Glow plugs are made to be ran on 1.2 volts. 1.5 volts would be the VERY maximum. Voltage isn't exactly what you are after though, it is current. Currrent is what makes the element glow. What you need is a glow driver that supplies 1.2 volts with an adjustable current. I would say that MOST plugs will go bad on 2 volts.

Without getting into great detail, OHM's law explains this.