Propanal oxidation at engine-relevant conditions: Autoignition and species evolution in an RCM

Abstract

Understanding the oxidation of propanal, a toxic carbonyl compound formed during the oxidation of bio-derived fuels and hydrocarbons, is critical for emission control in combustion engines. Prior low-temperature jet-stirred reactor (JSR) studies were limited to either low-pressure conditions with moderate fuel loadings, or higher pressures at highly diluted conditions. To bridge the gap between JSR and engine operation, ignition delay times (IDTs) were measured in a rapid compression machine (RCM) over a wide range of conditions (10-40 bar, 609-957 K, and $\varphi$ = 0.5-1.5), alongside simultaneous measurements of species concentrations using a single-valve gas sampling system and Gas Chromatography-Mass Spectrometry techniques. The propanal subset of the C3MechV4.0.1 model was revised using rate analogies to propane and our recent acetaldehyde mechanism. The latter contains fuel-specific radical reactions that were absent in the original model. The revision also incorporated literature-based rate refinements and thermochemistry updates. Our kinetic analysis indicates that, at low-temperature RCM conditions, propanal oxidation is governed by fuel-specific peroxyl pathways and alkylperoxyl reactions. At higher temperatures or lower oxygen concentrations, the former channels are disfavored, while alkylperoxyl pathways compete with concerted elimination steps and reactions with hydroperoxyl radicals. Literature models significantly underpredict low-temperature reactivity at RCM conditions, including those performing reasonably well against JSR targets. These discrepancies arise from missing fuel-specific peroxyl pathways or an underestimation of alkylperoxyl chemistry. The updated mechanism reproduces most measured targets and generally outperforms the original model against literature data. Remaining discrepancies in the medium-temperature RCM regime and at low-temperature, highly-diluted JSR conditions highlight fundamental knowledge gaps in alkylperoxyl and fuel radical chemistry. This work provides a basis for understanding propanal oxidation at engine-relevant conditions.

Novelty and significance statement: This work presents the first investigation of low-temperature propanal oxidation at both high pressure and high reactant concentrations using a rapid compression machine, providing IDTs and simultaneous species measurements at engine-relevant conditions. These data extend prior low-temperature targets that were limited to either low pressures (Zhang et al., 2019) or low fuel loadings (Veloo et al., 2013). A systematic reassessment of propanal oxidation chemistry using rate analogies to propane and our recently developed acetaldehyde mechanism (Caravaca-Vilchez et al., 2025), together with refinements constrained by the new data, resulted in a mechanism with improved predictive performance compared to existing models.

This study contributes to the advancement of the mechanistic understanding of propanal at practical combustion conditions by resolving the competition between fuel-specific and alkylperoxyl pathways at high pressures. As a key intermediate in the oxidation of bio-derived fuels and hydrocarbons, these findings support improved modeling of toxic carbonyl emissions and reveal limitations in alkylperoxyl and carbonyl-containing radical kinetics.