Comparison of Honda FC stack output density. Click to enlarge.

V Flow FC Stack. The new FC stack is smaller and more powerful, increasing the volume output density by 50% and the weight output density by 67% from the previous stack. The new stack more than quadruples the volume output density from the first stack delivered in 1999.

Aspects of the V Flow cell structure. Click to enlarge.

In prior generations of Honda fuel cell stacks, the hydrogen and air flowed horizontally through the cells. The V Flow FC Stack introduces a cell structure in which hydrogen and air flow vertically, and gravity is used to facilitate more efficient drainage of the water byproduct from the electrical generating layer. The result is greater stability in power generation. The new structure also allows for a 17% thinner flow channel and reduction in the stack’s size and weight.

Honda’s wave flow-channel separators also provide a more even and efficient supply of hydrogen, air and coolant to the electrical generating layer, resulting in higher generating performance, optimal cooling characteristics and major reductions in size and weight. More compact, the new stack has far fewer parts and can fit into a single box. It’s also much easier to manufacture, according to Honda.

The wave flow-channel separators are a critical element. Click to enlarge.

Wave flow-channel separators. The wave flow-channel separators are a critical element of the new stack, enabling a smaller design. A fuel cell consists of a membrane electrode assembly (MEA)—an electrolytic membrane sandwiched between the pairs of electrode layers and diffusion layers forming the hydrogen and oxygen electrodes—which are in turn enclosed between separators containing flow channels for hydrogen, air and coolant.

The V Flow FC Stack incorporates wave-shaped vertical flow channels for the hydrogen and air, with horizontal coolant flow channels weaving between them. The wave flow channels provide greater flow length per channel than straight channels, while the resulting turbulent flow within the channel promotes improved hydrogen and air distribution.

As a result, the hydrogen and air are spread over the entire electrode layer, making more efficient use of the compact electrical generation layer and achieving approximately 10% higher generating performance than with straight flow channels.

The horizontal coolant flow also ensures more even cooling over the entire electrical generation layer, allowing for a reduction in the number of cooling layers to half that of previous stacks. The previous stack had one cooling layer for each cell. The new stack needs only one cooling layer per two cells. This results in a 20% reduction in stack length and a 30% weight reduction—a major breakthrough in compact, lightweight stack design.

The improved water drainage due to the V Flow cell structure facilitates better output immediately after startup. The reduced coolant volume and single-box design made possible by the wave flow-channel separators results in heat mass 40% lower than previous stacks. As a result, the amount of time required to achieve 50% output after startup at -20°C (-4°F) is only one-quarter that of the previous stack. Startup is now possible at temperatures as low as -30°C (-22°F).

Unified coaxial drive motor/gearbox and PDU. An innovative configuration, with drive motor and gearbox oriented coaxially, achieves a 162-mm reduction in length as measured along the drive axis from the 2005 FCX. This coaxial motor/gearbox is further combined with the Power Drive Unit (PDU) in a unified configuration that saves another 240 mm in height. These innovations make possible the FCX Clarity’s stylish, short-nose design.

Lithium-ion battery. The vehicle’s auxiliary power source, the new lithium-ion battery, delivers improved performance and energy recovery in a more lightweight, compact package. The new battery is 40% lighter and 50% smaller than the ultra-capacitor of the 2005 FCX, allowing it to be stowed under the rear seat. This gives the car more passenger space and a bigger trunk.

The battery captures 11% more kinetic energy from regeneration than the ultra-capacitor used in the 2005 FCX. Some 57% of the energy of deceleration is regenerated with the new system.

As a result of increased energy storage capacity and a broader range of regeneration control, Honda has been able to implement a system that regulates acceleration and reduces the need for pedal operation in downhill driving. Assessing incline and vehicle speed, the system regulates acceleration when the driver first releases the accelerator pedal, minimizing the need for frequent braking.

The system simultaneously adjusts the amount of regenerative braking to help maintain constant vehicle speed after brake pedal inputs. The function is similar to engine braking in a gasoline-powered vehicle, but is more intelligent, smoother and easier to use.

Hydrogen storage. Whereas the 2005 FCX carries two hydrogen tanks, the FCX Clarity has only one. This creates more space for the rear seats and trunk. The shut-off valve, regulator, pressure sensor and other components in the refueling and supply system were integrated into a single in-tank module, reducing the number of parts by 74%. Tank capacity is greater, installation space efficiency is 24% better, and vehicle range is increased.

Cooling. Increased powerplant efficiency, vehicle weight reduction and improved aerodynamics have resulted in a major reduction in heat generation. The cooling air vent for the motor bay has been reconfigured and the cooling capacity of the radiators increased. These improvements made it possible to integrate the fuel cell radiator, the drive train radiator and the air conditioning condenser into a single three-layer unit. The new radiator unit requires 40% less space, contributing to the stylish short-nose design.

Drive motor. The new drive motor offers higher output, high torque, higher rpms and quieter operation in a more compact design. A new rotor and stator feature a combined reluctance torque, low-loss magnetic circuit and full-range, full-digital vector control to achieve high efficiency and high output over a wide speed range.

The innovative shape and layout of the magnets in the rotor result in high-output, high-torque, high-rpm performance, according to Honda. These innovations deliver a maximum output of 100 kW along with substantive torque and power output density. At the same time, resonance points in the high frequency range have been eliminated for quieter operation.

A newly designed rotor features an Interior Permanent Magnet (IPM) to lower inductance, improving reluctance torque for high-torque performance. The magnet’s high-energy characteristics also contribute to high torque and a more compact design. These innovations result in 50% higher output density and 20% higher torque density.

The number of poles has also been reduced and the magnet widened to better withstand stress, allowing the yoke to wrap around the outside of the IPM. A center rib has been installed for greater rigidity. This more robust construction allows for operation at higher rpm.

A new stator features a low iron-loss electrical steel sheet and higher density windings that decrease resistance and contribute to high torque and higher output. Honda reduced the number of poles from 12 to 8 to eliminate resonance points and produce quieter operation within the operating rpm range.

Coaxial gearbox. Click to enlarge.

The motor’s rotor shaft features a hollow construction, with the driveshaft passing through its center in a coaxial configuration. This arrangement, unique to electric vehicles, allows the motor and gearbox to be combined into a single, compact unit, while providing highly efficient transmission of the motor’s high-output, high-rpm power to the driveshaft. Innovative bearing design and fewer rotor oil seals result in lower friction for higher transmission efficiency, creating driving performance with a more direct feel.

Layout. The smaller fuel-cell stack provided Honda designers with additional layout flexibility. The V Flow FC Stack is located inside the center tunnel, and the lithium-ion battery is placed under the rear seat. The result is a free-flowing, full-cabin design with a long wheelbase that provides spacious and comfortable seating.

The drive motor, gearbox and PDU are combined for major space savings in the drive train system. A more compact radiator unit contributes to the short-nose design. Reducing the number of parts in the hydrogen tank and modifying its shape result in a more efficient use of space, creating ample room in the rear seating and trunk areas. Improvements both to the hydrogen tank and the FC stack layout result in a low floor and low overall height.