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Volkswagen Group shows 3 hydrogen fuel cell concepts at LA Show: Audi A7 Sportback h-tron; Golf Sportwagen HyMotion; Passat HyMotion
20 November 2014
|Audi A7 Sportback h-tron. Click to enlarge.|
Audi and Volkswagen, both members of the Volkswagen Group, unveiled three hydrogen fuel-cell vehicle demonstrators at the Los Angeles Auto Show: the sporty Audi A7 Sportback h-tron quattro, a plug-in fuel-cell electric hybrid featuring permanent all-wheel drive and the Golf Sportwagen HyMotion, a fuel-cell hybrid, both received a formal introduction in the companies’ press conferences. Further, Volkswagen brought two Passat HyMotion demonstrators for media drives. (The Golf and Passat models have identical hydrogen powertrains and control software.)
All three incorporate a fourth-generation, 100 kW LT PEM (Low Temperature Proton Exchange Membrane) fuel cell stack developed in-house by Volkswagen Group Research at the Volkswagen Technology Center for Electric Traction. (Volkswagen is tapping some expertise from Ballard engineers under a long-term services contract, earlier post.) The Group is already at work on its fifth-generation version, said Prof. Dr. Ulrich Hackenberg, Member of the Board of Management for Technical Development at Audi, during a fuel cell technology workshop held at the LA show, and may be ready to talk about that technology by the end of next year.
In visual terms, the fuel cell vehicles basically resemble their production counterparts, reflecting the Volkswagen Group’s strategic approach of developing alternative drivetrains so as to increase the powertrain options available to customers within the high-volume model lines. This is the opposite of the approach taken by Toyota with its Mirai fuel cell vehicle (earlier post) and Honda with its new FCV Concept (earlier post).
|“Fuel cell technology is running in competition with long-range battery electric vehicles. We don’t know which technology will be the winner.”|
—Dr. Ulrich Hackenberg
The technology developed and chosen for implementation in these demonstrators also reflects the Group’s focus on leveraging the capabilities of its modular toolkit approach (modularen Baukästen). (Earlier post.) Put another way, the fuel cell technology is being developed so as to work as components in the MQB (transverse) kit, the development of which is led by the Volkswagen brand, and the MLB (longitudinal) kit being driven by Audi.
The ultimate goal—one that Volkswagen Group and brand executives consistently emphasize—is to enable “bumper-to-bumper” production of brand models equipped with different drive systems (gasoline, diesel, natural gas, plug-in hybrid, battery-electric and fuel cell) using the same production line. (Earlier post.)
The Group is not—unlike Toyota, Honda and Hyundai—announcing production dates and initial markets for its fuel cell vehicles.
In 2009, we forecast that a breakthrough in hydrogen fuel cells could not be expected before the year 2020. We are still convinced of this. The fuel cell is and will remain an important an important supplement to our electrification strategy. We wanted to show you that we will be ready to launch when all of the issues related to hydrogen infrastructure have been solved.—Dr. Heinz-Jakob Neußer, Member of the Board of Management at Volkswagen responsible for the Development Division
Those issues include not only the availability of refueling stations, but also the ability to produce hydrogen from renewables, Dr. Neußer said in his remarks introducing the Golf Sportwagen HyMotion.
|VW Technology Center for Electric Traction|
|Since the 1990s, Volkswagen has been researching the potential of hydrogen fuel cells and transferring this drive technology to production cars. At the end of the past decade Volkswagen decided to build a dedicated Technology Center for Electric Traction near its headquarters in Wolfsburg, to further advance its capabilities in fuel cell development.|
|The Isenbüttel site was chosen for this center and construction of a special research center for electric drivetrains began in 2001. The infrastructure of the technology center includes a dedicated hydrogen fuel station. Volkswagen produces the hydrogen for the pressure tank station from renewable solar-generated electricity. A photovoltaic array was installed at the site for this purpose.|
The Fuel Cell Stack
The fuel cell system comprises more than 300 individual cells that together form a stack. The core of each of these individual cells is a polymer membrane, with a platinum-based catalyst on both sides of the membrane.
In a PEM fuel cell, hydrogen is supplied to the anode, where it is broken down into protons and electrons. The protons migrate through the membrane to the cathode, where they react with the oxygen present in air to form water vapor. Meanwhile, outside the stack the electrons supply the electrical power. Depending on load point, the individual cell voltage is 0.6 to 0.8 volts. The entire fuel cell operates in the voltage range of 230 to 360 volts.
The main auxiliary assemblies include a turbocharger that forces the air into the cells; a recirculation fan which returns unused hydrogen to the anode, thus increasing efficiency; and a coolant pump. These components have a high-voltage electric drive and are powered by the fuel cell.
There is a separate cooling circuit for the essential cooling of the fuel cell. A heat exchanger and a thermoelectric, self-regulating auxiliary heating element maintain pleasant temperatures in the cabin. The fuel cell, which operates across a temperature range of 80 degrees Celsius, places higher demands on the vehicle cooling than an equivalent combustion engine but achieves superior efficiency of as high as 60 percent—almost double that of a conventional combustion engine. Its cold-starting performance is guaranteed down to -28 degrees Celsius.
During the fuel cell workshop, Dr. Neußer said the Group is focused on two major areas of focus in the fuel cell stack to get the efficiency as high as possible with the goal of maximizing range. The first is to bring pressure losses as low as possible.
The second, and the key issue, he said, is the membrane technology itself. Volkswagen is working on nanostructuring the platinum coating to achieve as high a surface area as possible while also reducing the thickness.
(In an aside, Dr. Hackenberg noted that the nanostructuring work for the membrane assemblies has synergies on the battery side, where Volkswagen is exploring the use of very thin layer nanostructures very similar to what is being done on the fuel cell side.)
Audi A7 Sportback h-tron quattro plug-in fuel cell hybrid
|Click to enlarge.|
The Audi A7 Sportback h-tron quattro fuel-cell plug-in hybrid demonstrator features the fuel-cell stack in the engine compartment and an 8.8 kWh battery pack and an additional electric motor in the rear. The drive configuration gives the zero-emission Audi A7 Sportback h-tron quattro 170 kW of available power—a new level of performance in fuel cell cars. There is no mechanical connection between the front and rear axles; as an e quattro, the A7 Sportback h-tron quattro features fully electronic management of torque distribution.
Because the exhaust system only has to handle water vapor, it is made of weight-saving plastic.
The A7 Sportback h-tron quattro is a genuine Audi—at once sporty and efficient. Conceived as an e-quattro, its two electric motors drive all four wheels. The h-tron concept car shows that we have also mastered fuel cell technology. We are in a position to launch the production process as soon as the market and infrastructure are ready.—Prof. Dr. Ulrich Hackenberg, Member of the Board of Management for Technical Development at Audi
In the fuel cell mode, the A7 Sportback h-tron quattro needs only about one kilogram (2.2 lb) of hydrogen to cover 100 kilometers (62.1 mi); the energy content of 1 kg of hydrogen is equivalent to that of 3.7 liters (1.0 US gal) of gasoline. The tanks can store around five kilograms of hydrogen at a pressure of 700 bar—enough to drive more than 500 kilometers (310.7 mi). The range is boosted by up to 50 kilometers (31.1 mi) by a battery with a capacity of 8.8 kilowatt-hours, which is recharged by recuperation or alternatively from a power socket.
Like a car with combustion engine, refueling takes no more than around three minutes. The Audi A7 Sportback h-tron quattro accelerates from 0 to 100 km/h (62.1 mi) in 7.9 seconds and on to a top speed of 180 km/h (111.8 mph).
|Top left. Hydrogen fuel cell system. Top right. High-voltage components. Bottom left. quattro drive. Bottom right. Packaging. Click to enlarge.|
The 8.8 kWh Li-ion battery in the h-tron is adopted from the Audi A3 Sportback e-tron plug-in hybrid. (Earlier post.) The pack is located beneath the trunk and has a separate cooling circuit for thermal management.
The high-performance battery can store energy recovered from brake applications and supply powerful full-load boosting, enabling the impressive acceleration. Both the front and rear axles have no mechanical connections for the transmission of power. In the event of slip, the torque for both driven axles can be controlled electronically and adjusted continuously.
On battery power, the Audi A7 Sportback h-tron quattro covers as much as 50 kilometers (31.1 mi).
The battery operates at a different voltage level than the fuel cell; hence, there is a DC converter (DC/AC) between the two components—this tri-port converter is located behind the stack. Under many operating conditions, it equalizes the voltage, enabling the electric motors to operate at their maximum efficiency of 95 percent.
The power electronics in the front and rear of the vehicle convert the direct current from the fuel cell and battery into alternating current for the electric motors to drive the front and rear axles separately.
The two electric motors, which are cooled by a low-temperature circuit together with the voltage converters, are permanently excited synchronous machines. Each of them (the same motor used in the eGolf, earlier post) has an output of 85 kW, or up to 114 kW if the voltage is temporarily raised. The peak torque is 270 N·m (199 lb-ft) per electric motor.
The electric motors’ housings incorporate planetary gear trains with a single transmission ratio of 7.6:1. A mechanical parking lock and a differential function round off the system.
Switching from automatic transmission mode D to S increases the level of energy recovery when braking, so that the battery is charged up effectively during sporty driving. Brake applications, too, are almost always accomplished fully electrically: The electric motors then act as alternators and convert the car’s kinetic energy into electrical energy that is stored in the battery. The four disk brakes only become involved if more forceful or emergency braking is required.
The four hydrogen tanks of the Audi A7 Sportback h-tron quattro are located beneath the base of the trunk, in front of the rear axle, in the center tunnel. An outer skin made from carbon fiber reinforced polymer (CFRP) encases the inner aluminum shell.
Since 2013 Audi has been operating a pilot plant (earlier post) in which renewable wind power is used to produce hydrogen by electrolysis. At present, this hydrogen is still used in an additional production process to obtain synthetic methane (Audi e-gas). A future move to feed this hydrogen into a hydrogen supply and filling station network would make it available for refueling fuel-cell vehicles.
Golf Sportwagen HyMotion fuel cell hybrid
The Golf Sportwagen HyMotion is a full cell hybrid, that functions very similarly to a gasoline- or diesel-electric hybrid, except that the primary propulsion is electric, powered by the fuel cell.
|Click to enlarge.|
The hydrogen Golf highlights the potential of the MQB approach. The fuel cell, as noted above, is shared with the hydrogen A7; the 100 kW, 270 N·m (199 lb-ft) electric drive motor comes from the e-Golf, and the 1.1 kWh, 36 kW Li-ion battery pack comes from the Jetta Hybrid.
Volkswagen essentially is showing the Golf SportWagen HyMotion to demonstrate how a hydrogen fuel cell could be implemented in an MQB-based vehicle.
The motor and coaxial two-stage 1-speed transmission are located at the front of the engine compartment; also in the engine compartment are the fuel cell stack; cooling system; tri-port converter and the turbo compressor.
The power electronics are located in the center tunnel area; they convert the direct current (DC) into three-phase alternating current (AC) which is used to drive the motor. The power electronics also integrate a DC/DC converter, which converts energy from the high-voltage battery to 12 volts to supply the 12-volt electrical system.
The high-voltage lithium-ion battery is mounted close to the trunk and rear suspension. The 12-volt battery is also mounted at the rear. Two of the total of four carbonfiber composite hydrogen tanks are housed compactly under the rear seat and the other two in the luggage compartment floor. The hydrogen is stored in the tanks at a pressure of 700 bar. As in all other Volkswagen vehicles, the tank filler neck is located on the right side at the back of the car.
|Energy flow display showing battery (blue), fuel cell, triport connector, auxiliaries (NV), fuel tank and motor. Pressing any of the powertrain elements brings up a display with more information. (Screen from the Passat.) Click to enlarge.|
The lithium-ion battery is the second powerplant in the vehicle, and it plays an important role in the drive system. In addition to storing the energy recovered during regenerative braking, it is also an important component in all phases during which the chemical reaction needs to be initiated by feeding oxygen and hydrogen to the fuel cell (the latter via the turbo compressor), such as when driving off from a start.
At this point in time, the fuel cell has not built up enough electrical power to drive the motor by itself. In these phases, the lithium-ion battery jumps into action and supplies energy to the electric motor. The high-voltage battery also operates like a turbocharger during fast acceleration and while accelerating to top speed—i.e., boosting to supply overall system power of 100 kW or 134 hp.
The front-wheel-drive Golf SportWagen HyMotion accelerates from 0 to 62 mph (100 km/h) in 10.0 seconds; driving range is about 310 miles (500 km).
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