Synergy Drive is at the heart of the Toyota Prius. A traditional combustion engine works with an electric engine--powered by a battery--to keep the car running at maximum efficiency. The electric engine is the default, but when the Prius calls for more power, or the battery is in need of a charge, the engine will kick on. When maximum power is called for--such as highway acceleration--both the electric and the gas engine will power the car. One mode of the onboard computer shows a perpetual diagram of when and how the two engines are working. Also, the driver can occasionally feel a slight rumble when the gasoline engine engages.