How Do Hydrogen Vehicles Work?

While we have gotten used to reading about hybrid cars, plug-in hybrid cars, and more recently, electric cars, there is another option to reducing or eliminating our greenhouse gas emissions: hydrogen. The most plentiful element on earth, hydrogen, when used as fuel, produces only water from the vehicle’s tailpipe.

Of course, just because hydrogen produces only water from the tailpipe doesn’t make it completely clean. Approximately 33 percent of total hydrogen power generation in the U.S. still uses coal. However, alternative and more sustainable hydrogen-processing techniques can be cost-prohibitive. Manufacturers of hydrogen cars have considered this and many offer free fueling for the first years of ownership.

There are two major kinds of hydrogen vehicles – hydrogen combustion and hydrogen fuel cell – but both rely on hydrogen stored under pressure in an on-board tank, which can be refueled in a similar amount of time to gasoline. Quick refueling is one of the major advantages of hydrogen cars.

Storage Under Pressure

While hydrogen is significantly more energy-dense than batteries of the same weight, it is less energy-dense than gasoline or diesel – meaning an equal volume of hydrogen gas holds less power. Therefore, in order to deliver decent driving range, hydrogen gas is stored in a compressed form. Indeed, the pressure under which hydrogen is stored can reach 10,000 psi or more – roughly 300 times the pressure of the average car tire.

High-pressure storage means that a car’s hydrogen fuel tank needs to be very strong – not only to withstand the enormous internal pressure, but also to remain safe in a car crash. Hydrogen is highly combustible, which may bring to mind visions of exploding Zeppelins – but automakers have designed plenty of fail-safe mechanisms along with engineering exceptionally robust tanks. And don’t forget that gasoline is highly flammable as well, and you’ll sometimes see a haze of fuel vapor when filling up.

Both hydrogen combustion cars and hydrogen fuel cell cars store fuel in similar tanks – it’s what happens after fueling where they differ.

Hydrogen Combustion

Hydrogen-burning internal combustion engines generate energy through the combustion of hydrogen instead of gasoline. Unlike fuel-cell systems, they do not rely on a chemical reaction to generate electricity; instead, they burn hydrogen directly as a fuel – just like a gasoline engine.  

Except for the combustion of trace amounts of engine oil during driving, hydrogen engines emit no carbon dioxide. Unfortunately, since they burn hydrogen along with oxygen found in the air, as in the case with gasoline engines, a certain amount of nitrogen oxide is created. However, in general, they are far cleaner than both hybrids and plug-in hybrids.

Because hydrogen is delivered in pressurized gas form, special fuel injectors are necessary that can safely and efficiently introduce hydrogen into the engine. Hydrogen has a higher ignition temperature than gasoline and eight times the combustion speed, making hydrogen combustion engines very responsive.

Hydrogen combustion engines retain the performance and feel that drivers are accustomed to. They are easy to adapt to if you’ve ever driven a gasoline vehicle, with almost no learning curve or shift in lifestyle. Even the sound of the exhaust is nearly identical to a gasoline-powered car, despite the main emission being water!

Hydrogen Fuel Cells

On the other hand, hydrogen fuel cells drive more like an electric car – because they use a hydrogen fuel cell stack to produce electricity that powers an electric motor. In a fuel cell, hydrogen undergoes a chemical reaction with oxygen from the air to produce this electricity.

Indeed, you can think of a hydrogen fuel cell car as an electric car with its own on-board power station. Fuel cells were used in the sixties on-board America’s Gemini and Apollo spacecraft to produce electricity and water, using on-board hydrogen and oxygen.

The construction of a hydrogen fuel cell is similar to a battery. Hydrogen gas from the pressurized tank enters the anode, where it comes into contact with a catalyst that promotes the separation of hydrogen atoms into an electron and a proton.

The electrons are then gathered by a conductive collector, which is connected to the hydrogen car’s high-voltage circuitry. Fuel cells are typically grouped into a “stack” – an aggregate of a number of fuel cells combining oxygen and hydrogen – to produce the energy necessary to power the car.

This process feeds an on-board battery the energy for storage, the electric motor directly, or both. The motor then powers the wheels. The on-board battery is typically far smaller than the on-board battery for an all-electric vehicle, as the fuel cell stack provides the majority of the required electricity. The battery also captures energy from regenerative braking, which helps improve range.

Two Processes, One Result

Hydrogen combustion and hydrogen fuel cells are very different technologies, but the end result is similar: forward movement for the car and water from the tailpipe. Ultimately, hydrogen fuel cells are the cleaner technology – electric motors are more efficient and do not produce nitrogen oxide like combustion does. Fuel cell electric vehicles will also require less maintenance over time, while hydrogen combustion vehicles will still require oil changes, transmission service, and other fluid changes gasoline drivers are used to paying for.