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CFD simulations of reacting fuel sprays were conducted to identify temperaturedependent physical properties of the liquid fuel that should be emulated by diesel and jet fuel surrogates within simulations of compression ignited combustion. Using a validated CFD model for an n-dodecane spray under diesel-relevant conditions, six physical properties of the liquid phase fuel (density, vapor pressure, viscosity, surface tension, heat of vaporization, and specific heat capacity) were perturbed covering the minimum and maximum property range of hydrocarbons widely used in recent diesel and jet fuel surrogates. Liquid fuel density, viscosity, vapor pressure, and specific heat had significant impact on liquid penetration length, causing a 4 % ~ 16 % change from the baseline n-dodecane case. The changes resulted from the liquid fuel's influence on various physical phenomena, including droplet breakup, air entrainment and evaporation. For ignition delay, specific heat and density effects were most significant, with up to 10 % changes from the baseline case. Specific heat perturbations affected the thermal energy necessary for fuel vaporization, hence local temperature development and mixture reactivity. Liquid density influenced the velocity of the fuel injection event, which modified turbulent mixing rates, low temperature heat release characteristics and the transition to high temperature ignition. The results of this study indicate that liquid density, specific heat, viscosity, and vapor pressure should be considered for surrogate development to properly capture the liquid penetration and ignition delay characteristics of the target fuel.

Recent developments in ignition, boosting, and control systems have opened up new opportunities for highly dilute, high-pressure combustion regimes for gasoline engines. This study analytically explores the fundamental thermodynamics of operation in these regimes under realistic burn duration, heat loss, boosting, and friction constraints. The intent is to identify the benefits of this approach and the path to achieving optimum engine and vehicle-level fuel economy. A simple engine/turbocharger model in GT-Power is used to perform a parametric study exploring the conditions for best engine efficiency. These conditions are found in the mid-dilution range, a result of the tradeoff between fluid property benefits of lean mixtures and friction benefits of higher loads. Dilution with exhaust gas is nearly as effective as air dilution when compared using a 'fuel-to-charge' equivalence ratio defined as F 0 [ F (1-RGF) where RGF is the total residual gas fraction. Optimal brake efficiencies are obtained over a range 0.45 4 F 0 4 0.65 for operation up to 3 bar manifold pressure, yielding peak temperatures under 2100 K and peak pressures under 150 bar. These conditions are intermediate between homogeneous charge compression ignition and spark-ignition regimes, and are the subject of much current research on advanced combustion modes. An engine-vehicle drive train simulation shows that accessing this thermodynamic sweet spot has the potential for vehicle fuel economy gains between 23% and 58%.

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The effects of thermal and compositional stratification on the ignition and duration of homogeneous charge compression ignition combustion In this work, the effects of thermal and compositional stratification on the ignition and burn duration of homogeneous charge compression ignition (HCCI) combustion are studied with full-cycle 3D computational fluid dynamics (CFD) simulations with gasoline chemical kinetics performed during the closed portion of the cycle, from intake valve closing (IVC). The stratification was varied through the use of negative valve overlap (NVO) and positive valve overlap (PVO) breathing strategies. To remove charge energy and phasing effects from the simulation results, the fuel mass and ignition timing were held constant, while mean composition effects were isolated from those of local stratification by maintaining the same mean oxygen concentration, fuel-oxygen equivalence ratio and charge heat capacity. Fuel was premixed with the intake to avoid potential stratification effects arising from direct injection. With NVO, the incomplete mixing of the fresh charge with the large mass fraction of product gases retained within the cylinder from the previous combustion cycle leads to a 23% increase in the preignition thermal stratification, and an order of magnitude increase in the levels of stratifications in fuel to oxygen equivalence ratio and oxygen mole fraction relative to the PVO strategy, which employs a premixed mixture of fresh and product gases. Under the conditions studied, the use of NVO resulted in a 30% increase in the 10-90% burn duration (CA10-90) compared to the PVO condition. Two additional analyses were performed to decouple the effects of thermal and compositional stratification. The first examined the reaction space (based on the ignition delay distribution within the charge prior to ignition) for both breathing strategies to quantify the inherent reactivity stratification. The second examined the more stratified NVO case with a quasidimensional multi-zone model. These analyses revealed that under the conditions studied, HCCI reactivity and combustion duration are governed primarily by thermal stratification and are largely insensitive to compositional stratification at the time of ignition.