As the federal government defines it, a hybrid electric vehicle (HEV) is a vehicle that has two sources of motive energy. What this means in practice is the use of some type of internal combustion engine combined with an electric motor (or motors) getting its (their) power from storage batteries. Unlike so-called "pure electric" vehicles, the batteries are not charged by an outside source—for example, plugging them into an electrical socket in your garage. Instead, their batteries are charged by an in-vehicle charging system. Thus, they are self-contained except for the need to refuel their internal combustion engines.
The internal combustion engine used in a typical HEV can be sized to deal with average load, not peak load, because the auxiliary stored power, usually electric battery power used to activate the electric motor, is used to deal with higher loads such as hill climbing. This has the benefit of allowing the installation of a smaller, lighter, and less fuel-thirsty engine. In addition, hybrid vehicles have regenerative braking capability, which means that during deceleration some of the energy that in conventional cars is simply dissipated as heat off the brakes is used to recharge the storage batteries.
The regenerative braking process uses the electric motor as a generator to spin out electricity when the vehicle is slowing to a stop or simply coasting down a hill. In a conventional car this energy is wasted.
The automatic start-shutoff feature used by most current hybrid cars easily could be installed on conventional gasoline-powered vehicles. The United States DOE estimates this step alone could increase fuel economy 8 percent.
A fuel-saving feature offered by hybrids is the automatic start-shutoff. When you stop the vehicle, the system turns off the internal combustion engine. Then when you lift your foot off the brake pedal to accelerate again, the engine is restarted. Idling the engine while you're sitting still wastes energy, so this is a very sensible and reasonably easy-to-engineer step. Some sophisticated hybrid systems don't start the IC
engine until your vehicle is well underway, sometimes not until it is traveling at 30 mph or so. This maximizes the fuel economy and environmental benefits of the electric drive system, and it also alleviates the possibly irritating factor of having the gasoline engine stop and start repeatedly while you are crawling through bumper-to-bumper traffic.
The combination of automatic stop-start, regenerative braking and the use of smaller, less powerful engines enables hybrid vehicles to achieve fuel efficiency that is significantly better than with gasoline-powered vehicles, while emissions are greatly decreased. The key to making all this work seamlessly is a very sophisticated computerized transmission that can channel power from two sources, the internal combustion engine and the electric motor or motors. HEVs usually operate using only the gasoline engine power, the electric motor power, or both, depending on the situation. Enabling these transitions to occur without making them irritatingly noticeable to drivers and passengers is important for doing this well.
The major difference between HEVs and "pure" EVs is the use of an internal combustion engine using conventional gasoline as fuel. By using an IC engine to keep batteries topped up and to provide some motive force, instead of relying completely on a motor-storage battery combination, HEVs can conquer the range problem that has haunted electric vehicles since time immemorial. In addition, fuel economy is phenomenal because the engine needs only to propel the vehicle at cruising speeds on flat ground. In more challenging situations, like accelerating from a stop or passing another vehicle on the highway, the battery-powered electric motor offers a significant and almost instantaneous supplement. Plus HEVs use conventional fuels, which means virtually no change in infrastructure is required for their use. You can fill the fuel tank at any local gas station.
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