Just what is a hybrid electric vehicle?

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Basically, a hybrid electric vehicle combines an internal combustion engine and an electric motor powered by batteries, merging the best features of today's combustion engine cars and electric vehicles.

The combination allows the electric motor and batteries to help the conventional engine operate more efficiently, cutting down on fuel use. Meanwhile, the gasoline-fueled combustion engine overcomes the limited driving range of an electric vehicle.

In the end, this hybridization gives you the ability to drive 500 miles or more using less fuel and never having to plug in for recharging. Gasoline-fueled HEVs are among a select few vehicle technologies that can provide dramatically increased fuel economy and extremely low levels of smog-forming and cancer causing emissions, while delivering the safety and performance the public has come to expect. But that all depends on how well automakers apply the technology.

To help you navigate the hybrid market, let's take a closer look at what's under the hood that sets hybrids apart.

But remember, when looking at hybrids, no matter what the technology, the clearest and most direct way to evaluate the environmental performance of a hybrid electric vehicle is by its fuel economy and emissions.

How we classify Hybrids: Understanding the technology

Not all hybrids are created equal. In fact, there are degrees of hybridization such as 'mild' and 'full' and even different drivetrains utilized depending on which hybrid you're looking at. If we approach hybrids by looking at five technology steps that separate conventional vehicles from battery electric vehicles, we can better evaluate how a particular hybrid operates.

To be a true hybrid, a vehicle needs the first three steps. The fourth and fifth create the potential for hybrids with superior energy and environmental performance, but remember, don't just rely on the type of hybrid, always check the fuel economy and emissions data available at our Hybrid Consumer Center.

5 Steps to Hybridization

  1. Idle-off capability
  2. Regenerative braking capacity
  3. Power Assist and Engine downsizing (at this step you reach a 'mild' hybrid)
  4. Electric-only drive (at this step you reach a 'full' hybrid)
  5. Extended battery-electric range (at this step you become a 'plug-in' hybrid)

1. Idle-off capability

Like the switch that turns off the refrigerator light bulb when the door is closed, this feature allows a vehicle to turn off its gasoline engine when stopped, saving fuel. In a well-designed system, the engine will turn back on and be ready to go in less time than it takes for you to move your foot from the brake to the gas pedal. However, while hybrids use a full function electric motor operating above 100 Volts to accomplish this, conventional vehicles accomplish this same thing by using a beefed up 12 Volt or 42 Volt starter motor (often called an integrated starter-generator). So, this ability alone does not define a hybrid even though all hybrids can do this.

Some automakers are trying to take advantage of idle-off provided by beefed up starter-motors to claim they are actually putting hybrids on the road, garnering an undeserved green image. Claiming these vehicles are hybrids simply rings hollow because they don't take the next two steps, which are necessary to qualify as a real hybrid. Be wary, these are, at best, half-hearted attempts at hybridization.  Find out more information in our Hybrid Watchdog.

2. Regenerative braking capacity

The energy associated with a car in motion is called kinetic energy-the faster a car moves, the more kinetic energy it has. To slow down or stop a car, you have to get rid of that energy. In a conventional car, you use the friction of your mechanical brakes to stop, turning the kinetic energy into hot brakes and thereby throwing away the energy. 'Regen,' or regenerative braking takes over some of the stopping duties from the friction brakes and instead uses the electric motor to help stop the car.

To do this, the electric motor operates as a generator, recovering some of the kinetic energy and converting it into electricity that is stored in the battery so it can be used later to help drive the vehicle down the road. In order for the system to actually improve fuel economy, however, the vehicle must have a large enough electric motor operating at a high enough voltage to efficiently capture the braking energy.

Also, the vehicle requires a battery pack with enough capacity to store this energy until it is needed. Some automakers claim to have regenerative braking on conventional vehicles with integrated starter-generators, but their system cannot recover enough energy to actually help power the vehicle or cut fuel use beyond what is achieved with their idle-off ability.

3. Power Assist and Engine downsizing (at this step you reach a 'mild' hybrid)

The most basic definition of a hybrid vehicle is one that uses two methods of providing power to the wheels. As a result, the ability of an electric motor to help share the load with a gasoline engine is the technology step that, on top of the first two, truly qualifies a vehicle as a hybrid. A vehicle meets this classification only if it has a large enough motor and battery pack such that the motor can actually supplement the engine to help accelerate the vehicle while driving.

This power assist ability reduces the demands on the gasoline engine, allowing for the use of a smaller, more efficient gasoline engine while maintaining the same performance as a vehicle with a larger engine. This engine 'downsizing' may be achieved by using physically smaller engines with less cylinders or smaller displacements, or may be achieved using more efficient combustion cycles.

For example, Toyota and Ford use the same size engine in many of their hybrids, but employ an Atkinson combustion cycle to improve the gasoline engine efficiency over the conventional engine configuration. The implementation of the Atkinson cycle is a technique in which the intake and compression stroke of a four-stroke engine are shorter than the power and exhaust stroke. This typically lowers the power output of the engine but improves overall engine efficiency. Typically vehicles containing these first three features are categorized as a 'mild' hybrid like the Insight, Civic, and Accord hybrids from Honda.

4. Electric-only-drive

This technology step allows the vehicle to drive using only the electric motor and battery pack, thus taking full advantage of electric side of the dual system. With this step, we separate out 'full' hybrids such as the Toyota Prius and Ford Escape Hybrid. This is the reason why Prius owners are sometimes shocked when they start their car and don't even realize it's on-only the quiet battery system is operating the car rather than the traditional rumble of the combustion engine.

5. Extended Battery-Electric Range

Hybrids can boast better 'low end torque' than comparable conventional vehicles-meaning that the gasoline-electric drive will actually deliver better acceleration at low speeds.

The final level of hybridization extends the electric motor's capacity to drive the car by recharging the battery from a clean energy grid (i.e. 'plug in'). This would allow the hybrid to operate solely as a battery-electric vehicle for as much as 20-60 miles, thus improving their environmental performance if they are using clean sources of electricity.

A Plug-in can operate as a typical full hybrid if it is not recharged from the power grid, so the benefits of this feature are largely dependent on how often the consumer plugs in. The biggest challenge with these hybrids is cost-they have the highest up-front costs because they require larger motors and battery packs to ensure good vehicle performance and sufficient all-electric range. To date automakers have not offered any of these hybrids for passenger vehicles, though DaimlerChrysler is currently testing a commercial van-based plug-in hybrid.

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