  We have developed a simple model to investigate the modifications of the
hydrodynamics and non-equilibrium ionization X-ray emission in young supernova
remnants due to nonlinear particle acceleration. In nonlinear, diffusive shock
acceleration, the heating of the gas to X-ray emitting temperatures is strongly
coupled to the acceleration of cosmic ray ions. If the acceleration is
efficient and a significant fraction of the shock ram energy ends up in cosmic
rays, compression ratios will be higher and the shocked temperature lower than
test-particle, Rankine-Hugoniot relations predict. We illustrate how particle
acceleration impacts the interpretation of X-ray data using the X-ray spectra
of Kepler's remnant, observed by ASCA and RXTE. The thermal X-ray emission
provides important constraints on the efficiency of particle acceleration, in
complement to nonthermal emission. X-ray data from Chandra and XMM Newton, plus
radio observations, will be essential to quantify nonlinear effects.
