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Extending electric vehicle range

05 August 2010

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Range anxiety is what stands between many car drivers and electric vehicles. Range-extender technologies for EVs are seen as panacea to the problem and expected to play a prominent role in the broad uptake of electric vehicles and governmental carbon-reduction strategies. The industry is working on different technological solutions, optimised combustion engines are just one among others.
For full electric vehicles the driving range is limited by the capacity of currently available batteries. Range extenders can extend the driving range for an electric vehicle while providing a redundancy in propulsion and also possibly a lower weight (trading battery weight with the weight of the range extender). The challenge is to produce a range extender at a low cost and with good noise, vibration and harshness (NVH) properties. The range extenders are composed of a small system of combustion engine and electric generator which produces the required electricity for charging the batteries in time.

The electrification of the automotive powertrain necessitates an adaptation and in many cases technical modification of all related components. The integration of diverse technologies in range extender, transmission, e-motor, battery and control systems in one harmoniously working system constitutes a real challenge to automotive engineers. In particular e-drive and range extender need to be optimally fine-tuned to each other in order to provide attractive vehicle performance.

The different forms range extender can take

Latest trends in the manufacturing of range extenders lean to monoblock engines, reducing the number of parts and total weight as well as reducing costs and improving reliability. Lotus, for example, has developed a tiny “range-extending engine”. The "monoblock"weighing 124 pounds is still an internal combustion engine with the limitations of fuel choice, reliability etc that this implies. But it implements numerous money and weight saving features. The block, cylinder heads, and exhaust manifold have all been built into a single, lightweight casting. This sheds weight and 17 parts.

A UK consortium led by micro gas turbine company Bladon Jets and also including luxury car maker Jaguar Land Rover, on the other hand, is currently developing a range extender in form of a gas-turbine generator specifically designed for automotive applications. Their work is partially funded by the UK Technology Strategy Board which supports in the development of Ultra Lightweight Range Extender (ULRE) for next-generation electric vehicles.

The consortium's ULRE will incorporate a Bladon Jets patented axial-flow gas turbine engine coupled to a high-speed generator using SR Drives’ proprietary switched-reluctance technology. Here again there is a monoblock approach to reduce the number of parts and cost plus improve reliability and weight but you only have to look at a jet aircraft to realise that turbines take us into a whole new realm of reliability and fuel versatility. Design of the ULRE’s packaging for integration into the vehicle will be overseen by engineers at Jaguar Land Rover.

The India Tata as well seems to see the future of range extenders in gas turbines while the German Audi, on the other hand, is bringing back the Wankel motor and trying a mini version as range extender in their electric concept car. The range extender, developed by the Austrian AVL List Gmbh, is an independent auxiliary-power unit which can be added to pure electric vehicles to extend the operational-driving range beyond what is possible in pure battery operation.

To reduce costs and vehicle weight, the battery capacity is optimised for the majority of driving schedules. Longer driving distances, which are driven less frequently, will not be covered by the battery alone, but supported by an IC engine range extender that, depending on the usage profile, can be designed either as two- or four-stroke reciprocating piston engine, or for very low NVH as rotary engine.

Because of the specific drivetrain system layout, the electrical system and the IC engine can be optimised for single-point operation. All the range extender functionalities are integrated in a modular package to minimise interfaces with the vehicle.

Industry voices speculate that hybrid and pure electric vehicle technology might converge in the form of long range vehicles with small-sized range extenders that are far more than just optimised versions of conventional combustion engines. Ideas are not amiss, as we see.


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