What happens under the bonnet of a hydrogen vehicle? Although many people use the term “hydrogen engine”, in reality, hydrogen vehicles use electric engines powered by hydrogen. Hydrogen trucks, cars, tractors and so on are indeed electric vehicles or more precisely Fuel Cell Electric Vehicles (FCEV).
However, unlike the “conventional” electric vehicle (which should really be referred to as a battery electric vehicle or BEV) – whose electricity is stored in the battery during charging – the “hydrogen electric vehicle” generates its own electricity using a hydrogen fuel cell.
To use a hydrogen vehicle, the user must fill the hydrogen tank rather than recharging the battery.
There are two types of hydrogen engine technology:
- The FCEV: an electric vehicle with an electric engine powered by a fuel cell – most hydrogen vehicles available today use this technology,
- The ICE: A vehicle with a hydrogen internal combustion engine. This technology, which is not yet on the market, is in the R&D phase and interests manufacturers such as Toyota, Porche, and MAN. This type of engine would keep much of the current engine’s operating system the same and would require minimum adjustments to vehicles.
What defines the power of a hydrogen engine?
FCEVs have several design parameters that define a vehicle’s power:
- Battery capacity (in kWh)
- Fuel cell power (in kW)
- Hydrogen storage capacity (in kg)
- Overall engine power in (kW)
Depending on the vehicle’s purpose, manufacturers design various configurations of these four elements.
Gas or liquid hydrogen?
These are the two means of storing hydrogen and each has its advantages and disadvantages, depending on the vehicle’s purpose. Liquid hydrogen allows you to store four times more hydrogen but liquefying hydrogen results in a lower energy yield due to the additional liquefaction operation. What’s more, once it is in the car, it must be used very quickly before it turns back into gas and expands. This means it must be used or evacuated ... and lost!
How exactly does a fuel cell work?
There are several types of fuel cells, which convert fuel into energy. The most common is the hydrogen fuel cell, which converts hydrogen (the fuel) into electricity (the energy), through the electrochemical reaction of hydrogen and oxygen.
This process, which occurs instantaneously, powers the electric engine while releasing only water.
Technically, the fuel cell uses a “redox” reaction to produce electricity. It comprises two electrodes: an oxidising anode (emitting electrons) and a reducing cathode (collecting electrons), separated by an electrolyte, which directly conducts the ions from one electrode to the other and blocks the electrons, forcing them to pass through the external circuit of the battery.
The tank continuously supplies the anode with hydrogen, while the cathode receives oxygen (from the air).
The anode causes the fuel to oxidise and release electrons, which are forced by the ion-charged electrolyte (which separates the two electrodes) to pass through an external circuit. This external circuit therefore provides a continuous electric current which powers the engine.
Once reunited in the cathode, the ions and electrons recombine with oxygen. This combination causes reduction, which – in addition to the electric current – generates heat and pure water vapour, which escapes in the form of gas through a small tube placed under the vehicle, either while driving or by pressing a button at the end of the trip.
- The tank continuously supplies the anode with hydrogen, while the cathode receives oxygen (from the air).
- The anode causes the fuel to oxidise and release electrons, which are forced by the ion-charged electrolyte (which separates the two electrodes) to pass through an external circuit. This external circuit therefore provides a continuous electric current which powers the engine.
- Once reunited in the cathode, the ions and electrons recombine with oxygen. This combination causes reduction, which – in addition to the electric current – generates heat and pure water vapour, which escapes in the form of gas through a small tube placed under the vehicle, either while driving or by pressing a button at the end of the trip.
Running a hydrogen engine therefore emits only pure water and is completely clean, provided that a completely clean hydrogen fuel is used.
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