On the Mechanism of Controlled Auto Ignition

Controlled auto ignition (CAI) is a form of combustion which uses an auto-ignited homogeneous air/fuel mixture but is controlled (or moderated) by regulating the quantity of internal exhaust gas residuals. In this paper, using a fully variable valve train and a newly developed exhaust valve control strategy, we substituted EGR with hot nitrogen or hot air. We found that the internal exhaust gas residuals have both thermal and chemical effects on CAI combustion. To investigate the thermal effect, nitrogen was used as it is a chemically inert gas. Although its temperature was raised to that of the internal exhaust gas residuals during testing, CAI combustion could not be promoted without assistance from a spark in a form of hybrid CAI, thus indicating that exhaust gas residuals have a chemical effect as well. Conversely, with the introduction of oxygen (which in air is a reactant of combustion), no auto ignition combustion occurred until its temperature was increased to 120°C, proving that a minimum thermal condition of an added gas is required to generate auto-ignition. Comparing this EGR introduction, we found that nitrogen has the ability to delay combustion and smooth the pressure increase rate, while oxygen accelerates combustion and turns CAI combustion into an uncontrollable form normally associated with knock. These effects help to explain the contribution of the chemical effect of the EGR on CAI combustion since exhaust gas residuals contain nitrogen as well as chemically active species.