Understanding the Oxidation Behavior of Automotive Liquefied Petroleum Gas Fuels: Experimental and Kinetic Analyses

Liquefied petroleum gas (LPG) is a low-carbon fuel with an existing fuel supply infrastructure. As compared to petroleum-based gasoline, it features a higher octane rating. As compared to port fuel injection (PFI) systems, the direct injection (DI) of LPG engines reveals significant advantages in modern spark-ignition, such as higher efficiency. LPG primarily consists of C₃ and C₄ hydrocarbons, but the composition can drastically vary according to the current European LPG fuel standard EN 589. Several studies have focused already on understanding the oxidation process of its primary components. In this study, the focus will be on the autoignition behavior of different LPG compositions. Thereto, four different LPG fuels according to the current European LPG fuel standard EN 589 have been investigated. They cover a wide range of compositions and thus different autoignition behaviors. The fuels involve an LPG with a maximum propene/propane content, a typical winter-grade LPG with propane/n-butane/isobutane content, a high propane content, and high n-butane/isobutane content. These fuels also contain minor fragments of C₂ and other C₄ hydrocarbons. A rapid compression machine (RCM) has been used in this study to measure ignition delay times primarily in the low-to-intermediate temperature regime at stoichiometric conditions with a final compression pressure of 20 bar. Zero-dimensional simulations, including the facility effects of the RCM, have been performed with the help of detailed chemical kinetic mechanisms reported in the literature. The Aramco Mech 3.0 mechanism was chosen on the basis of its ability to represent the experimental data investigated in this study and additionally on the basis of the criteria that the major species in the mechanism are available and validated at application-relevant conditions. The mechanism is further used to understand the oxidation behavior of the fuel. Sensitivity analyses with the selected mechanism at application-relevant conditions were performed for the different LPG mixtures to reveal the most sensitive reactions, which affect the reactivity of the fuel. Similarly, to monitor the rate of production and consumption of species at experimental conditions of interest, flux analyses were performed at the point where 20% of the fuel is consumed. From the performed kinetic analyses, it is observed that the production of HȮ₂ radicals by the subchemistry of the primary fuel component is consumed by propene subchemistry, leading to more ȮH radical production, which controls the global reactivity in the investigated regime.