Hydrogen engines are considered by many people to be an excellent alternative to fossil fuel engines. There are two types of hydrogen engines, and they rely on different principles for power. Combustion hydrogen engines work in a manner similar to petroleum-burning internal combustion engines. A hydrogen fuel cell engine works by mixing hydrogen and oxygen, generating electricity during the chemical reaction.
Combustion hydrogen engines are just like other internal combustion engines, except that they use hydrogen instead of fossil fuel, making it a simple matter to convert the manufacturing process from petroleum burners to hydrogen engines. These hydrogen engines burn liquid hydrogen to move pistons and generate power. Hydrogen delivers high energy without producing harmful exhaust.
There are, however, some limitations to the combustion hydrogen engine that make it impractical. In order to hold enough fuel to be useful, hydrogen must be stored in liquid form, which requires chilling to extremely low temperatures. Temperatures this low would lead to strain and cracking, not only on the fuel tank, but on any surrounding structure. Insulating and reinforcing a vehicle to withstand these temperatures raises the manufacturing cost to prohibitive levels.
An alternative to this model is the fuel cell model. Hydrogen and oxygen mix inside the fuel cell, combining to form water. This chemical reaction also releases electricity, which can be stored and used to power the engine. Water is the only exhaust produced by this engine, making it a good option for reducing air pollution.
Inside the fuel cell, compressed hydrogen gas passes through a platinum-coated catalyst, where electrons are pulled away, generating electricity and creating positive hydrogen ions. Oxygen injected into the cell through a cathode bonds with the ions, producing water. This water can then be released as exhaust. Energy generated by a single fuel cell would not be sufficient to power a vehicle, but a series of cells could be linked to provide sufficient energy.
There also are limitations to fuel cell hydrogen engines. They are exceptionally fragile and might not be sturdy enough to survive use in a vehicle. They use expensive components and precious metals such as platinum in their construction, driving up the manufacturing cost.
Fuel cells are also prone to freezing, especially before startup. After the hydrogen engine is running, sufficient heat is generated by the chemical process to keep the cells from freezing. They do not, however, produce as much heat as combustion engines, and fuel cell engines take considerably longer to warm up.