The paper aims to provide an assessment of the Homogeneous Charge Compression Ignition (HCCI) combustion, compared to a well-established alternative such as Direct Injection Compression Ignition (DICI) combustion, under the criteria of CO2 emission reduction potential. The assessment is performed by reviewing the relevant literature and analyzing the commercial products available on the market that are featuring these two technologies. DICI engines have demonstrated in the real world the ability to deliver top fuel conversion efficiencies of about 50%, and fuel conversion efficiencies largely above 40% over most of the load and speed range. Research-only HCCI engines have delivered fuel efficiencies well below 40% in the very few carefully selected map points where they working during carefully performed laboratory experiments. As a further contribution, the model of a reference DICI diesel engine is modified to accommodate port fuel injection (PFI) of n-heptane and a reference HCCI combustion model to explain the HCCI downfalls. HCCI combustion is extremely delicate, and difficult to be phased at TDC. Even when it is occurring at TDC, it is characterized by larger heat loss, incomplete combustion, higher peak pressure, and a higher rate of pressure build-up vs. the DICI combustion, that more efficiently occurs at the center of the chamber surrounded by a cushion of air at a reduced, controllable, rate, with peak pressure conveniently located during the expansion stroke. The review explains why, despite the huge efforts produced in research and development for the HCCI technology over now many decades, not racing, nor a production engine, that is featuring this technology, has been proposed.