Clarity Fuel Cell Powertrain

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Hope for Tomorrow in a 5-seater Sedan Package

Part 1 From Under the Cabin to Under the Hood

To realize the world’s first 5-seater sedan FCV - the Clarity Fuel Cell - fitting the fuel cell powertrain under the hood was a necessity.

Fitting the Fuel Cell Stack Under the Hood

E-Drive designer Teruaki Kawasaki

Fuel cell stack tester Daisuke Okonogi

2-cell cooling structure

Narrower gas passage

Cell thickness comparison

In 2008, Honda began lease sales of the FCX Clarity, gaining a wealth of experience, leading to the Clarity Fuel Cell. The team endeavored to not only develop a higher-performing FCV, but a more attractive sedan.

“We aimed to maximize cabin space, to realize a 5-seater sedan package. The FCX Clarity was designed as a 4-seater, as the fuel cell stack was positioned within the center tunnel, making it less spacious than we would have preferred. So, we decided to position the fuel cell stack under the hood.” (Teruaki Kawasaki, E-Drive Engineer)

To do so, the fuel cell powertrain had to be the same size as a V6 engine. But…

“In the early stages of development, we found that trying to position the fuel cell stack and fuel cell voltage converter unit (FCVCU) on top of the motor was not feasible, since the driver could not see anything in front, let alone close the bonnet. As engineers, we then battled on to make the fuel cell stack, hydrogen/air supply system and FCVCU smaller.” (Kawasaki)

Fitting the fuel cell powertrain under the hood required a smaller fuel cell stack.

“Honda’s fuel cell consists of three metal plates called separators, and two Membrane Electrode Assembly (MEA) plates which generate electricity. Each unit has two cells, and by cooling both cells, rather than each cell, we managed to reduce the fuel cell’s size.” (Daisuke Okonogi, Fuel Cell Stack Engineer)

“We had to make the fuel cell stack smaller to fit in under the hood. We considered reducing the number, or thickness, of the cells.” (Okonogi)

The cell (in a fuel cell) is where electricity is generated by reacting hydrogen and oxygen in the air at the MEA. By stacking multiple cells (making a fuel cell stack), the required amount of electricity can be generated. The fuel cell stack can be made smaller by increasing the output of each cell, allowing the number of cells to be decreased while generating the same amount of electricity.
The key to increasing cell output is handling the water generated. In the fuel cell, hydrogen reacts with oxygen, generating water at the MEA’s oxygen pole. Air flow can be improved, leading to more efficient electricity generation, by efficiently removing this generated water.

“With the previous design, water generated tended to accumulate around the section where the MEA faces the airflow-forming separator, preventing gas from reaching the electricity generator. By designing a new cell with a narrower flow path to prevent water accumulation, generated water became easier to discharge, and improve electricity generation.” (Okonogi)

These changes improved electricity generation for each cell by 1.5 times, allowing the engineers to reduce the number of cells by 30%.
Structural evolution also reduced the size to fit under the hood.

“With the previous model fuel cell stack, humidification was increased to improve electricity generation. This would cause part of the water generated to condense and accumulate within the flow path, so cells were vertically oriented with hydrogen and air running parallel from the top, allowing gravity to discharge the water. The new design, in addition to MEA improvements, has opposing hydrogen and air flows, homogenizing humidity distribution over the electrolytic membrane, and reducing humidification. Gravity is no longer needed to discharge generated water without condensation, allowing the fuel cell stack layout to be horizontal, and placement under the hood becomes possible.” (Okonogi)

Reducing Condensed Water Contributes to Smaller Cells

“Reduction of condensed water allowed us to efficiently use the gas flow path, which could now be narrower, leading to a 20% thinner, 1mm-thick cell.” (Okonogi)

Less cells, and 20% thinner cells, realized a fuel cell stack 33% smaller. There were, however, new issues.

“A thinner and shallower gas flow path meant air was harder to supply to the cells. This required an air compressor to push air into the fuel cell stack at a higher pressure than before.” (Okonogi)

Part 2 Technology Supporting Size Reduction

A smaller stack alone was not enough to fit the fuel cell powertrain under the hood: A smaller, more powerful air compressor and drive unit were also necessary.

Electric Turbo Air Compressor Contributing to a Smaller Fuel Cell Stack

To realize a smaller, higher-performing fuel cell stack, an air compressor to pump air in at a high pressure was vital.

Fuel cell system designer Ryoichi Yoshitomi

Two-stage supercharging

“We initially considered a different type of air compressor. However, we needed to pump air at a higher pressure to make the fuel cell stack smaller. Other devices and piping supplying hydrogen, air and coolant had to be fitted under the hood in addition to the air compressor. Our conclusion was to design an electric turbo air compressor.” (Ryoichi Yoshitomi, Fuel Cell System Engineer)

The Clarity Fuel Cell’s compressor is smaller, yet 1.7 times more powerful.

“Two different superchargers, one at each end of the air compressor motor’s axis, are connected by piping to increase air pressure in two stages. We hadn’t mass-produced an electric turbo air compressor before, and we also considered the high costs, but the possibilities were too attractive not to try.” (Yoshitomi)

The world’s first production car with Honda’s unique two-stage supercharging electric turbo air compressor was to be born. And the advantages it would have, are not only high pressure air compression.

“Two stage supercharging presented advantages in a wide range of situations. The air compressor, for example, can easily handle an idling stationary car taking of on an uphill, high-load climb, up to high-speed cruising. And, it is also very quiet.” (Yoshitomi)

With previous models, Lysholm-type compressors with two gear-powered wings, not matter how well soundproofed, could sometimes be heard from the outside.

“FCVs are quiet compared to conventional cars, so we always had problems with reducing air compressor noise. Compressed air discharge for the new electric turbo air compressor is quiet, allowing us to reduce the size of the sound-suppressing resonator, and to do away completely with the air compressor silencer, halving the size of sound-suppressing equipment.” (Yoshitomi)

The decision to use the electric turbo air compressor not only helped reducing the fuel cell stack size, but helped reduce the size, and noise, of the entire fuel cell system.

From Large Current to High Voltage

The smaller fuel cell stack has a lower voltage than previous models, but by increasing the current, is powerful enough to drive a vehicle.

“By changing the number of cells stacked in a fuel cell, we can control the voltage, similarly to adding ordinary batteries in serial to increase voltage. Increasing the number of cells for a higher voltage, however, increases the size of the fuel cell stack, which is not ideal, considering the future of vehicles with fuel cell powertrains. We chose to increase current to realize a higher output.” (Kawasaki)

This caused new challenges, contrary to creating a smaller package, for circuitry handling power from the fuel cells.

Fuel Cell Powertrain

Motor Voltage Comparison

Impact-resistant Cell Fastening Structure

“A larger current means thicker wiring, and circuit components need to be more durable to cope. For the drive motor to realize more power with the higher current, coils must be thickened, which increases the size of the motor, which led us to use the FCVCU to reduce the current generated by the fuel cell stack, and raise the voltage to up to 500V.” (Kawasaki)

In the fuel cell powertrain, the FCVCU is positioned above the fuel cell stack.

“By positioning the FCVCU on top of the fuel cell stack, the length of wires conducting large currents can be minimized, and the size benefits through using the FCVCU can be maximized. The FCVCU enables a high voltage to be supplied to the motor, equivalent to increasing the cells in the fuel cell stack. As a result, the motor’s maximum output could be increased from 100kW to 130kW while keeping the size the same as the previous model.” (Kawasaki)

The FCVCU itself was scrutinized to reduce its size.

“Existing technologies alone could not fit the FCVCU into a thickness of 100mm. We achieved a high frequency-driven compact package by using, for the first time in a production vehicle, high-performance intelligent power modules with silicon carbide (SiC).” (Kawasaki)

The fuel cell powertrain under the hood is, of course, designed to be safe.

“Many are concerned with safety, because of the use of hydrogen. Being under the hood, collision impact on the fuel cell stack is much greater than that for previous models. The largest concern was to create a structure to protect the hydrogen from impact, with cells only 1mm thick. Hydrogen would leak if the stacked cells were shaken out of place with each other. After coming up with the idea to prevent cell movement by using a fastening bar with matching grooves, we spent a long period destroying cells by conducting tests (dropping cells from a few meters above). We had destroyed quite a number of fuel cell stacks during research and development, but in the end we managed to create a highly safe structure that could withstand an impact, leaving the cells completely unscratched, which we confirmed further in collision tests using test cars.” (Okonogi)

And, efforts to reduce fuel cell stack cost were also made.

“Although the MEA, which generates electricity, uses expensive materials, we were concerned with low material efficiency for the previous model, which used the same materials for complex-shaped components that were not used to generate electricity, as those that did. With the new design, the electricity generator is rectangular, making it efficient to cut from a single roll, and resin is used for components that do not generate electricity. Reducing the number of cells, and total components, also helped reduce cost. In addition, by a resin frame with the MEA, we could design independent gas distribution for hydrogen and air flows on both sides of the MEA. The previous model was designed with embossed separators to balance hydrogen and air flows on each side of the MEA. By using resin, we also managed to design a more efficient gas distribution within the cells.” (Okonogi)

The smaller fuel cell stack not only contributed to realizing a 5-seater sedan package, but also helped in reducing the cost of FCVs. By fitting the fuel cell powertrain under the hood, new model variations are now possible.

“Looking at the completed fuel cell powertrain, it is filled with functional beauty. We are proud that we managed to build into the fuel cell powertrain, new possibilities to open up the next step.” (Kawasaki)

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