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Lean combustion is an approach to achieving higher thermal efficiency for spark ignition engines. However, it faces low burning velocity and unstable combustion problems near the lean flammability limits region. The current work is attempting to investigate the combustion characteristics of iso-octane flame with 0% and 30% H2 up to near lean limits (λ = 1.7) at 100-300 kPa and 393-453 K. The flame appeared spherically by 37 mJ spark energy at λ = 0.8-1.2, whereas the ultra-lean mixtures, λ ≥ 1.3, ignited at 3000 mJ under wrinkles and buoyancy effects. The impact of initial pressure and temperature on the lean mixture was stronger than the stoichiometric mixture regarding flame radius and diffusional-thermal instability. The buoyancy appeared at the highest burning velocity of 27.41 cm/s.

Soot and carbon dioxide released from internal combustion engines became the key issues when using fossil fuels. The use of zero-carbon fuel, ammonia, with hydrocarbon fuels may play an important role in reducing the exhaust effect on the environment and mitigating the reliance on nonrenewable energy resources. However, ammonia reduces the flame speed of hydrocarbon fuels. A numerical approach was executed to study the ammonia impact on n-heptane, a diesel surrogate, flame. A kinetic mechanism was prepared by adding the sub-mechanism of ammonia, NO2 and NO3 emissions, and soot precursors to the n-heptane kinetic mechanism. The modified Arrhenius equation and soot surface reactions were used to study the soot formation with NOx emissions. The results showed that ammonia decreased the fractions of carbon-related species and raised the concentration of non-carbon-related species.

Soot and carbon dioxide released from internal combustion engines became the key issues when using fossil fuels. Ammonia and hydrogen having zero-carbon species can reduce carbon-related emissions and enhance the reliance on renewable fuels. A comparative study of ammonia and hydrogen impact on combustion and emission characteristics of iso-octane flame was performed under different combustion conditions. Arrhenius equation, soot surface reactions, and modified kinetic mechanism were used to study the flame growth, soot nucleation, and surface growth rates. The results show that hydrogen increased the temperature about 20.74 K and 59.30 K, whereas ammonia reduced it about 82.17 K and 66.03 K at premixed and counterflow conditions, respectively. The flame speed of iso-octane was increased 43.83 cm/s by hydrogen and decreased 34.36 cm/s by ammonia. A reduction in CH2O caused a reduction in CO and CO2 emissions.