Theoretically a glow engine can start without having to put a source to heat the glow plug, but you're talking about somewhere in the vicinity of 10,000 to 1 that you can actually get it started. In the real world you're going to have apply 1.5 volts (not 9 volts) to heat the glow plug so that you can get the motor started. 1.5 volts will be provided by the smaller cordless glow drivers available, while power panels use a higher voltage that is "pulsed" so that it doesn't burn out the glow plug. If you apply a full 9 volts directly to the glow plug you will burn it out. Once the motor is running the heat of combustion keeps the glow plug hot.

Hope this helps

Ken