by Mike Millikin
In a demonstration of its ongoing development of advanced alternative powertrains, Audi unveiled the new h-tron quattro fuel cell concept vehicle at the North American International Auto Show (NAIAS). Like its production-bound sibling the e-tron quattro battery-electric vehicle, the h-tron concept is based on Audi’s second-generation modular longitudinal platform (MLB evo, earlier post).
The Audi h-tron quattro concept combines an Audi fifth-generation fuel cell stack delivering up to 110 kW with a power-optimized 1.8 kWh HEV battery that can provide a temporary boost of 100 kW for combined peak system power of 210 kW. The car can be fully refueled with hydrogen in around four minutes, and is then ready to drive for up to 600 kilometers (372.8 miles). Unlike the 3-motor e-tron quattro, the h-tron uses two electric motors, one on each axle, and so drives like a “conventional” electric vehicle, notes Audi Head of Electric Powertrain Siegfried Pint—i.e. without the potential for the type of advanced dynamics control offered by the e-tron quattro. (Earlier post.)
The decision to go with two motors was based on the power capability of the fuel stack, Pint said. However, in a demonstration of the flexibility of the MLB evo platform components, the Audi engineers took the 140 kW motor from the e-tron quattro front axle and used it for the rear axle of the h-tron; and took one of the 90 kW rear axle motors from the e-tron quattro to use for the front axle in the h-tron.
|“When we planned the e-tron [quattro], we already had in mind the h-tron [quattro], to show the flexibility of the MLB evo platform.”
Each motor powers one set of wheels—as on the technology demonstrator Audi A7 Sportback h-tron quattro (a plug-in hybrid design with an 8.8 kWh pack), which Audi presented in November 2014. (Earlier post.) The torque can be varied continuously for both axles.
An intelligent management system controls the interplay between the two motors as appropriate for the situation, placing maximum emphasis on efficiency. At low loads only the front axle receives propulsive power, and at very low speeds the power is supplied solely by the battery.
A heat pump for the interior air conditioning and a large solar roof that generates up to 320 watts, equivalent to adding up to 1,000 kilometers (621.4 mi) to the range annually, also boost efficiency.
A Cd value of 0.27 makes a major contribution to maximizing range and efficiency. Aerodynamic elements down the flanks, on the underbody and at the rear improve the way air flows around the car at higher speeds. Cameras take the place of exterior mirrors, further enhancing aerodynamics and efficiency.
With 550 N·m (405.7 lb-ft) of system torque, the Audi h-tron quattro accelerates from 0 to 100 km/h (62.1 mph) in less than seven seconds; its top speed is governed at 200 km/h (124.3 mph). According to the New European Driving Cycle, fuel consumption of the h-tron quattro is around one kilogram (2.2 lb) of hydrogen per 100 kilometers (62.1 mi).
(As two rough comparisons, Toyota’s production Mirai fuel-cell sedan, with a 114 kW fuel cell stack and a 113 kW traction motor, has an NEDC fuel consumption—calculated from the JC08 homologated value—of 0.76 kg H2/100 km, according to Toyota. The Hyundai iX (Tucson) Fuel Cell Vehicle SUV, with a 100 kW stack and 100 kW motor, has an NEDC fuel consumption of 0.95 g H2/100 km. )
Fuel cell stack. The fifth generation of fuel cell technology from Audi and Volkswagen uses lighter materials to reduce the weight and to improve performance, responsiveness, service life and efficiency compared to the fourth generation fuel cell technology applied in the A7 Sportback h-tron. With an efficiency rating in excess of 60%, the fuel cell now surpasses any combustion engine. The stack, comprising 330 individual cells, is housed in the forward structure. Depending on the load point, the stack operates in the range of 220 to 280 volts.
The focus of development for the fifth generation stack (Ballard is Audi and Volkswagen’s development partner, earlier post) is on new materials for the diaphragms and the bipolar plates of the membrane electrode assemblies (MEAs) that guide the gases in the stack while keeping the cells separate. They help to make the entire unit lighter, smaller, stronger and also more economical, especially as the content of the precious metal platinum catalyst has been reduced. The operating life and responsiveness are improved, and hydrogen consumption is cut.
The fifth-generation fuel cell technology operates at a temperature level of up to 95 ˚C, an increase of 15 degrees compared with the previous generation. The higher thermal operating range enables a reduction in the balance of plant (BoP), Pint noted, as the cooling requirements are not as demanding. A high-efficiency heat pump absorbs the waste heat from the electrical components and a thermoelectric auxiliary heating element maintain pleasant temperatures inside the car. The car starts down to -28 ˚C.
Battery. Complementing the 110 kW fuel cell is a compact lithium-ion battery designed for optimum power output. The battery, weighing less than 60 kilograms (132.3 lb), is located beneath the passenger compartment to optimize the center of gravity. It supplies up to 100 kW of power, ample for a temporary, forceful burst when accelerating. When the car is braked it stores the recovered energy.
The driver can influence the degree of recuperation by selecting either gliding mode or coasting recuperation. The four wheel brakes only cut in if harder or emergency braking is required.
The storage battery operates over the range of 220 to 460 volts—a 3-way DC/DC converter in the engine compartment equalizes the difference compared with the fuel cell’s voltage level.
A separate low-temperature circuit keeps the driveline and high-voltage battery components sufficiently cool even when the car is driven sportily.
3-way power DC/DC converter. One of the key features of the h-tron quattro concept is the 3-way power DC/DC converter. The 3-way converter connects the battery, fuel cell and power electronics. By doing so, the h-tron can maintain the high output voltage level without power loss from voltage drop, Pint said.
Hydrogen storage. The three hydrogen tanks are located beneath the passenger compartment or luggage compartment but do not impinge on the interior. At a pressure of 700 bar, they store enough hydrogen for a range of up to 600 kilometers (372.8 mi)—i.e., about 6 kg.
Every tank is made up of several layers: the inner tank made with gas-tight polyamide is wrapped in carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP). Like a car with combustion engine, refueling takes about four minutes. at a maximum rate of 1.5 kg/minute.
The hydrogen tanks differ in size. The front one is installed longitudinally beneath the center console, and the two other tanks transversely beneath the rear seats and luggage compartment respectively. Together with the battery, they are secured to a structural frame. Neither on the fuel cell drive version nor on the Audi e-tron quattro concept do the tanks or battery impinge on the interior. This also demonstrates the versatility of the new MLB evo platform.
The three tanks communicate with the refueling system by infrared interface and equalize the pressure and temperature levels. The stainless steel hydrogen tank cap is located on the front right wing of the sporty SUV; its flap opens electrically.
zFAS and domain architecture. The h-tron quattro uses the zFAS domain controller for driving assistance systems, as does the e-tron quattro. When the latter moves into production in 2018, it will use the second-generation of the technology, shown at CES 2016. (Earlier post.)
All available sensor information is processed by the zFAS. It computes a complete model of the car’s surroundings in real time and makes this information available to the assistance systems and the piloted driving systems. They can assume driving tasks during parking or in stop‑and‑go traffic on freeways at speeds of up to 60 km/h (37.3 mph).
The movement to a domain-controlled architecture, with zFAS being an example of one of the domain controllers relieves a tremendous burden from current powertrain controllers, Pint noted. For the e-tron quattro, he said, Audi will have a torque domain controller to handle the intricate interplay between the 3-motor systems to deliver optimal performance and dynamics.
Renewable hydrogen and market prospects. While Audi is strongly committed to the commercialization of battery-electric e-tron platforms (Audi of America is targeting 25% of its sales to be e-tron by 2025, earlier post), there is less of an obvious current case for hydrogen.
On the one hand, Pint observed, hydrogen offers a long-distance solution with a fast refill—almost a diesel-like use case, in other words, compared to the shorter ranges and longer fill times for most current EVs. As battery technology improves, and as fast charging infrastructure develops for longer trips, hydrogen value proposition in that area may erode.
While Audi will not commercialize a hydrogen vehicle in this decade, Pint said, the ultimate decision as to whether or not to add the h-tron to showrooms will come down to the “two Ifs”: if there is a sufficient hydrogen infrastructure in place and if there is customer demand.
Unlike electric vehicles, for which there is already a massive slow-speed recharging infrastructure in place and for which rapid charging networks are being built (including Audi’s commitment to 150kW charging using the CCS standard connectors), the hydrogen infrastructure still needs to be built.
Furthermore, from a greenhouse gas point of view, the hydrogen ultimately needs to be derived from renewable sources if there is to be sufficient environmental benefit. The Audi e‑gas facility in Werlte, Germany is demonstrating the production of hydrogen with green power, and there are other green hydrogen approaches, but these would need to be scaled up aggressively.