Visualization Research on Low-Temperature Ignition and Combustion Characteristics of Diesel/Gasoline Blends under Cold-Start Conditions

Although engines fueled with diesel/gasoline blends show excellent combustion and emission performance, its low-temperature flame development characteristics under cold-start conditions remain to be further verified. To clarify the details, experiments were conducted in an optical constant volume combustion chamber using Mie-scattering and direct photography methods at different ambient temperatures. Results show that the ignition delay of pure diesel during spray combustion shows a zero temperature coefficient (ZTC) region, and the addition of gasoline weakens the ZTC behavior until it disappears. The cool flame initiates the ignition, and the hot flame tends to far from the base of the cool flame as the gasoline content increases. In addition, the addition of gasoline to diesel increases the ratio of cool flames because the high evaporation reduces the temperature in the mixing zone, so only cool flame occurs in the G45 blends. Consequently, the total flame intensity presents an order of magnitude decrease. At lower ambient temperatures, the addition of gasoline significantly increases ignition instability. It is difficult to convert a cool flame into a hot flame due to the inhomogeneity of temperature and species field, which results in various unstable ignition phenomena, such as a short flash cool flame and intermittent cool and hot flame. Therefore, it is essential to directly target the cool flame and pay attention to the intrinsic mechanism of the evolution from the cool flame to the hot flame during the spray combustion process.