Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion engine in which, well mixed fuel and oxidizer (typically air) are compressed to the point of auto ignition.
HCCI has the characteristics of a gasoline engine and diesel engine. The gasoline engine combines homogeneous charge (HC) with spark ignition (SI) abbreviated as HCSI. The diesel engine combines stratified charge (SC) with compression ignition (CI) abbreviated as SCCI.
Engineering Explained has defined HCCI engines as the ones that run on gasoline but ignite their fuel using compression like a diesel engine.
Here is a video on details of the world’s most efficient internal combustion engine:
(Video Courtesy: Engineering Explained)