Schematic of the optically accessible HCCI research engine.

Homogenous charge compression ignition (HCCI) engines use fuel mixed with a substantial amount of excess air or recirculated exhaust gas that is compressed in the combustion chamber until it autoignites. The resulting combustion is a flameless and low-temperature burn that consumes less fuel than traditional spark-ignition internal combustion engines, and therefore, produces less carbon dioxide.

HCCI engines have diesel-engine efficiency but without the accompanying nitrogen oxide and particulate emissions. One of the main obstacles to HCCI development is that the maximum loads these engines can produce are below those of spark-ignition and diesel engines. At the higher loads desired, the pressure rise rate becomes too high, eventually resulting in engine knock.

The Combustion Research Facility (CRF) team, John Dec (Transportation Energy Center), Nicolas Dronniou, Wontae Hwang, and Magnus Sjöberg (Engine Combustion Dept.), used chemiluminescence in an optically accessible engine to investigate the homogeneity of the charge. Their results show that even when the air-fuel mixture was homogeneous, factors such as heat transfer to the combustion chamber walls and residual hot pockets from the previous cycle actually created thermal inhomogeneities, or thermal stratification, in the chamber.

This work was supported by the DOE Vehicle Technologies Program.

Read more about this work at the CRF website.