Variations of Tempo, the ritardando, accelerando, and tempo rubato, are all legitimate aids demanded by Expression.
"Style in Singing"
W. E. Haslam
She noticed his large, protuberant Adam's apple throbbing with the accelerando of pleasure, and a thaw set in between them.
"Star-Dust A Story of an American Girl"
Fannie Hurst
The end is fortissimo and accelerando, with a curious sequence of passing notes in the melody against a very rough chord, repeatedly struck by the rest of the orchestra.
"Shakespeare and Music"
Christopher Wilson