How Hybrid Cars and Trucks Work

Hybrid electric vehicles ("HEVs") combine an internal combustion engine with an electric motor powered by batteries. This combines the best features of today's electric vehicles with the advantages of traditional combustion engines. The combination allows the electric motor and batteries to help the conventional engine operate more efficiently, reducing fuel use. Meanwhile, the gasoline-fueled combustion engine overcomes the limited driving range of some electric vehicle.

As a result, hybrid technology lets you drive 500 miles or more, using relatively less fuel than traditional counterparts and without recharging.

In fact, gasoline-fueled HEVs are among a select few vehicle technologies that can dramatically increase fuel economy, while reducing pollution and delivering top safety and performance.

To help you navigate the hybrid market, let's take a closer look at how hybrids work.

Understanding hybrid technology

Not all hybrids are created equal. In fact, there are degrees of hybridization—such as "mild" and "full"—and often different drivetrains, depending on the hybrid. To evaluate how a particular hybrid operates, it's best to look at the five technological characteristics that separate conventional vehicles from battery electric vehicles.

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.

5 characteristics of 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

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.

2) Regenerative Braking

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. Regenerative braking, or "Regen," 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

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. The greater flexibility of full hybrids allows the vehicle to spend more time operating its engine only when it is at its most efficient. At low speeds and at launch, the electric motor and battery powers the car and at high speeds the engine takes over.

5) Extended Battery-Electric Range

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. 


Now that we've covered the basic technology that defines hybrid vehicles, let's take a look at how they are put together to move the vehicle. The drivetrain of a vehicle is composed of the components that are responsible for transferring power to the drive wheels of your vehicle. With hybrids there are three possible setups for the drivetrain: the series drivetrain, the parallel drivetrain, and the series/parallel drivetrain.

Check out the feature below to see different drivetrains in action.

Series Drivetrain

This is the simplest hybrid configuration. In a series hybrid, the electric motor is the only means of providing power to get your wheels turning. The motor receives electric power from either the battery pack or from a generator run by a gasoline engine. A computer determines how much of the power comes from the battery or the engine/generator set. Both the engine/generator and regenerative braking recharge the battery pack. The engine is typically smaller in a series drivetrain because it only has to meet average driving power demands; the battery pack is generally more powerful than the one in parallel hybrids (see below) in order to provide remaining peak driving power needs. This larger battery and motor, along with the generator, add to the cost, making series hybrids more expensive than parallel hybrids.

While the engine in a conventional vehicle is forced to operate inefficiently in order to satisfy varying power demands of stop-and-go driving, series hybrids perform at their best in such conditions. This is because the gasoline engine in a series hybrid is not coupled to the wheels. This means the engine is no longer subject to the widely varying power demands experienced in stop-and-go driving and can instead operate in a narrow power range at near optimum efficiency. This also eliminates the need for a complicated multi-speed transmission and clutch. Because series drivetrains perform best in stop-and-go driving they are primarly being considered for buses and other urban work vehicles. 

Parallel Drivetrain

With a parallel hybrid electric vehicle, both the engine and the electric motor generate the power that drives the wheels. The addition of computer controls and a transmission allow these components to work together. This is the technology in the Insight, Civic, and Accord hybrids from Honda. Honda calls it their Integrated Motor Assist (IMA) technology. Parallel hybrids can use a smaller battery pack and therefore rely mainly on regenerative braking to keep it recharged. However, when power demands are low, parallel hybrids also utilize the drive motor as a generator for supplemental recharging, much like an alternator in conventional cars.

Since the engine is connected directly to the wheels in this setup, it eliminates the inefficiency of converting mechanical power to electricity and back, which makes these hybrids quite efficient on the highway. Yet the same direct connection between the engine and the wheels that increases highway efficiency compared to a series hybrid does reduce, but not eliminate, the city driving efficiency benefits (i.e. the engine operates inefficiently in stop-and-go driving because it is forced to meet the associated widely varying power demands). 

Series/Parallel Drivetrains

This drivetrain merges the advantages and complications of the parallel and series drivetrains. By combining the two designs, the engine can both drive the wheels directly (as in the parallel drivetrain) and be effectively disconnected from the wheels so that only the electric motor powers the wheels (as in the series drivetrain). The Toyota Prius made this concept popular, and a similar technology is in the new Ford Escape Hybrid. As a result of this dual drivetrain, the engine operates at near optimum efficiency more often. At lower speeds it operates more as a series vehicle, while at high speeds, where the series drivetrain is less efficient, the engine takes over and energy loss is minimized. This system incurs higher costs than a pure parallel hybrid since it needs a generator, a larger battery pack, and more computing power to control the dual system. However, the series/parallel drivetrain has the potential to perform better than either of the systems alone.