Alternative Fuel Vehicles or AFVs are not powered by conventional diesel or gasoline. These vehicles usually use either natural gas, hydrogen, or electricity. Due to increasing gasoline prices and environmental concerns like climate change and pollution, AFVs have been gaining popularity (see Figure 1).
In order to understand the cost, please see comparing gasoline, diesel, natural gas, and electricity prices. In order to understand comparative greenhouse gas emissions please see GHGs for AFVs. The convenience of alternative fuel vehicle infrastructure must also be taken into consideration.
An electric vehicle. This model is the 2013 Nissan Leaf.
A hydrogen car. This model is the Mazda RX-8.
Compressed natural gas in a vehicle.
Natural gas vehicles use natural gas as fuel. Natural gas mainly consists of methane and has an energy density of 47.2 MJ/kg. When natural gas is burned, the end products are energy, carbon dioxide, and water. This is a hydrocarbon combustion reaction just like burning normal gasoline, but produces less carbon dioxide for a given amount of energy extracted.
In many ways, light duty natural gas engines work very similar to that of a gasoline engine. Natural gas is stored at the rear of the vehicle under high pressure cylinders. When in use, gas is transferred from the tank to the engine through high pressure lines. Gas pressure is reduced from storage pressure down to required fuel injection pressure via gas regulators. Upon entering the engine compartment, the gas is mixed with air (which has the needed oxygen) and ignited to release energy.
Fuel cell vehicles use hydrogen as their energy currency, which is used as a 'fuel' for the vehicle (which is not the same as a fuel in the primary energy sense). Hydrogen has the highest energy density at 142.0 MJ/kg. The end products generated by FCVs are heat and water resulting in zero carbon dioxide emission while being driven. Of course, FCVs have the same long tail-pipe problem that electric vehicles have.
In FCVs electric motors are attached to the wheel as propulsion system. Electricity used to run these motors are produced from polymer electrolyte membrane (PEM) fuel cells. It consists of an anode and a cathode. When hydrogen is introduced here, it releases electrons which run through the electric circuit producing an electric current. The hydrogen then combines with oxygen in the atmosphere to make water. Extra energy produced in the system can be stored in on-board batteries depending on the design.
Electric vehicles, similar to FCVs, use electric motors. Batteries supply electricity to power them. When depleted these batteries are charged by plugging them into a power source. Since EVs use stored electricity, no emission is produced by this system while operating. Primary energy used to make this electricity leads to what is known as the long tail-pipe problem. Various types of batteries are available in the market. Depending on the type, stored energy can vary.