What kind of internal energy? By mechanical energy, do you mean work? For example, thermal energy can be converted into work, but there are limits due entropy.
Yes, there is a rather famous example of this... a steam engine. Heat causes the water to turn to steam and the rising of the steam turns a turbine (and that turbine, in turn could be attached to a dynamo motor to produce electricity). Voila, usable energy from heat (although the efficiency of such processes suck).