It decides what mode is best to run it in. It defaults to using EV mainly. That's kinda the purpose. Once the battery drains then it'll kick the ICE on to charge it. Even then, the engine isn't driving the wheels. The only time it goes into parallel mode (where the ICE connects directly to the wheels) is at higher speeds when the battery is depleted or for a short burst of power to get up to speed etc. If you really just want the ICE to run then you can choose "Save" mode and get it to constantly run.