Transport is a major use of energy. Most transport burns hydrocarbons. If partially burned, these create pollution. Though vehicles in the United States have been getting cleaner because of environmental regulations, this has been offset by more vehicles and more use of each vehicle.
Low-pollution fuels can reduce pollution. The most popular low-pollution fuel at this time is liquified natural gas. Hydrogen is an even lower-pollution fuel.
Another tack is to make vehicles more efficient, which reduces pollution and waste by reducing the energy use. If electricity can be gotten to the vehicle, electric motors are the most efficient of all. Another method is to generate energy using fuel cells, which are two to five times as efficient as the heat engines traditionally used in vehicles. A trivial, but very effective method is to streamline ground vehicles, which spend up to 75% of their energy on air-resistance. Another method is to recycle the energy normally lost to braking, but this leads to a more complex vehicle.
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