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

Abstract

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 C 3 and C 4 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 C 2 and other C 4 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 HO ̇2 radicals by the subchemistry of the primary fuel component is consumed by propene subchemistry, leading to more O ̇H radical production, which controls the global reactivity in the investigated regime.

A chemical kinetic perspective on the low-temperature oxidation of propane/propene mixtures through experiments and kinetic analyses

“…The primary goal of this study is to develop a consistent mechanism that can represent the auto-ignition behavior of propane, propene, and their mixtures over a wide range of temperature, pressure, mixture composition, and dilution. The mechanism development is a continually evolving process, and the experimental work established in this work and from previous studies [9,10] has contributed to the development of NUIGMech1.0. The study on automotive LPG fuels [9] and the previous study on propane [10] showed that mechanisms available in the literature do not capture the reactivity for the neat components and their mixtures in a consistent way.…”

“…The mechanism development is a continually evolving process, and the experimental work established in this work and from previous studies [9,10] has contributed to the development of NUIGMech1.0. The study on automotive LPG fuels [9] and the previous study on propane [10] showed that mechanisms available in the literature do not capture the reactivity for the neat components and their mixtures in a consistent way. In this study, the experimental work on propene is extended to higher pressures (80 bar) for stoichiometric mixtures in a rapid compression machine (RCM).…”

“…In the introduction, we indicated that the goal of this work is to obtain a consistent mechanism that can predict both the neat components of propane and propene, along with their mixtures in the investigated regime. For this purpose, many mechanisms from the literature were considered, and simulations against the high-pressure IDT experiments for propane [10] and four different automotive LPG mixtures [9] were performed and analyzed. At the same time, the recent study by Burke et al [13,14] investigated the oxidation of propene for a wide range of experimental conditions using different reactors and established a mechanism that represents the oxidation of propene well.…”

“…Propene and propane are significant components of LPG [6][7][8][9], and they also serve as representative species for larger alkanes and alkenes. Therefore, investigating these species and their mixtures can provide new insights into their oxidation behavior and lead to a mechanism that can capture the auto-ignition behavior of LPG fuels containing significant concentrations of propane and propene.…”

“…Burke et al [13,14] also developed a detailed kinetic mechanism describing propene oxidation; however, there is a quantitative gap in the ability of the mechanism to replicate experimental data for propane and for propane/propene mixtures. Apart from a recent study by Ramalingam et al [9] of different automotive LPG fuels, there is still a great void in experimental data for propane/propene mixtures. Figure 1 shows the pressure-temperature plot for propene experiments that are available in the literature from different facilities [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”