"Accel." is a common abbreviation for the noun "acceleration" in physics, but I've never seen it used for the verb "accelerate." I tried it in a few sample sentences, and it seems unnatural. If at all possible, don't abbreviate this word. (If it's in a title, avoid abbreviations completely.)
If this program is a computer program, though, you can use any variable names you want. Computer programs do not follow the normal rules of English usage.