I've heard of that technique, but I can't see how it would provide enough heat. Professional growers use smudge pots which throw great heat and smoke.

Spraying water on the tree before the freeze adds a protective layer of ice to the blossoms.

On the thread linked below, "evonline" has some information and an interesting approach to growing fruit trees in cold zones. Holding back the flowering period should help with fruit trees that are otherwise hardy, but don't bear fruit because of cold weather during the flowering period.

Here is a link that might be useful: Apricots in Zone 4