Visualization Of Ignition And Combustion Characteristics Of Diesel In Low-reactivity Fuel Ambient

RCCI has attracted much attention due to its low Soot,NOx emissions,and the flexibility to control the heat release rate.The author researched the effects of equivalent ratio of isooctane,ambient temperature,oxygen concentration and diesel injection strategy on the ignition and combustion characteristics of diesel in isooctane ambient based on the dual-fuel constant volume combustion bomb platform.What’s more,author also writed algorithm which extracts flame topography,basing on the idea of local binary patterns.Specific conclusions are as follows:Isooctane has an inhibitory effect enhancing with increasing equivalence ratio on diesel ignition which is manifested by weakening flame brightness,reducing flame area,and increasing flame lift-off length.One reason is isooctane consumes OH in the free radical pool,and another reason is isooctane poorer ignition characteristics.Isooctane has axial and radial diffusion motion inside CVCB and the main-injection interval affects the spatial distribution of isooctane inside CVCB.There is an optimal injection interval at a given equivalence ratio so that the amount of isooctane in the center of CVCB is the most at SOC of diesel,and its impact on diesel ignition is also greatest.As the temperature increases,the burning rate of isooctane increases dramatically.Low oxygen concentration reduces overall reactivity and the chance of isooctane selfignition.Under the premise that the fuel injection quantity and the injection pressure are constant,the relationship between TIIN and ignition delay can be divided into three zones: the downhill zone,the linear zone,and the gradual zone.Under isooctane ambient,pilot injection mainly affects main injection by increasing the local temperature and equivalence ratio which is determined by the amount of isooctane ignited by pilot injection.Pilot-injection quantity determines whether pilot injection could ignite isooctane and the amount of isooctane ignited by it.The pilot-injection interval determines the amount of isooctane in the center of the CVCB at the time of pilot-injection.As for pure diesel mode,under specified pilotinjection quantity,there exists one pilot-injection interval which makes main-injection ignition delay shortest.When the pilot-injection interval is too short,the pilot-injection quantity cannot burn completely,heat release is not obvious and main-injection ignition delay is prolonged.When pilot-injection interval is too long,heat loss increases,the pilot-injection combustion position becomes lower,the influence on main injection is weakened,and main-injection ignition delay is prolonged.The main-injection interval affects the spatial distribution of isooctane in CVCB.Under the short main-injection interval,the combustion exhibits a three-stage exotherm,which is pilot combustion,main combustion,post combustion of isooctane,respectively.The introduction of pilot injection raises the local temperature/pressure which shortens ignition delay of main injection,increases the amount of soot,the average flame brightness,the flame area,and shortens the flame lift-off length.Comparing with isooctane,methanol has a stronger ignition inhibitory effect on diesel,and the average flame brightness is weaker and the ignition delay is longer.When the injection interval is 12 ms,the average flame brightness strengthens with the increase of the methanol equivalent ratio which means it is the local equivalent ratio rather than the overall equivalent ratio that inhibits ignition of diesel.Camera shooting conditions effects the threshold which distinguish the linear zone and the gradual zone.The larger the aperture is,the more light the camera can capture,and the threshold distinguishing the linear zone and the gradual zone increases.