Kinetic Modeling of the Influence of NO on the Combustion Phasing of Gasoline Surrogate Fuels in an HCCI Engine

摘要

In this work, a chemical kinetic model describing the nitric oxide (NO) sensitization effect for the oxidation of toluene reference fuels has been developed. The influence of NO on combustion phasing in a homogeneous charge compression ignition (HCCI) engine has then been studied by kinetic modeling and compared to experiments. An iso-octane-heptane blend, a toluene-heptane mixture, and a full boiling range gasoline using a three-component surrogate fuel consisting of 55% iso-octane, 23% toluene, and 22% n-heptane by liquid volume have been simulated under two different operating conditions with NO concentrations from 4 up to 476 ppm. All three fuels have the same research octane number of 84. The first operating condition has a high intake pressure (2 bar absolute) and low intake temperature (40 ℃). The other operating condition has a high intake temperature (100 ℃) and atmospheric intake pressure. The model predicts in accordance with experimental observations that, at high intake pressure in the primary reference fuels (PRF) case, the ignition delay is retarded beyond the baseline case (absence of NO in intake) for a high concentration of NO, while for toluene reference fuels (TRF) and the full boiling range gasoline, the combustion phasing is advanced with an increasing NO concentration. The reason for the differences between TRF and PRF fuels is that the promoting effect of toluene is stronger than the one for iso-octane when the NO concentration is increased. This is further explained in terms of reaction kinetics. For PRF, there is a net production of HONO (nitrous acid), which is chain-terminating, whereas for TRF, there is a net consumption of HONO, which is chain-branching. Calculations of fuel sensitivity on the ignition delay time for a gasoline surrogate fuel indicate that it is possible to control the combustion phasing in a gasoline HCCI engine by simultaneously varying the amount of NO (EGR) and the fraction of aromatics and iso-paraffins.