Andy Pye reviews the TREV electric vehicle from China that was widely regarded to be the most interesting exhibit at the Geneva motor show.
The performance and environmental impact of future electric vehicles could be revolutionised by an innovative turbine recharging technology from China, which was presented for the first time at the 86th Geneva International Motor Show in March.
Techrules is a new automotive research and development company based in Beijing, China. A subsidiary of TXR-S, a research and development company which has other subsidiaries operating in the fields of new materials development, biogas production and aerospace, its stated mission is to develop automotive powertrain technologies to help next generation vehicles be more efficient, more environmentally-friendly, easier to use and deliver a better user experience. In Geneva, the company showcased its patented Turbine-Recharging Electric Vehicle (TREV) system in an all-new concept supercar.
Introducing TREV technology in its own supercar is intended to allow Techrules to demonstrate its credentials as an environmentally-friendly technology that can deliver exceptional dynamic performance. As it evolves quickly from an automotive research and development business into a car manufacturer, Techrules aims to lead a symbolic shift by the Chinese car industry towards world-class automotive quality and performance standards.
The TREV system delivers very high efficiency and very low emissions, and is a sealed-for-life powertrain which requires almost zero maintenance throughout the ownership cycle. The only service item is the air intake filter.
TREV is presented in two designs, the AT96 and GT96. These designs – each offering an alternative configuration of the TREV system – are two variations of a vision of how turbine-recharging supercars might look when the technology enters production in China’s first supercar. “AT” refers to “Aviation Turbine”, indicative that the turbine is configured to run on a liquid fuel such as aviation kerosene, diesel and gasoline. The AT96 is a vision of a track-focused version of the supercar and features a large rear wing, which provides both straight-line stability as well as downforce to aid high speed cornering. The GT96 – for gas turbine – is designed to run on a gaseous fuel such as biogas and natural gas and is styled as a road-going hypercar.
Projections, based on initial testing, indicate that the range of a future production supercar under battery power alone will be up to 150km. Where charging points are unavailable TREV technology can recharge batteries anywhere, either while underway or when parked – eliminating range anxiety. It is envisioned that this parked recharging process could be completed unsupervised, overnight for example. Maximum range – based on the battery configuration in the concept supercar presented at Geneva – is projected to be over 2,000 km from 80 litres of aviation kerosene (in urban driving conditions), or a fuel with equivalent calorific value.
In addition, because Techrules plans to launch China’s first supercar – featuring Turbine-Recharging Electric Vehicle (TREV) technology – within three years, it is investigating locations for its European production base. It then plans to begin production of higher volume city cars a few years later. Through a small in-house team, Techrules plans to have a shortlist established and a final decision will be made by the end of the year. The team is looking at existing sites that are currently or will soon become vacant, and also at possible sites where it could develop an all-new site from the ground up.
Meanwhile, the company’s engineering team is focused on finalising the development and manufacturing processes of the production version of its TREV micro turbine hybrid range extender technology. And it will continue to progress vehicle engineering and styling programmes for the AT96 and GT96 variants of its supercar with specialist partners in the UK and Italy.
TREV is a proprietary, patent-protected series hybrid powertrain range extender system that uses a micro-turbine to generate electricity that charges a battery pack. The battery powers the motors that drive the wheels. Newly developed battery management technologies enable superior charging efficiency. The high efficiency of the TREV range extender results in a requirement for fewer batteries, saving weight and space.
Techrules showcased its technology in a plug-in hybrid range extender TREV concept – the first step towards production of China’s first supercar. A development prototype started testing in February 2016 at the world-famous Silverstone race circuit in the UK.
Grid capacity challenges
A fundamental challenge to the feasibility of mass adoption of EVs in several markets is the massive draw on the electricity grid that would be impossible to meet within the current grid capacity. If the Chinese market, for example, adopted plug-in EVs, the result would be a massive increase in pollution from coal-fired power stations. And many markets in the western world, too, are precariously close to the limit of the electricity generation capacities of their power generation infrastructures. These markets would not be able to sustain a widespread adoption of plug-in EVs.
Since the significant uptake of EVs is dependent on the availability of charging point networks, which require major investments in new infrastructure, harnessing the existing fuel distribution infrastructures reduces the absolute reliance on these networks.
With a common core architecture, the TREV system can be tailored to run on one of a variety of fuels. This means that the configuration of the TREV system can be matched to the fuel which is already prevalent in a specific market with a comprehensive supply and distribution infrastructure. As a result, adoption of the TREV system by the fuel supply industry, vehicle manufacturers and consumers requires no major investment in new networks – as with plug-in EVs or hydrogen fuel cells. The TREV system’s turbine has been tested in various guises, with alternative versions running natural gas, biogas, diesel, gasoline and aviation kerosene.
Unlike many previously developed turbine powertrain systems, there is no direct electrical feed from the generator to the electric motors: the TREV system is purely a series hybrid range extender system.
Air drawn into the micro turbine is passed through a heat exchanger where heat from the exhaust air is transferred to the cold intake air after it has been compressed. Ignition of the compressed and heated fuel-air mixture generates enormous energy which is channelled at very high speeds to turn the turbine vanes. As this hot exhaust gas is expelled, it passes through the heat exchanger to ensure the heat energy is recuperated and transferred to cold intake air.
Techrules Chief Technology Officer, Matthew Jin, explains: “In a conventional car, the combustion engine that converts a fuel’s chemical energy into useful mechanical energy is also the driving an engine that turns the wheels. Because turbines have always been a very inefficient way to convert chemical energy into useful wheel turning mechanical energy, only a few have tried to use a turbine in the powertrain system, and none have ever succeeded commercially.”
But, with electric vehicles, an electric motor is used to drive the wheels, which effectively frees the combustion engine to exclusively convert chemical energy into mechanical energy and finally into electric energy. This is a major breakthrough, making it possible to use the highly efficient turbine engine as a range extender.
Micro turbines are significantly more efficient than piston engines in range extender applications, because significantly less energy is sacrificed in frictional losses, meaning more of the fuel’s chemical energy is harnessed.
The turbine shaft powers a generator that produces electricity to charge the battery cells. In Techrules’ TREV configuration, the turbine and the generator share the same shaft and rotate at the same speed of over 96,000 revolutions per minute producing an output of 36kW. Of this, 30kW powers the generator, with 6kW directly powering auxiliary equipment, such as the inverters. The 30kW electrical output from the generator is used to charge the battery pack.
The total weight of the TREV range extender system (micro-turbine, inverters, fuel pumps, air pumps and generator, but excluding batteries and motors) is approximately 100 kg.
TREV system features several proprietary innovative technologies that make it approximately 50 per cent more efficient than range extender systems using petrol engines, which dramatically increases the viability of its commercial series production.
Low frictional losses
The high rotational speeds that the shaft requires in order to draw in the required volume of air means that achieving low friction is paramount to the efficiency of the TREV system. Instead of a traditional oil lubricant film to separate the shaft from the bearing, Techrules employs air bearing technology – a high pressure feed of compressed air. This results in fewer frictional energy losses, since it eliminates the parasitic losses of a mechanical bearing.
The use of an air bearing system is not unique, but how Techrules uses the air bearing is. Of particular note is that the air bearing is also supported by a magnetic field that allows for precise adjustment of the high speed shaft. Both bearing systems work together to maintain exceptional stability. The magnetic bearing allows a far greater clearance between the shaft and its wall lining, which delivers significant advantages for the long-term durability of the system.
This is an especially important consideration in automotive applications of turbine systems because – unlike stable power generation conditions – the entire assembly must be able to be capable of withstanding volatile operating conditions that result from, for example, vertical shocks from uneven road surfaces and lateral forces in cornering. Techrules’ hybrid bearing system is also more economic to produce, because the built-in extra clearance space reduces the extreme tolerances usually required.
Next, a new design of internal foil – an intrinsic component within an air bearing – is used for the bearing liner that supports the air pressure and flow. It is made of a new compound material that is more durable. Of equal importance is that the new foil enables the mass production of the bearing liner at the required production tolerances to be achieved at a high volume scale at low cost.
Heat exchanger innovation
Techrules has also introduced a new and innovative heat exchanger design that is more thermally efficient than conventional designs. A new material has been introduced in the hybrid heat exchanger which greatly increases the efficiency of heat recuperation from the exhaust gases.
Using readily-available cylindrical 18650 Lithium-Manganese-Oxide battery cells, Techrules is focusing its capabilities on the efficiency of the battery management rather than the battery chemistry itself. The battery pack comprises 2,376 individual 18650 cylindrical cells that can be charged by the turbine generator in approximately 40 minutes.
Unlike most electric vehicle (EV) development programmes, because the TREV system incorporates a series hybrid range extender, Techrules is prioritising power density – the capability of the batteries to deliver peak power – ahead of energy density – the capability of the batteries to hold maximum energy.
Smart battery management
The system employs an innovative smart battery management system that optimises the efficiency of battery charging and power balancing between battery cells. In a conventional lithium-ion battery management system, to avoid cells being damaged by overcharging, the cells – which each charge at a slightly different rate – must be balanced as they charge. This balancing is conventionally achieved by actively discharging the cells that are charging more quickly in order to enable the other cells to “catch up”. This process sees a proportion of energy wasted during the charging process and increases the time required to charge all cells fully.
To address this shortcoming, Techrules has introduced an innovative new charge balancing strategy. The smart battery balancing system harnesses the “excess” voltage in cells that are charging more quickly, sharing their charge with slower-charging neighbouring cells to achieve the required balance. As a result, the entire pack charges more quickly, and there is no energy wasted in actively discharging the best-performing cells.