The objective of this study is to develop a practical technique to achieve HCCI operation with wide operation range. To attain this objective, the authors previously proposed the blowdown supercharge (BDSC) system and demonstrated the potential of the BDSC system to extend the high load HCCI operational limit. In this study, experimental works were conducted with focusing on improvement of combustion stability at low load operation and the reduction in cylinder to cylinder variation in ignition timing of multi-cylinder HCCI operation using the BDSC system. The experiments were conducted using a slightly modified production four-cylinder gasoline engine with compression ratio of about 12 at constant engine speed of 1500 rpm. The test fuel used was commercial gasoline which has RON of 91.To improve combustion stability at low load operation, the valve actuation strategy for the BDSC system was newly proposed and experimentally examined. With the newly proposed valve actuation strategy, only intake valve lift was varied and EGR and exhaust valve lifts were fixed with variation in target load. Compared to the previously proposed valve actuation strategy for the BDSC system, relatively late EGR valve opening timing with early intake valve closing timing is applied at low load condition in the proposed strategy. This provides high in-cylinder temperature and relatively rich fuel concentration at low load operation while providing a large amount of diluted mixture at high load operation. Additionally, effect of coolant water temperature on combustion stability was experimentally investigated. Coolant water temperature was varied from 85°C to 105°C. Experimental result showed that for low load HCCI operation, the valve timing with early intake valve closing and relatively late EGR valve opening was effective to increase combustion stability due to increases in in-cylinder temperature and fuel concentration. With early intake valve closing and relatively late EGR valve opening, a stable HCCI operation at net IMEP of 200 kPa was achieved. However, brake thermal efficiency was slightly deteriorated due to increase in pumping work utilized to increase in in-cylinder temperature. The increase in coolant water temperature also improved combustion stability and extends the low load operational limit due to increase in in-cylinder temperature. With a cooling water temperature of 105°C, the low load HCCI with net IMEP of 134 kPa was attained.In addition, to reduce the cylinder to cylinder variation in ignition timing which restricts HCCI operating range with multi-cylinder operation, the secondary air injection system in which air is injected into an exhaust port of each cylinder to control recharged EGR gas temperature using gas fuel injector was proposed and examined to control ignition timing of each cylinder independently. As a result, the cylinder to cylinder variation in ignition timing was successfully reduced and the high load HCCI operational limit with four-cylinder operation was extended up to net IMEP of 570 kPa. Compared to conventional SI operation, brake thermal efficiency was improved by about 15% using the newly proposed strategy with more than 99% reduction in NOx emission.