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Fuel Cell Powertrain in the Honda FCX Clarity Is Significant Advancement over Predecessors

The advances in fuel-cell powertrain technology reflected in the FCX Clarity reduce the size of the system and provide designers with more flexibility. Click to enlarge.

Honda’s production fuel cell vehicle, the FCX Clarity, introduced at the Los Angeles Auto Show and due to begin limited leasing in the spring (earlier post), represents significant advancements in all areas of the powertrain from Honda’s earlier fuel cell vehicles.

The FCX Clarity utilizes Honda’s V Flow Stack in combination with a new compact and efficient lithium-ion battery pack and a single hydrogen storage tank to power the vehicle’s electric drive motor. The fuel cell stack operates as the vehicle’s main power source.

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.


Comparison of Honda Fuel Cell Stacks
Output 60 kW 86 kW 100 kW
Volume 134 L 66 L 52 L
Weight 202 kg 96 kg 67 kg
Vol. density 0.448 kW/L 1.3 kW/L 1.92 kW/L
Weight density 0.30 kW/kg 0.90 kW/kg 1.5 kW/kg
Electrolytic membrane Flourine
Max op. temp. 80°C
Max op. temp. 95°C
Max op. temp. 95°C
Stack construction,
cell structure
Bolt-fastened, machined carbon separators, separate seals Panel box, stamped metal separators, unitized seals V flow structure, 1-box stack, wave flow channel separators
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.


Mark A

Great news, by a forward thinking company. These are the solutions we need in these adapting times. Solutions such as these, as opposed to band aid fixes such as E85 ideas (aka gasohol, goals that we will never be able to meet, while at the same time disrupting our critical food supply).

Ford, GM, Mercedes, and yes even Toyota will appear to be needing to play catch-up. Perhaps we all should have bought Honda stock in the past. They are set to change our way of thinking.


Mark A,

Don't get all gushy. The car may be a great improvement over past FCVs, but there are huge questions about hydrogen infrastructure. Not to mention that depending on the hydrogen source, this car could actually be dirtier well-to-wheels than many efficient ICE hybrids.

BEVs and PHEVs still have a big conceptual and practical advantage over fuel cells. Honda is extremely short-sighted in this regard. It is they who may be playing catch-up.


Heh. We're going to have flying hydrogen cars on the moon, and people will still be moaning about batteries and the third decimal place on the well-to-wheels efficiency numbers.

Jim G.

BlackSun: At the very least, you have to acknowledge that no small part of this, e.g. the motor, the brakes, etc., is EV technology and is useful even if you ripped out the hydrogen and fuel cells. This is a pretty impressive design improvement.


Technically very impressive, but they will still have to reduce the amount of platinum used by an order of magnitude before this can be considered a serious contender for car of the future. Some recent work (Argonne labs etc..) shows promise in this regards, but, AFAIK, that work is still in its infancy. Perhaps fuel cells will make good range extenders for PHEVs in a future decade.

Harvey D

PHEVs buyers could eventually be given various choices for the power generator range extender.

Fuel cell, flex fuel ICE, bio-diesel, micro-turbine, NG, solar panels, etc.

In the short term, hydrogen fuel cells will not have the essential fuel supply stations making it a limited choice.


Nice motor/gearbox design. I had the hollow core idea myself a while back - I guess Honda beat me to it though :)

Not so crazy about the hydrogen aspect, though.


This is a very impressive list of improvements for such a short time span. I hope they will sell these Hondas in a PHEV version also with a scaled down fuel cell and a larger battery for example 10 kWh. That would be a better car I think and it would not cost more to produce.


Hydrogen will not be a solution for transportation any time soon, the problem of production, storage, distribution are beyond imagination, the cost is unmanageable, on top of this a transportation system based on H would double our energy consumption for the same number of miles because of poor well to wheel efficiency.

We don't need an hydrogen civilisation, we need sustainable energy civilisation, certainly not what hydrogen can do.


you can buy the home hydrogen generation unit IV from Honda. This doesn't solve the fueling infrastructure problem totally but it reduces the problem to a point of almost no contention. I like the idea of the LitIonBat and the fuel cell together.


yeah, but how big is the REAL market for a home fuel kit? How many people drive a CNG civic or converted vehicle and use the home CNG kits?

I am not a proponent of Hydrogen but I do recognize that you can't get batteries meeting the power and range of the fuel cell at even close to the same weight of the complete fuel cell system Honda has.


So they added an advanced lithium ion battery, and a more effective electric motor.

And thats their magical improvement ;D

Hydrogen is the most efficient method of energy storage of excess renewable energy and nuclear energy output in the fall, spring and summer for use in the winter. There will be an excess of solar energy in the summer, and excess of wind energy in spring and fall. Even as constant and as reliable as nuclear energy output, there will be an excess of it in certain season, and a shortage of it in another. The extremely high cost of building a nuclear plant means that it will not be cost-effective to oversize it.


==but I do recognize that you can't get batteries meeting the power and range of the fuel cell at even close to the same weight of the complete fuel cell system Honda has.==

Thats not true at all.

The ONLY way that hydrogen gets any significant *long range* capabilities is with cryogenic freezing.

Cryogenic freezing is a very dirty/energy-intensive technology.

You're basically keeping your fuel tank about 20 degrees above absolute zero. Takes about 1/3rd the energy contained in the fuel tank just to compress it into the tank. And the fuel tank runs empty in a few days due to the tank warming up.


Besides which,
Batteries CAN get the long range already, so I really don't know what you're talking about.

The Clarity has a range of 270 miles.
The Tesla has a range of 240 miles.
And the ZapX is expected to get a range of 350 miles range.



You really don't know what I am talking about because, like so many others in modern society, you are looking for the crib notes and skimming past the majority of the content. Re-read my statement: I recognize you can't get an equal battery WITH THE SAME FLIPPIN' WEIGHT of Honda's fuel cell system.

So, with your lengthy response and multiple links where is the battery SYSTEM (include cooling and encasement) that achieves 250miles with 100kW max power at under 150kg? (67kg for stack and just taking a shot in the dark that the remainder of the system of tanks, hoses, cooling etc is roughly 83kg)


This is the first time I've run across a fuel cell car announcement that gives significant attention to the battery. It's clear that any viable fuel cell vehicle is going to be a "hybrid" since the cost of a fuel cell that can drive the car on its own is prohibitive. I predict that the first mass-produced fuel cell car will have an even larger battery and a much smaller fuel cell; perhaps 10-20KW. One problem the hydrogen proponents face is that the presence of a battery will eventually lead their drivers to ask "why can't I plug it in?" But once the car is pluggable, many of hydrogen's supposed advantages are dissipated.



Here I did a quick search:

Based on this the best battery (Altairnano) would pack close to 27kW-hr of energy into 150kg

According to this site:

GM's EV1 was getting roughly 6 miles per kW-hr so this battery would get the EV1 162 miles with a 150kg battery pack (cell weight only).

This site shows:

0.17 to 0.37kW-hrs per mile (cited from an Idaho National lab report) or roughly 160 miles at the best case from 150kg of Altair batteries (cell weight only)



I'd hesitate to call Altair the "best" battery in terms of capacity. At the cell level their gen1 EV cells hold about 79Wh/kg. Their claim to fame is high cycle life and fast recharge.


"This is the first time I've run across a fuel cell car announcement that gives significant attention to the battery. It's clear that any viable fuel cell vehicle is going to be a "hybrid""

More importantly, each fuel cell vehicle is for all intents and purposes a battery-buffered series hybrid. Companies like TOYota have lambasted the series concept - which is appearing increasingly in buses and concept commuter cars - claiming their parallel method is the alpha and omega. Nonetheless, they've been making series hybrids for years in the form of their hydrogen fuel cell vehicles.


Looks like some very impressive engineering achievements... my hat is off.

However, I'd love to see any of these car companies making a projection about how long it will be before they can profitably sell a hydrogen car for some stated price.

Also, it looks like far less progress is being made on the infrastructure or well to wheels CO2 issue for hydrogen.


I'd really like to know what this car would cost to buy outright.

They've clearly made some very impressive advancements. Enough to actually put the vehicle into limited production. Pity there are so many other problems yet to be solved.


AES: GM/Opel boasted it a huge success, that their HydroGen 5 was capable of running without a buffer battery. Even the project head said so in an email exchange I had with him.

Funny, that out of th ~20 motor journalists invited to drive that thing across europe, a significant number complained about the lack of engine braking (they needed to use the brakes with the HydroGen constantly, in order to have the same coast-down performance as an orderary car, which was seen as rather wasteful - even by motorhead journalists at the time....

Beside, a 100 kW(sustained) fuel cell - what is that for? Even here in germany the government starts to feel the pressure to finally put decent speed limits on their autobahnen (120-140 km/h are being discussed).

A med-sized car, operating at 100 kW would have a top speed of >220 km/h (could be 270 km/h, depending on cW) - which, in my book, is an insane speed for public roads. For quick take-off and accelleration, use a battery or ultra-caps, for christ's sake... (they are cheaper, more durable, and better (power/weight))

hampden wireless

They came much further along in two years then I had expected. The 250 mile range is excellent. I hear the home fueling system can make hydrogen cheaper then gasoline per mile making it a good solution if the car was priced right.

Two things still stick out as being a problem with this and other hydrogen cars:

1) The fuel cell does not last forever, in fact I hear the lifetimes are far shorter then the life of the car.

2) The actual price of the car is probably greater then $100,000! Honda is only leasing them. For that price you could get a Tesla or other electric exotic.



Considering I've seen million-dollar ranges for FCVs in the past, even if it's $200k, that would be a vast improvement. I wonder if you could use biogas as a feedstock to get the hydrogen.


==@Patrick@: I recognize you can't get an equal battery WITH THE SAME FLIPPIN' WEIGHT of Honda's fuel cell system.==

Well lets see.

AltairNano can do 35-70Wh/Kg

On the other hand an Electrovaya MN series battery can field 330Wh/kg

So for a 150kg weight, we're talking a 49.5KWh pack
Which by your numbers would get 291 miles of range.


This of course would be better if it were using a high efficiency electric motor, like Honda is claiming.

For instance, like ones designed by Raser Technologies.


Certainly would be rather interesting if Raser and Electrovaya were to work together.


If for instance we were to use these Plugin-Prius figures.
Of 0.1154 KWh/mile

We would end up with a 429 mile range for 150kg.

Just love to see how high it would go with some high efficiency motors like the Clarity is sporting.

Can you say 500+ mile range?

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