So how does the Saturn Hybrid system work? Essentially, it follows the steps outlined below:

• Shuts off the engine when the vehicle is stopped, to minimize idling
• Restarts the engine promptly when the brake pedal is released
• Enables early fuel shut-off during vehicle deceleration
• Captures vehicle kinetic energy during deceleration (regenerative braking) to charge an advanced nickel metal hydride battery
• Performs intelligent battery charging when it is most efficient

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Source: Saturn