for tractor pulls i'm not sure of the exact setup, but it includes a box with ice, and i'm guessing that they plumb the compressed air through it.

 

A very cold liquid does a better job than a solid like Dry Ice can because of the surface area contact with the heat exchanger. Liquid can physically "touch" more on the surface area thereby absorbing more heat. You could chill the diesel fuel on hot days though to keep it as dense as possible, but it turns into a wax if too cold!