Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releases energy that can be transformed in an engine into work and heat. HCCI combines characteristics of conventional gasoline engine and diesel engines. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. Diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI.

As in gasoline engines, HCCI injects fuel during the intake stroke. However, rather than using an electric discharge (spark) to ignite a portion of the mixture, HCCI raises density and temperature by compression until the entire mixture reacts spontaneously. HCCI is also similar to diesel engines in that it relies on a drastic rise in temperature and density which results in self-ignition. So in both gasoline and diesel engines, combustion occurs at the boundary of the fuel and air, either producing higher emissions or else a loss in energy conversion efficiency. With HCCI, however, there is no boundary between fuel and air because ignition occurs at all points simultaneously (homogeneously) resulting in a complete burning of the fuel.

The greatest and yet unsolved problem with HCCI is that auto-ignition is difficult to control due to varying engine conditions. Many schemes for controlling HCCI have been tested, but none are reliable enough to be used commercially on a wide scale. What's proposed then is a method of finely controlling the ignition process in HCCI engines by using a supplemental supply of oxygen. Basically, an onboard oxygen concentrator makes and stores oxygen that can then be added to the fuel and air mixture as needed to control ignition under varying engine conditions. For instance, with a "cold" internal combustion engine, a microprocessor will determine that a higher amount of supplemental oxygen is needed to increase the partial pressure in the engine and so ignition occurs at a lower temperature and pressure. With a "hot" engine, less oxygen is needed. The point is that the relatively small amount of supplemental oxygen added to the fuel and air mixture totally controls the entire ignition process to a high degree. Energy needed to run the oxygen concentrator will consume 5% or less of total energy output. However, an HCCI process makes an internal combustion engine 30% or more efficient and with 40% less environmentally harmful pollutants emitted!