In reality this cycle is never to 100% closed since we are not able to utilize the energycontent of the crops to 100 %. During the processing and transport from the field and to the tank of the car we also have losses and need to put in energy. Just as we have to invest energy to extract and process raw-oil to the petrol we otherwise would fill in our cars.
Different feedstocks and different plants vary in energycontent and efficiency. Thus the GHG savings vary depending on how the bioethanol is produced.