Pre-chamber jet ignition is a promising technology for spark ignition engines. In this paper, classical pre-chamber jet ignition and a new pre-chamber jet ignition method named flame accelerated ignition are investigated. Utilizing the connecting nozzles to generate the jets is the classical form of pre-chamber jet ignition. Two combustion modes were found by the RCM experiments with optical method: double stage combustion and single stage combustion. Double stage combustion mode takes place in the condition with relatively small nozzle dimension, showing long ignition delay and extremely short combustion duration. The jets cannot ignite the mixture directly. Instead, ignition happens at a central position in the main chamber after a lag time followed by the rapid development of the flame with similar speed in each direction. However, the double stage combustion mode has poor combustion stability due to the high randomness of the ignition process inside the main chamber. With single stage combustion mode, the ignition delay and the combustion duration can be shortened simultaneously with satisfying combustion stability. The combusti ... mehron processes inside the pre-chamber and the main chamber take place continuously. The flame jet develops from the nozzle, composed of thin fire near the nozzle and approximately conical fire in the tip. The speed of flame jet exceeds 15 times than that of conventional flame propagation. According to the concept of pre-chamber jet ignition and the phenomenon of flame acceleration in tunnel, a new ignition method named flame accelerated ignition (FAI) is proposed. The flame acceleration tunnel can be regarded as a pre-chamber, where flame acceleration happens. Then the combustion in the main-chamber can be induced by the flame jet rushed out of the tunnel. The RCM experiments indicated that the combustion could be evidently enhanced by FAI. The flame jet maintains nearly cylindrical with favorable speed characteristic. Higher indicated thermal efficiency is gained by applying FAI compared to conventional spark ignition in the natural gas engine. In addition, introducing residual gas cavity into the flame acceleration tunnel could expand the misfire limit and improve the defect of FAI mode in lean-burn conditions.