The homogeneous charge compression ignition combustion process was performed using chemiluminescence imaging to characterize the flame structure and its length scales and fluctuations. A mixture of 50% n-heptane and 50% iso-octane was used as fuel. Cool flames existed at about 20 crank angles before TDC. The main heat released peaks around TDC. Throughout the combustion process the average standard deviation was below 50% of the average intensity, indicating less variation compared to the port fuel injection case. Around peak intensity, the average standard deviation was below 25% of the average intensity. The cycle-to-cycle variations were larger for the port injected case and they were largest at the start of combustion. This is an abstract of a paper presented at the IFP International Congress (11/26-27/2001, Rueil-Malmaison, France).