Simulations and Optical Diagnostics for Internal Combustion Engines

In the last few decades, application of advanced combustion technologies, engine design modifications, after-treatment techniques, etc. have improved engine performance and reduced exhaust emissions. However, advancement in technologies introduced a large number of parameters, which need to be optimised for satisfactory results. This increases the system complexity therefore it become very difficult to perform experiments for optimization of each parameter. In recent years, simulation techniques have shown their potential to resolve this issue. Simulation techniques can optimize such complex systems and provide optimized solution with less effort involvement. Optical diagnostic techniques are the other important area for internal combustion engines. These techniques can provide information about in-cylinder conditions, spray parameters, fuel-air mixing and combustion, which is not possible with conventional techniques. This book is based on these simulation and optical diagnostic techniques and provides case studies to resolve several technical issues involved with internal combustion engines. Few chapters based on advanced topics such as microwave-assisted plasma ignition, laser ignition, etc. are the important aspect of this book. Chapters are based on review of state-of-the-art models for combustion, with special focus on the theory, development and applications of these combustion models in internal combustion systems make this book suitable for researchers, working in the area of alternative fuels and advanced combustion techniques.

Deteriorating environment and stricter emission norms are motivating researchers for finding sustainable transport solutions. Researchers are focusing on two approaches namely adaptation of alternative fuels, and exhaust gas after-treatment. Utilization of alternate fuels such as methanol, ethanol, and biodiesel etc. in internal combustion (IC) engines reduces inherent chemical components present in conventional fossil fuels. These chemical species are a major source of harmful pollutants such as particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), PM bound trace metals, etc. Advancement in after-treatment technologies such as optimization of hexagonal cells of substrate, use of noble metals, etc. are also effective in reducing pollutants from engine tail-pipe. However, developments for adaptation of these technologies in existing engines is a challenging task. For adaptation of any alternative fuel, engine components need to be modified according to fuel properties. However, optimization of design parameters of thousands of engine components is a tedious task, which cannot be done experimentally. This can be done easily using modelling techniques, in which a prototype engine can be developed to investigate the effect of engine design parameters and fuel properties on the engine performance and emission characteristics. In last few years, 1-D and 3-D simulation tools have been extensively explored for engine design and performance optimization. This chapter discusses basic modelling techniques, which can be used for engine research. This chapter also presents heat transfer models, which are important for in-cylinder combustion analysis. Few fluid-flow models have also been discussed in this chapter, which are mainly used for in-cylinder air-flow investigations, fuel flow in the fuel injection system, etc. Overall, this chapter discusses modelling aspects related to engine design so that alternative fuels can be adapted.

Enhancing the predictability of diesel spray numerical simulation, a droplet breakup model has been developed and optimized under non-evaporative diesel spray with a large-eddy simulation. In the spray simulation community, the model called Kelvin–Helmholtz and Rayleigh–Taylor model has been widely used even nowadays, both of which are modeled for high Weber number conditions. While upstream region of spray considers as high Weber number thanks to high injection pressure, downstream of the spray represents by low Weber number region. In this study, a hybrid breakup model which combines the Kelvin–Helmholtz and modified Taylor analogy breakup has been proposed. The Kelvin–Helmholtz and modified Taylor analogy breakup models are used to the primary and secondary breakup models, respectively. For validating the breakup model, one of the unique optical diagnostics techniques has been introduced to capture both macroscopic and microscopic spray characteristics at the same time. The system called super high spatial resolution photography lens is able to capture sufficient area of the spray with having a 5 µm spatial resolution. Spray simulations under non-evaporative condition were performed to validate the Kelvin–Helmholtz–modified Taylor analogy breakup model. It is found that the simulation results of Kelvin–Helmholtz–Modified Taylor analogy breakup are in good agreement with experimental measurements of droplet distribution under non-evaporative spray.

The depleting oil resources and harmful emissions from fossil fuel have raised keen attention whole world wide. Emulsified fuel technology might prolong availability of earthborn fossil fuel along with the reduction of the pollutants. It may also be seen as a useful technique to meet the latest stringent Bharat Stage VI norms in diesel engine. Emulsification requires mixing fossil fuel with water in the presence of a surfactant. Oxygenated fuels like alcohols may also be blended due to its renewability. Oxides of nitrogen tend to reduce owing to cooling effect of water content in the emulsified fuel. At the same time decrease in the amount of particulate matter in the tail pipe was observed because of reduced molecule size of emulsified fuel. The combustion and exhaust emissions characteristics have been elaborated numerically with the help of simulation software in this chapter. Acceptability of emulsified fuels has also been highlighted for its use in compression ignition engines.

In order to comply with the current and future emission norms applicable to diesel engines, understanding the fuel-air mixing phenomena in depth is quite crucial. Fuel spray inside the cylinder of an engine in operation interacts with in-cylinder gases as well as with solid boundaries. Fuel spray impinging on the cylinder wall and piston top, may subsequently enhance soot formation and hence, study and analysis of fuel spary characteristics can help to minimize these effects. However, study of the physics of spray evolvement and dynamics demands advanced diagnostics and numerical techniques. Many attempts have been made in developing computational models for analyzing the fuel-air and fuel-wall interactions. Despite those efforts it remains an exciting area of research to accurately model the spray behavior under dynamic conditions inside the engine cylinder. These models need continuous inputs from experimental studies for validation and for further development purposes. For experimental investigations point of view, several optical methods have been adopted viz. Phase Doppler Interferometry (PDI), Shadowgraphy, Schlieren photography etc. However, deployment of these techniques for acquiring precise and reliable data requires certain expertises. The aim of this chapter is to confine various optical diagnostics techniques applicable to diesel engines. A critical review of these methods has been presented for further advancement in the field.

Maximum fuel injection pressure in gasoline direct injection engine is expected to increase because of its potential to reduce emissions while maintaining a high efficiency in spark ignition engine. Present gasoline injectors in the market operates in the range of 20–30 MPa. Because of many positive effects of high injection pressure for the emission reduction and fuel efficiency, an interest has been developed to investigate the spray behavior at around 40 MPa, 60 MPa and even more higher injection pressure. A fundamental investigation of spray characteristics at high-pressure injection will help to develop the understanding of spray behavior at such elevated pressure. In the present study, a gasoline fuel spray was studied through the numerical model at an injection pressure ranging from 40 to 150 MPa. A numerical simulation was performed in an optical accessible constant volume chamber. The chamber was effectively non-reacting and non-vaporizing condition since the focus was on the spray droplets. In the numerical model, gas flow was calculated by large-eddy simulation (LES) method and the liquid phase was accounted by a standard Lagrangian spray model. The fuel spray atomization was modelled using the Kelvin Helmholtz—Rayleigh Taylor (KH-RT) model, and droplet size distribution followed the Rosin-Rammler distribution function. Simulation results were validated by comparing the liquid penetration length of spray with the experimental data at different fuel injection pressures. Then, the mean droplet sizes such as arithmetic mean diameter and Sauter mean diameter of the spray droplets were compared with the measure droplet sizes as a function of pressure. The spray droplet size distribution was also shown along with measured droplet sizes. The result shows that the liquid length penetration of the spray was significantly increases together with the higher probability of smaller droplet by increasing the fuel injection pressure. Moreover, the mean droplet sizes were also reducing by increasing the fuel injection pressure, such as the droplet SMD was reduced from 13.5 to 7.5 \( \upmu \)m by injecting the fuel at pressure 150 MPa instead of 40 MPa.

Fuel Injection Equipment (FIE) are an integral component of modern Internal Combustion Engines (ICE), since they play a crucial role in the fuel atomization process and in the formation of a fuel/air combustible mixture, consequently affecting efficiency and pollutant formation. Advancements and improvements of FIE systems are determined by the complexity of the physical mechanisms taking place; the spatial scales are in the order of millimetres, flow may become locally highly supersonic, leading to very small temporal scales of microseconds or less. The operation of these devices is highly unsteady, involving moving geometries such as needle valves. Additionally, extreme pressure changes imply that many assumptions of traditional fluid mechanics, such as incompressibility, are no longer valid. Furthermore, the description of the fuel properties becomes an issue, since fuel databases are scarce or limited to pure components, whereas actual fuels are commonly hydrocarbon mixtures. Last but not least, complicated phenomena such as phase change or transition from subcritical to transcritical/supercritical state of matter further pose complications in the understanding of the operation of these devices.

Due to the depletion of petroleum resources and environmental concerns, automobile industry has been developing new engine technologies with acceptable cost range to consumers. Among many new technologies, application of non-thermal plasma ignition system is considered as a promising path to achieve high-efficiency clean gasoline vehicles. In this study, we developed a microwave-assisted plasma ignition using 3 kW, 2.45 GHz magnetron with customized electric components and ignitor. This system was tested in a constant volume combustion vessel to investigate the effects of microwave ejection on ignition kernel growth. High-speed shadowgraph imaging and hydroxyl (OH) radical imaging were carried out under various air-fuel ratio, ambient pressure, and ignition strategy conditions. The in-cylinder pressure measurement was also performed to compare combustion phase between conventional spark and microwave-assisted plasma ignition system. The experimental result showed that the microwave ejection on the thermal plasma created by conventional discharge had a significant improvement on initial flame development. The microwave-assisted plasma ignition system indicated advanced combustion phase with extended lean limit where conventional spark ignition failed to achieve flame propagation. The OH imaging on propagating flame presented much higher intensity with microwave-assisted plasma ignition case. The analysis on light emission spectrum showed 7,000 K higher electron temperature in the plasma created with microwave ejection. This implies that chemical reactions which could not be progressed with conventional spark ignition was enabled with additional non-thermal plasma induced by electro-magnetic wave. On the other hand, however, the enhancement in flame development was decreased under high pressure condition due to lower reduced electric field.

Most SI engine globally use conventional electric spark plug as an ignition source. Electric spark plugs have limitations in achieving higher efficiency and reducing emissions from gaseous fuelled automotive engine. In contrast to conventional electrical spark plugs, laser spark plugs are apt for use at higher in-cylinder pressures. Laser Ignition technology is capable of igniting leaner fuel-air mixture, which cannot be successfully ignited by conventional spark plugs. Laser ignition has proven its worth in defence and rocketry industry by replacing traditional spark ignition systems globally. Laser ignited combustion can be controlled by only few critical parameters, which makes it useful for implementation in variety of practical applications. Controlled combustion using laser plasma which requires minor engine hardware modifications is the key to implement it for variety of applications. Laser pulse generated plasma at the focal point, which is much more intense than conventional electrical spark plasma, can successfully ignite lean fuel-air mixtures. Main advantages of laser ignition technology include the possibility of igniting leaner fuel-air mixtures and flexibility to freely choose location of the igniting plasma. These interventions lead to lower NOx emissions, increased efficiency while avoids quenching effects due to electrodes, reduced electrode wear and consequently increased the lifetime of the electrodes. Laser ignition can be used for multi-cylinder engine by employing fiber optics and a single laser source. This chapter reviews laser ignition of gaseous fuel-air mixture as well as technology adaptation for implementation of laser ignition in automotive sector.