While Homogeneous Charge Compression Ignition (HCCI) is a promising combustion mode with significant advantages in fuel economy improvement and emission reductions for vehicle engines, it is subject to a number of limitations, for example, hardware and control complexity, or NOx and NVH deterioration near its operating upper load boundary, diminishing its advantages. Conventional spark-ignition combustion mode is required for higher loads and speeds, thus the operating conditions near the HCCI boundaries and their corresponding alternatives in Sl mode must be studied carefully in order to identify practical strategies to minimise the impact of the combustion mode transition on the performance of the engine. This paper presents the results of an investigation of the combustion mode transitions between Sl and HCCI, using a combination of an engine cycle simulation code with a chemical kinetics based HCCI combustion code. It investigates and discusses the key issues concerning the combustion mode transition and its control. A new approach has been adopted in the modelling of the HCCI engine gas exchange and combustion, which allowed a parametric study of the transient process in the transition on a cycle-by-cycle basis and provided guidelines in the design of the engine hardware and its control strategies.