Using DC in transformers?
Transformers are AC only. 

Running DC through a transformer basically gives you a heater.

Critically, transformers work through the fact that a change in magnetic field induces a voltage in a wire. The critical portion is that the change is required.

In an ignition coil, the change is created by simply connecting and disconnecting the ignition coil from the battery. The disconnection of power from the coil produces a collapse of the magnetic field produced by the current-flow through the coil, and results in a high-voltage pulse on the output, and subsequently a spark.

The connection and disconnection of the ignition coil from the battery converts (some of) the DC battery voltage to AC.