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Automobile R&D Center, Honda R&D Co., Ltd.

Fitting the entire powertrain under the front hood—a seemingly impossible goal

From ‘sedan-type’ to full-fledged ‘sedan’

  After his sojourn in the US, Shimizu was appointed to LPL (large project leader) of the next Honda FCV. After hearing raw feedback from American users, Shimizu was now being tasked with leading development of the next-generation model.
  That model, the fourth generation in Honda's line of FCVs, would no longer be for testing or test marketing, but rather for bringing FCVs to the masses. The goal would be impossible without a level of convenience and value comparable to gasoline engine vehicles. With American users’ comments still echoing in his mind, Shimizu drew up a new set of development goals:

"To engineer an FCV that rivals gasoline engine vehicles in every category: driving performance, cruising range, riding comfort, trunk space, convenience, and design."

  Like the FCX Clarity, the next-generation model would be a midsize sedan, a popular segment in the US, the world's largest FCV market. To achieve these goals, the new FCV would have to be equipped with the same level of convenience and added value as a sedan of the same class—in other words, a five-seat package with ample cargo space would be absolute requirements of the next model. To do that, the project team "decided on development goals that would achieve the same packaging as a gasoline-engine sedan. We would further downsize the fuel cell stack that was placed in the central tunnel of the FCX Clarity so the entire powertrain could fit under the front hood, like an engine, securing maximum space for the cabin and trunk."
  The time had come for the Honda FCV to leave behind its "sedan-type" name and proudly proclaim itself as a genuine sedan.

‘If someone’s going to do it, let it be us.’

  Current technology, however, made fitting the entire powertrain under the front hood seem like nothing but a dream. Chief Engineer Kenichiro Kimura, who was appointed to powertrain assistant large project leader (A-LPL), looks back.
"There's no way it’ll fit. That’s honestly what I thought. After all, when we tried, it got so high it blocked half the windshield and you couldn't see out," he says, laughing.

  Though skeptical at first, Kimura felt increasingly ambitious as he listened to Shimizu's dreams of creating an FCV that rivals gasoline vehicles.
  As long as FCVs continue to evolve, Kimura reasoned, making fuel cell stacks smaller will always be an objective. That means someone will eventually succeed in installing the powertrain in the front end. In that case it might as well be Honda. If we could bring it down to the size of a V6 engine, he concluded, the fuel cell powertrain could fit inside under the front hood of a midsize sedan. Kimura thus began the work of downsizing all of the powertrain components.

  The biggest hurdle was the fuel cell stack. A single stack consisted of 500 layers of cells “stacked” on top of one another. This number had to be reduced by about 30 percent for the stack to fit under the hood. And for that, the electric potential of each cell would have to be increased by 50 percent.
  There was no magical, one-time solution to this problem. Kimura instructed his team to carry out a comprehensive redesign of the cell structure to make efficiency improvements in all areas, including water removal, gas diffusion, and uniform quantity of electricity. Very slowly but steadily, generation capacity improved.

"We just kept searching for areas to improve," says Kimura. After continuing this process for two years, they became confident they could reach their goal. And a year after that, they had a fuel cell powertrain about the size of a V6 engine.

Clarity Fuel Cell: Powertrain placement

Clarity Fuel Cell: Powertrain placement Kenichiro Kimura, Chief Engineer and powertrain assistant large project leader (A-LPL) of the Clarity Fuel Cell

Kenichiro Kimura,
Chief Engineer and powertrain assistant large project leader (A-LPL) of the Clarity Fuel Cell

Comparison of powertrain sizes

Clarity Fuel Cell powertrain

Clarity Fuel Cell powertrain

3.5-liter V6 gasoline engine

3.5-liter V6 gasoline engine

Tackling the noise paradox: greater quietness increases sensitivity to noise

Kenji Uchibori, Chief Engineer and fuel cell design project leader (PL) of the Clarity Fuel Cell

Kenji Uchibori,
Chief Engineer and fuel cell design project leader (PL) of the Clarity Fuel Cell

Electric turbo air compressor

Electric turbo air compressor

The newly developed coaxial two-stage air compressor delivers higher air pressure and flowrate in a more compact size. It employs air bearings, using compressed air to lift the rotors from their bearings, and can spin at up to 100,000 rotations per minute.

graph

A perplexing requirement for near-silent performance

  While Kimura was grappling with the powertrain's excessive size, Chief Engineer Kenji Uchibori was focused on improving the powertrain's noise performance.
  Uchibori, who joined Kimura in powertrain development, decided to change the design of the air compressor, the part that delivers air to the fuel cell stack, from the conventional rotary-screw type to a new electric turbo type.
"The air compressor is an important component that can influence the fuel cell stack's generation capacity," says Uchibori. "We weren’t sure if we could reach the performance required from the new FCV simply by improving the existing compressor’s design. So we tested different types of compressors from various angles and determined that an electric turbocharger type would do well at satisfying all of the requirements."

  Uchibori got to work on developing a compressor for the new FCV. But he soon ran into an unexpected problem.

"They said it was too noisy."

  That can't be, Uchibori thought, questioning his own hearing ability. Structurally speaking, the electric turbocharger type is quieter than the conventional rotary-screw type. Skeptical, Uchibori dug a little further and discovered an important phenomenon.
"FCVs are quiet to begin with, but the Honda team had taken that quietness to the next level with the new FCV. That made even the smallest sounds coming from electric turbo air compressor stand out," he explains.
Any quieter and the compressor would practically be silent. Is such a thing possible? Feeling unsure of himself, Uchibori consulted the manufacturing partners with whom Honda was working on development.
"We had trouble seeing eye to eye at first. Turbochargers are typically used when the engine is running, so noise when idling is rarely a problem. The level of quietness we were hoping to achieve, therefore, was an order of magnitude beyond what common knowledge of turbochargers could provide. From their perspective, they didn’t have a clue about what could be done."

  Uchibori relentlessly continued to explore and test out various solutions. He also sought input from turbocharger experts in the company. The manufacturing partners eventually came around to Uchibori's persistent enthusiasm, lending some momentum to the development effort. The end result, an innovative coaxial two-stage compressor, is not only quieter but also led to a smaller fuel cell stack, smaller body apertures and, by extension, improved aerodynamic performance.

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