Hybrid transportation is the use of vehicles propelled by more than one form of energy. The term is usually used today to refer to hybrid road vehicles such as trucks, buses, and cars which release energy by burning a fuel—often gasoline—and converting as much of that energy as possible into electricity to drive the vehicle using electric motors.
Hybrid vehicles are distinct from electric vehicles, which carry only electrical systems. The electricity for all-electric vehicles, whether trains, cars, or other, is generated in distant power plants and may either be transmitted directly to the vehicle (as by overhead wires or a third rail, in the case of electric trains) or stored by the vehicle in batteries (as in the case of electric cars). Since the early twentieth century, some submarines, a few ships, and most locomotives have relied on hybrid diesel-electric drives, using diesel engines to drive electric generators and using the generator’s electric output to drive electric motors, charge batteries, or both.
Hybrid drives might seem at first glance to be illogical. Every internal combustion engine (an engine that burns fuel in cylinders to drive pistons) can turn only a fraction of the energy into its fuel into useful mechanical motion. Likewise, a generator does not turn all of the mechanical energy it uses into electricity, and an electric motor does not turn all of the electrical energy it uses into mechanical motion. How can it be an advantage to add the extra losses and complexities of a generator, batteries, and electric motor? Yet hybrid automobiles not only get better mileage than non-hybrid gasoline-burning cars of equivalent size, they can produce as little as a tenth as much air pollution.
The answer lies partly in the properties of electric motors. Electric motors can apply large torque (twisting force) to a vehicle’s drive train even when turning at low speed. Internal-combustion engines, in contrast, cannot run at low speeds at all: their fast rotation must be stepped down through a gear and clutch system to get a car (or locomotive) into motion or to run it slowly. This adds complexity and losses. Electric motors are, therefore, more effective when accelerating a vehicle from a stop or moving it at low speeds. In fact, in the early 2000s, some automakers began adding hybrid features to large consumer vehicles such as sport utility vehicles (SUVs) not primarily to improve mileage, but to improve performance (mostly acceleration). This is also the primary reason for the use of diesel-electric drives in locomotives rather than straight diesel drives.
Also, an internal combustion engine can be made to run more efficiently and with less pollution if required to run only at a constant speed. An internal combustion engine that powers a vehicle must (despite gearing and clutching) run over a wide range of speeds, forcing efficiency compromises at every stage of design. By hitching a gasoline engine to an electric generator and powering the vehicle primarily with the electric output of the generator, the internal-combustion engine design can be optimized for a constant speed. This allows it to be more efficient, overcoming the conversion losses of adding an electricity-generating step and reducing pollution (in hybrid automobiles) by up to 90%.
Furthermore, when an bus, truck, or automobile applies its brakes, friction between the moving wheels and brake pads is used to convert some of the kinetic energy of the vehicle into heat, which is radiated into the environment and lost. In contrast, the use of storage batteries and electrical generators built into the braking system allows an electric bus or automobile to recover some of the energy that would otherwise be lost braking. A 2005 model Toyota Prius hybrid car, for example, recovers about 30% of braking energy as electric energy and stores it in its batteries.
Hybrid automobiles were first introduced to the United States consumer market in 1997. As of October 2006, hybrids made up 1.54% of the U.S. new car market. Toyota Corporation, one of the world’s largest car makers, announced in 2005 that it intended to offer a hybrid option for its entire line of vehicles by 2030. By that time, oil giant ExxonMobile estimated in 2005, about 30% of U.S. car sales will be hybrids.