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Homogeneous Charge Compression Ignition (HCCI) engines offer high fuel efficiency and some emissions benefits. However, it is difficult to control and stabilize combustion over a sufficient operating range because the critical compression ratio and intake temperature at which HCCI combustion can be achieved varies with operating conditions such as speed and load as well as with fuel octane number. Replacing part of the base fuel with reformer gas, (which can be produced from the base hydrocarbon fuel), alters HCCI combustion characteristics in varying ways depending on the replacement fraction and the base fuel auto-ignition characteristics. Injecting a blend of reformer gas and base fuel offers a potential HCCI combustion control mechanism because fuel injection quantities and ratios can be altered on a cycle-by-cycle basis.

Homogeneous Charge Compression Ignition (HCCI) combustion offers high fuel efficiency and some emissions benefits. However, it is difficult to control and stabilize combustion over a significant operating range because the critical compression ratio and intake temperature at which HCCI combustion can be achieved vary with operating conditions such as speed and load as well as with fuel octane number. Replacing part of the base fuel with reformer gas, (which can be produced from the base hydrocarbon fuel), alters HCCI combustion characteristics in varying ways depending on the replacement fraction and the base fuel auto-ignition characteristics. Because fuel injection quantities and ratios can be altered on a cycle-by-cycle basis during operation, injecting a variable blend of reformer gas and base fuel offers a potential HCCI combustion control mechanism.

At certain operating conditions, Homogeneous Charge Compression Ignition (HCCI) can provide ultra-low NOx emissions with good combustion efficiency. However, using HCCI operating modes in a SI-based engine still requires some means to control HCCI ignition over a range of operating conditions. Amongst various possible control techniques, altering fuel ignition quality by blending a reformer gas mixture with base fuel is attractive, primarily because of the capability to alter fuel injection ratios on a cycle-by-cycle basis. As well as fuel blending, the mixture composition is defined by equivalence ratio (ϕ) and exhaust gas recirculation (EGR) ratio. The effects of changing such parameters have been widely studied both experimentally and with models. However, adjusting any variable has multiple effects on the mixture's thermodynamic and chemical properties so a detailed understanding of how these variables affect combustion is generally difficult to achieve.