Effects Of In-cylinder Components And Temperature Inhomogeneities On Conrolled Auto Ignition Combustion Of A Gasoline Engine

Controlled auto-ignition (CAI) combustion has the advantage in fuel economyand NOx emissions for engines. However, there is no direct ignition control in CAIengines, as the spark in gasoline engines or the fuel injection in diesel engines, whichis a great challenge for combustion control. The present paper explores the method tocontrol CAI auto-ignition through adjusting in-cylinder components and temperaturedistribution based on a fully variable valve technology. That is significant to thepractical application of CAI combustion.A gasoline engine model with four vavles and intake and exhaust ports is built inKIVA-3V. The Shell model is used to simulate the auto ignition, and thecharacteristic time model is used to simulate the combustion process. Multi-cyclesimulation is applied to reduce the sensitivity of the results in the initial conditionsand the boundary conditions. The validity of ignition delay is validated. The errors ofCA10, CA50and CA90of the simulation results and the experimental results arebetween1-2degrees crank angle. The "inhomogeneity" is defined as the standarddeviation to quantify the in-cylinder residual gas fraction (RGF) and temperaturedistributions.The effects of exhaust valve closing (EVC) timing, intake valve opening (IVO)timing and intake valve lift (IVL) on the in-cylinder flow, RGF and temperaturedistributions are studied by CFD simulation. The inhomogeneity of residual gas andtemperature has a wide range of variation under CAI mode, and the inhomogeneitycontrol can be achieved through the adjustment of both intake valve open timing andintake valve lift. The in-cylinder inhomogeneity increases with the intake valve opentiming delaying and intake valve lift increasing. However, the intake valve opentiming control strategy is only effective when applied to high in-cylinder RGF cases,whereas the intake valve lift control strategy is effective in controlling the in-cylinderinhomogeneity under the whole CAI range. The inhomogeneity of RGF andtemperature increases as the RGF increases.The abilities of the in-cylinder RGF and temperature distribution to control CAIignition and combustion processes are investigated by CFD simulations. The resultsshow that the temperature inhomogeneity can effectively control the CAI ignition timing with the same in-cylinder RGF and average temperature. However, thestratification of the RGF and temperature caused by the valve strategies is not strongeenough to influence the CAI combustion duration. The CAI auto-ignition timingadvances with the temperature inhomogeneity increasing in a wide range of the32%?85%RGF. Therefore, the auto-ignition timing can be controlled by thetemperature inhomogeneity.In order to validate the effects of the in-cylinder distribution on ignition andcombustion, the intake backflow is introduced by controlling the intake valve timing,which could lead to some RGF and temperature inhomogeneities. The distribution isrevealed through CFD simulation and the ignition characteristic is researched viasingle-cylinder gasoline engine experiments. The results show that the hightemperature zone and high RGF zone are separated by intake backflow, which is aadvantage for the ignition in low loads. A new concept for the realization of CAIcombustion in low load is provided.