Design Comparison

         Hybrid Drive Design Comparison
There are 3 current manufacturer designs for hybrid engines/drivetrains:
  1. Synergy Drive --- used by Toyota, Lexus, and Nissan hybrids
  2. Integrated Motor Assist (IMA) --- used by Honda hybrids
  3. Mild Hybrid --- used by GM for the Chevy and Saturn hybrids
We'll go through each of these designs in detail.

1. Toyota's Synergy design

The Synergy design uses two electric motors --- an electric drive motor and an electric motor-generator. Both electric motors are connected to a 1.5-liter gas engine and a planetary-gear-type variable speed transmission via a two-way power splitter. Here is a diagram of how Synergy works:

Hybrid Drive - Toyota Synergy

When accelerating or going uphill, both electric motors plus the gas engine power the car. When coasting or travelling at constant speed, the electric motor solely powers the car. If the 250-volt-DC nickel-metal-hydride battery (used to run the electric motor) needs charging, the gas engine is turned on and used to turn the motor-generator which charges the battery.

At speeds less than 25MPH (and also when starting from a full stop), the gas engine is turned off unless the battery needs charging. When braking or going down a steep hill, the motor-generator is engaged with the planetary gears via a proprietary power splitter device, and performs regenerative braking, while at the same time charging the battery.

For the 4WD version of the Toyota Highlander Hybrid, there is an a third electric motor powering the rear wheels.

2. Honda's IMA design

The IMA (Integrated Motor Asssist) design uses one electric motor-generator plus a 1.3-liter gas engine. The gas engine is hard-connected to the motor-generator. That pair connects to a belt-type continuously variable transmission (CVT).

Hybrid Drive - Honda IMA

When starting or quickly accelerating, both the electric and gas motors power the car. When mildly accelerating or cruising at high-speed, only the gas engine operates. When coasting or low-speed cruising, the gas engine's spark plugs, fuel injectors, and valves are all shut off, allowing the electric engine to power the car with very little drag from the gas engine. The electric engine only uses enough current to maintain the speed requested by the driver's pedal position.

When braking or decelerating, the gas engine is also shut off and the motor-generator is used to charge the 120-volt-DC nickel-metal-hydride battery using regenerative braking.

3. GM's Mild Hybrid design

The Mild Hybrid is similar to a conventional car engine, but with an oversize starter. The gas engine is turned off when coasting, braking, or if stopped. During that time an oversize battery powers any accessories that are turned on.

Hybrid Drive - GM Mild Hybrid

Since there is no electric motor connected to the drive train, this design is not as fuel-efficient as either the Toyota Synergy or Honda IMA designs. For example, there is no regenerative braking performed, so when decelerating or braking, the potential energy of that action is not recouped.

Note that there are several hybrid cars coming out in the future which may have different drive designs from the above, such as the Fisker Plug-in Hybrid. Also several major manufacturers are releasing 100% electric cars in the 2009-2010 timeframe.

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