Environmental, Thermodynamic and Chemical Factor Effects on Heptane- and CNG-fuelled HCCI Combustion with Various Mixture Compositions 2008-01-0038

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.

This study uses a numerical model confirmed by engine experiments to study the effects of base fuel chemistry, reformer gas (RG) blending, external (cool) EGR and internal (hot) EGR on HCCI combustion of both high- and low- octane fuels. Adjusting these parameters significantly affects HCCI combustion as an integrated result of changed in-cylinder conditions and the changes in mixture thermodynamic and chemical properties. (Thermodynamic properties are defined as those which affect temperature during compression, while chemical properties are those which affecting chemical reaction rate at a given temperature). This study adjusts mixture composition by various means, (fuel blending, equivalence ratio and exhaust gas recirculation ratio), and examines the sensitivity of combustion due to the individual and combined effects of thermodynamic, chemical and in-cylinder environment changes.

The results illustrate differences in how HCCI combustion is affected by various mixture composition adjustments. Changing in-cylinder conditions provide the greatest effect on HCCI combustion characteristic when the amount of fuel contents changes. Chemical and thermodynamic changes are more responsible for combustion behavior when the mixture fuel blend is altered by RG blending additives. (In this study, RG used was a simulated 75%H₂-25%CO RG). The predominance of thermal or chemical effects depends on the base fuel auto-ignition properties. Finally, the level of external EGR mostly affects compression temperature rise through diluted mixture strength, (chemical effect). However, with internal EGR, the environmental effects of higher pre-compression temperature override all others, advancing SOC.