW, for example, has announced plans to join forces with Sanyo to develop Li-ion batteries, while Nissan is working with NEC. And in Germany, Bosch is setting up a specific unit to develop them for automotive applications.

Chrysler's Jeep Renegade, a small dune buggy-type vehicle, is claimed to achieve 110mpg fuel economy by linking a small diesel engine to a lithium-ion battery pack

Already one of the world's largest suppliers of automotive technologies, Bosch sees Li-ion technology as a part of its drive to develop low-carbon technologies. 'Nearly 400 Bosch engineers are active in this field,' said chairman Franz Fehrenbach. He added that the new unit will focus the company's efforts on the further development of the core competence needed for the increased use of electrical motors in drive systems. 'Our development efforts are directed at bringing Li-ion batteries — which we ourselves pioneered in power tools — on to the road,' he said.

The TSB's low-carbon vehicle projects, which will receive £23m of government funding, also include a Li-ion component. Involving FiFe Batteries, which operates a lithium-ion R&D centre in Oxfordshire, and Anglo-American firm ABSL Power Solutions, which is providing Li-ion batteries to NASA for powering space-suit life support, the project aims to develop a high-efficiency, low-cost, ultra-safe battery for vehicles. The project is particularly concerned with improving the energy density of the batteries — the amount of energy each battery can hold.

The TSB is awarding 16 grants in this funding round and, although not all of them are aimed at REEV development, this is clearly one of the goals that has been identified. For example, Axon Automotive — a relatively new company that is aiming to develop fuel-efficient cars using carbon-fibre technology — is to work with Warwick University, Powertrain Technologies and composites specialist Scott Bader to produce a plug-in hybrid of its 500kg vehicle, powered by a 500cc engine and performing at better than 100mpg.

Land Rover is also involved in a project that could improve the much-maligned 'gas-guzzler' image of SUVs. Working with Ricardo, the company is leading the project to develop a modular electric and diesel-electric powertrain suitable for a variety of vehicle types. This will be tested on a 'large premium SUV', in a configuration that will emit 120-130g/km of carbon, and will have a zero-emissions range of at least 12 miles. The project is also to investigate providing a series of recharging facilities for a fleet of REEVs for E.On, which is also part of the consortium.

Land Rover will not comment on the project, although it is already working on a hybrid concept vehicle known as the Land-E. Based around a lightweight aluminium chassis to reduce weight, the vehicle will be powered by an electric system around the rear axle that will be able to drive all the wheels separately. The goal is to reduce emission levels by 30 per cent, compared with equivalent conventional Land Rovers, without compromising off-road efficiency.

Another TSB project is also aimed at transforming an icon — the London taxi. Fuel cell specialist Intelligent Energy, which was responsible for the power unit of the ENV concept scooter in 2006, is working with Lotus, TRW Conekt and London Taxi International to produce an electric hybrid cab in time for the 2012 London Olympics.

Taxis are ideal candidates, because central depots can provide the hydrogen needed, freeing them from the existing fuelling infrastructure. The new taxis — a fleet of 50-100 vehicles is the target for the project —will run for an entire day on a single tank of hydrogen, and will be capable of a top speed of 75 mph. in fact, IE claims they will be capable of better acceleration than a conventional taxi. 'The Black Cab is an internationally recognised London icon, and this partnership is an important part of making low-carbon transport a practical and near-term reality,' said Henri Winard, IE chief executive.

His company's part in the project is to develop a proton-exchange membrane fuel cell capable of producing in excess of 25kW of power. Lotus will package the components of the system and build a control system to optimise the performance of both fuel cells and motors. And TRW Conekt is to lead the safety analysis aspect of the project.

'The taxi project will provide further evidence of our ability to put commercially available hydrogen fuel cell vehicles on the market,' said Winard. 'What better stage to prove the credentials of UK innovation than on the way to the 2012 Olympics?'