Engine-like Operating Conditions Research Articles

Exploiting the potential of large eddy simulations (LES) for ducted fuel injection investigation in non-reacting conditions

The diesel combustion research is increasingly focused on ducted fuel injection (DFI), a promising concept to abate engine-out soot emissions in compression-ignition engines. A large set of experiments carried out in constant volume vessel and numerical simulations, at medium-low computational cost, showed that the duct adoption in front of the injector nozzle activates several soot mitigation mechanisms, leading to quasi-zero soot formation in several engine-like operating conditions. However, although the simplified CFD modelling so far played a crucial role for the preliminary understanding of DFI technology, a more accurate turbulence description approach, combined with a large set of numerical experiments for statistical purposes, is of paramount importance for a robust knowledge of the DFI physical behaviour.In this context, the present work exploits the potential of large eddy simulations (LES) to analyse the non-reacting spray of DFI configuration compared with the unconstrained spray. For this purpose, a previously developed spray model, calibrated and validated in the RANS framework against an extensive amount of experimental data related to both free spray and DFI, has been employed. The tests have been carried out considering a single-hole injector in an optical accessible constant volume vessel, properly replicated in the simulation environment. This high-fidelity simulation model has been adapted for LES, firstly selecting the best grid settings, and then carrying out several numerical experiments for both spray configurations until achieving a satisfying statistical convergence. With this aim, the number of independent samples for the averaging procedure has been increased exploiting the axial symmetry characteristics of the present case study.Thanks to this approach, a detailed description of the main DFI-enabled soot mitigation mechanisms has been achieved, shrinking the knowledge gap in the physical understanding of the impact of spray-duct interaction.

Ducted Fuel Injection: Experimental and numerical investigation on fuel spray characteristics, air/fuel mixing and soot mitigation potential

Enhancing mixture preparation upstream of the premixed autoignition zone is a solution to reduce soot emission formation in compression ignition engines. With this aim, in recent years, Ducted Fuel Injection (DFI) concept has been developed: DFI is based on the idea of injecting the fuel spray through a small cylindrical pipe within the combustion chamber at a certain distance from the nozzle injector hole. Recent research studies have highlighted the high potential of this innovative concept for soot mitigation in both constant volume vessel and engine-like operating conditions. However, the mechanisms driving the soot reduction have not yet been fully understood. The aim of this research work is to further investigate the DFI concept, evaluating its impact on the spray characteristics, on the air/fuel mixing and, therefore, on the soot formation phenomena. Firstly, an experimental activity was carried out by means of a constant volume vessel test bench with optical accesses to compare the spray evolution and sizing with and without duct adoption, over a wide range of vessel thermodynamic conditions and injection pressures. After that, a simulation setup was defined in the commercially available 3D-CFD software CONVERGE reproducing the experimental test bench, calibrating and validating the spray model. Firstly, the spray model was calibrated and validated considering the same non-reacting conditions exploited in the experimental analysis. Then, the calibrated spray model was used as a virtual tool to investigate the air entrainment process. As a results, the DFI adoption increases the air entrainment in the near-nozzle region caused by the high velocity spray that generates a pumping effect at the duct inlet. Moreover, mixing process is also enhanced by the turbulence distribution at the duct exit resulting in a narrower distribution of equivalence ratio at the ignition. As a consequence of the more effective air entrainment and improved turbulent mixing, DFI remarkably mitigates soot emissions, with a reduction up to 80% with respect to free spray configuration in all tested operating conditions.

Time-resolved 3D imaging of two-phase fluid flow inside a steel fuel injector using synchrotron X-ray tomography

The multiphase flow inside a diesel injection nozzle is imaged using synchrotron X-rays from the Advanced Photon Source at Argonne National Laboratory. Through acquisitions performed at several viewing angles and subsequent tomographic reconstruction, in-situ 3D visualization is achieved for the first time inside a steel injector at engine-like operating conditions. The morphology of the internal flow reveals strong flow separation and vapor-filled cavities (cavitation), the degree of which correlates with the nozzle’s asymmetric inlet corner profile. Micron-scale surface features, which are artifacts of manufacturing, are shown to influence the morphology of the resulting liquid-gas interface. The data obtained at 0.1 ms time resolution exposes transient flow features and the flow development timescales are shown to be correlated with in-situ imaging of the fuel injector’s hydraulically-actuated valve (needle). As more than 98.5% of the X-ray photon flux is attenuated within the steel injector body itself, we are posed with a unique challenge for imaging the flow within. Time-resolved imaging under these low-light conditions is achieved by exploiting both the refractive and absorptive properties of X-ray photons. The data-processing strategy converted these images with a signal-to-noise ratio of ~ 10 into a meaningful dataset for understanding internal flow and cavitation in a nozzle of diameter 200 μm enclosed within 1–2 millimeters of steel.

Rate of heat release characteristics of supercritical sprays of dieseline blend in constant volume combustion chamber

In present experimental study, combustion characteristics of homogeneous supercritical (SC) fuel spray injection system are analyzed in cylindrical constant volume combustion chamber (CVCC) at experimental conditions similar to diesel engine-like operating conditions. A comparative study is also performed between SC fuel spray and normal diesel spray combustion under similar conditions. Combustion characteristics of SC fuel sprays includes rate of heat release (ROHR) and peak pressure (PP). Operating parameters include hot surface temperature range (673 ​K–723 ​K), pre-firing cylinder air pressure range (20 ​bar–40 ​bar) and injection pressure (IP) range of 100 ​bar–300 ​bar. Piezoresistive pressure transducer is employed for indicating and recording combustion processes. Diesel fuel and dieseline blend (50% gasoline and 50% diesel by v/v) are used for normal diesel spray and SC spray combustion respectively. It is found that ROHR in SC fuel spray combustion is smaller than in normal diesel spray combustion at all operating conditions. In SC fuel spray combustion, ROHR increases with increase in IP but decreases with increase in HST and CP due to homogeneous and hot air combustion nature of SC fuel sprays. Percentage reduction in ROHR of SC fuel spray combustion as compared to ROHR in normal diesel spray combustion is found to be nearly 34% at normal operating conditions of diesel engine. PP reached during SC spray combustion is noted to be less than PP reached during normal diesel spray combustion in all cases and is responsible for reduced NOx formation in SC spray combustion.

Dual-fuel injection fundamentals: experimental – numerical analysis into a constant-volume vessel

Dual-fuel combustion mode in compression ignition engines has been tested thoroughly, showing high potential for the reduction of emissions (especially nitric oxides and particulate matter) while keeping unchanged the fuel conversion efficiency compared with conventional Diesel engines. Controlling the reactivity of the secondary fuel is crucial for this kind of application.To this aim, a combined experimental/numerical approach is proposed in this study to provide, on one side, experimental data in controlled conditions for the calibration of the numerical models; on the other side, a numerical framework for the accurate simulation of the dual-fuel injection in engine-like operating conditions. More in detail, a constant-volume combustion vessel has been used to simulate and analyze the injection process varying the characteristic control parameters. Detailed high-resolution images of the injection and combustion processes were acquired for the validation of the numerical framework.Numerical simulations, carried out by means of the CONVERGE CFD code using a Reynolds Average Navier Stokes (RANS) approach allow for understanding the key differences between the nominal and off-design settings. Results have been compared with the experimental data in terms of liquid spray penetration. A comparison with high resolution images has also been done to prove the accuracy of the model to describe the spray evolution in terms of spray characteristics. In the provided picture, this contribution aims at demonstrating the robustness of the experimental/numerical framework that is essential for further development of such engine solution.

Experimental and computational approach to the transient behaviour of wall-flow diesel particulate filters

The implementation of tight vehicle emission standards has forced manufactures to use aftertreatment systems extensively. In addition to pollutant emissions abatement, these devices have a noticeable impact on the wave pattern. This fact affects the muffler design criteria. All monolithic aftertreatment devices produces a damping effect because of the honeycomb structure and the narrow channels. However, this response is more marked in wall-flow diesel particulate filters (DPF) because of the alternatively plugged ends and the dissipative properties of the porous substrate.The main goal of this paper is to assess the transient fluid-dynamic behaviour of wall-flow DPFs using experimental and modelling techniques. The experimental data were gathered in clean and loaded conditions. The DPF was subjected to a variety of pressure excitations to characterise its transient behaviour in the time and frequency domains. Afterwards, the DPF response was evaluated under engine-like operating conditions in an unsteady flow gas stand. Once the main characteristics of the response were known, a non-linear gas-dynamics model was proposed for analysis and prediction. The model accounts for space and time gradients, combining the thermo-and fluid-dynamic solution with a model based on a packed bed of spherical particles that defines the meso-structure of the loaded substrate.

On the rate of injection modeling applied to direct injection compression ignition engines

Modern engine design has challenging requirements toward maximum power output, fuel consumption and emissions. For engine combustion development programs, the injection system has to be able to operate reliable under a variety of operating conditions. Today’s legislations for quieter and cleaner engines require multiple-injection strategies, where it is important to understand the behavior of the system and to measure the effect of one injection on subsequent injections. This study presents a methodology for zero-dimensional modeling of the mass flow rate and the rail pressure of a common rail system, constructed from a set of experimental measurements in engine-like operating conditions, for single- and multiple-injection strategies. The model is based on mathematical expressions and correlations that can simulate the mass flow rate obtained with the Bosch tube experiment, focusing on the shape and the injected mass, using few inputs: rail pressure, back pressure, energizing time and so on. The model target is to satisfy two conditions: lowest computational cost and to reproduce the realistic injected quantity. Also, the influence of the rail pressure level on the start of injection is determined, especially for multiple-injection strategies on the rate shape and injected mass. Good accuracy was obtained in the simulations. The results showed that the model error is within the 5%, which corresponds at the same time to the natural error of the injector and to the accuracy of the measures which had been done. The benefits of the model are that simulations can be performed quickly and easily for any operation points, and, on the other hand, that the model can be used in real-time on the engine test bench for mass estimations when doing additional experiments or calibration activities.

Fuel temperature influence on the performance of a last generation common-rail diesel ballistic injector. Part II: 1D model development, validation and analysis

A one-dimensional model of a solenoid-driven common-rail diesel injector has been developed in order to study the influence of fuel temperature on the injection process. The model has been implemented after a thorough characterization of the injector, both from the dimensional and the hydraulic point of view. In this sense, experimental tools for the determination of the geometry of the injector lines and orifices have been described in the paper, together with the hydraulic setup introduced to characterize the flow behaviour through the calibrated orifices.An extensive validation of the model has been performed by comparing the modelled mass flow rate against the experimental results introduced in the first part of the paper, which were performed for different engine-like operating conditions involving a wide range of fuel temperatures, injection pressures and energizing times. In that first part of the study, an important influence of the fuel temperature was reported, especially in terms of the dynamic behaviour of the injector, due to its ballistic nature. The results from the model have allowed to explain and further extend the findings of the experimental study by analyzing key features of the injector dynamics, such as the pressure drop established in the control volume due to the control orifices performance or the forces due to viscous friction, also assessing their influence on the needle lift laws.

Fuel-Substitution Method for Investigating the Kinetics of Low-Volatility Fuels under Enginelike Operating Conditions

The combustion properties of two low-volatility diesel fuels were characterized under engine-relevant conditions using a new fuel-substitution strategy that minimizes biases associated with unknown boundary conditions. The engine is operated in a homogeneous charge compression ignition mode with port fuel injection of the primary reference fuels, n-heptane and isooctane, and an upstream prevaporizer for the diesel-like heavy test fuels. The engine is first operated on the primary reference fuels (PRFs). The intake conditions are then fixed, and the heavy fuel is introduced in increasing amounts while the mass of port-injected fuel is reduced to maintain engine load and the n-heptane-to-isooctane ratio is adjusted to maintain constant combustion phasing. It is shown that this provides nearly constant in-cylinder thermodynamic and boundary conditions. The baseline condition, which uses well-characterized fuels, can be used to adjust the trapped-gas initial conditions and the heat transfer rates of a computational model, but these parameters do not change with the subsequent addition of heavy test fuel. The test fuel combustion characteristics are described in terms of the PRF number of the port-injected mixture. A simple linear blending method was found to adequately represent the heavy test fuel in terms of an effective PRF number. Testing under different operating conditions showed no significant change in the effective PRF number of the test fuels. The fuels investigated in this study were F-76, a high-sulfur-content diesel fuel, and HRD, a hydroprocessed renewable diesel fuel composed primarily of alkanes. Effective PRF numbers of 35 and −25 were found for F-76 and HRD, respectively.

Evaluation and optimisation of phenomenological multi-step soot model for spray combustion under diesel engine-like operating conditions

In this work, a two-dimensional computational fluid dynamics study is reported of an n-heptane combustion event and the associated soot formation process in a constant volume combustion chamber. The key interest here is to evaluate the sensitivity of the chemical kinetics and submodels of a semi-empirical soot model in predicting the associated events. Numerical computation is performed using an open-source code and a chemistry coordinate mapping approach is used to expedite the calculation. A library consisting of various phenomenological multi-step soot models is constructed and integrated with the spray combustion solver. Prior to the soot modelling, combustion simulations are carried out. Numerical results show that the ignition delay times and lift-off lengths exhibit good agreement with the experimental measurements across a wide range of operating conditions, apart from those in the cases with ambient temperature lower than 850 K. The variation of the soot precursor production with respect to the change of ambient oxygen levels qualitatively agrees with that of the conceptual models when the skeletal n-heptane mechanism is integrated with a reduced pyrene chemistry. Subsequently, a comprehensive sensitivity analysis is carried out to appraise the existing soot formation and oxidation submodels. It is revealed that the soot formation is captured when the surface growth rate is calculated using a square root function of the soot specific surface area and when a pressure-dependent model constant is considered. An optimised soot model is then proposed based on the knowledge gained through this exercise. With the implementation of optimised model, the simulated soot onset and transport phenomena before reaching quasi-steady state agree reasonably well with the experimental observation. Also, variation of spatial soot distribution and soot mass produced at oxygen molar fractions ranging from 10.0 to 21.0% for both low and high density conditions are reproduced.