  Based on time-dependent radiation-hydrodynamics simulations of the evolution
of Planetary Nebulae (PNe), we have carried out a systematic parameter study to
address the non-trivial question of how the diffuse X-ray emission of PNe with
closed central cavities is expected to depend on the evolutionary state of the
nebula, the mass of the central star, and the metallicity of stellar wind and
circumstellar matter. We have also investigated how the model predictions
depend on the treatment of thermal conduction at the interface between the
central `hot bubble' and the `cool' inner nebula, and compare the results with
recent X-ray observations. Our study includes models whose properties resemble
the extreme case of PNe with Wolf-Rayet type central stars. Indeed, such models
are found to produce the highest X-ray luminosities.
