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Results of computations of flows, sprays and combustion performed in an optically- accessible Diesel engine are presented. These computed results are compared with measured values of chamber pressure, liquid penetration, and soot distribution, deduced from flame luminosity photographs obtained in the engine at Sandia National Laboratories and reported in the literature. The computations were performed for two operating conditions representing low load and high load conditions as reported in the experimental work. The computed and measured peak pressures agree within 5% for both the low load and the high load conditions. The heat release rates derived from the computations are consistent with expectations for Diesel combustion with a premixed phase of heat release and then a diffusion phase. The computed soot distribution shows noticeable differences from the measured one.

In this paper, a wall-modified interactive flamelet model is developed for improving the modeling of Diesel combustion. The objective is to include the effects of wall heat loss on the transient flame structure. The essential idea is to compute several flamelets with several representative enthalpy defects which account for wall heat loss. Then, the averaged flamelet profile can be obtained through a linear fit between the flamelets according to the enthalpy defect of the local gas which results from the wall heat loss. The enthalpy defect is estimated as the difference between the enthalpy in a flamelet without wall heat loss, which would correspond to the enthalpy in the gas without wall heat loss, and the gas with wall heat loss. The improved model is applied to model combustion in a Diesel engine. In the application, two flamelets, one without wall heat loss and one with wall heat loss, are considered.

In this paper, results from multidimensional computations in which a flamelet model is employed to model heat release rates and NO in transient jets under Diesel conditions are presented. These results are compared with those obtained by employing a Local Equilibrium Characteristic Time (LECT) model which is a combination of mixing-limited and kinetic-limited submodels. The LECT model has been widely employed in Diesel engine computations in prior work. Several variables, arising in the implementation of the flamelet model, are considered in detail to determine the sensitivity of the computed results to the variables themselves. These include probability density functions (PDFs), strain rates and kinetics. It is shown that the heat release rate results are not significantly sensitive to the PDFs selected and the strain rates. It is also shown that the heat release rates are relatively insensitive to the choice of detailed or reduced kinetics.

The standard model for predicting the outcome of drop-drop collisions in sprays is one developed based on measurements in rain drops under atmospheric pressure conditions. This model includes the possible outcomes of grazing collisions and coalescence. Recent measurements with hydrocarbon drops and at higher pressure (up to 12 bar) indicate the possibility of additional outcomes: bounce, reflexive separation and drop shattering. The measurements also indicate that the Weber number range over which bounce occurs is dependent on the gas pressure. The probability of a drop-drop collision resulting in bounce increases with gas pressure. A composite model that includes all these outcomes as possibilities is employed to carry out computations in a constant volume chamber and in a Diesel engine. A sub-model for bounce that includes the pressure effects is also part of the composite model.

In recent years, there has been an interest in early-injection Diesel engines as it has the potential of achieving a more homogeneous and leaner mixture close to top-dead-center (TDC) compared to standard Diesel engines. The more homogeneous mixture may result in reduced NOx and soot emissions and higher efficiency. Diesel engines in which a homogeneous mixture is achieved close to TDC are known as Homogenous Charge Compression Ignition (HCCI) engines. PREmixed lean DIesel Combustion (PREDIC) engines in which the start of fuel injection is considerably advanced in comparison with that of the standard Diesel engine is an attempt to achieve a mode of operation close to HCCI. Earlier studies have shown that in a PREDIC engine, the fuel injection timing affects the mixture formation and hence influences combustion and pollutant formation.

A multidimensional model is employed to model ignition and heat release rates in a Diesel engine. An interactive flamelet model is employed to model combustion. Nheptane is used as a representative fuel for Diesel fuel in the computations. Comparisons of computed and measured results are presented for a range of engine operating conditions: speed 1200 rpm, start of injection 12.5 degrees before top dead center to 9.5 degrees after top dead center and intake air temperature of 340-360 K. The primary objective of this work is to assess the ability of the model to reproduce ignition timings. The flamelet model uses detailed chemical kinetics and it is shown that it can reproduce the qualitative trends of changes in ignition delay and heat release rates with respect to changes in operating conditions of the engine. The capability to reproduce the measured changes in ignition delay is important because changes in injection timing lead to changes in ignition timing.

In Diesel engines, the vapor phase of the fuel jet is known to impinge on the walls. This impingement is likely to have an effect on mixing characteristics, the structure of the diffusion flame and on pollutant formation and oxidation. These effects have not been studied in detail in the literature. In this work, the structure of a laminar wall jet that is generated from the impingement of a free laminar jet on a wall is discussed. We study the laminar jet with the belief that the local structure of the reaction zone in the turbulent reacting jet is that of a laminar flame. Results from non-reacting and reacting jets will be presented. In the case of the non-reacting jets, the focus of the inquiry is on assessing the accuracy of the computed results by comparing them with analytical results. Velocity profiles in the wall jet, growth rates of the half-width of the jet and penetration rates are presented.

In this paper, computations of pulsating flows in a duct with multiple inlets using the lattice Boltzmann method (LBM) are reported. As future emissions standards present a significant challenge for Diesel engine manufacturers, several options are being investigated to identify strategies to meet such regulations. Exhaust gas aftertreatment is one of the most important among them. As the performance of the various aftertreatment devices is sensitive to the flow conditions in the exhaust, a greater understanding of the flows under pulsating conditions in the presence of multiple cylinders is needed. The Lattice Boltzmann Method (LBM) is a relatively new and promising computational approach for applications to fluid dynamics problems. Two advantages of the method relative to traditional methods are ease of implementation and ease of parallelization and performance on parallel computers.

A recently developed stochastic particle approach for computing soot particle dynamics is implemented in a three-dimensional model for flows, sprays, combustion and emissions in Diesel engines. The model is applied to study the distribution of soot particles in a direct-injection Diesel engine. In particular, the effect of thermophoresis on soot distribution is examined. It is shown that thermophoresis could be important once the soot particles are brought close to the walls, i.e. within the boundary layer, by turbulent eddy convection or as a result of the orientation of the sprays. Thermophoresis does not appear to result in a change in the distribution of soot in the regions outside the boundary layer as the characteristic time associated with turbulent eddy convection is at least an order of magnitude shorter than that associated with thermophoresis and it and bulk convection are by far the dominant factors in determining the soot distribution.

In this work, a multidimensional model that the authors have previously developed for computing radiant heat loss in an internal combustion engine is applied to study radiant heat loss characteristics in a Diesel engine. The model is applied to study the effects of load and speed on radiant heat transfer in the engine. It is shown that as load is increased the radiant heat loss increases and the fraction of radiant to total heat loss increases from about 12% at an overall equivalence ratio of 0.3 to 16% at an overall equivalence ratio of 0.5. As speed is increased, the radiant and total heat loss again increase but the ratio of radiant to total heat loss remains about the same for the cases considered. It is shown that there is a strong correlation between the radiant heat loss characteristics and soot concentration and temperature in the chamber.

The spray submodel is an important component in multidimensional models for Diesel engines. The satisfactory representation of the spray is dependent on adequate representation of turbulence in the jet which, in part, determines its spread and penetration. In this work, the RNG k-ϵ model is evaluated relative to the standard k-ϵ model for computing turbulent jets. Computations are made for both gas jets and sprays. The gas jet is computed with an adequately high degree of numerical spatial resolution of the order of the orifice diameter. In the case of the spray, achieving such a high resolution would be challenging. Since the spray has similarities to the gas jet, and the gas jet may be computed with such high resolution and adequate accuracy, firm conclusions may be drawn for it and they may be applicable to sprays. It is concluded that the RNG k - ϵ model, in general, results in predictions of greater mixing in the jets relative to the standard model.

It has been shown recently that the maximum penetration of the liquid phase in a vaporizing Diesel spray is relatively short compared to the overall jet penetration and that this maximum is reached in 2 - 4°CA after start of injection. This implies that the drops that are formed by atomization vaporize in a short characteristic time and length relative to other physical processes. This paper addresses an important question related to this observation: Are the vaporizing fuel drops disappearing because they reach a critical state? Related to this question is another: Under what conditions will vaporizing fuel drops reach a critical state in a Diesel engine? Single drops of pure component liquid hydrocarbons and their mixtures vaporizing in quiescent nitrogen or carbon dioxide gas environments with ambient pressures and temperatures at values typically found in Diesel engines are examined.

In this paper, results from an experimental and computational study relating NO and soot emissions in a Diesel engine to heat release rate characteristics are reported. The experiments were carried out in a Cummins N-14 single-cylinder Diesel engine. The computations were carried out for the same engine. It is shown that, more than any significant feature of the heat release rate itself, the NO appears to be related to the temperature of the reactants with higher temperatures resulting in higher NO emissions. Relationships of NO to the heat release rates are secondary to this primary dependence. In general, the soot-NO trade-off relationships appear to hold. However, for the range of conditions studied, soot and NO are found to simultaneously decrease with decreasing air temperatures. It is also found that at the most retarded timings, NO and soot simultaneously decrease but with a severe penalty in fuel consumption.

NO and soot emissions from Diesel engines are dependent on several parameters related to the engine design and operating conditions. Multidimensional models are increasingly employed to study the effect of these parameters. In this paper, a multidimensional model for flows, sprays and combustion in engines is employed to study the dependence of NO and soot formation and oxidation on injection timing, injection pressure, chamber temperature, EGR and ignition delay, and compare the computed trends with those observed in experimental studies reported in the literature. Computations are carried out in a typical heavy-duty Diesel engine and additional computations in a constant volume chamber are used to clarify the engine results when appropriate. For several parametric changes, the experimentally observed trends are reproduced. However, several limitations are identified. The structure of the computed combusting jet has differences with those suggested from recent experiments.

Accurate modeling of the transient structure of reacting diesel jets is important as transient features like autoignition, flame propagation, and flame stabilization have been shown to correlate with combustion efficiency and pollutant formation. In this work, results from Reynolds-averaged Navier-Stokes (RANS) simulations of flame lift-off in diesel jets are examined to provide insight into the lift-off physics. The large eddy simulation (LES) technique is also used to computationally model a lifted jet flame at conditions representative of those encountered in diesel engines. An unsteady flamelet progress variable (UFPV) model is used as the turbulent combustion model in both RANS simulations and LES. In the model, a look-up table of reaction source terms is generated as a function of mixture fraction Z, stoichiometric scalar dissipation rate Xst, and progress variable Cst by solving the unsteady flamelet equations.