  The identification of pre-acceleration mechanisms for cosmic ray ions in
supernova remnant shocks is an important problem in astrophysics. Recent
particle-in-cell (PIC) shock simulations have shown that inclusion of the full
electron kinetics yields non-time-stationary solutions, in contrast to previous
hybrid (kinetic ions, fluid electrons) simulations. Here, by running a PIC code
at high phase space resolution, ion acceleration mechanisms associated with the
time dependence of a supercritical collisionless perpendicular shock are
examined. In particular the components of $\int \mathbf{F} \cdot \mathbf{v} dt$
are analysed along trajectories for ions that reach both high and low energies.
Selection mechanisms for the ions that reach high energies are also examined.
In contrast to quasi-stationary shock solutions, the suprathermal protons are
selected from the background population on the basis of the time at which they
arrive at the shock, and thus are generated in bursts.
