IMHO, you are expecting too much of your PHEV. It is not a pure electric car. At it's core it is an HEV with some added battery capacity that will enable short trips on electric power, as long as you provide a plug in charge. It has a more powerful traction motor that allows a little more electric torque, but if you accelerate hard, the ICE motor will still kick in like any other hybrid. It is likely that as the traction battery discharges, the ICE may kick in a little sooner than at a higher charge. It needs that reserve traction battery capacity to be able to run as a hybrid. Also, the traction battery doesn't just power the electric motor. It also charges your 12 volt system. The ICE has to come on to recharge the traction battery, and in turn the traction battery supplies the 12 volt. If you ran your traction battery down to nothing, the whole system would shut down because the starter generator wouldn't be able to keep up with the demand.