Clarity of purpose

  • 21-Mar-2017 05:11 EDT
Clarity Fuel Cell_007.jpg

The 2017 Clarity Fuel Cell is an electric vehicle with a combined fuel-economy rating of 68 mpg and a driving range of up to 366 mi. (image: Honda).


We expected some pop and sizzle–at least something vaguely adventurous—to mark the occasion of refueling Honda’s all-new 2017 Clarity Fuel Cell sedan with compressed hydrogen pumped in at a heady 10,153 psi (700 bar).

Instead, standing under the stylish translucent sunshade protecting this hydrogen refueling station—one of about 26 such public facilities currently operating in Southern California—the overwhelming experience is exactly how Honda and hydrogen-fuel interests want it to be: nearly indistinguishable from the gas station a stone’s throw away.

Even the time to fully fill the Clarity Fuel Cell is about the same, maybe five or six minutes to fully replenish the fuel-cell car’s two-tank capacity of 141 liters (37.3 gal). Five minutes and you’re on your way in this large and comfortable sedan for another maximum of 366 miles (589 km). We cruise the California freeways and breeze through snaky backroads in utter silence and with an unassailable refinement that, if we shut our eyes, convinces us the Clarity Fuel Cell could be an all-electric Accord that Honda’s had in production for years. The EPA-estimates this all happens at a combined fuel-economy rating of 68 mpg (3.5 L/100 km).

So what’s not to like about these fuel-cell cars, anyway?

Technical challenges resolved

Kiyoshi Shimizu, chief engineer and development leader for the 2017 Clarity Fuel Cell, has led Honda’s FCV Powertrain Development Dept. since 1997. He seems almost the personification of why the new Clarity Fuel Cell need make no excuses as a fully viable “everyday” vehicle. The company’s dogged decades of R&D have progressed fuel-cell technology from 1998’s “one-and-a-half-passenger” minivan he almost sheepishly describes as a “chemical plant on wheels” to today’s Clarity that packages the entire powertrain—fuel-cell stack, drive unit and all associated power and control electronics—neatly under the hood. The entire propulsion system requires less physical space than Honda’s 3.5L V-6.

Honda claims that tidy packaging is a world-first. And considering arch-rival Toyota’s Mirai is the only other production FCV sedan on the road. Mirai locates the fuel-cell stack under the passenger-compartment floor, much like the Clarity Fuel Cell’s predecessor, the FCX Clarity, launched in 1998. The only other series-production FCV offered for sale to the public is Hyundai’s Tucson Fuel Cell, although Hyundai recently revealed a concept crossover model using fourth-generation fuel-cell technology that the company promises will impart a 2018 production version with 800 km (497 mi) of driving range.

As the Clarity Fuel Cell’s almost miraculous packaging demonstrates, the eight years between fuel-cell generations has delivered demonstrable advance in almost every sense.

The powertrain’s packaging advance—including a 34% cut in motor height—comes from several development areas, chief being a 30% reduction in the number of cells in the stack. The new cells themselves are 20% thinner. And thanks to the new stack design, the unit now can be situated horizontally rather than vertically.

Despite the decrease in the number of cells, each now has 1.5 times more electrical output, for a total of 103 kW, or a power density of 3.1 kW for each of the stack’s 33L of volume. More voltage from the stack means more power available to the traction motor.

Drivetrain: more power, less noise

But while engineers aimed to improve system power and efficiency, they were similarly bent on cutting noise, both from the “intake” portion of the fuel cell and the drive unit itself.

First, oxygen supply is critical to allow the more power-dense fuel-cell stack to do its job; a considerable advance—both technically and in terms of NVH—comes from the adoption of a new, electrically-driven two-stage air compressor to shove air into the stack. Not only does this ICE-type turbocharger produce less sound, and of a higher frequency that can be muffled by a simpler silencer than the former Roots-style compressor—but it pushes 1.7 times the air volume. The compressor body is about 40% smaller, too.

The quiet and more-powerful electric turbocharger is an energy-hungry devil, though: chief engineer Shimizu told Automotive Engineering it typically might draw between 1-2 kW of power. But the heightened energy density of the new fuel-cell stack allowed for the draw attributable to the electric turbo that is the result of a joint development between Honda and “a supplier” Shimizu smilingly would not name.

Also new to the Clarity Fuel Cell’s powertrain is a “fuel cell voltage control unit,” a 4-inch-thick booster (and the uppermost component one sees when lifting the hood), steps up power from the cell stack to a maximum of 500 volts via a silicon-carbide power semiconductor material that vastly improves switching frequency while necessitating a much-smaller heat sink than would a more-conventional silicon-only semiconductor.

The upgraded 500V input—the most power the previous FCX Clarity fuel-cell stack could deliver was 330V—to the AC synchronous electric motor means its power output is hiked by 30% to 174 hp, while torque climbs to a robust 221 lb·ft (300 N·m) compared to the previous 189 lb·ft (256 N·m). Maximum motor rpm also is increased slightly from 12,500 rpm to 13,000 rpm and top speed climbs 4 mph to 103 mph (166 km/h).

The motor itself enjoyed incisive development tweaks to cut noise. The rotor now is sliced into four sections instead of two, which Honda said reduces torque fluctuation, while the stator is optimized to reduce vibration and the motor housing has additional structural ribs. Honda calculates that motor noise transmitted to the cabin at 0.2 g of acceleration is reduced by 25%.

The quieted motor and the new electric turbocharger team to all but nullify one of the ongoing NVH bugaboos that has lingered for most FCVs: loud compressor noise and motor whine. Automotive Engineering’s hard standing-start acceleration could wring little more than a nearly inaudible high-pitched whine in the cabin with the entertainment system turned off. The same was true for floored-throttle roll-on acceleration at freeway speeds.

New underpinnings, mass-optimization innovations

The 2017 Clarity Fuel Cell’s cutting-edge driveline propels an all-new platform that Honda doesn’t seem to be calling anything in particular, but said its fundamental “straight frames” structure was designed to maximize cabin space, cut weight and deliver optimized driving dynamics for a fuel-cell vehicle (and the pending battery-electric and hybrid-electric Clarity-family variants). Designed specifically to handle the weight of the aluminum-lined carbon-fiber/fiberglass storage tank (said to be a world-first), the new platform’s center of gravity is some 4 in (10 cm) lower than an Accord Hybrid.

The four-wheel independent suspension uses a few clever lightweighting techniques: the front strut setup uses forged-aluminum lower arms that are 30% lighter than a common pressed-steel arm and hollowing the knuckle saves 10%. At the rear, the multilink arrangement’s arms all are aluminum (worth a 40% cut compared with steel) and the tie rods come from what Honda said is the world’s first high-strength aluminum forging, allowing the rods to be 20% lighter than a conventional aluminum forging.

Another world-first for the Clarity Fuel Cell: a hollow die-cast aluminum front subframe eliminates the welding of multiple pieces and saves 20% in weight; the technique, borrowed from motorcycle-frame development, also creates a seamless and exceptionally rigid structure.

Want some more claimed world-firsts? Look to the Clarity Fuel Cell’s body, where Honda has the first glass-fiber reinforced plastic (GFRP) rear bumper beam. Meanwhile, there’s a “hybrid” plastic bulkhead at the front that replaces steel and Honda said 40% of the vehicle’s platform is comprised of super-high-tensile steel and the company said the car marks the world’s first (again) use of high-formability 980 MPa-class steel for automotive application. Along with aluminum body panels and other lightweight advanced materials, the Clarity Fuel Cell’s structure is claimed to be stronger than that of a conventional midsize sedan yet is 15% lighter.

In terms of size and weight, the Clarity Fuel Cell has a 108.3-in (2750-mm) wheelbase (Honda’s Accord: 109.3) and is 192.7 in (4895 mm) in overall length, nearly the same as the Accord and also quite dimensionally similar to Toyota’s Mirai, although the Clarity seats five and the Mirai is a 4-seater. The Honda fuel-cell sedan’s interior volume is listed at 102 cubic feet and the most spacious Accord variant offers 103.2 cubic feet of passenger volume. The Clarity Fuel Cell weighs 4134 lb (1875 kg), while a typical automatic-transmission Accord is about 700 lb (318 kg) lighter.

In terms of size and utility, then, the 2017 Clarity Fuel Cell gives up nothing (okay, maybe some sheer trunk usability) to a conventional midsize sedan. But the green-oriented in California have further incentive: Honda’s opening “deal” for this FCV is compelling: a $369/month lease for 36 months with $2,500 down. But factor in a California HOV-lane sticker, the state’s instant $5000 rebate and Honda’s debit card for $15000 worth of fuel over the lease term and the monthly out-of-pocket outlay is almost laughably skimpy. Many California dealers have waiting lists for the car that for now is being built in Tochigi, Japan, but is moving to a mass-production plant somewhere (the U.S., perhaps?) early next year, said a Honda source.

Ah, that fuel, that infrastructure

You’ll find plenty of contradictory opinion about the environmental friendliness of hydrogen fuel. Critics say its production and subsequent compression is energy-intensive and balloons hydrogen’s carbon profile (one prominent study concluded, however, the well-to-wheels CO2 emissions of hydrogen produced from natural gas for an FCV is approximately half that of gasoline).Chief engineer Shimizu told us the well-to-wheels energy efficiency of hydrogen derived from natural gas is better than a hybrid-electric vehicle—and reminds that an FCV has zero tailpipe emissions.

The carbon-dioxide argument will continue, but few can argue the almost complete lack of a national refueling network makes FCVs, for now, strictly a California “compliance” play (see sidebar). Stephen Ellis, Honda’s veteran of alternative-fuel implementation, said fast refueling times, the Clarity Fuel Cell’s plump driving range and the expanding “hydrogen highway” of conveniently-located refueling stations make FCVs utterly practical in California. He then said to expect imminent news (likely at New York’s auto show in April) regarding the initiative to extend refueling strategies to the northeastern U.S., where many states have adopted California’s emissions standards.

After a full day’s sampling of the refinement and brisk performance of the 2017 Clarity Fuel Cell, few could question the car’s potential for mainstream acceptance. And Honda and other FCV proponents are correct in pointing out fuel-cell advantages over battery-electric vehicles. If and when hydrogen fuel becomes more widely available, the marvelously-developed Clarity Fuel Cell is proof the fuel-cell approach could give batteries a run for the money.

For a related perspective on this topic, see this accompanying story: http://articles.sae.org/15334/

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