A vehicle is a hybrid if it utilizes more than one form of on board energy to achieve propulsion. In practice, that means a hybrid will have a traditional internal-combustion engine and a fuel tank, as well as one or more electric motors and a battery pack.
Hybrid cars are sometimes mistakenly confused with electric vehicles. Hybrids are most often gasoline-burning machines that utilize their electric bits to collect and reuse energy that normally goes to waste in standard cars. Theoretically, diesel-electric hybrids would be even more fuel-efficient, but hybrid systems and diesel engines both represent extra cost. So far, installing both in the same vehicle has proven to be prohibitively expensive.
Motor-generator: The more accurate term for the electric motor. It provides supplemental acceleration "oomph" when operating as a motor by drawing electricity from the battery. Several hybrids have two, and a few models employ three.
Stop-start: Present on all hybrids, the engine's traditional starter motor is absent because the motor-generator takes on that function, too. Hybrid-control software shuts the engine off while stopped at traffic signals and automatically restarts it again with the electric motor when the driver releases the brake pedal. Eliminating the fuel waste of an idling gas engine causes overall mpg to climb significantly and tailpipe emissions to drop, especially in town.
Regenerative braking: An important function of the motor-generator is to generate electricity to recharge the battery as it absorbs a portion of the vehicle's momentum when slowing or coasting downhill. Normal cars waste all of their excess momentum as heat in the brakes. Regenerative braking is insufficient to stop a car quickly, so conventional hydraulic brakes are still necessary.
Electric drive: Operating the vehicle on electric power alone is possible if the hybrid system has enough electrical capacity. The maximum speed and distance over which electric-only operation can be sustained varies from essentially zero to a handful of miles, and has everything to do with the weight and aerodynamics of the vehicle, the strength of the motor-generator and, more than anything else, the capacity of the battery.
Hybrid Layouts
Not all hybrids possess these attributes in equal measure, nor do they operate the same way. It all begins with the layout of the system.
Series hybrids: This is the oldest hybrid type. Diesel-electric locomotives and ships using this layout appeared in the last century. In a series hybrid car, electric motors alone turn the drive wheels, so the motors must be large and powerful. But a series hybrid is not a "pure" electric vehicle. It has a dedicated engine that burns fuel and expels emissions. The engine powers a generator to produce the electricity on board the vehicle.
The 2014 BMW i3 and the out-of-production 2012 Fisker Karma use series hybrid systems. But these cars from BMW and Fisker are also plug-in hybrids. More about that flavor of hybrid a little later.
Nearly every carmaker also has a series hybrid demonstration vehicle that uses a hydrogen-powered fuel cell instead of a gasoline engine to generate the electricity. Those cars, usually called fuel-cell electric vehicles (FCEVs), are expected to start entering the retail market in small numbers by 2015. Look for them initially in limited areas, such as the Los Angeles basin, where some sort of retail hydrogen fuel system is present.
Parallel hybrids: These are the simplest and least costly type in current automotive use. Here the output of the engine and the electric motor are blended together upstream of the transmission. The engine dominates, never doing anything except propelling the vehicle. An electric motor provides an extra boost, and if it's large enough, it may be the car's only source of propulsion for short distances. In conventional parallel hybrids, such as the Civic Hybrid from Honda, regenerative braking is the sole source of recharging power for the battery.
Series-parallel hybrids: As the name implies, these cars contain elements of both types. Conceptually, the engine and the electric motor feed into the transmission via separate paths, enabling fully independent propulsion via the engine or electricity. In parallel fashion, the motor-generator
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Hybrid
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