2012 Toyota Prius PHV, 'Plug-in Vehicle' prototype on display during the Green Drive Expo in Richmond, California © Kim Kulish/Corbis via Getty Images
Asked by: Julie Blakemore, Hereford
In a hybrid car, electric motors are used most when they're travelling at slower speeds around town. Petrol is more efficient at high speed, so the conventional engine is in charge on the motorway. For much of the time, however, the engine and electric motors run together with the car's power control unit, varying the relative amount of drive from each. So there's no set point where electric power switches off and petrol kicks in.
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