The control of nitrogen oxide and smoke emissions in diesel engines has been one of the key researches in both the academia and industry. Nitrogen oxides can be effectively suppressed by the use of exhaust gas recirculation (EGR). However, the introduction of inert exhaust gas into the engine intake is often associated with high smoke emissions. To overcome these issues there have been a number of proposed strategies, one of the more promising being the use of low temperature combustion enabled with heavy EGR. This has the potential to achieve simultaneously low emissions of nitrogen oxide and smoke. However, a quantitative way to identify the transition zone between high temperature combustion and low temperature combustion has still not been fully explored. The combustion becomes even more complicated when ethanol fuel is used as a partial substitution for diesel fuel.In the work reported here, the heat release rate (HRR) of diesel low temperature combustion at different engine loads and various intake oxygen levels was analyzed. The high temperature combustion zone, the transition zone and the low temperature combustion zone were initially defined from the smoke emissions and then identified on the corresponding HRR characteristics. The HRR-shape changes were observed for different combustion zones. A new parameter named combustion acceleration was proposed which was the first derivative of HRR to capture the HRR-shape change. It was observed that the peak of combustion acceleration can potentially be used as an indication of the transition zone. The same analytical procedure was applied to the ethanol combustion initialized by diesel direct injection. The corresponding results were compared with the diesel single-shot low temperature combustion.