  We study a one-dimensional hamiltonian chain of masses perturbed by an energy
conserving noise. The dynamics is such that, according to its hamiltonian part,
particles move freely in cells and interact with their neighbors through
collisions, made possible by a small overlap of size $\epsilon > 0$ between
near cells. The noise only randomly flips the velocity of the particles. If
$\epsilon \rightarrow 0$, and if time is rescaled by a factor $1/{\epsilon}$,
we show that energy evolves autonomously according to a stochastic equation,
which hydrodynamic limit is known in some cases. In particular, if only two
different energies are present, the limiting process coincides with the simple
symmetric exclusion process.
