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In Detail: Honda FCX Clarity Fuel Cell Stack

By / 9 years ago / Features / No Comments

Instead of carrying a large on-board battery, hydrogen fuel cells use a chemical reaction to generate their own electricity, and this then powers an electric motor. Refuelling takes less than five minutes, the range is close to a conventional car and hydrogen is an abundant, renewable fuel with water as the only by-product of the fuel cell’s electricity generation.

With modern fuel cells shrinking in size and weight, upping efficiency and performance and shedding manufacturing costs, it’s becoming a viable option for motorists held back only by the difficulties compressing the gas and building an infrastructure for the vehicles in the UK.  So how does the system work?


-          Power Drive Unit (PDU) – FCX Clarity uses a high output coaxially-mounted electric motor and gearbox which is a full 180kg lighter and 40% smaller than the unit in its prececessor, allowing more cabin space. But it’s also more powerful, offering acceleration times close to a 2.4-litre petrol or diesel engine with the characteristic instant torque delivery of electric drive.

-          V-Flow Fuel Cell Stack – Essentially a tiny power station, the fuel cell unit generates its own electricity using a chemical reaction that produces water as its only by-product. Located in the transmission tunnel, Honda’s latest unit drains water vertically to improve efficiency and operates down to -30C. It is also lighter and smaller than its predecessor, and easier to manufacture too, cutting costs.

-          Lithium Ion Battery – Similar to the units found in electric and plug-in hybrid vehicles, FCX Clarity has an on-board battery pack which stores excess electricity from the fuel cell and is topped up using a regenerative braking system. This is used to power on-board systems while the fuel cell is inactive, and provides a power boost to the PDU during acceleration.

-          Hydrogen tank – Honda’s latest fuel cell system uses a single hydrogen tank, offering a range of up to 354 miles from a five-minute refuel stop. The hydrogen is stored as a high-pressure gas, and the tank features a crash protection structure and leak detection systems for added safety. Honda has also built a check valve into the filler neck, which avoids reverse flow or fuel contamination.

How it works:

-          Fuel Cell Electricity Generation – The fuel cell stack contains individually cooled cells arranged like slices of toast, which each generate electricity through a chemical reaction. Hydrogen from the tank is expanded and squeezed into one side of the cell, where the atoms are split into positive and negative ions using a catalyst.  These Negative ions flow into an external circuit, providing electricity for the motor and battery, while the positive ions go through a central membrane and are combined with oxygen from the air. On the other side of the cell, all three components are recombined, producing nothing but water vapour which drains out through the car’s exhaust system.

-          Startup and acceleration – When the motor is under heavy load, the FCX Clarity uses both the fuel cell stack and electricity stored in the battery to provide power as efficiently as possible. This allows the car to use the battery pack for an additional boost while accelerating hard.

-          Cruising – The fuel cell stack provides enough electricity to propel the FCX Clarity without assistance, allowing efficient motorway cruising without using the battery pack. Honda says the car is three times the efficiency of a petrol car, twice that of a hybrid and a 10% improvement over the old FCX.

-          Deceleration – Deceleration, particularly through braking, loses energy through heat. The FCX Clarity features a brake energy regeneration system, which uses the traction motor as a generator, converting this kinetic energy into electricity and storing it in the battery. Excess electricity from the fuel cell stack is also stored while decelerating.

-          Idling – To avoid wasting hydrogen, the FCX Clarity shuts off its fuel cell stack like a conventional stop/start system while in stationary traffic. Power for on-board systems is supplied by the lithium ion battery.

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Alex Grant

Trained on Cardiff University’s renowned Postgraduate Diploma in Motor Magazine Journalism, Alex is an award-winning motoring journalist with ten years’ experience across B2B and consumer titles. A life-long car enthusiast with a fascination for new technology and future drivetrains, he joined Fleet World in April 2011, contributing across the magazine and website portfolio and editing the EV Fleet World Website.

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