- One of the most commonly investigated alternative fuel for hybrid cars is hydrogen. Hydrogen can be used in hybrid cars in two ways: combustion or fuel-cell conversion. The combustion method burns hydrogen similar to traditional gasoline cars. Fuel-cell conversion uses advanced fuel cells which convert the hydrogen into electricity. When accomplishing this, the only byproduct from the spent hydrogen is water.
- Biofuels were implemented in both hybrid and traditional cars during the late 1990s and early part of the 21st century. The fuel comes from recently dead biological material rather than long-dead material such as fossil fuels. Biofuels can be made from various plants such as corn, switch grass, palm and soy. Biofuels can also be made from recycles cooking oils such as that used in deep frying.
- According to the U.S. Department of Transportation, nearly 190,000 vehicles in the country use propane as a fuel method, the most common being forklifts. However, the technology to implement this fuel into hybrid vehicles only began to emerge in the early 21st century. Propane is one of the most abundant fossil fuels in the world according to the U.S. Department of Energy, making it a viable option for replacing traditional gasoline and diesel.
- Countries such as Argentina and Brazil have begun to use compressed natural gas as a fuel within their car fleets. Natural gas can be compressed to one percent of its size and used as a substitute for gasoline. Hybrid cars in these countries that use compressed natural gas release less carbon dioxide into the atmosphere than other fossil fuels according to the Environmental Protection Agency.
- A proposed alternative fuel for hybrid cars is ammonia. Ammonia can be used as a substitute for both gasoline and diesel with only minor modifications to the present combustion system. Opponents of this option point to the fact that ammonia is generally considered a toxic elements. However, the EPA finds that after use only nitrogen and water vapor still remain.
- Liquid nitrogen is produced by liquefying the nitrogen in air. When it is exposed to small levels of heat, it evaporates into gaseous nitrogen which can power a piston or turbine engine. This can be added to a hybrid car to further streamline the negative environmental impact. According to the Energy Department, the costs of liquid nitrogen per mile are approximately identical to that of gasoline engines. This means that this method is most efficient in cutting carbon dioxide emissions and offsetting the high cost of fossil fuels.
