The motivation of this modeling study was to understand the effect of increased boost pressure on the fuel chemistry in an HCCI engine. In this work we examined 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). Homogeneous Charge Compression Ignition (HCCI) technology offers major advancement in high efficiency, low emission engines. This technology is highly desirable, since it combines the high fuel efficiency of Diesel or compression ignition (CI) engines with the less polluting abilities of spark-ignition engines. As in spark ignition engines (SI), the charge is well mixed which minimizes particulate emissions, and as in CI engines, it is compression ignited leading to high fuel efficiency. In HCCI, the fuel is diluted with air or exhaust gases to reduce the peak charge temperature and consequently, NOx emissions. Premixing of the fuel and air is usually achieved by injecting the fuel early in the intake stroke. HCCI can operate using a variety of fuels - gasoline, diesel, natural gas, bio-fuels and hydrogen. Ignition in the HCCI engine occurs nearly simultaneously when the homogeneous fuel and ‘air' mixture is compressed - near top dead centre (TDC). The ignition is controlled primarily by chemical ignition kinetics, with little influence from effects such as turbulence and mixing. Several technical difficulties with HCCI exist, and need to be resolved before HCCI can be implemented in production engines. Reducing the amount of unburned hydrocarbons emitted, expanding the load range of the engine and controlling the phasing of HCCI combustion are current problem areas. HCCI combustion is difficult to manage since it is achieved by controlling the temperature, pressure, exhaust gas recirculation and composition of the fuel and air mixture so that it spontaneously ignites close to top dead centre so the maximum engine efficiency is realized. HCCI engines have been shown to operate well at low to medium loads however, due to this difficulty in controlling the autoignition event, obstacles have been encountered at high load conditions, at cold starts and also during transient operating conditions. Controlling the combustion phasing and limiting the heat-release rate (HRR) at high loads continue to be major issues. One way to increase the load range is to increase boost pressure. In this work, we are concerned with the simulation of an HCCI research engine, previously described by Sjöberg and Dec in many papers. The aim is to probe the low temperature chemistry (LTC) region and examine the chemistry responsible for this effect under the conditions ranging up to and including increased boost pressure. We have performed calculations for number of pure fuel compounds, n-heptane, iso-octane, methylcyclohexane, and cyclohexane, as well as a surrogate gasoline mixture and a primary reference fuel (PRF) mechanism. We have examined the effect of increased fueling, as well as reaction rate variation and the inclusion of residuals in our calculations in an effort to understand how the fuel chemistry is affected by increased boost pressure. Results of this study will be presented and discussed, with particular relevance to the detailed calculations performed using the well validated and chemically understood LLNL PRF mechanism.

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