A Computational Study of Abnormal Combustion Characteristics in Spark Ignition Engines

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Super-knock that occurs in spark ignition (SI) engines is investigated using two-dimensional (2D) numerical simulations. The temperature, pressure, velocity, and mixture distributions are obtained and mapped from a top dead center (TDC) slice of full-cycle three-dimensional (3D) engine simulations. Ignition is triggered at one end of the cylinder and a hot spot of known temperature was used to initiate a pre-ignition front to study super-knock. The computational fluid dynamics code CONVERGE was used for the simulations. A minimum grid size of 25 μm was employed to capture the shock wave and detonation inside the domain. The Reynolds-averaged Navier-Stokes (RANS) method was employed to represent the turbulent flow and gas-phase combustion chemistry was represented using a reduced chemical kinetic mechanism for primary reference fuels. A multi-zone model, based on a well-stirred reactor assumption, was used to solve the reaction terms. Hot spots introduced inside the domain at various initial temperatures initiated a pre-ignition front, which resulted in super-knock due to detonation of the end gas. The detonation was induced for temperatures greater than 1000 K during the start of pre-ignition flame propagation. The detonation speed was around 2000 m/s, at temperatures higher than 1000 K. For temperatures between 800 K and 1000 K, detonation was observed near the end of combustion. The laminar pre-ignition flame front speed calculated from the simulations was an order of magnitude higher than the one-dimensional laminar flame speed, which is characteristic of sequential auto-ignition. Multiple auto-ignition sites in the end-gas region were observed at higher temperatures. The auto-ignition location initiated an auto-ignition front of higher velocity that later transitioned into detonation. Interaction between the detonating fronts generated local pressure peaks inside the domain. End-gas reactivity was characterized by the formation of formaldehyde (CH2O) and was an indicator for occurrence of auto-ignition/detonation. Negative temperature coefficient regimes (750 K-850 K) exhibited higher mass fraction of CH2O indicating enhanced reactivity of end gas, leading to highest peak pressures during detonation onset. The low-temperature case, 700 K, exhibited a deflagration mode of flame propagation without detonation development. The results were analyzed and reported by comparison with Bradley diagram, which predicted a deflagration mode of combustion for the lowest temperature case, and developing a detonation mode for all other cases considered in this study.
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DOI
https://doi.org/10.4271/2018-01-0179
Pages
14
Citation
Mubarak Ali, M., Hernandez Perez, F., Sow, A., and Im, H., "A Computational Study of Abnormal Combustion Characteristics in Spark Ignition Engines," SAE Int. J. Engines 11(6):743-755, 2018, https://doi.org/10.4271/2018-01-0179.
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Published
Apr 3, 2018
Product Code
2018-01-0179
Content Type
Journal Article
Language
English