  A novel extension of a rapid compression machine (RCM), namely a Rapid
Compression Expansion Machine (RCEM), is described and its use for studying
chemical kinetics is demonstrated. Like conventional RCMs, the RCEM quickly
compresses a fuel/air mixture by pushing a piston into a cylinder; the
resulting high temperatures and pressures initiate chemical reactions. In
addition, the machine can rapidly expand the compressed gas in a controlled way
by pulling the piston outwards again. This freezes chemical activity after a
pre-defined reaction duration, and therefore allows a convenient probe sampling
and ex-situ gas analysis of stable species. The RCEM therefore is a promising
instrument for studying chemical kinetics, including also partially reacted
fuel/air mixtures. The setup of the RCEM, its experimental characteristics and
its use for studying chemical reactions are outlined in detail. To allow
comparisons of RCEM results with predictions of chemical reaction mechanisms, a
simple numerical model of the RCEM process is described. As a first
application, ex-situ measurements of the temporal evolution of species in
partially reacted dimethyl ether/methane-mixtures are presented, and the
results are compared to predictions of a reaction mechanism.
