A patented VCC (variable combustion chamber) piston mechanism is presented, by which excess in-cylinder pressure would be able to be limited effectively based on each working cycle while a spark-ignition engine running with higher CR (compression ratio) of 12.0:1. A conventional engine can be converted to a VCC engine just by replacing its typical pistons with VCC pistons.Besides the benefits that VCR (Variable Compression Ratio) has been still pursued so far, there would be other advantages for VCC engine, such as excellent fuel economy at each load not only at light loads, and the improvement of cycle-by-cycle variation of in-cylinder pressure, and high reliability with simple structure.The innovative design of VCC piston is introduced. The main design features of VCC piston are a VCC mechanism assembled between the piston crown and the piston skirt, and a special reset cam assembled at the wrist-pin end of the connecting rod. While VCC piston works during combustion, it behaves somehow as a typical valve-cam does.There are several differences between VCC technology and VCR technology, including the control modes of in-cylinder pressure, and the effects on the engine behaviors and the simulations of the engine working process etc. Based on the software of AVL Boost and the software of ADAMS, the coupled simulation of both Boost and ADAMS is conducted to validate the technical feasibilities of VCC engine.The results from the coupled simulation indicate that VCC piston technology could meet the demands of vehicle engines at various different working conditions including transient loads and speeds, and VCC piston would be one of the best substitutes for VCR technology due to its simple structures with fast transient response, and the fuel economy of VCC engine is increased up to 8%-10% at heavy load and the fuel economy is increased up to 25% at light loads with CR 12:1 from CR 9.5:1. Finally, cycle-by-cycle variation of in-cylinder pressure would be decreased by means of VCC piston.