Many sources of ignition exist, including open flames, hot gases, hot surfaces, mechanical sparks and electrical sparks, among others. In the process of ignition, once a propagating flame is provided enough energy from an ignition source, it turns into a self-sustaining flame.
The strongest ignition source is the open flame because all it has to do is make contact with flammable objects to become self-sustaining.
Other ignition sources are less likely to become self-sustaining flames, but are still dangerous. Hot gases from exhaust, warm heating pipes and mechanical sparks from abrasive cutting are all capable of ignition. Electrical sparks from electrical equipment are also a common ignition source.