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Understanding the Chemical Effects of Increased Boost Pressure under HCCI Conditions
Abstract:
One way to increase the load range in an HCCI engine is to increase boost pressure. In this modeling study, we investigate the effect of increased boost pressure on the fuel chemistry in an HCCI engine. Computed results of HCCI combustion are compared to experimental results in a HCCI engine. We examine the influence of boost pressure using a number of different detailed chemical kinetic models - representing both pure compounds (methylcyclohexane, cyclohexane, iso-octane and n-heptane) and multi-component models (primary reference fuel model and gasoline surrogate fuel model). We examine how the model predictions are altered by increased fueling, as well as reaction rate variation, and the inclusion of residuals in our calculations. In this study, we probe the low temperature chemistry (LTC) region and examine the chemistry responsible for the low-temperature heat release (LTHR) for wide ranges of intake boost pressure.
Results of this study are discussed, particularly focusing on the detailed calculations using the well-validated and chemically understood LLNL PRF mechanism. Inclusion of the effect of residuals improved the model's prediction of the onset and phasing of LTHR in PRF80 simulations. For the residual-containing simulations, the LTHR was advanced by the inclusion of residual to the fuel and air charge. This is due to the fact that by including residual in the calculation, the fuel and air charge contains reactive species (partial-burn products) from the onset of the calculation, and these species have the ability to advance the reactivity of the system.