  We present the results of relativistic hydrodynamic simulations of the
collision of two dense shells in a uniform external medium, as envisaged in the
internal shock model for BL Lac jets. The non-thermal radiation produced by
highly energetic electrons injected at the relativistic shocks is computed
following their temporal and spatial evolution. The acceleration of electrons
at the relativistic shocks is parametrized using two different models and the
corresponding X-ray light curves are computed. We find that the interaction
time scale of the two shells is influenced by an interaction with the external
medium. For the chosen parameter sets, the efficiency of the collision in
converting dissipated kinetic energy into the observed X-ray radiation is of
the order of one percent.
