  The dynamical reaction of the particles accelerated at a shock front by the
first order Fermi process can be determined within kinetic models that account
for both the hydrodynamics of the shocked fluid and the transport of the
accelerated particles. These models predict the appearance of multiple
solutions, all physically allowed. We discuss here the role of injection in
selecting the real solution, in the framework of a simple phenomenological
recipe, which is a variation of what is sometimes referred to as thermal
leakage. In this context we show that multiple solutions basically disappear
and when they are present they are limited to rather peculiar values of the
parameters. We also provide a quantitative calculation of the efficiency of
particle acceleration at cosmic ray modified shocks and we identify the
fraction of energy which is advected downstream and that of particles escaping
the system from upstream infinity at the maximum momentum. The consequences of
efficient particle acceleration for shock heating are also discussed.
