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In addition to the Spark Ignited and Compression Ignited modes of reciprocating internal combustion engine operation, a third option exists. In this class of engine, elevated charge temperatures and (in some cases) active chemical species are employed to achieve a Homogeneous Charge Compression Ignition (HCCI) mode of operation. This is typically accomplished using copious amounts of charge dilution by exhaust gases, which can elevate charge temperatures into autoignition regimes, yet simultaneously temper runaway combustion rates which would otherwise lead to destructive knocking. Engine operation in the HCCI mode has been described as efficient, stable, and low in the production of engine-out emissions. However, with the large amounts of charge dilution required, such engines to date suffer from relatively low energy density, and are difficult to control over wide ranging speed/load conditions.