  We calculate the X-ray emission from both constant and time evolving shocked
fast winds blown by the central stars of planetary nebulae (PNs) and compare
with observations. Using spherically symmetric numerical simulations with
radiative cooling, we calculate the flow structure, and the X-ray temperature
and luminosity of the hot bubble formed by the shocked fast wind. We find that
a constant fast wind gives results that are very close to those obtained from
the self-similar solution. We show that in order for a fast shocked wind to
explain the observed X-ray properties of PNs, rapid evolution of the wind is
essential. More specifically, the mass loss rate of the fast wind should be
high early on when the speed is ~300-700 km/s, and then it needs to drop
drastically by the time the PN age reaches ~1000 yr. This implies that the
central star has a very short pre-PN (post-AGB) phase.
