•An quasi-dimensional multi-component vaporization model for wall film was developed.•The thermal and mass diffusion in the film was described by high-order polynomials.•Effect of the thermal and mass diffusion on diesel film vaporization was discussed.•The relative error of various vaporization models for wall film was discussed.•Computational accuracy and efficiency of models were evaluated in CFD application.An improved multi-component quasi-dimensional vaporization model for wall film was proposed with considering the finite thermal and mass diffusions within the liquid film. In the improved model, high-order polynomials were introduced to describe the profiles of the temperature and component concentrations within the film. The results show that the predictions from the present quasi-dimensional model agree well with those predicted by the one-dimensional model. By investigating the effect of the thermal and mass diffusions on the vaporization of the diesel film, it is found that the thermal diffusion plays a more dominant role in the multi-component film vaporization. Compared with the linear temperature model with the linear temperature and uniform component distributions in the film, the application range of the quasi-dimensional model is considerably wider and the computational error is significantly reduced. Finally, the linear temperature, quasi-dimensional, and one-dimensional models were integrated into a Computational Fluid Dynamics (CFD) code for the simulations of film vaporization in the flow over a backward facing step and in a practical diesel engine. The results indicate that the improved model gives much better agreement with the one-dimensional solutions than the linear temperature model, while maintaining high computational efficiency under different operating conditions.

The evaporation of the fuel wall film considerably affects the performance and exhaust emissions of internal combustion engines. An analytical model for wall film heating and evaporation was developed and applied in this paper for predicting the temporal and spatial temperature distributions of the liquid film. The effects of the heat conduction between the fuel film and the wall, the convection between the film surface and the surrounding gas, and the film evaporation were taken into account in the present analytical model. This analytical solution was validated by the predictions from the discrete numerical vaporization model, and it is found that accurate predictions can be obtained by the present model. In order to understand the evolution behavior of the wall film, the influence of the wall temperature, the ambient pressure, and the initial film thickness were investigated. The results indicate that the evolution of the wall film evaporation can be divided into two distinct stages, i.e., an initial rapid heating stage and a slow cooling stage. The lifetime of the wall film can be shortened by increasing the wall temperature, decreasing the ambient pressure and the initial wall film thickness. The purpose of this work is to reproduce the transient behaviors of the wall film heating and evaporation with an analytical solution, which is easy to setup and solve.

•Parametric study was performed on a heavy-duty LTC engine under a wide load range.•Effect of LIVC coupled with the related operating parameters were investigated.•The first and second law of thermodynamics were used to analyze the engine efficiency.By using a multi-dimensional computational fluid dynamics (CFD) code, the combustion process of a heavy-duty diesel engine with low temperature combustion (LTC) at different loads was investigated. Based on the optimization results, the potential of the late intake valve closing (LIVC) strategy coupled with boosted intake pressure, as well as the influence of fuel injection timing and exhaust gas recirculation (EGR) rate on the fuel consumption and emissions was discussed. The energy and exergy analyses were further performed using the first and second law of thermodynamics. The results indicate that when the LIVC strategy is applied, boosted intake pressure is needed to improve the thermal efficiency and reduce the soot emissions, especially at high load. However, retarding IVC timing leads to increasing exergy destruction as the global equivalence ratio remains constant. The exergy destruction at mid load is the lowest owing to the highest combustion temperature. At low and mid load, with advanced fuel injection, high EGR rate is required to reduce the nitrogen oxides (NOx) emissions. At high load, with retarded fuel injection, relatively lower EGR rate is required for reducing NOx emissions because of the retarded combustion phasing and more H2O and CO2 contained in the exhaust gases.

A new skeletal oxidation mechanism for the primary reference fuel (PRF) was established with a decoupling methodology. The mechanism is composed of n-hexadecane and iso-cetane submechanisms, containing 44 species and 139 reactions. Using the present mechanism, the relationship between cetane number and the ignition delay in shock tubes was investigated. First, based on the ignition delay data in shock tubes, the cetane number of various fuels was estimated using the present PRF mechanism and a weighted least-squares method. The prediction of cetane number investigated in this study primarily focused on the operating conditions of practical diesel engines (i.e., the equivalence ratio of 1.0 and pressures from 19–80 atm), which encompass the cetane number from 15 to 100. Under the test operating conditions, the mean absolute deviation of the predicted cetane number is within 3.327. Furthermore, according the cetane number of different fuels, the ignition delays in shock tubes were reproduced by the present mechanism focusing on a wide range of equivalence ratios (0.5–3.0) and pressures (20–50 atm). The results indicated that the predicted IDs of alkanes were more accurate than those of other types of fuels and blended fuels because of the consistent molecular structure of the n-hexadecane/iso-cetane used in the present mechanism. Because of the compact size of the skeletal mechanism, its application can considerably reduce the computational time for 3D combustion simulations, especially for practical fuels with complicated compositions.

A series of skeletal oxidation mechanisms for the saturated fatty acid methyl esters (FAMEs) from methyl butanoate to methyl palmitate were developed on the basis of a decoupling methodology with special emphasis on engine-relevant conditions from low to high temperatures at high pressures. When detailed H2/CO/C1, reduced C2–C3, and skeletal C4–Cn submechanisms are introduced, the final mechanism consists of 42 species and around 135 reactions for each methyl ester. Both the high-temperature reactions of the methyl ester moiety and the low-temperature reactions of the aliphatic chain of the ester are included in the mechanism. The skeletal mechanisms were verified against experimental data in shock tubes, jet-stirred reactors, flow reactors, and premixed and opposite flames over the temperatures from 500 to 1700 K at pressures of 1–50 atm from fuel-lean to fuel-rich mixtures. An overall satisfactory agreement between the measurements and computational results was achieved for all of the saturated methyl esters, especially for the large saturated methyl esters with a long aliphatic main chain. The results also indicate that the ignition delay time and the consumption of reactants could be reproduced by employing a skeletal C4–Cn submechanism reasonably well. In addition, the evolution of major products and flame propagation and extinction characteristics were satisfactorily reproduced because the detailed H2/CO/C1 mechanism was used. The compact size makes it easy to integrate the mechanism into multi-dimensional computational fluid dynamics (CFD) simulation.

•A reduced n-dodecane-PAH mechanism is proposed for combustion and soot predictions.•A multi-step soot model was coupled with the mechanism to predict the soot formations.•Effects of ambient conditions (temperature, O2, density) on soot were discussed.•The simulations capture the important characteristics of the soot formation process.•More effort is needed to refine chemical kinetic mechanisms.A reduced n-dodecane-polycyclic aromatic hydrocarbon (PAH) mechanism is presented for modeling the combustion and soot formation processes of n-dodecane and compared with the optical studies conducted in the constant volume vessel from the engine combustion network (ECN). The proposed reduced n-dodecane-PAH mechanism consists of 100 species and 432 reactions, and it is also validated with available ignition delays and species concentration profiles from shock tubes and jet stirred reactors (JSR). The fuel–air mixing process of non-reacting n-dodecane sprays was validated with available liquid and vapor penetration, and fuel mixture fraction distribution data. The proposed mechanism was coupled with a multi-step soot model to predict the combustion and soot formation processes. The effects of ambient temperature, density, oxygen concentration, injection pressure and also reaction mechanism on soot formation are analyzed and discussed. The results show that the simulations capture the important characteristics of the soot formation process. Increasing injection pressure and ambient oxygen concentration helps soot reduction by either enhancing the mixing process to suppress the soot formation rate, or by increasing the available oxygen to accelerate the soot oxidation rate, while higher ambient temperature and density have negative effects on soot formation. In addition, comparisons between results with the proposed n-dodecane mechanism and a reduced n-heptane mechanism show that the general combustion characteristics are quite similar. However, the n-heptane mechanism tends to predict longer ignition delays and lift-off lengths under spray combustion conditions, although the predicted ignition delay of n-heptane is very close to or even shorter than n-dodecane under homogeneous conditions. This observation shows that although the simulations provide reasonable combustion and soot predictions under a wide range of operating conditions, more effort is still needed to refine chemical kinetic mechanisms.

A new chemical mechanism with 12 species and 26 reactions for formation of polycyclic aromatic hydrocarbons (PAHs) was developed and integrated into a skeletal mechanism for oxidation of primary reference fuel (PRF). Coupled with the new skeletal PRF-PAH mechanism, an improved phenomenological soot model was further constructed based on our previous work. By validating against the experimental data on the related PAHs in four premixed laminar flames of n-heptane/iso-octane and three counterflow diffusion flames of n-heptane, it is indicated that the major species concentrations were well reproduced by the model. Moreover, validations of the new soot model show that the soot yield, particle diameter, and number density were predicted with reasonable agreement with the experimental data in a rich n-heptane shock tube over wide temperature and pressure ranges. Compared with the soot model with acetylene as precursor species, the new model agrees better with the measurement, which proves the necessity of including PAHs chemistry for soot modeling. Finally, the model was applied to simulate the soot distributions in n-heptane sprays in the Sandia constant-volume combustion chamber under high EGR conditions, as well as the evolutions of PAH and soot concentrations in an engine fueled with n-heptane. It is also found that the experimental data was reasonably well reproduced by the model.

A series of skeletal mechanisms was developed based on a decoupling methodology to describe the oxidation of n-alkanes from n-octane to n-hexadecane. In the decoupling methodology, a fuel oxidation mechanism is divided into two parts: one is an extremely simplified model for species with a carbon atom number larger than two to simulate the ignition characteristics of n-alkane; the other is a detailed mechanism for H2/CO/C1 to predict the concentrations of small molecules, laminar flame speed, and extinction strain rate. The new skeletal mechanism includes only 36 species and 128 reactions for each n-alkane from n-octane to n-hexadecane. The mechanism was extensively validated against the experimental data in a shock tube, jet-stirred reactor, flow reactor, counterflow flame, and premixed laminar flame. Good agreements on ignition delay, the concentrations of major species, laminar flame speed, and extinction strain rate between the predictions and measurements were obtained over wide ranges of temperature, pressure, and equivalence ratio, which demonstrates the capability of the decoupling methodology to build skeletal oxidation mechanisms for n-alkanes. Due to the compact size of the new skeletal mechanism, it can be easily integrated into the computational fluid dynamics (CFD) simulation.

A full-cycle three-dimensional computational fluid dynamics (CFD) model coupled with detailed chemical kinetics has been developed to investigate the effect of late intake valve closing (IVC) on combustion and emission characteristics in a diesel engine with premixed charge compression ignition (PCCI) combustion. The application of late IVC was demonstrated to provide efficient control of ignition timing and significant reduction of nitrogen oxides (NOx) and soot emissions by decreasing the effective compression ratio and increasing premixing, but it possibly led to increases of hydrocarbon (HC) and carbon monoxide (CO) emissions due to low combustion temperature and insufficient oxygen amount. Parametric studies by varying intake pressure, exhaust gas recirculation (EGR) rate and start of injection (SOI) timing with varied IVC timing were conducted to explore the potential of late IVC for emission reduction in diesel PCCI engines. The results showed that, with assistance of increasing intake pressure, late IVC could reduce NOx, soot, HC, and CO emissions simultaneously. A certain EGR rate and optimized SOI timing were always necessary to maintain satisfactory NOx and soot emissions for diesel PCCI combustion.

A new skeletal surrogate model including methyl decenoate (MD), methyl 5-decenoate (MD5D), and n-decane was proposed. In the surrogate model, MD and MD5D were chosen to respectively represent the saturated methyl ester and unsaturated methyl ester in biodiesel, and n-decane was included to match the energy content and C/H/O ratio of actual biodiesel fuel. Based on a decoupling methodology, an oxidation mechanism for the biodiesel surrogate was constructed by integrating the skeletal large-molecule sub-mechanisms for n-decane, MD and MD5D, a reduced C2–C3 mechanism, and a detailed H2/CO/C1 mechanism. The final mechanism for the biodiesel surrogate is composed of 60 species and 172 reactions. The mechanism was validated against experimental data, including ignition delay times in shock tubes and major species concentrations in jet-stirred reactors over wide operating conditions. Moreover, the mechanism was employed to simulate the combustion and emission characteristics of an engine operated in a low temperature combustion mode with SME as fuel. The overall agreement between the predictions and measurements is satisfactory.

•The present study was performed on a PCCI engine under a wide load range.•The VVT strategy at different load conditions was systematically optimized.•The operating parameters under various IVC timings were revealed.By coupling a multi-dimensional computational fluid dynamics (CFD) code with genetic algorithm (GA), the combustion of a heavy-duty diesel engine with LTC (low temperature combustion) was optimized under a wide load range. At each load, a comprehensive optimization of the operating parameters including IVC (intake valve closing) timing, SOI (start of injection) timing, EGR (exhaust gas recirculation) rate, the initial in-cylinder pressure and temperature at IVC was conducted in order to simultaneously minimize ISFC (indicated specific fuel consumption), NOx (nitrogen oxides) and soot emissions, and seek the optimal control strategies. Furthermore, by employing the one-dimensional simulation, the correlation between the initial in-cylinder conditions at IVC and the intake conditions was developed. The optimization results indicate that the range of the operating parameters narrows considerably with increasing load. At low load, both early and late IVC timing can be employed. As late IVC is introduced, high intake pressure and high EGR rate up to 70% are needed to realize low NOx emissions, whereas low intake pressure and moderate EGR rate (around 40%) are necessary for early IVC. For both late and early IVC, the optimal SOI timing is 10–20 °CA BTDC (before top dead center) at low load to simultaneously avoid serious spray/wall impingement and diffusion combustion. At mid load, IVC timing should be advanced to 104–110 °CA BTDC with a moderate EGR rate (40%–50%) and slightly high intake pressure, and SOI is similar with that of low load. In contrast, at high load, the optimal IVC timing is fixed at around 114 °CA BTDC and EGR rate is reduced to about 20%, while a late SOI (2.9 °CA ATDC) is needed to avoid overly high in-cylinder peak pressure and pressure rise rate.