Abstract:

This study investigated the auto-ignition and knock behavior of stoichiometric n-heptane/air mixtures under spark-ignition conditions using an optical rapid compression machine (RCM) to reveal the evolution characteristics of knock induced by end-gas auto-ignition of highly reactive fuels and the factors that affect knock intensity. High-speed photography synchronized with high-frequency pressure acquisition was employed to identify the knock characteristics of n-heptane. In addition, zero-dimensional chemical kinetic calculations were conducted to analyze the thermodynamic trajectory of the end-gas during auto-ignition. The results show that the knock intensity of n-heptane is positively correlated with the initial energy density. At the same initial temperature, increasing the initial pressure enhances the knock intensity; the knock intensity decreases with increasing spark-ignited/auto-ignited consumption ratio, and the sensitivity of knock intensity to this ratio becomes stronger at higher initial temperatures. At the same initial pressure, increasing the initial temperature advances auto-ignition but weakens the pressure oscillation; as the initial temperature increases from 615 K to 685 K, the temperature rise experienced by the end-gas before auto-ignition decreases from approximately 135 K to 55 K, and the combustion process shifts from being significantly affected by flame propagation to being dominated by auto-ignition. The results indicate that the high reactivity of n-heptane causes the end-gas to auto-ignite before further compression by flame propagation, making its thermodynamic trajectory at the auto-ignition timing difficult to enter the negative temperature coefficient (NTC) region. Therefore, knock analysis of highly reactive fuels should comprehensively consider energy density and the relative contribution of flame propagation and auto-ignition.

Key words: rapid compression machine (RCM), detonation, knock intensity, n-heptane, mixture auto-ignition