The ignition delay of n-heptane homogeneous charge compression ignition(HCCI) combustion under high levels of carbon dioxide addition was quantitatively measured at elevated pressure from low to intermediate temperatures in a rapid compression machine.The experiments were conducted in the compressed temperature range 613-750 K.Both the compression ratio and fuel/air equivalence ratio were varied to investigate their effects on the ignition delay of n-heptane.Carbon dioxide was subsequently added to study the influence of the carbon dioxide level on the ignition delay of n-heptane under low-temperature conditions.It was found that carbon dioxide had different effects on the two-stages of ignition delay of n-heptane under low-temperature conditions:the concentration of carbon dioxide had little effect on the first-stage ignition time;a certain concentration of carbon dioxide accelerated the first-stage ignition but had a significantly larger impact on the second-stage ignition delay,thus increasing the overall ignition delay time.The results also showed that the first-stage ignition delay of n-heptane is only a function of temperature under low-temperature conditions.The mass of n-heptane in the combustible mixture,the equivalence ratio,and the pressure at the top dead center had little effect on the first-stage ignition time of n-heptane. The ignition delay of n-heptane homogeneous charge compression ignition (HCCI) combustion under high levels of carbon dioxide addition was quantitatively measured at elevated pressure from low to intermediate temperatures in a rapid compression machine. The experiments were conducted in the compressed temperature range 613- 750 K.Both the compression ratio and fuel / air equivalence ratio were varied to investigate their effects on the ignition delay of n-heptane. Carbon dioxide was added to study the influence of the carbon dioxide level on the ignition delay of n-heptane under low-temperature conditions. It was found that carbon dioxide had little effects on the two-stages of ignition delay of n-heptane under low-temperature conditions: the concentration of carbon dioxide had little effect on the first-stage ignition time; a certain concentration of carbon dioxide accelerated the first-stage ignition but had a significantly larger impact on the second-stage ignition delay, thus increasing the overall ignition delay time. The results also showed that the first-stage ignition delay of n-heptane is only a function of temperature under low-temperature conditions. The mass of n-heptane in the combustible mixture, the equivalence ratio, and the pressure at the top dead center had little effect on the first-stage ignition time of n-heptane.