People like to categorize the world. It helps our brains figure stuff out, because we can only manage so much information. Lumping things into bins helps get information down to a manageable level. Unfortunately, most things don't readily fall into discrete categories. Hybrid drive technology is one of them, but we'll try anyway. On the continuum that is hybrid technology, we typically break things down into strong, mild and micro-hybrids. Strong hybrids include systems like Toyota's hybrid synergy drive, Ford's hybrid system and General Motors two-mode system. Micro-hybrids are really nothing more than automatic start stop systems.
Somewhere in between those groups lies the mild hybrid. The basic premise of the mild hybrid is the same as the strong hybrid. An electric motor/generator operate in parallel with the internal combustion engine to provide additional drive torque as well as regenerative braking. The primary difference lies in the power and energy capacity of the electrical side of the system. Continue reading about mild hybrids after the jump.
Why would an automaker create a mild hybrid system? Mainly to get some of the benefits of a hybrid system at a significantly lower cost and weight. Mild hybrids typically have a much smaller battery than a strong hybrid and a smaller, weaker motor/generator. The first manufacturer to build a system that fits into the mild category is generally considered to be Honda with it's Integrated Motor Assist (IMA) system as used in the Insight and Civic.
Since the Insight debuted in 1999, General Motors has released two different mild hybrid systems, one that was briefly offered in the Silverado hybrid in 2005-6 and the more recent belt-alternator-starter (BAS) system offered on several models. Mercedes-Benz and BMW have also co-developed a system of their own that will debut this summer on the S400 BlueHybrid and then on the new 7-series.
The GM system uses what is essentially a beefed up alternator and modified belt drive system to provide some additional drive torque to the engine as well as re-start it. Other systems, such as those from Honda and Mercedes, use a disk-shaped motor-generator sandwiched between the engine and transmission to provide the same functionality. The motor also takes the place of the torque converter in the transmission.
All of these mild hybrid systems have motor/generators with output of anywhere from 5-15 kW, significantly lower than the 50-70 kW found on strong hybrids. The result is that the electric drive doesn't have enough power to propel the vehicle on its own. Instead, the motor operates more like an electric turbocharger, providing an on-demand transient power boost. This can be particularly useful at lower speeds because the electric motor provides its maximum torque from zero speed. When the vehicle comes to a stop, the motor also provides automatic start-stop functionality to prevent the engine from idling.
During deceleration the mild hybrid system can also provide some regenerative braking capability. Because the battery pack is generally smaller than what you find on a strong hybrid, the ability to store energy to drive the vehicle is limited. However, the Mercedes-BMW mild hybrid is using the electrical power in a different way. The energy stored in the battery is being used to drive vehicle electrical systems such as audio, windows, HVAC and others. By using recaptured kinetic energy, the load on the alternator is reduced, cutting parasitic losses.
The Mercedes mild hybrid system will be the first mainstream hybrid to use a lithium ion battery. The Continental-supplied battery is mounted under-hood and is the same size as the traditional lead-acid starter battery, which is replaced by the lithium unit. In 2010, GM plans to launch a second-generation version of its mild hybrid. The new version increases the motor power from 5 to 15 kW and the current nickel metal hydride battery is replaced by a higher capacity, lighter lithium ion unit.
While a mild hybrid system can't drive the vehicle on electricity alone, it still provides benefits. Like direct injection and turbocharging, it allows the automaker to downsize the base engine while maintaining the same performance level. The combination of the reduced peak output of the engine and eliminating engine idle can contribute fuel consumption savings of up to 15 percent in urban driving and 8-10 percent overall. While not as significant as a strong hybrid, these benefits come at a much lower cost in mild hybrid form.