It's now been just over a decade since the first hybrids, the original Honda Insight and Toyota Prius, were introduced. After a slow start, rising fuel costs caused consumer interest to take off mid-decade. Today, most consumers have some idea of what a hybrid is, but many are unaware that hybrid systems from competing manufacturers have entirely different hardware and function in dramatically dissimilar ways. That's why, for instance, you can't drive a Honda Civic hybrid on electric power alone, but you can in a Prius. Even greater variations on the hybrid theme will become available in the next few years, as automakers attempt to reduce costs, improve efficiency, and steer clear of patents held by their competitors. How do you make sense of it all? Click below to launch our gallery.

  • Defining Hybrids- Hybrid systems are typically described as micro, mild or strong depending on the amount of electrical power available to drive the vehicle. The most basic function of hybrid systems is automatic start-stop to shut the engine down when the car stops. Preventing idling in this way conserves fuel and lowers emissions. In Europe there are a number of these micro-hybrids that really aren't hybrids at all because they only provide the start-stop function but no electric drive or regenerative braking capability.

  • Image Credit: Ford
Mild and Strong Hybrids
  • Mild and Strong Hybrids
  • Mild and Strong Hybrids - The mild and strong hybrids add an electric boost or drive motor to the package. Since electric motors are generally able to produce electricity when they are driven mechanically, this also allows for regenerative braking. In these hybrids, lifting off the accelerator and coasting causes the wheels to turn the motor to produce electricity and recharge the battery. While this is happening it increases drag and the vehicle slows, just like it does under engine braking in a conventional vehicle. The battery powers the electric motor during acceleration or cruising. This cuts the workload on the gasoline engine and reduces its fuel consumption. In a mild hybrid, the motor typically has an output of 15 kilowatts or less, which is enough to provide boost and start-stop capability, but not enough to drive the vehicle on electricity alone. Strong hybrids typically have motor/generators with 30 kW or more, which allows them to be driven on electricity alone for short distances (1-2 miles) at speeds under 65 miles per hour.

  • Image Credit: GM
  • Parallel Hybrids - All the hybrids on the market today are so-called "parallel" hybrids, so named because multiple propulsion sources can drive the wheels at the same time. These systems consist of a conventional internal combustion engine combined with one or more electric motor/generator(s) and a battery. Let's take a look at representatives of each hybrid camp, and see how they differ.

  • Image Credit: Ford
Power-Split Strong hybrid
  • Power-Split Strong hybrid
  • Power-Split Strong hybrid - Over the past 12 years Toyota has sold far more hybrid vehicles than all the other manufacturers combined, and as a result its power-split strong hybrid configuration is the most common type by a wide margin. The same basic hardware configuration is also used by Ford for the Escape and Fusion. GM's two-mode hybrids, though different in design, also fall under this category. Each of these systems consist of an electronically controlled, variable-ratio transmission with motor/generators integrated into the transmission case. The variable ratio gears allow torque output from the engine speed to be continuously adjusted relative to vehicle speed so that the electric motor torque can be seamlessly blended in when extra power is needed. In these strong hybrids, the electric components have enough power to propel the vehicle on battery power alone for limited distances. The electric drive speed varies from about 25 mph for the Chevrolet Tahoe hybrid to up to 47 mph for the Ford Fusion. Some enthusiasts have hacked the control software in the Prius allowing it to go up to 60 mph on electricity alone.
  • Image Credit: Toyota
Power-Split Strong hybrid, continued
  • Power-Split Strong hybrid, continued
  • Power-Split Strong hybrid, continued - Besides being highly efficient, electric motors have another important functional advantage over internal combustion engines. Motors produce their maximum torque from idle and basically maintain that torque through most of their operating range. When incorporated into a hybrid powertrain, this allows the engine designers to modify the behavior of the internal combustion engine for improved efficiency. Gasoline engines in conventional vehicles run on what is known as the Otto four-stroke cycle, while hybrids use the Atkinson cycle. The main difference between the Otto and Atkinson is that the intake valves close later in the cycle to reduce pumping losses. This makes Atkinson more fuel-efficient but produce less torque. Normally drivers wouldn't like the way this feels but the electric motor fattens up the combined torque curve. Power-split hybrids have proven to be efficient but the transmission integrated with the motors is mechanically complex and expensive to build.

  • Image Credit: Toyota
Engine-Mounted Mild Hybrid
  • Engine-Mounted Mild Hybrid
  • Engine-Mounted Mild Hybrid - The 1999 Honda Insight was actually the second modern hybrid to come to market after the Prius (although it did beat the Toyota to U.S. by about six months). Honda has since used its hybrid architecture one each of its successive hybrid models. It's so-called "Integrated Motor Assist" uses a lower power motor mounted directly on the output end of the engine's crankshaft. When energized, the motor applies torque directly to the crankshaft upstream of the clutch. In 2005, GM briefly offered a similar system in its full-size pickups and BMW and Mercedes-Benz have jointly developed a system of their own that went on sale in late 2009. Because the motor is directly coupled to the engine and has lower output than a strong hybrid, these mild hybrids are unable to drive on electricity alone. Instead the motor provides extra boost and regenerative braking capability and also acts as the starter. While motor output is inadequate to drive the vehicle, it is sufficient to provide the torque needed to run the engine on an Atkinson cycle for improved efficiency. Since mild hybrids use less powerful motors, they consequently have smaller batteries as well.
  • Image Credit: Honda
Engine-Mounted Mild Hybrid, continued
  • Engine-Mounted Mild Hybrid, continued
  • Engine-Mounted Mild Hybrid, continued - These engine-mounted mild hybrids are less expensive to build than power-split systems and have the advantage of being modular. Separating the electric motor from the transmission makes it easier to adapt the technology to different vehicles by combining an internal combustion engine and electric motor with any type of gearbox including manuals (Honda CR-Z), conventional planetary gear automatics (BMW ActiveHybrid 7) and continuously variable transmissions (Honda Insight).
  • Image Credit: Honda
Transmission-Mounted Strong Hybrid
  • Transmission-Mounted Strong Hybrid
  • Transmission-Mounted Strong Hybrid - Several new strong hybrids from Hyundai, Volkswagen and Porsche are coming to market this year. Unlike the earlier strong systems, these new variants are in many ways closer to the Honda/Mercedes-Benz/BMW systems, with their electric motor/generator separated from the transmission internals. However, because the higher output motors are able to adequately drive the vehicle electrically, they are connected to the input side of the transmission.
  • Image Credit: Porsche
Transmission-Mounted Strong Hybrid, continued
  • Transmission-Mounted Strong Hybrid, continued
  • Transmission-Mounted Strong Hybrid, continued - These systems use an automatically controlled clutch between the engine and electric motor that is opened up when the battery has sufficient power and the driver's acceleration demand is low or the vehicle is stopped. In the case of the Volkswagen/Porsche system, there is an additional clutch between the electric motor and transmission to help improve the smoothness of operation during shifts. Putting the motor at the input of the transmission allows it to spin at a lower speed so that it can provide electric drive at higher vehicle speeds for better highway economy. The 2011 Hyundai Sonata, Volkswagen Touareg and Porsche Cayenne hybrids can all cruise with the engine off at over 60 mph.
  • Image Credit: Porsche
Belt-Alternator-Starter Mild Hybrid
  • Belt-Alternator-Starter Mild Hybrid
  • Belt-Alternator-Starter Mild Hybrid - GM offered a belt-alternator-starter (BAS) mild hybrid system in the Saturn Vue and Aura, as well the Chevrolet Malibu. Rather than having the electric motor feed power directly to the transmission input, GM developed a system with what looked like a conventional alternator. However, in addition to producing electricity, this alternator could also act as a drive motor.
  • Image Credit: GM
Belt-Alternator-Starter Mild Hybrid, continued
  • Belt-Alternator-Starter Mild Hybrid, continued
  • Belt-Alternator-Starter Mild Hybrid, continued - GM engineers added a second idler pulley to the drive belt, which allowed the alternator-motor to restart the engine after a stop and feed some extra torque to the engine crankshaft when the driver wanted more acceleration. Unfortunately, the first generation BAS hybrid only had a 5 kW output (about 6.7 horsepower) and a 36-volt nickel-metal-hydride battery, which limited its usefulness. At the 2008 Geneva Motor Show, GM announced a second-generation version of this system with a more powerful 15 kW (about 20 hp) alternator-motor and a lithium-ion battery. This system will debut in early 2011, likely on the Buick Regal.
  • Image Credit: GM
Through-the-Road Strong Hybrid
  • Through-the-Road Strong Hybrid
  • Through-the-Road Strong Hybrid - Through-the-road (TtR) hybrids are likely to be the least familiar to most drivers since there aren't any currently available for sale. Unlike all of the other hybrid systems described here, the engine and electric drive systems are completely independent in the vehicle. The name is derived from the way that power blending is handled "through the road." The first production TtR hybrids will go on sale in Europe in 2011 when Peugeot introduces this system in its 508 sedan and 3008 crossover. A typical TtR hybrid will have a front-engine driving the front wheels, with an electric motor mounted at the rear axle along with a battery somewhere in the back of the vehicle. In this way, the electric drive can provide instant on-demand, all-wheel-drive when extra power or traction is needed. At lower speeds, if the battery has enough charge, the system can also drive the car electrically by shifting the front transmission into neutral.
  • Image Credit: Peugeot
Through-the-Road Strong Hybrid, continued
  • Through-the-Road Strong Hybrid, continued
  • Through-the-Road Strong Hybrid, continued - The advantage of a TtR hybrid is that the cost can be significantly lower because the conventional drivetrain can be essentially stock. Only the control software would need to be changed in order to allow engine shutoff at idle or lower speeds. All-wheel-drive can be achieved without having to run a driveshaft down the middle of the vehicle. On the other hand, the electric motor has to be separately packaged and a through-the-road hybrid might not be as efficient at as some of the other systems because it cannot capture as much kinetic energy during regenerative braking at the rear axle.
  • Image Credit: Peugeot

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