FEATURE: Electric vehicle range extenders – rival approaches
As automotive manufacturers announce new projects, Dr Peter Harrop, Chairman IDTechEx, takes a look at the latest developments in the burgeoning field of the EV 'range extender'.
Led by the Japanese and with Hyundai of Korea, the Europeans and the Americans coming up fast, we are in the decade of the hybrid vehicle, with much more being spent on them than on pure electric vehicles. This is because traction batteries alone cannot affordably provide sufficient range in most of the large applications by land, sea or air. On-road vehicles are the primary focus with the quick win being to add a conventional off-the-shelf piston engine to work in parallel or simply in series to charge the battery as the vehicle goes along. The next stage has been to use the newer lithium-ion batteries with greatly simplified piston engines that supply almost steady power and therefore dispense with the complications of a conventional engine designed for hugely varying power demands it no longer encounters because the battery now copes with those.
Notably in Europe, at Lotus, DLR and elsewhere, the mechanical engineers have designed simplified, piston-engined range extenders that are only required to charge the battery at almost steady load. To produce the electricity, a separate generator is attached. The nearly silent Polaris Industries REX single cylinder design from Switzerland is among the most impressive of these piston range extenders.
Improving the internal combustion range extender
As a next stage, quite radically different internal combustion engines are being attached to generators. Audi in Germany is still working on the previously unsuccessful Wankel rotary combustion engine. There have been problems with sealing and emissions in the past but Austro Engines of Austria demonstrated its Wankel engine at the Paris Air Show in 2011 in what was claimed to be the world's first aircraft with a serial hybrid electric drive system employing an internal combustion engine. Fuel consumption is very low since the combustion engine always runs with a constant low output of 30kW. The two-seater motor glider successfully completed its maiden flight on June 8 at the Wiener Neustadt airfield in Vienna, Austria. It was built by Siemens, EADS (which recently demonstrated a pure electric stunt aircraft with very limited range) and Diamond Aircraft using its DA36 E-Star motor glider.
"A serial hybrid electric drive can be scaled for a wide range of uses, making it highly suitable for aircraft as well," said Dr. Frank Anton, the initiator of electric aircraft development at Siemens Germany which is developing much lighter weight electric motors for such purposes. "The first thing we want to do is test the technology in small aircraft. In the long term, however, the drive system will also be used in large-scale aircraft. We want to cut fuel consumption and emissions by 25%, compared to today's most efficient technologies. This will make air travel more sustainable."
"The serial hybrid electric drive concept makes possible a quiet electric takeoff and a considerable reduction in fuel consumption and emission," said Christian Dries, the owner of Diamond Aircraft. "It also enables aircraft to cover the required long distances."
An equally radical improvement on the piston engine has been provided by the Capstone mini turbines made in the USA and deployed in over 1000 DesignLine hybrid buses. Bladon Jets in the UK is miniaturising and simplifying the turbine as a range extender: it has them designed into the planned Jaguar supercar, though much development remains to be done. Both of these turbines have only one moving part and they can accept a wide variety of fuels without retuning. Altria Controls in the US now offers bus and truck hybrid powertrains based on turbine range extenders and Langford Performance Engineering in the UK has demonstrated a similar powertrain in a Ford S Max. The mechanical engineers are therefore capturing huge gains in range extender performance, reliability, life, emissions and cost as the years go by.
Inherent electricity generation
However, the physicists note that all these devices have an extended shaft or transmission with a separate electricity generator attached to it – legacy thinking. The physicists seek a more elegant solution where the power source is its own generator. For example, in December 2010, the patents of Dr. Herbert Hüttlin of Innomot AG, in Germany and Switzerland, revealed a new range extender that does not have a shaft and separate electrical generator. He seeks to, “combine the special advantages of the three-dimensional kinematics of the Hüttlin-Kugelmotor (spherical engine) with those of an electrical aggregate as far as possible as a single assembly, with the electrical aggregate functioning as starter, motor or generator. Our aim was: smaller, lighter, simpler, more efficient, more economical and cheaper. To succeed in this it was necessary to move away from the shape and form of conventional aggregates. In principle, the Hüttlin-Kugelmotor is a 4-stroke engine, which, however, follows a completely new kinematical principle. The mechanical aggregate part of the Hüttlin-Kugemotor with its innovative three-dimensional kinematics is set inside, while the ring-shaped electrical aggregate is positioned in axial symmetry around it.”
It was shown at the 2011 Geneva Motor Show, with a piston cage rotating around the system axis. There are two pistons in the same plane, positioned in the longitudinal center and orthogonal to the system axis. Guided by two hollow spheres, they roll on the bilateral sides of a sinusoidal member fixed to the outer casing, thus executing the 4-stroke ventilation cycle but with integral magnets and coils to generate electricity. The Hüttlin Range-Extender transfers power exclusively as electrical energy through buffer batteries to two to four wheel hub motors.
Integral electricity generation is also being pursued by the German Aerospace Center and others in developing the free piston internal combustion engine which has coils around the unattached piston that generate electricity directly. On the other hand, Clarian Laboratories in the USA has a Wankel-like rotary piston engine with no shaft that inherently generates electricity through its windings.
Even more integral is the fuel cell because it makes electricity directly from a chemical reaction, not through windings, usually by turning hydrogen fuel into water and energy by reacting it with oxygen in the air in the Proton Exchange membrane PEM version. Unfortunately, for many decades, the fuel cell for mainstream vehicles has been ten years away but now the proponents are making it easier for themselves by focussing on range extender versions offering as little as 0.5 to 10 kW of steady power, notably on the smaller vehicles such as motor bikes, cars, taxis, small manned aircraft, Unmanned Air Vehicles UAV and Autonomous Underwater Vehicles AUV. Price is one of the big difficulties remaining but there are advances in eliminating the currently used platinum for example. Intelligent Energy, Mercedes and Proton Power in Europe and Nuvera, Ballard, Protonex Technology and Enerfuel in the USA are among the many fuel cell developers in North America and Europe carrying out trials in vehicles. There is much work in East Asia as well.
Mercedes says it will soon have a fuel cell car in production – claimed to be the world’s first - and the German and Korean governments both plan 500,000 on-road vehicles with fuel cell range extenders deployed by the end of the decade. Unfortunately, that does not amount to a promise to put in the necessary hydrogen fuelling infrastructure. No one wants to pay that huge cost in advance, so, short of a breakthrough, the coming decade may see fuel cells in volume produced vehicles confined mainly to fleets where hydrogen distribution is not a problem.
The coming decade
In the new IDTechEx report, “Range Extenders for Electric Vehicles 2011-2021” the situation is fully analysed and forecasted. There will definitely be huge sales of hybrid vehicles in the next decade and most will be series hybrids with range extenders. A high proportion of those range extenders will consist of piston and turbine combustion engines designed to purpose, with a generator attached. The more radical approach of power sources that integrally generate electricity should also see some volume production later in the decade – maybe as much as 10% of hybrid sales by then.
Even in the larger vehicles, few range extenders will need to generate more than 50 kW unlike the conventional engines they replace, not least because of the future availability of top-up fast charging and increasing use of third generation lithium-ion batteries with higher energy density doing more of the work. Indeed, multi-mode on-board energy harvesting from solar panels, active suspension, heat and so on will also ease the load on the range extender. For land, sea and air, the harvesting options and experience are detailed in the new IDTechEx report, “Energy Harvesting for Electric Vehicles 2011-2021”.
For more on the support from new charging infrastructure see the new IDTechEx report, “Electric Vehicle Charging Infrastructure 2011-2021”.