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KAUST team develops computational model for simulating soot production in gasoline direct injection engines

Researchers at KAUST have developed a computational model capable of simulating soot production inside gasoline direct injection (GDI) engines.

Although today’s passenger vehicle engines are cleaner and more fuel efficient, GDI exhaust can still contain significant numbers of nanoscopic soot particles that are small enough to penetrate the lungs and bloodstream. This new computer model could help car makers improve their engines to cut soot formation.

Particulate matter emissions are presently a concern in gasoline fueled direct-injection spark-ignition (DISI) engines. To better understand the combustion behavior of refinery gasoline fuels and their emission characteristics, surrogate fuels are typically adopted for the development of predictive tools. Gasoline contains straight and branched-chain aliphatics, aromatics, alkenes, and cycloalkanes. It has been shown that toluene primary reference fuels (TPRFs), the ternary mixtures of n-heptane, iso-octane, and toluene can match the ignition quality of gasoline fuels. The presence of aromatics in gasoline fuels and their surrogates enhances the formation of polycyclic aromatic hydrocarbon (PAHs) and soot, which may cause adverse health and environmental problems; however, fundamental studies on the formation of PAHs and soot from real fuel combustion are rather limited. In particular, direct fuel injection strategies aimed at enhancing fuel efficiency in gasoline engines often result in increased emission of smaller sized particulate matters, which are being regulated under new stringent emission laws.

An accurate prediction of PAH concentrations, which are molecular precursors for soot formation and growth, is required to understand the effects of gasoline fuel composition on sooting characteristics, such as soot volume fraction, number density, and particle size distribution.

… the purpose of the present study is set to expand the experimental data available for gasoline surrogate fuels in diffusion flames to better understand the effects of fuel blending on PAH formation, with a special focus on TPRF mixtures. Furthermore, an updated detailed chemical kinetic model for TPRF gasoline surrogate mixtures was also developed, with an emphasis on the formation of PAHs, to simulate and elucidate the experimental observations.

—Park et al.

S. Mani Sarathy from the KAUST Clean Combustion Center, and his coworkers tackled the problem by burning chemically simplified “gasoline surrogate” mixtures comprising n-heptane, iso-octane, and toluene in an experimental setup called a counterflow diffusion flame.

By shining lasers into this open flame, they could monitor soot and its precursors as the fuel burns. Such experiments have been done previously with gaseous fuels, but this is the first time they have been done with gasoline-relevant liquid fuels, Sarathy said.

The team varied the composition of the fuel and observed particle production to build a model of the basic chemical reactions through which soot particles form and grow.

Once we have this basic kinetic model that works well in simple flames, we can utilize the model in an engine simulation.

—S. Mani Sarathy

An engine combustion simulation is essentially an ensemble of many tiny flamelets, which are combined to give a complete picture of how soot is formed in an engine.

The model accurately captures the synergistic PAH formation characteristics observed experimentally for n-heptane/toluene and iso-octane/toluene binary mixtures. Furthermore, the present experimental and modeling results also elucidated different trends in the formation of larger PAHs and soot between binary n-heptane/iso-octane and ternary n-heptane/iso-octane/toluene mixtures.

They found that propargyl radicals (C3H3) were important in the formation and growth of PAHs for n-heptane/iso-octane mixtures when the iso-octane concentration increased; however, reactions involving benzyl radicals (C6H5CH2) played a significant role in the formation of PAHs for n-heptane/iso-octane/toluene mixtures. These results indicated that the formation of PAHs and subsequently soot was strongly affected by the composition of gasoline surrogate mixtures.

Car makers could use Sarathy’s model in their own simulations to test whether changes, such as altering engine geometry or the timing of fuel injection, might cut soot production.

We also have industrial partners that utilize the model to see how different fuels and engine combustion strategies affect soot production.

—S. Mani Sarathy

Resources

  • Sungwoo Park, Yu Wang, Suk Ho Chung, S. Mani Sarathy (2017) “Compositional effects on PAH and soot formation in counterflow diffusion flames of gasoline surrogate fuels,” Combustion and Flame, Volume 178, Pages 46-60 doi: 10.1016/j.combustflame.2017.01.001

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