Homogeneous charge compression ignition (HCCI) combustion requires high EGR rate and high intake temperature. HCCI combustion has not yet been made to operate at conditions other than low speed and low load in a four-stroke engine. Two stroke engine, however, have produced reasonable power in the HCCI combustion or active thermo-atmosphere combustion (ATAC) mode. In this paper, the nature of ATAC is discussed by spectroscopic observation to determine why the ATAC (under favorable condition) produces very low cyclic irregularity and low NO emission. ATAC low heat rejection engine and ATAC with alternative fuels are discussed.