With the heightened pressure on car manufacturers to increase the efficiency and reduce the carbon emissions of their fleets, more challenging engine operation has become a viable option. Highly dilute, boosted, and stratified charge, among others, promise engine efficiency gains and emissions reductions. At such demanding engine conditions, the spark-ignition process is a key factor for the flame initiation propagation and the combustion event. From a computational standpoint, there exists multiple spark-ignition models that perform well under conventional conditions but are not truly predictive under strenuous engine operation modes, where the underlying physics needs to be expanded. In this paper, a hybrid Lagrangian-Eulerian spark-ignition (LESI) model is coupled with different turbulence models, grid sizes, and combustion models. The ignition model, previously developed, relies on coupling Eulerian energy deposition with a Lagrangian particle evolution of the spark channel, at every time-step. The spark channel is attached to the electrodes and allowed to elongate at a speed derived from the flow velocity. The LESI model is used to simulate spark ignition in a non-quiescent crossflow environment at engine-like conditions, using CONVERGE commercial CFD solver. The results highlight the consistency, robustness, and versatility of the model in a range of engine-like setups, from typical with RANS and a larger grid size to high fidelity with LES and a finer grid size. The flame kernel growth is then evaluated against schlieren images from an optical constant volume ignition chamber with a focus on the performance of flame propagation models, such as G-equation and thickened flame model, versus the baseline well-stirred reactor model. Finally, future development details are discussed.

Although many studies have been carried out using control-oriented engine models for advanced combustion technologies, methods of adapting model parameters for these models have still not been considerably studied. In this paper, we propose a method to adapt model parameters for an ignition model and a combustion model of a physics-based discrete diesel engine model using neural networks. Using experimental data on an engine bench as well, we examined how to perform adapting and training, such as optimization algorithms, how to select training data, appropriate feature values, and algorithms for switching the neural networks according to the operating regions. As a result, this adaptation method was able to improve the prediction accuracy of the heat release rate of pre-combustion better than when adapted using the previous adaptation method and when the model parameters were fixed.

The ring-pack lubrication is a complicated physical process involving multiple physical phenomena. This paper presents an attempt to model the ring-pack lubrication in three-dimensional space, considering the ring-bore structure interaction, bore distortion, ring-twist, piston secondary motion, non-Newtonian lubricant behavior, and ring/bore asperity contacts. The physics of the model includes the interface between the structure of the ring, oil lubricant, and the structure of the cylinder liner. The ring is modeled as a three-dimensional FEA model with the nodes along the ring circumference. The ring face orientation changes circumferentially depending on ring geometry as well as piston tilt angle and three-dimensional ring twist angle at every crank angle degree. The oil lubrication is modeled with the Reynolds equation with shear thinning and temperature dependent oil viscosity and with or without the flow factors. The cylinder liner description allows three-dimensional bore distortion and ring/liner asperity contact to be modelled. The key of the analysis is solving simultaneously at every crank angle increment a set of coupled linear and non-linear equations of ring structure, ring face lubrication, bore distortion, and asperity contact. The model predicts variations of the ring-pack lubrication in the axial and circumferential directions. Using the hydrodynamic lubrication model coupled with the asperity contact model allows calculations of the friction forces due to asperity contact (boundary and mixed lubrication) and oil film interactions (hydrodynamic and mixed lubrication). The transition from hydrodynamic lubrication to boundary lubrication through mixed lubrication is determined interactively based on ring / liner surface properties, ring loads, and lubrication properties. The new friction sub-module calculates axial and circumferential variation of both types of friction forces as well as total friction. The asperity contact induced friction forces and asperity contact pressure can further be used for ring wear calculations. The developed model has been applied to determine the performance of a production engine ring-pack. The influence of different phenomena affecting the ring-pack performance has been analyzed and compared.

With the engine technology moving towards more challenging (highly dilute and boosted) operation, spark-ignition processes play a key role in determining flame propagation and completeness of the combustion process. On the computational side, there is plenty of spark-ignition models available in literature and validated under conventional, stoichiometric SI operation. Nevertheless, these models need to be expanded and developed on more physical grounds since at challenging operation they are not truly predictive. This paper reports on the development of a dedicated model for the spark-ignition event at non-quiescent, engine-like conditions, performed in the commercial CFD code CONVERGE. The developed methodology leverages previous findings that have expanded the use and improved the accuracy of Eulerian-type energy deposition models. In this work, the Eulerian energy deposition is coupled at every computational time-step with a Lagrangian-type evolution of the spark channel. Typical features such as spark channel elongation, stretch, attachment to the electrodes are properly described to deliver realistic energy deposition along the channel during the entire ignition process. The numerical results are validated against schlieren images from an optical constant volume chamber and show the improvement in the simulation of the spark channel during the entire ignition event, with respect to the most commonly used energy deposition approach. Further development pathways are discussed to provide more physics-based features from the developed ignition model in the future.

The Los Alamos turbulent reactive flow researchers, our modelers and simulation code developers have succeeded in providing the engine research and development community an encompassing, robust, accurate and easy to use software for engine modeling or simulations. This software is now known as the FEARCE Toolkit. In this paper we discuss the physics present in the engine by discussion the methods we’ve employed to solve the model equations within the toolkit. Provided are background on what has been developed recently at LANL for internal combustion engine modeling.

Shear-driven cavitation plays an important role in many technological applications, including fuel injectors and power generators. Cavitation affects the performance of components and hence it is desirable to understand and predict its behavior since it can have favorable as well as adverse consequences. Although there have been a vast number of studies, a full understanding or theoretical framework describing its behavior has not yet been achieved. This is in part due to the complexities associated with cavitating flows including, internal flow physics, turbulence, two-phase flow and non-equilibrium thermodynamics. Further, experimental techniques are limited in their ability to visualize the phenomena with sufficient resolution for a detailed analysis. In this work, an unstructured, finite volume, computational fluid dynamic (CFD) code coupled to the Eulerian-Eulerian multi-fluid model is utilized to study cavitation phenomena in a nozzle. The well-reported Winkhlofer nozzle at a range of conditions including ΔP = 20, 40, 60, 70, 75, 80, and 85 bar is modeled using n-dodecane reference fuel properties. Three cavitation sub-models were investigated and the results compared with previous experimental and simulation flow data. The flow turbulence was modeled using Reynolds Averaged Navier Stokes Equation (RANS) and Large Eddy Simulation (LES) models and the results evaluated. A mesh sensitivity analysis was conducted with minimum cell sizes of 13.40, 9.48, 7.55, and 6.13 μm were considered to show grid convergence. Further, a novel erosion model was also integrated to identify the potential vulnerability damage zones with respect to the nozzle flow operating conditions. The results were in good agreement with experimental data from optical nozzles as well as previous simulation results. The models capture the cavitation near the solid boundary region and were able to predict the critical cavitation as well as the chocked flow regions. This was consistent with all the models. The results from the erosion model revealed a direct relationship between surface erosion, in terms of Mean Depth of Penetration Rate (MDPR) and incubation time, to higher pressure drops across the nozzle. These findings can be useful to develop future injector nozzle designs that can better mitigate cavitation induced material damage for improved engine endurance.

For past decades, substantial developments have been accomplished in internal combustion (IC) engine technology, but there still remain some possible improvements. The combustion in an IC engine is a highly intricate phenomenon, thus, numerous factors correlated with different forms of loss decides the efficiency of an engine. In spark-ignition (SI) engines, the combustion duration is considered important because it plays a key role in determining the combustion phasing for best possible energy conversion. The geometry of engine components may directly change the burning rate of air-fuel mixture, therefore, it should also be considered as significant as other aspects like exhaust gas recirculation (EGR) rate or boosting in investigation of the engine performance. This is the reason the development engineers are putting their effort to design an engine with optimized flow motion. Tweaking the flow dynamics via design modification or use of auxiliary device influences the turbulence level inside the combustion chamber, thus, the burning rate as well. Intake port orientation, masking, and piston shape are one of the typical design parameters manipulated for such purpose, and profound understanding on the effect of these design parameters on burning rate is encouraged in order to assist the optimization process. The design optimization process should be based on a fundamental understanding of how the design parameters affect the flow motion and combustion characteristics. This study aims for a simpler and faster method to investigate the consequences of design modifications. As a base model, a physics-based quasi-dimensional (QD) engine model is developed for simulation of SI combustion phenomenon. It is modeled to consider the change in flow motion and turbulence properties via simplified modeling. The advantages of such QD model is that it requires much less computational resource compared to 3D CFD model, and allows a greater degree of freedom within the simulation process which facilitates parametric studies. A zero-dimensional (0D) turbulence submodel is used to describe energy cascade mechanism, and turbulence intensity is calculated reflecting the effect cause by design modification. According to the sensitivities drawn from parametric study, the results of each effect on burning rate and other engine performance properties are compared individually and collectively. A qualitative analysis suggests how sensitive each effect are at given operating conditions. The result infers that the flow concentration by port design modification boosts the burning rate, but it is advantageous in terms of fuel economy to enhance the breathing ability by valve masking. The product of this comparative study assists an intuitive understanding on how the design modification would affect the engine operations, and it is encouraged to develop the model further via validation with experiment data to provide more reliable output. It is believed that it can be utilized as a good reference in engine design process.

Improved internal combustion (IC) engine simulations of natural gas (NG) combustion under conventional and advanced combustion strategies have the potential to increase the use of NG in the transportation sector in the United States. This study focused on the physics of turbulent flame propagation. The experiments were performed in a single-cylinder heavy-duty compression-ignition (CI) optical engine with a bowl-in piston that was converted to spark ignition (SI) NG operation. The size and growth rate of the early flame from the start of combustion until the flame filled the camera field-of-view were correlated to combustion parameters determined from in-cylinder pressure data, under low-speed, lean-mixture, and medium-load conditions. Individual cycles showed evidence of turbulent flame wrinkling, but the cycle-averaged flame edge propagated almost circular in the 2D images recorded from below. More, the flame-speed data suggested a different flame propagation inside a bowl-in piston geometry compared to a typical SI engine chamber. For example, while the flame front propagated very fast inside the piston bowl, the corresponding mass fraction burn was small, which suggested a thick flame region. In addition, combustion images showed flame activity after the end of combustion inferred from the pressure trace. All these findings support the need for further investigations of flame propagation under conditions representative of CI engine geometries, such as those in this study.

Volume of Fluid (VOF) and Eulerian-Lagrangian (EL)/Discrete Droplet Methods (DDM) are two of the most widely used methods in spray simulations. It is well known that these two methods have their pros and cons. VOF is good at capturing the transient detailed flow physics, while it is usually very expensive. EL is very efficient; however, to inject spray parcels, some experimental/pre-computed information is needed, such as rate of injection, and/or the parcel radius distributions, etc. It is often the case, the detailed fluid flow information at the nozzle exit, which is essential for downstream droplet breakup and coalescence, cannot be accounted in the EL method. In this paper, we developed a one-way coupled approach, in which VOF is employed to compute the detailed fluid field in the injector and this fluid information is then utilized by EL for the injection of parcels at the nozzle exit. The one-way coupled approach is used to calculate some ECN (Engine Combustion Network) spray cases, such as Spray A and Spray H. The simulated results are compared to the experimental data, and satisfactory agreement is obtained.

Stringent emission regulations require spark ignited (SI) engines to operate at stoichiometry to enable the use of a three way catalyst (TWC). Thus, accurate prediction of the intake charge mass flow rate is paramount. Current speed-density air mass-flow prediction techniques require extensive calibration for predicting volumetric efficiency, while mass air flow (MAF) meter based approaches suffer from a loss of accuracy during transients. This work aims to provide an alternative, i.e. a model based air charge estimation algorithm that can reduce calibration effort and provide a universal solution across engine platforms. An additional objective is to minimize the number of required sensors and associated cost. The foundation is established with a 0-D physics-based air charge model, where air flow through intake and exhaust valves is modeled on a crank-angle basis, without the need to measure in-cylinder pressure. The proposed algorithm solves differential equations for cylinder pressure and mass flow rate in/out of the cylinder to simultaneously obtain instantaneous pressure and mass-flow estimations, hence eliminating the need to install cylinder pressure transducers. An additional benefit is the robustness of the new model, due to its ability to self-compensate for an error in the intake runner pressure or initial estimation of the cylinder pressure. The model has been validated with GT-Power simulations and steady-state engine tests with multiple actuator sweeps. Transient tests and real-time implementations were performed as well.

Its inherent economic and environmental advantages as an internal combustion engine fuel make natural gas (NG) an attractive alternative to diesel fuel as the primary energy source for some compression ignition (CI) engine applications. Diesel pilot-ignition of NG is an attractive fueling strategy as it typically requires minimal modification of existing CI engines. Furthermore, this strategy makes use of the highly developed direct injection (DI) diesel fuel systems already employed on modern CI engines for to control dual-fuel (DF) combustion. Despite the increasing popularity of the dual-fuel NG engine concept, the fundamental understanding of the fuel conversion mechanisms and the impact of the fueling parameters is still incomplete. A conceptual understanding of the relevant physics is necessary for further development of fueling and pilot-ignition strategies to address the shortcomings of dual-fuel combustion, such as low-load emissions and combustion stability. An experimental facility supporting optical diagnostics via a Bowditch piston arrangement in a 2-litre, single-cylinder research engine (Ricardo Proteus) was used in this study to consider the effect of fueling parameters on the fuel conversion process in a dual fuel engine. Fueling was achieved with port injected CH 4 and diesel direct injection using a common rail system. Simultaneous, high-speed natural luminosity (NL) and OH* chemiluminescence imaging was used to characterize dual-fuel combustion and the influence of pilot injection pressure (300 bar vs. 1300 bar) and relative diesel-CH 4 ratios (pilot ratio, PR), as these have been noted as key operating dual-fuel control metrics. The pilot injection pressure was observed to have a significant impact on the fuel conversion process. At higher pilot injection pressures, the auto-ignition sites were concentrated around the piston bowl periphery and the reaction zone propagated towards the center of the bowl. At lower pilot injection pressures, ignition initiated in the vicinity of the pilot fuel jet structures and resulted in a more heterogeneous fuel conversion process with regions of intense natural luminosity, attributed to particulate matter. An increase in the pilot ratio (i.e., increased diesel fraction) resulted in a more aggressive combustion event, due to a larger fraction of energy released in a premixed auto-ignition event. This was coupled with a decrease in the fraction of the combustion chamber with significant OH* or NL light emission, indicating incomplete fuel conversion in these regions. The insight to the dual-fuel conversion processes presented in this work will be ultimately used to develop dual-fuel injection strategies, as well as provide much needed validation data for modeling efforts.

Modern engines with increasing power densities have put additional demands on pistons to perform in incrementally challenging thermal environments. Piston cooling is therefore of paramount importance for engine component manufacturers. The objective of this CFD study is to identify the effect of a given piston cooling nozzle (PCN) geometry on the cooling oil jet fanning (spreading) phenomenon. The scope of this study is to develop a numerical set-up using the open-source CFD tool OpenFOAM ® for measuring the magnitude of oil jet fanning and comparing it to experimental results. Large eddy simulation (LES) turbulence modeling is used to capture the flow physics that strongly affects the inherently unsteady jet break-up phenomenon. The oil jet fanning width is the primary metric used for comparing the numerical and experimental results. The results of simulation are validated for the correct applicability of LES by evaluating the quality metric (according to Pope [1]) at various probe locations and also by performing turbulent kinetic energy (TKE) spectral analysis. CFD results appear promising since they correspond to the experimental data within a tolerance (of ±10%) deemed satisfactory for the purpose of this study. Further generalization of the set-up is underway, towards developing a tool that predicts the aforementioned metric — thereby evaluating the effect of nozzle geometry on jet fanning and hence on the oil catching efficiency (CE) of the piston cooling gallery. Such a tool would act as an intermediate step in defining the boundary conditions for determining the filling ratio (FR) and subsequently the heat transfer coefficients (HTCs) in the piston cooling gallery.

Recent experimental observations show that lifted diesel flames tend to propagate back towards the injector after the end of injection under conventional high-temperature combustion conditions. Earlier studies have referred to this phenomenon as “flashback,” but more recently the term “combustion recession” has been adopted to reflect findings that the process appears dominated by “auto-ignition” reactions upstream of the lifted flame after the end of injection. Since this process is only initiated after the end of injection, it is also closely linked to the end-of-injection entrainment wave and its impact on the transient mixture-chemistry evolution upstream of the lift-off length. A few recent studies have explored the physics of combustion recession with experimental and simplified modeling approaches, but the details of the chemical kinetics and convective-diffusive transport of reactive scalars in this phenomenon are still largely unexplored. There are also uncertainties in the capability of engine computational fluid dynamics (CFD) simulations to accurately capture entrainment wave and combustion recession phenomena. In this study, highly-resolved numerical simulations have been employed to explore the mixing and combustion of a diesel spray after the end of injection and the influence of modeling choices on the prediction of these phenomena. The simulations are centered on a temperature sweep around the Engine Combustion Network (ECN) Spray-A conditions, from 800–1000 K, where different combustion recession behaviors are observed experimentally. Reacting spray simulations are performed in the open-source CFD software OpenFOAM, using a Reynolds-Averaged Navier-Stokes (RANS) approach with a traditional Lagrangian-Eulerian coupled formulation for two-phase mixture transport. Two reduced chemical kinetics models for n-dodecane by Yao et al. and Cai et al. are used to evaluate the impact of low-temperature chemistry and mechanism formulation on predictions of combustion recession behavior. Observations from the numerical simulations are consistent with recent findings that a two-stage auto-ignition sequence drives the combustion recession process; self-sustained reacting mixtures arise in distinct regions that are spatially separated from the lifted flame. Simulations with two different chemical mechanisms indicate that low-temperature chemistry reactions drive the likelihood for second-stage ignition and combustion recession that in turn strongly influence local entrainment in these mixtures and likelihood of combustion recession.

Diluting Spark-Ignited (SI) stoichiometric combustion engines with excess residual gas improves thermal efficiency, and allows spark to be advanced towards Maximum Brake Torque (MBT) timing. However, flame propagation rates decrease and misfires can occur at high Exhaust Gas Recirculation (EGR) conditions and advanced spark, limiting the maximum level of charge dilution and its benefits. The misfire limits are often determined for a specific engine from extensive experiments covering a large range of speed, torque and actuator settings. To extend the benefits of dilute combustion while at the misfire limit, it is essential to define a parameterizable, physics-based model capable of predicting the misfire limits, with cycle to cycle varied flame burning velocity as operating conditions change based on driver demand. A cycle averaged model is the first step in this process. The current work describes a model of cycle averaged laminar flame burning velocity within the early flame development period of 0 to 3 percent mass fraction burned. A flame curvature correction method is used to account for both the effect of flame stretch and ignition characteristics, in a variable volume engine system. Comparison of the predicted and the measured flame velocity was performed using a spark plug with fiber optical access. The comparison at a small set of spark and EGR settings at fixed load and speed, shows an agreement within 30% of uncertainty, while 20% uncertainty equals ± one standard deviation over 2,000 cycles.

This paper presents steady state power optimization for an Organic Rankine Cycle (ORC) waste heat recovery (WHR) system in heavy-duty diesel (HDD) applications and examines attainable power levels. Both recirculated and tail pipe exhaust gas streams are utilized as heat sources via a parallel evaporator configuration. A dynamic, physics-based ORC model is utilized to determine optimized net power production. The optimization variables include: working fluid pump speed, mass flow distribution ratio for the parallel evaporators, turbine speed, and the pump speed of condenser cooling fluid. A sensitivity analysis is conducted for all optimization variables. Working fluid pump speed and expansion turbine speed are demonstrated as the most sensitive variables. Sensitivity results indicate increasing system power production as the working fluid mass flow rate increases. A map is developed to identify optimal and safe ORC operating regions for a given engine cycle. Based on the sensitivity analysis working fluid pump speed is selected as the variable of interest in the steady state optimization and results indicate that system power generation could be improved by as much as 25%. Analysis of two quasi steady operational cycles, a multi-mode and a constant speed variable load are conducted. ORC power results indicate that a speed-constrained, mechanically coupled turbine expander operates in its peak efficiency zone to produce maximum power output for the steady-state cycles tested. These steady state power optimization results can be used for future power optimization during engine transients.

Durability is a prime concern in the design of hydraulic systems and fuel injectors [1–3] thus an accurate prediction of impact velocities between components and the flow through them is essential to assessing concepts. Simulation of these systems is difficult because the geometries are complex, some volumes go to zero as the components move, and the flow at a single operating condition generally spans Reynolds numbers less than 1 to more than 10 4 [4–8]. As a result of these challenges, experimental testing of prototypes is the dominant method for comparing concepts. This approach can be effective but is far more costly, time consuming, and less flexible than the ability to run simulations of concepts early in the design cycle. A validated model of a fuel injector built from publicly available data [1] is used to present a new approach to modelling hydraulic systems which overcomes many of these obstacles. This is accomplished by integrating several commercially available tools to solve the physics specific to each area within the fuel injector. First, the fuel injector is simulated using a 3D CFD simulation integrated with a 1D CFD system model. The flow in various regions of the injector is then analyzed to determine if the fluid models in these areas can be simplified based on the flow regime. Based on this analysis, a combination of models is assembled to improve the quality of the simulation while decreasing the time required to run the model. The fuel injector is simulated using a multibody dynamics model coupled to a reluctance network model of the solenoid and several fluid models. The first is a 3D CFD simulation which uses novel mesh refinement techniques during runtime to ensure high mesh quality throughout the motion of components, to resolve the velocity profile of laminar flows, and to satisfy the requirements of the RNG k-ε turbulence model and wall functions. This approach frees the analyst from defining the mesh before runtime and instead allows the mesh to adapt based on the flow conditions in the simulation. Due to the highly efficient meshing algorithm employed, it is possible to re-mesh at each timestep thus ensuring a high quality structured mesh throughout the simulation duration. Then a 3D FEM solution to the Reynolds Equation and a statistical contact model is employed to solve for the squeeze films between components and to allow separation and contact between bodies in the control valve. These detailed simulations are integrated with a 1D flow model of the fuel injection system. The results from the detailed coupled simulations are compared to the results from simpler 1D models and measured data to illustrate under which operating conditions a more advanced technique incorporating 3D CFD is worth the additional computational expense versus a traditional 1D model.

High fidelity, three-dimensional CFD was used to model the flow, fuel injection, combustion, and emissions in a large bore medium speed diesel engine with different levels of natural gas substitution. Detailed chemical kinetics was used to model the complex combustion behavior of the premixed natural gas, ignited via a diesel spray. The numerical predictions were compared against measured multiple cycle pressure data, to understand the possible factors affecting cyclic variation in experimental data. Under conditions with high natural gas substitution rates, diesel was injected much earlier than firing-TDC. This additional mixing time allowed the active radicals from diesel dissociation to initiate combustion from the cylinder wall and propagate inwards. 0%, 60%, and 93% natural gas substitution rates (by energy) were tested in this study to develop computational capabilities needed to accurately model and understand the underlying physics. Several innovative computational methods such as adaptive mesh refinement (which automatically refines and coarsens the mesh based on the existing solution parameters), and multi-zoning (which groups chemically similar cells together to reduce combustion calculation time) were utilized to obtain accurate predictions at a lower computational cost. Important engine emissions such as NOx, CO, unburnt HC, and soot were predicted numerically and compared against measured engine data.

A simple single injection scheme is used to understand the fundamental processes of diesel engine ignition. Two different combustion regimes, partially premixed combustion (PPC), and conventional direct injection compression ignition (DICI), are computationally achieved with the single injection scheme in a 3-D CFD program. An ignition phase curve covering the two combustion regimes is proposed and verified by numerical simulation. The ignition phase curve is used to reveal the underlying physics of each regime. It is found that the interaction among piston motion, chemical kinetics, fuel-air mixing, and injection event differs the two combustion regimes. The conventional DICI mode ignition is dominated by injection timing and affected by the mixture pressure and temperature during the flame induction period. In the PPC mode, the over-mixing effect of the fuel affects largely the ignition process. The variations of the moment of cool flame onset and high temperature ignition are discussed in detail. The differences between the proposed and calculated ignition phase curve are due to the specific piston and injector design of the test engine for which calculations are done. Finally, the effects of intake temperature on the ignition phase curve are explained based on numerical results.

The previous nine-step phenomenological soot model was revised by including the oxidation effect on soot number density. Using KIVA-3V Release 2 code coupled with this revised phenomenological soot model, multi-dimensional computational fluid dynamics (CFD) simulations of diesel spray combustion in a constant volume chamber was conducted to investigate the combustion physics and soot emission characteristics. Meanwhile, experiments were conducted in an optical constant volume combustion chamber under different ambient temperatures (800, 900, 1000 K), from which the combustion characteristics and soot distributions were obtained for validation. The results indicate that ignition retards with the decrease of ambient temperature, which results in more air-fuel mixing controlled diffusion combustion at high ambient temperature, and more premixed combustion at low ambient temperature. The corresponding soot formation and distribution shows that the soot emission is strongly related to the local equivalence ratio, which leads to lower soot emission in the lower initial temperature case with more homogeneous mixture. Compared to previous nine-step model, the revised model predicted lower soot number and bigger soot particles size.

The purpose of this study was to characterize combustion behavior for n -heptane using experimental measurements in a direct-injection constant-volume combustion chamber (CVCC) to validate chemical-kinetic mechanisms. This work is focused on compression-ignition ( i.e. , diesel) combustion, primarily because mechanisms for larger-chain diesel-relevant species are not well developed and require significant attention. The CVCC used in this work can be pressurized and heated to create engine-relevant conditions that enable study of autoignition behavior. In addition, the chamber is equipped with a high-pressure, common-rail diesel injector, making the study of autoignition and combustion in this system highly relevant to modern diesel engines. By varying injection pressure and duration, it is possible to control global equivalence ratio as well. Chamber pressure during injection and combustion is measured using a piezoelectric transducer, and can be subsequently used to infer heat-release rates. Experimental measurements for n -heptane mostly displayed expected trends. As initial chamber pressure increased, ignition delay decreased and peak pressure increased. As injection duration increased, ignition delay decreased due to faster ignition of richer mixtures, and peak pressure increased due to higher total heat release. The effect of temperature on ignition delay, however, was more complex and suggested some amount of NTC (negative temperature coefficient) behavior. For all conditions, heat-release rates indicated entirely premixed combustion with no hint of mixing-controlled combustion. Experimental data were compared with results from CHEMKIN-PRO simulations. The model simulated zero-dimensional combustion using a detailed n -heptane mechanism developed at Lawrence Livermore National Laboratory. These computations were used to infer local equivalence ratio information, based on equivalence ratio required in the model to match experimental ignition delay. For most test cases, the model required an equivalence ratio that was at least ∼2× richer than the global value. In addition, equivalence ratios in the model ranged a full order of magnitude, from ∼0.6 to 6, suggesting that local mixture equivalence ratios varied considerably as experimental conditions were varied. Results suggest that improved models that include details of spray physics are required in order to properly predict local equivalence ratios and resulting autoignition characteristics.