Acetaldehyde reactivity at engine-relevant conditions: An experimental and kinetic modeling study

Understanding the combustion chemistry of acetaldehyde, a carcinogenic by-product formed during the low-temperature oxidation of various hydrocarbons, is essential for reducing harmful emissions in engines. Previous acetaldehyde experimental works have largely focused on low-pressure conditions, with a few exceptions. Some studies report a clear negative temperature coefficient (NTC) behavior for acetaldehyde and highlight the need for further low-temperature, high-pressure experiments to fully characterize it. In this context, acetaldehyde ignition delay times were measured using a rapid compression machine and a shock tube over a wide range of conditions (580–1410 K, 10–40 bar, and equivalence ratios of 0.5–1.5), significantly extending the very limited IDT data available in the literature at 10 bar. At low temperatures, the most comprehensive kinetic models of acetaldehyde greatly underestimate its reactivity, even those that show reasonable performance for flow reactor species measurements from the literature in the same temperature regime. At high temperatures, model predictions were generally in better agreement with the measured data. To improve prediction accuracy, refinements were made within GalwayMech1.0 model, incorporating recently calculated thermochemistry from the literature and modified reaction rate parameters based on direct analogies and literature information. The resulting chemistry revealed that the acetyl peroxy radical is the primary driver of low-temperature reactivity at high pressures through a closed-loop fuel consumption pathway. Further adjustments in the peroxyl radicals chemistry, which is less relevant under low-pressure conditions, successfully separate first-stage and main ignition in the NTC region. At high temperatures, revised H-atom abstraction by $\dot{H}$ and O${}_2$ rates improved high-temperature predictions. Overall, the proposed model outperforms existing mechanisms over a wide range of conditions, but retains uncertainties in the formation of a few minor intermediates. This work highlights the importance of using high-pressure validation targets for comprehensive kinetic modeling and provides a solid foundation for future studies on acetaldehyde oxidation.