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

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/n-heptane blend, a toluene/n-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 °C). The other operating condition has a high intake temperature
(100 °C) 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.