A Computational Study of Abnormal Combustion Characteristics in Spark Ignition Engines 2018-01-0179

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.