On the mechanism of controlled auto ignition

Controlled auto ignition, CAI, is a combustion form, which uses auto-ignited homogeneous air/fuel mixture but controlled by regulating internal EGR introduction. It offers superior fuel economy and significantly reduced pollutants potentials, but still a distance away from practical application due to lack of knowledge into the mechanism of such combustion. In this paper, using a fully variable valve train and a newly developed exhaust valves control strategy, we substituted EGR with hot nitrogen and hot air. We found that the internal EGR has two aspects of effect towards CAI combustion: thermal effect and chemical effect. Nitrogen is a chemically inertial gas. Although its temperature was raised up to the level of internal EGR during the test, no CAI combustion occurred. This indicates that EGR has a strong chemical effect towards CAI combustion. Oxygen in air is part of reactant of combustion. With its introduction, no CAI combustion occurred till its temperature ramped up to 120oC. Such result proved that a minimum thermal condition of an added gas is required to generate auto-ignition. Comparing with EGR introduction, we found that nitrogen has the ability to delay the combustion ignition and smooth pressure increase rate while oxygen accelerates combustion and turned auto ignition combustion into an uncontrollable form normally experienced with knock. This explains the chemical effect of EGR’s contribution towards CAI combustion since it contains a large amount of nitrogen as well as some chemical active species.