Ricardo Novella

The role of detailed chemical kinetics on CFD diesel spray ignition and combustion modelling

Spray ignition and flame stabilisation in the frame of diesel-like combustion conditions combine fundamental and complex physical and chemical processes. In this work, a numerical investigation has been performed to evaluate the potential of integrating detailed chemistry into CFD calculations, in order to improve predictions and gain more insight in involved processes. This work has been carried out using the capabilities of OpenFOAM^(C)code, which provides an opensource framework for 3D-CFD simulations, including an ODE solver for solving chemical kinetics. As a general methodology, this study is based on simulating free n-heptane sprays injected into a constant volume vessel, corresponding to the conditions of the experimental database provided by Sandia National Laboratories. Calculations results have been compared to experiments, evaluating the effect of a wide range of ambient conditions on spray ignition and combustion characteristics. Specifically, this research checks the performance of some relevant n-heptane oxidation mechanisms found in the literature, with different degree of complexity, for modelling the chemical history of the fuel. The results of this investigation show the relative influence of chemical mechanism on spray/flame structure in terms of ignition delay and also ignition and flame stabilisation sites. The comprehensive mechanism performs generally better than more simplified chemistry models. However, its accuracy is also compromised for modelling advanced diesel-like combustion concepts based on injecting the spray into a low oxygen concentration environment.

Numerical simulation and extended validation of two-phase compressible flow in diesel injector nozzles

In this paper, the ability of a computational fluid dynamics code to reproduce cavitation phenomena accurately is checked by comparing data acquired by numerical simulations against those obtained from different experiments involving the mass flow, the momentum flux, and the effective injection velocity. Cavitation is modelled using a single-phase cavitation model based on a barotropic equation of state together with a homogeneous equilibrium assumption. In the research reported in this paper, the ability to use the code for actual diesel injector nozzle geometries and conditions has been checked and validated. The main contribution of the present investigation and what makes it different from previous work in the literature is the consideration of extended experimental data for validation purposes: the mass flow, the momentum flux at the nozzle exit, and the effective injection velocity. These are unique features in contrast with other publications, which normally take into account at the most, if at all, the cavitation morphology or the mass flow. The results obtained and their comparison with available experimental data show how the model is able to predict the behaviour of the fluid in such conditions with a high level of confidence.

Analysis of combustion concepts in a newly designed two-stroke high-speed direct injection compression ignition engine

Two research paths are being followed to develop compression ignition engines, the extreme optimization of the conventional diesel combustion concept and the development of alternative combustion concepts. The optimization of the conventional diesel combustion concept focuses on controlling the combustion development in an attempt to improve pollutant emissions and efficiency. Additionally, extensive research in four-stroke engines already demonstrated the benefits of the partially premixed combustion concept in terms of emissions and efficiency when using high volatility and low reactivity fuels, such as gasoline-like fuels, from medium-to-high engine loads. A detailed optimization of the conventional diesel combustion concept has been performed in an innovative two-stroke poppet valves high speed direct injection compression ignition engine, in order to find the real limits of this engine configuration. Later, its compatibility with the partially premixed combustion concept using a high octane fuel (Research Octane Number 95) with a triple injection strategy for reducing pollutant emissions at medium-to-low load conditions has been evaluated considering also the impact on engine efficiency. Results confirm the potential for attaining state-of-the-art emission levels operating with diesel combustion, and how emissions and efficiency can be optimized by adjusting the air management settings without facing any additional trade-off aside from that usual between NOX and soot. The feasibility of combining this engine configuration with the gasoline partially premixed combustion concept for controlling pollutant emissions has been also corroborated and, with a fine tuned triple injection strategy, engine efficiency even improves compared to that obtained operating with well-optimized diesel combustion.

Influence of injection conditions and exhaust gas recirculation in a high-speed direct-injection diesel engine operating with a late split injection:

Abstract This paper describes an investigation on low-temperature combustion in a small single-cylinder high-speed direct-injection diesel engine. This engine is representative of turbocharged passenger car diesel engines equipped with a particulate trap and an oxidation catalyst. A medium-load engine operation condition was evaluated using two fuel injection events near top dead centre and high levels of exhaust gas recirculation. The combustion was characterized by extremely low emission of nitrogen oxides (of the order of 10—15ppm) and low combustion noise. The study was carried out by means of a design of experiments, consisting of four factors varied on three levels according to a Box—Behnken design. This Box—Behnken design was extended with validation points, allowing all factors to be studied independently in a parametric approach. Additionally, some relevant two-factor interactions have been studied. The investigated factors were the exhaust gas recirculation rate, the combustion phasing, the injection pressure, and the dwell between the split injections. The results show how it is possible to achieve very low levels of nitrogen oxides and low combustion noise with reasonable fuel consumption, smoke emissions, hydrocarbons, and carbon monoxide emissions, with injection and combustion just after top dead centre.

Influence of Boost Pressure and Injection Pressure on Combustion Process and Exhaust Emissions in a HD Diesel Engine

The scope of this study is the analysis of the influence of boost pressure and injection pressure on combustion process and pollutant emissions. The influence of these parameters is investigated for different engine speeds. Fuel mass was kept constant for all the tests in order to avoid its influence on the analysis. A single cylinder research diesel engine, equipped with a common rail injection system capable of operating up to a maximum pressure of 150 MPa was used. Special attention was paid to NOx, smoke (which are the most important pollutants for legislation) and brake specific fuel consumption.

Investigation of the ignition and combustion processes of a dual-fuel spray under diesel-like conditions using computational fluid dynamics (CFD) modeling

Abstract Recent research activities in the field of diesel engines have shown the potential to reduce pollutant emissions and improve the thermal efficiency by controlling the fuel reactivity. However, understanding the impact of blending fuels with different physical and especially chemical properties on diesel-like spray mixing and combustion processes is still a challenge. Since the experimental techniques are still far from providing detailed temporal and spatial information about local spray conditions, computational fluid dynamics (CFD) modeling tools have become the key source of information for investigating the characteristics of these dual-fuel sprays. In this frame, the present research focuses on modeling a dual-fuel spray in diesel-like conditions, comparing different gasoline and diesel blends in terms of ignition characteristics and flame structure. The results confirm the suitability of the state of the art computational CFD modeling tools for reproducing the complex phenomena associated to dual-fuel sprays. Moreover, the important benefits provided by dual-fuel blends, considering the expected reduction in pollutant emissions as a consequence of the differences observed in terms of flame structure, are confirmed.

Investigation on Multiple Injection Strategies for Gasoline PPC Operation in a Newly Designed 2-Stroke HSDI Compression Ignition Engine

Partially Premixed Combustion (PPC) of fuels in the gasoline octane range has proven its potential to achieve simultaneous reduction in soot and NOX emissions, combined with high indicated efficiencies; while still retaining proper control over combustion phasing with the injection event, contrary to fully premixed strategies. However, gasoline fuels with high octane number as the commonly available for the public provide a challenge to ensure reliable ignition especially in the low load range, while fuel blends with lower octane numbers present problems for extending the ignition delay in the high load range and avoid the onset of knocking-like combustion. Thus, choosing an appropriate fuel and injection strategy is critical to solve these issues, assuring successful PPC operation in the full engine

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.

Evaluation of combustion models based on tabulated chemistry and presumed probability density function approach for diesel spray simulation

Two combustion models of different complexity have been implemented in a RANS solver in the CFD platform OpenFOAM®. Both models rely on the flame prolongation of ILDM (FPI) method, which allows the use of detailed chemistry mechanisms at relatively low computational costs. The homogeneous auto-ignition (HAI) model, directly using the chemical data from the FPI tabulation, does not take into account subgrid turbulence-chemistry interaction. Therefore, the second, more advanced model combines the FPI method with a presumed conditional moment approach. This auto-ignition-presumed conditional moment (AI-PCM) model accounts for the fluctuations of the mixture fraction and the progress variable caused by the turbulent flow. Both models have been evaluated by means of a parametric study of a single diesel spray at varying initial temperatures and oxygen concentration levels. The results obtained with the CFD models have been compared with experimental data from the engine combustion network (ECN). The comparison of the two models demonstrates the important role of the subgrid turbulence-chemistry interaction on the accuracy of the auto-ignition process and the diesel flame structure, as indicated by the agreement of the AI-PCM predictions with the measured data.

A New Methodology to Evaluate Engine Ignition Systems in High Density Conditions

In this research, a new methodology to evaluate the operation of spark ignition systems in high density conditions is presented. New requirements in engines and new combustion modes demand more from these systems. One of the most important new requirements is the increase in density. Thus, a better understanding of the effects of high density and the behavior of the ignition system in these conditions seems necessary. To carry out this work two experimental facilities have been used: a transparent constant volume vessel, and an optical engine to simulate real engine conditions. Thus, the study combines the electrical signals derivate parameters and images obtained with a high speed camera. The methodology has been applied for different cases of pressure, intake temperature, and other parameters that affect the density. Results show that an increase in density causes a decrease in integrated power. Additionally, the dispersion in this integrated power increases too. Finally, the methodology results offer a useful data base for the engineers willing to improve the design of the ignition system. Moreover, it is validated that the results of the ambient transparent constant volume vessel follow the same trends and values as the realistic ones.