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click above for a high res gallery of Honda's IMA hybrid system

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.


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    • 1 Second Ago
  • 7 Comments
      • 6 Months Ago
      I appreciate this explanation. It was very informative. However it really seems that it is a classification system that does not explain very much and is also in error as well. The Honda "weak" hybrid system actually does allow for electric only propulsion of the automobile in certain limited circumstances which the article does not acknowledge, apparently through ignorance or inconvenience to their classification sytem. Since electric only propulsion of "strong" hybrids is also under specified limited conditions, it does not seem to be very useful classification system.

      In addition the article reaches conclusions about the merits that may not be accurate. All you have to do is go to fueleconomy.gov and look at the mileages posted by owners and you will find that mild hybrids like Hondas and strong hybrids like Prius get almost the same fuel economy in the hands of real drivers. A mile or two per gallon difference is really not very significant.

      Why not call them what they are?
      Full hybrid: A completely flexible system, gasoline, electric and combined modes of operation, like the ones sold by Toyota and Ford.
      Electric assist hybrid: Honda and I suppose the GM. Belt drive? Really?
      Extended range electric hybrid: The GM Volt dream.

      Simple, accurate, understandable, with no definition key required.
        • 6 Months Ago
        Except then you could not call the GM a hybrid since its motor generator is not designed to move the vehicle alone and the elusive all electric mode of the Honda means that it cannot really be driven as an electric would disqualify it as a hybrid under your definition.

        It seems to me that if you have two energy systems involved in propelling the car it is a hybrid. That would disqualify no idle systems and systems that use a second energy means just to power accessories. My feeling is that we should be talking about hybrid DRIVE systems. A solar cell that helps charge the battery or runs a fan to circulate air while the car is parked is not make it a hybrid.

        A full hybrid would be one which can operate alone on either energy system.

        It seems to me that an electric motor assist to a gasoline engine makes it a hybrid. While I understand the reasoning for wanting to call the Volt an electric car because the engine does not add propulsion power, the common understanding of an electric car as something that requires a recharge when the battery is depleted means that you have to call it a hybrid too. The engine is necessary for the continued forward momentum of the vehicle.

        I would have suggested parallel/series because of its common use in electrical circuits, but that has already been totally confused and over complicated by its use so far. Honda/Volt would represent series, both energy systems going through the same pipe so to speak. Toyota/Ford would represent parallel which allows each energy system to operate independently to move the vehicle without interference from the other.

        You could make the series label more explanatory by designating the basic motive energy system. Honda/GM trucks would be gasoline/diesel series hybrids. Volt would be an electric series hybrid.

        So I think that full hybrid, electric assist hybrid, and extended range electric hybrid are easy to understand and remember and accurate.

        • 6 Months Ago
        There is a difference between the GM BAS system and the Honda IMA system, the BAS uses a 48 volt battery and motor, the IMA uses a 144 volt battery and motor with a much higher peak power rating. That means the IMA can provide quite a bit more "assist" to the engine and greater regenerative braking, both of which helps reach better fuel economy figures.

        As for "Series vs. Parallel" - A parallel system has mechanical power from both the IC engine and electric motor travelling "in parallel", to the wheels. The Honda IMA and GM BAS are examples of parallel hybrids. A series system has the power flowing in sequential steps - IC engine to generator, then to motor, then to the wheels. The GM Volt and diesel locomotives are examples of series hybrids. There is a third category, where power from the IC engine is split, part moving in a parallel path with a motor to the wheels, and part going in a series path to a generator to power a motor, then the wheels, These are referred to as "split path" or "parallel/series" hybrids, and they combine the best aspects of parallel and series hybrids. The Toyota and Ford hybrids are examples of split path hybrids.
        • 6 Months Ago
        Or you could say a hybrid is a vehicle that can be driven on more than one type of motor, in these cases, an internal combustion engine and an electric motor.
        This would mean the Volt is not a hybrid, since it cannot be driven by the ICE. The electric motor is the only means of propulsion in the Volt. The ICE is just a portable generator to produce electricity.
        That's why GM doesn't call the Volt a hybrid. I think they call it an extended range (by means of bringing along that ICE generator) electric vehicle.
      • 6 Months Ago
      There is a minor technical error in the article. It turns out that the Mild Hybrid system in the 1st gen Honda Civic _is_ enough to power the car. I know because my wife called me once telling me that there was something wrong with our Hybrid: it was very slow and would only drive at ~1 mph. When she got a slight incline at the entrance to a parking lot it didn't have enough power to get up the slope. It turns out that she had run out of gas! Oops.
      • 6 Months Ago
      And reducing the necessary size of the base engine is where its all about. I know that I was pretty surprised to realize that a small car often only needs ~10hp to propel it down the highway at a steady speed; having electric assist around for acceleration spikes allows an engine that's far more efficient at putting out that 10hp (at the expense of peak output) than before.
        • 6 Months Ago
        I disagree about reducing the size of the engine.

        Toyota recently INCREASED the size of their engine in order to help improve their MPG. The fact is that it is all about finding the right size engine for the job. Sure, it may only take 10 hp to keep a vehicle in motion at the same speed on a flat plane, but that is really fuel efficient already. The places where we lose fuel are when we are accelerating and when we are going uphills and the engine has to work harder. It is not sufficient to have a smaller engine - it is more efficient to find an engine that has the easiest time moving a vehicle.

        To find out the differences between the Prius and the Insight (and why the Prius is better) go to http://www.mullertoyotaprius.com and check the menu item Prius vs. Insight at http://www.mullertoyotaprius.com/prius-vs-insight/