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 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 speed was around 2000 m/s. The detonation was induced for temperatures greater than 1000 K during the start of pre-ignition flame propagation. For temperatures between 800 K to 1000 K detonation was exhibited when almost all the fresh gases are consumed by the propagating pre-ignition front. Multiple auto-ignition sites in the end-gas region were observed at higher temperatures. High peak pressures were generated during the 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 propagation mode for the lowest temperature case and developing detonation mode for all other cases considered in this study.