随着内燃机缸内工作压力的升高，传统液态燃料喷雾转捩为超临界射流的现象已广泛存在。实际燃料通常是含众多组分的混合燃料。为解析多组分燃料液滴的超临界转捩特性及机理，本文采用分子动力学（MD）模拟方法，对类发动机工作条件下多组分燃料液滴的超临界转捩机理和判据、多组分燃料液滴的亚/超临界蒸发特性、蒸发模式的影响因素及蒸发模式分区图谱等关键问题开展系统研究。建立了多组分碳氢燃料液滴的静态/非静态MD蒸发模型，研究对象包括单组分（正十六烷）、三组分和六组分燃料，构建了氮气、空气、带燃烧废气的空气和燃烧废气等环境气体模型。研究了燃料液滴发生超临界转捩（从常规蒸发到扩散混合的蒸发模式转捩）的环境条件。将混合时间除以蒸发寿命得到无量纲混合时间τ，提出基于每个燃料原子的平均位移增量（ADI）的蒸发模式转捩通用微观判据τ0.9P（每个燃料原子随时间变化的ADI曲线首次达到其峰值的90%时对应的无量纲混合时间），临界值τ0.9Pc = 0.5。结果显示，超临界蒸发时的扩散混合模式与亚临界常规蒸发模式相比，超临界蒸发时每个燃料原子的平均位移增量以及对熵总和增量的峰值明显提前，对熵总和随时间变化曲线整体抬升。解析了多组分燃料液滴的亚/超临界蒸发特性。在扩散混合模式下，多组分液滴中不同燃料组分蒸发速率间的相对差异显著减小，相较常规蒸发模式，环境气体的溶解量则显著增大。同一时刻燃料组分间的内力随着环境温度或环境压力的升高而减小；燃料的原子平均动能/原子平均势能随着环境温度或环境压力的升高而增大。随着环境温度的升高，蒸气中较大燃料分子团簇的尺寸、数量占比和蒸气总的团簇率均减小；环境压力升高时，情况则相反。研究了燃料成分对超临界转捩的影响，提出了一种可定性比较两种燃料的超临界转捩可能性的关联式。解析了环境气体成分、流场相对速度等因素对超临界转捩的影响规律。燃料在燃烧废气中发生超临界转捩的可能性比在纯氮气中更大。对于接近或处于扩散混合模式的多组分燃料液滴，液滴相对速度越大，轻/重燃料组分蒸发速率间的相对差异越小。基于纳米尺度下的液滴/液膜的蒸发研究，提出了无量纲尺寸因子SF，其可有效反映某种燃料液滴/液膜的初始尺寸对蒸发的影响。基于ADI判据，MD模拟预测的液滴超临界转捩条件的结果与宏观试验结果能够匹配。建立了单/多组分燃料液滴蒸发模式分区图谱，并提出了对多组分碳氢燃料液滴超临界转捩过程实施调控的方法。

With the increase of the operating pressure in the cylinder of internal combustion engines, the transition of conventional liquid fuel sprays to supercritical jets has become widespread. The actual fuel is usually a mixture composed of many components. In order to resolve the supercritical transition characteristics and mechanism of multicomponent fuel droplets, this thesis systematically investigated three key issues using molecular dynamics (MD) simulations, including the supercritical transition mechanism and criterion of multicomponent fuel droplets in the context of internal combustion engines, the subcritical/supercritical evaporation characteristics of multicomponent fuel droplets, and the influencing factors and transition maps of evaporation modes.Molecular dynamics models for quiescent/non-quiescent evaporation of multi-component hydrocarbon fuel droplets were developed for single-component (n-hexadecane), three-component and six-component fuels, respectively. Four ambient gas models such as nitrogen, air, air with combustion exhaust and combustion exhaust were established. The environmental conditions of the supercritical transition (evaporation mode transition from conventional evaporation to diffusive mixing) for multicomponent hydrocarbon fuels were studied. The absolute time t was divided by the evaporation lifetime tL to obtain the dimensionless time τ. Based on the average displacement increment (ADI) of fuel atoms, an universal microscopic criterion for mode transition of evaporation τ0.9P (the smaller dimensionless time corresponding to 90% of the peak of the time-dependent ADI profile for fuel atoms) was proposed, and the critical value τ0.9Pc was equal to 0.5. It was revealed that compared with the subcritical conventional evaporation mode, in the supercritical diffusion mode, the peak of average displacement increment of each fuel atom and the peak of the increment of sum of pair entropy appeared earlier, and the time-dependent pair entropy curve were generally raised.The subcritical and supercritical evaporation characteristics of multicomponent fuel droplets were resolved. It was found that in diffusion mode, the relative differences of evaporation rates among fuel components in the multicomponent droplet were greatly reduced, and compared with the case of conventional evaporation mode, and the dissolution process of ambient gases in the droplet was significantly enhanced. On the whole, the internal force of the fuel components decreased with increasing pressure or temperature. The per-atom translational kinetic energy or pair potential energy of fuel atoms increased with increasing temperature or pressure. In the case of supercritical temperature studied here, with increasing ambient temperature, the size and the proportion of large clusters in the fuel vapor as well as the whole cluster rate all decreased, while the opposite happened with increasing ambient pressure. The effects of fuel components on supercritical transition were investigated. A correlation was proposed to qualitatively compare the possibility of supercritical transition for two fuels.Effects of ambient multicomponent gases on supercritical transition of fuels were investigated. Effects of the relative velocity between the droplet and the ambient on supercritical transition were also revealed. The possibility of supercritical transition for fuels in combustion exhaust was greater than that in pure nitrogen. For multicomponent fuel droplets close to or in diffusion mode, with larger relative velocities, the relative differences between evaporation rates for light/heavy fuel components were reduced. Based on the study of evaporation of droplets/films at an atomic level, a dimensionless size factor SF was proposed, which could effectively reflect the effect of the initial size of the fuel droplet/film on evaporation. Based on the ADI criterion, the conditions of the supercritical transition for fuel droplets predicted by MD simulation could match the macroscopic test results. Transition maps of evaporation modes of single- and multi-component fuel droplets were established. The methods of regulating and controlling the supercritical transition process of multicomponent hydrocarbon fuel droplets were proposed.