Making Sense of the Newest Technologies
There are two types of hybrid electric vehicles, series and parallel. In a series hybrid, a small gas or diesel engine is employed as a generator, creating power to drive the electric motor and recharge the small battery pack. Thus, the gasoline engine never directly powers the vehicle. The batteries also store electricity generated through a process called regenerative braking, which is a brilliant method of recuperating up to half the motion energy (i.e. heat that warms the brake pads) vehicles traditionally waste during the decelerate-and-stop cycle. This type of braking allows the running gas motor to become a recharging generator, an especially useful feature for those of us who live in stop-and-go city traffic.
By contrast, parallel hybrids, such as in the Honda Insight, have two discreet power sources, through a fuel tank that supplies fuel to the engine and also a set of batteries supplying power to an electric motor. Both the electric motor and the engine can drive the transmission, and in turn power the wheels. When they work together, they can provide the kind of power needed to keep up with and even surpass the flow of traffic.
One type of parallel hybrid has the car starting out in battery-powered mode, and then automatically firing up its internal-combustion engine when the batteries have lost 40 percent or more of their charge. The parallel hybrids computer picks which form of power is on line, and makes barely noticeable switches between gas and electric mode, or merges them for quick acceleration. Another parallel type runs mostly on a small gas motor, switching on the electric for added boost. Many hybrids also have a continuously variable automatic transmission (CVT), which cuts down on energy lost during shifting.
The system in the Toyota Prius incorporates elements of both the parallel and series hybrid. The engine, a fuel-efficient, low-revving (its power peak is at 4,000 rpm) 1.5 liter four-cylinder, delivers its output both to a generator and the wheels, through an electronic power-splitter. When the car is stopped, the engine shuts down automatically. This means it wont pollute when stuck in traffic jams. An engine management system decides how the power is divided, and if it makes the right decision the Prius (which also uses regenerative braking) can achieve 52 mpg in the city and 45 mpg on the highway.
Basically, when the car first starts up or is moving slowly, the engine is turned off and the batteries are in charge. At normal running speeds, the gas motor starts up and sends power to the generator and to the road. When the Prius accelerates abruptly, horsepower from the engine is aided by extra power extracted from the batteries. As the car slows down or brakes the motor becomes a generator, capturing the kinetic energy from the wheels and recharging the batteries.
While only parallel hybrids, or combinations of the two, have been produced for mass consumption thus far, serial hybrids arent far away. Either way, hybrid gas/diesel-electric powertrains are here to stay, the forerunners to fuel cell vehicles.