Researchers at KAUST in Saudi Arabia, with colleagues from Saudi Aramco and the National University of Ireland Galway, have developed a new method for investigating the auto-ignition behavior of bioethanol-blended gasolines.

Their work, reported in a paper in the journal Combustion & Flame, explores the oxidation behavior of two oxygenated certification gasoline fuels and the variation of fuel reactivity with molecular composition.

Many advanced combustion engine technologies have been proposed in recent years to achieve higher efficiencies and lower emissions. Most of these technologies revolve around low-temperature combustion (LTC) concepts and include variants such as homogeneous charge compression ignition (HCCI), partially pre-mixed combustion (PPC) and reactivity controlled compression ignition (RCCI). New combustion modes used for advanced combustion engines are mainly controlled by the autoignition of fuel/air mixture. Therefore, fundamental understanding of the chemical kinetics of fuel combustion is essential in the development and optimization of these new engine technologies.

Gasolines, one of the most widely used light duty automotive fuels, are complex mixtures containing hundreds of different chemical compounds such as alkanes, aromatics, naphthenes and olefins. Thus, to develop a detailed reaction mechanism comprising all of the components present in gasoline in order to predict gasoline ignition behavior is intractable. Owing to this complexity, a popular approach to describe the combustion behavior of gasoline is to use a surrogate fuel composed of several reference components.

Various gasoline surrogates, ranging from single to multi-components, have been proposed in the literature. The primary reference fuels (PRFs), which are binary blends of n- heptane and iso-octane, are widely adopted, given their relatively well-validated chemistry. However, previous studies have shown that PRFs may not be suitable surrogates over wide range of engine operating conditions.

… The objectives of this study are twofold. First, we aim to investigate the autoignition characteristics of two oxygenated gasoline fuels over a wide range of pressure (10-40 bar), temperatures (650– 1250K) and equivalence ratios in a shock tube and a rapid compression machine. Second, we compare the ignition delay times of gasolines with ternary, quaternary and multi-component surrogates to suggest suitable surrogates for ethanol-containing gasolines. A chemical kinetic model is developed to simulate ignition delay times of the multi-component surrogate and to assess the fidelity of the surrogate model in capturing the auto-ignition behavior of gasoline.

—Lee et al.

Gasoline anti-knock quality, defined by the research and motor octane numbers (RON and MON), is important for increasing spark ignition (SI) engine efficiency. Gasoline knock resistance can be increased using a number of blending components. For over two decades, ethanol has become a popular anti-knock blending agent with gasoline fuels due to its production from bio-derived resources.

An approach to modeling the combustion characteristics of gasoline blended with biofuels provides valuable insights into the combustion and potential of this fuel combination.

As we move toward new engine technologies, fuel characteristics play a very important role in optimizing engine efficiency and minimizing emissions,” explains Farooq. Our aim was to produce a model for simulating fuel-engine interactions for fuels containing large fractions of bioethanol.

—Aamir Farooq, KAUST, corresponding author

The researchers first prepared two high-octane gasolines blended with different amounts of bioethanol and observed the autoignition behavior of the fuels over a wide range of pressures, temperatures and air-to-fuel ratios. At high temperatures, the team used a high-pressure shock tube to observe the reaction. But combustion takes longer at intermediate and low temperatures, so the team used a rapid compression machine to observe reactivity.

Because gasolines are complex fuels containing hundreds of different chemical compounds, the team also used three types of surrogate fuels consisting of three, four and eight components, which allowed them to simulate the combustion of the bioethanol-blended gasolines and model their ignition delay times.

Among the findings of the study was that more complex surrogate mixtures are needed to emulate the reactivity of gasoline with higher octane sensitivity.

The simplest surrogate, TPRF (a mixture of n-heptane, iso-octane and toluene) is able to adequately capture the reactivity trends of the Haltermann gasoline. However, the four-component surrogate (n- heptane/iso-octane/toluene/ethanol) performs slightly better at low temperatures. For the higher sensitivity Coryton gasoline, the simple TPRF surrogate is too non-reactive due to the very high fraction of toluene which is needed to match the MON requirement of the Coryton gasoline. The four-component and eight-component surrogate simulations better reproduce the reactivity of the Coryton gasoline. For either gasoline, one key advantage of the multi-component surrogates is that these are able to better capture a large set of physical and chemical characteristics of the real fuels.

—Lee et al.

Despite the different rate of evolution of chemical species during combustion, the researchers found that at high temperatures the blended fuels exhibited similar autoignition characteristics, but at intermediate temperatures, the fuel with higher octane rating and higher ethanol content exhibited longer ignition delay times.

The researchers plan to explore the ignition characteristics of two more biofuels—methanol and dimethyl ether—after blending with regular gasoline and diesel, said Farooq.

The research was funded by Saudi Aramco under the FUELCOM program and by King Abdullah University of Science and Technology (KAUST).

Resources

• Lee, C., Ahmed, A., Nasir, E.F., Badra, J., Kalghatgi, G. Sarathy, S.M., Curran, H. & Farooq, A. (2017) “Autoignition characteristics of oxygenated gasolines” Combustion and Flame 186, 114-128 doi: 10.1016/j.combustflame.2017.07.034