  A canonical partition function for the two-component excluded volume model is
derived, leading to two different van der Waals approximations. The one is
known as the Lorentz-Berthelot mixture and the other has been proposed
recently. Both models are analysed in the canonical and grand canonical
ensemble. In comparison with the one-component van der Waals excluded volume
model the suppression of particle densities is reduced in these two-component
formulations, but in two essentially different ways. Presently used
multi-component models have no such reduction. They are shown to be not correct
when used for components with different hard-core radii. For high temperatures
the excluded volume interaction is refined by accounting for the Lorentz
contraction of the spherical excluded volumes, which leads to a distinct
enhancement of lighter particles. The resulting effects on pion yield ratios
are studied for AGS and SPS data.
