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It is known that the levels of hydrocarbon (HC) emissions from Homogeneous Charge Compression Ignition (HCCI) engines are relatively higher than that of Spark-Ignition (SI) engines because of the lower combustion temperature. In order to improve understanding of the mechanisms and products of HCCI combustion in comparison with SI combustion, a quantitative analysis of individual hydrocarbons in the C1 - C6 range emitted in the exhaust gases of gasoline direct injection V6 engine operating in SI and HCCI modes with cam profile switching has been carried out using gas chromatography - mass spectrometry (GCMS) apparatus attached on-line to engine exhaust. In this study, with a GC run time of 20 minutes all aliphatic and olefinic hydrocarbon species in the range C1 to C6 are resolved.

With the high auto ignition temperature of natural gas, various approaches such as high compression ratios and/or intake charge heating are required for auto ignition. Another approach utilizes the trapping of internal residual gas (as used before in gasoline controlled auto ignition engines), to lower the thermal requirements for the auto ignition process in natural gas. In the present work, the achievable engine load range is controlled by the degree of internal trapping of exhaust gas supplemented by intake charge heating. Special valve strategies were used to control the internal retention of exhaust gas. Significant differences in the degree of valve overlap were necessary when compared to gasoline operation at the same speeds and loads, resulting in lower amounts of residual gas observed. The dilution effect of residual gas trapping is hence reduced, resulting in higher NOx emissions for the stoichiometric air/fuel ratio operation as compared to gasoline.

It is well-known that gasoline is a poor fuel for HCCI operation due to its high autoignation temperature, while the major problem for diesel HCCI is that the ignition temperature of diesel fuel is too low so that diesel autoignites too early. Interestingly a blend of gasoline and diesel fuel could have desirable characteristics for HCCI operation. The negative valve overlap (NVO) is a practical and feasible control mode for production applications of the HCCI concept. At present, the most serious problem is the difficulty to control the moment of auto-ignition and extend the limited operating window of smooth HCCI operation. In this paper, the promotive effects of diesel fuel on gasoline HCCI combustion were experimentally examined. The diesel fuel as additive was added in advance in different proportion (10% and 20% by mass) into gasoline for the purpose of improving its ignitability. The experiments conducted on a gasoline HCCI engine which was naturally aspirated and unthrottled.