Promotive Effect of Diesel Fuel on Gasoline HCCI Engine Operated with Negative Valve Overlap (NVO) 2006-01-0633
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. The engine operated at a constant speed of 1500 rpm and the HCCI controlled using NVO (Negative Valve Overlap), without using external Exhaust Gas Recirculation (EGR). The experimental results showed that the addition of diesel fuel can be utilized to reduce the required negative valve overlap interval for suitable HCCI combustion under a given engine speed and compression ratio conditions. The HCCI combustion region was expanded dramatically without substantive increase in NOx emissions under a given inlet and exhaust valve timing due to the improvement of charge ignitability resulting from the addition of diesel fuel.
In addition, it was also found that due to the addition of diesel fuel, the possible scale of negative valve overlap was extended. A substantial increase in the lean limit of excess air ratio and the upper limit of load range can be achieved without auxiliary intake heating because of higher volumetric efficiency resulting from decreased required negative valve overlap and the presence of less residual gases in cylinder.