Modelling Spark Ignition Combustion

A two-zone combustion model consisting of burned and unburned masses was constructed to simulate an engine during combustion. The study of thick flame in turbulent flow in engines revealed there are microvolumes of unburned gases behind the flame front that progressively engulf the unburned gases at a rate determined by the turbulence intensity which in turn is a function of the inlet air velocity or the engine speed, where extra turbulence by squish at TDC is not generated. These small volumes are consumed at the rate of laminar burning velocity from their surfaces. Their typical size is found to scale with the dimensions of the combustion chamber and the turbulence Reynolds number. Two types of models exist. (a) A model which considers the delay and main combustion as a continuous phenomenon. Here, the rate of burning is directly proportional to the mass of unburned gases behind the flame front and inversely to a characteristic time. The time constant is again defined as the ratio of the characteristic size of the microvolumes or eddies to the laminar burning velocity, thus bringing the two important aspects of flow turbulence and chemistry into consideration. The size of the eddies is related to the inlet valve lift to account for the intake-generated turbulence. Later, it is argued that the size could be the Taylor microscale. (b) A model which describes two distinct phases namely the ignition delay period and main combustion duration. Here, the size of the eddies is considered of Taylor macro scale and the course of heat release itself is described using the familiar Wiebe or similar function that needs the durations of the two phases of combustion. The two-zone models with two engine-specific constants that are invariant with operating conditions successfully describe the rate of heat release and emissions of nitric oxides and hydrocarbons. They can also be used to predict knocking.

The fuel economy of a conventional spark-ignited gasoline engine falls short of that of a direct injection diesel engine at part loads mainly due to the heavy and irreversible pumping losses, introduced by the throttle. Also, the valve timings are optimum only at full load and mid-speed. At all other operating points, many important parameters like residual gases, volumetric efficiency, and thermal efficiency are compromised. Variable valve timings (VVT) systems are, therefore, implemented in different variations taking into consideration the system’s complexity, durability, and cost. However, VVT itself results in some throttling in the low-load range affecting the thermodynamic efficiency. Incorporating variable valve lift (VVL) could almost eliminate the throttle, reducing the pumping losses and improving combustion efficiency due to turbulence enhanced by higher flow velocities at the valves. VVL could be achieved by a relatively simpler switching between two cams with larger and smaller lifts or a more complicated continuously variable lift by changing the fulcrum of the valve rocker arm. By experiments and computer simulation using commercial software packages, the optimum timings and lifts can be determined for VVT and VVL. It has been found that varying inlet valve timings are more advantageous over exhaust valve timing variation either in conjunction with the inlet timings or alone. Therefore, VVT and VVL are applied for the inlet only in the majority of engines.

Modern-day IC engines face an ever-increasing challenge of reducing fuel consumption and tail-pipe emissions to mitigate environmental concerns and to increase engine performance in terms of higher torque and power output with a lower fluctuation to improve driveability. This has resulted in a renewed interest in further investigations into spark-ignition (SI) engines. Newer variants of SI engines like gasoline direct injection (GDI) and pre-chamber combustion engines have emerged over the years and have become quite popular due to their better emission and fuel economy performance than the traditional port fuel injection (PFI) counterparts. Lately, the emergence of optical engines has provided valuable optical access inside the combustion chamber resulting in the development of several laser-based in-situ measurement techniques. These laser diagnostic methodologies have resulted in the direct imaging of in-cylinder flow-field, fuel spray, mixture distribution and combustion flame fronts providing highly accurate data sets that have proved to be extremely crucial for the development of valid CFD models. These CFD studies have then provided valuable insights into several in-cylinder phenomena that have a direct impact on augmenting combustion. This chapter discusses the evolution of optical engines and compares them with the corresponding metal engines. Some very popular laser diagnostic methodologies are then discussed like particle image velocimetry (PIV) for flow field visualisation, planar laser-induced fluorescence (PLIF) for fuel distribution imaging, diffused backlight imaging (DBI) for spray morphology studies and finally combustion flame front imaging. The chapter concludes with a detailed discussion of several studies that have used these visualisation tools to develop CFD models and have provided various interesting results.

The in-cylinder air flow pattern in the combustion chamber of an internal combustion engine has a significant impact on engine performance and emission characteristics. It also affects engine noise, harshness, and durability. Inducting more air inside the combustion chamber at a sufficient turbulence level has always been the main concern for engine designers. In-cylinder charge motion is highly unsteady and non-uniform. It is very complex to visualize and understand it through mere experimental techniques available today. Nowadays, multi-dimensional computational fluid dynamics (CFD) has become a widely accepted and indispensable tool in the analysis of in-cylinder flows, combustion, emission prediction, and engine performance. The outcomes of CFD analysis are subject to the accurate modelling of engine geometry, mesh quality, and selection of sub-models to replicate the physics. The proper input boundary conditions also play a vital role in the accuracy of CFD simulations. The chapter presents insights on 1- and 3-dimensional modelling of the in-cylinder phenomenon in spark ignition engines. The general techniques and governing equations behind the CFD sub-models have been explained. The effects of combustion chamber shape, piston shape, fuel injection, valve lift, and water injection are thoroughly presented.

Spark ignition system is the most critical component for a spark-ignition (SI) engine. The ignition timing determines the combustion phasing of the engine and consequently affects all the performance indicators of the engine. The chapter reviews the history and the state of the art of spark ignition system models for three-dimensional computational fluid dynamics. The mathematical formulations of the DPIK model, AKTIM model, and a new ignition system model are presented. The ignition system model is validated by comparing it with the available experimental data. Their applications in engine combustion modelling are discussed as examples, such as the effects of spark plug orientation. The future directions of ignition system modelling are discussed.

The previous chapter discusses various models describing the spray breakup and mixing. Once the fuel is injected into the combustion chamber, it mixes with the surrounding air. The mixing process is mainly influenced by the shape of the combustion chamber, in-cylinder air motion, and turbulence. After the desired type of mixture is formed in the combustion chamber (lean stratified, stoichiometric, or rich mixture), ignition and subsequent combustion of the mixture take place. In GDI engines, combustion of the fuel–air mixture is initiated with the help of a spark plug. The spark plug is mounted at a location where the equivalence ratio is stoichiometric or slightly rich at the time of ignition, under all the available modes of operation in order to avoid misfire. For lean stratified operation, the location of spark plug needs to be precise as it is difficult to initiate ignition in an ultra-lean mixture. The engine operation under a homogeneous mixture condition is relatively less sensitive to the location of the spark plug from the point of view of mixture richness around it. Ignition leads to combustion of the air–fuel mixture which has been already prepared inside the combustion chamber. The flame front travels across the combustion chamber consuming the fuel–air mixture through the process of combustion. The process of ignition involves the generation of spark across the electrodes of the spark plug, whereas the process of combustion is nothing but the evolution of chemical reactions as the generated flame front travels across the chamber. Thus, ignition and combustion are conventionally modelled using individual models. Therefore, this chapter first discusses some of the popular ignition models and then the focus is directed to the study of a few combustion models. A spark is generated by the ignition system according to the command received by the electronic control unit (ECU). The ignition phase includes electrical discharge, plasma breakdown, and shock wave propagation. Ignition takes place in a small volume ranging from the spark gap to the turbulence integral length scale within a period of less than 1 µsec. This chapter provides insights into the sub-processes that take place during ignition and the different models available to simulate them. Once the spark is generated, it travels across the combustion chamber consuming the charge inside it. The rate of propagation is a strong function of the local equivalence ratio and temperature. This propagation of flame is modelled based on several approaches. One of the popular approaches is to use the chemical reaction mechanism of the corresponding fuel surrogate. However, a reduced chemical mechanism is defined for a certain range of the equivalence ratio and temperature thus limiting it applicability to a wide range of operating conditions. On the other hand, using a more detailed chemical reaction mechanism would result in longer computational time. Another approach is to model the combustion process in the premixed charge using the popular three-zone model. According to this approach, the charge inside the combustion chamber is divided into three zones. The burned and the unburned zones are separated by a thin reaction zone where the chemical reactions are taking place. Based on the three-zone model, some of the popular combustion models are discussed in this chapter. Apart from these two popular approaches, this chapter also discusses some high-fidelity combustion models based on large eddy simulation (LES). As it is not possible to explain all the models developed for simulating combustion in GDI engines, the interested readers are encouraged to consult the open literature to gain further insight in the subject matter.

With the development of low-temperature engine combustion strategies, the performance of gasoline-type fuels under compression ignition conditions has attracted extensive research interest. Meanwhile, for the sake of co-optimization of engines and fuels for future ground transportation, identification, and evaluation of general fuel properties should be a core research priority instead of endless testing of specific fuels. In this study, the roles of fuel octane sensitivity in characterizing the ignition performance of gasoline surrogates have been systematically investigated under typical gasoline direct ignition compression ignition (GDCI) engine conditions using 1D and three-dimensional combustion CFD simulation. The results of 1D simulations illustrate the combustion phasing of different fuel surrogates under HCCI conditions. Three-dimensional CFD simulations were employed to investigate the coupled effects of in-cylinder charge stratification and charge cooling. We considered two operating conditions. One is a beyond-RON case that has a high boost pressure and a low boost temperature. The other is a beyond-MON case that has a low boost pressure and high boost temperature. By comparing the three-dimensional CFD results with zero-dimensional chemical kinetic results, the effects of stratification and charge cooling on these combustion processes are highlighted through the analysis of chemical kinetics, fluid dynamics, and mixing. The results showed that due to its lower volatility, the fuel with higher fuel octane sensitivities leads to a slightly higher equivalence ratio stratification and stronger charge cooling. However, the chemical kinetics that depends on the fuel reactivity are still the more dominant factor than the stratification and charge cooling effects in determining combustion phasing.

Conventional spark ignition engines that use gasoline for ignition typically require a mixture to be prepared by the carburettor. Port fuel injection systems allow for fuel injection into the engine’s intake port, which then passes through the combustion chamber in the form of a mixture cloud. This mixture is then mixed with the air present in the combustion chamber. Recently, the development of electronically controlled fuel injectors has enabled the direct injection of gasoline at high pressure, similar to diesel engines, allowing for greater efficiency and reduced emissions. The direct injection of gasoline in the combustion chamber offers several advantages, including improved volumetric efficiency because of reduced in-cylinder gas temperatures resulting from direct injection of the fuel; precise control of the fuel–air mixture; locally stratified mixture formation; and improved fuel atomization, thus improving overall combustion efficiency.

Pre-chamber combustion system is a promising engine technology that allows ultra-lean combustion with high thermal efficiency and low emission. The chapter reviews the mechanism and the state of the art in modelling passive pre-chamber combustion systems for gasoline engines. The ignition mechanism in the main chamber is investigated numerically. A design of pre-chamber is proposed and optimized using the three-dimensional computational fluid dynamics (CFD) simulation. An efficient optimization method based on the Bayesian updating strategy is introduced, which is employed to optimize the pre-chamber geometrical design. Three-dimensional CFD simulation coupled with the design of the experiment (DOE), genetic algorithm, and machine learning methods is used to optimize the pre-chamber with desired combustion phasing. Regression analysis shows that the radius of the pre-chamber is the most influential design parameter.

Lean air–fuel mixture combustion is an effective way to reduce HC/CO/NO_x emissions significantly in premixed gasoline and gas engines. Slower flame speeds of lean mixture impact cycle-to-cycle IMEP variation and thereby emissions. This becomes much more challenging with large-bore gasoline and gas engines, leading to significant HC and CO emissions with lower efficiencies. This can be overcome with a prechamber combustion design which generates hot burning gas (and radicals) jets that come into the main chamber and provide higher ignition energy and turbulence along with a higher surface area for entraining and propagating flames which can engulf the premixed mixture in the main chamber much better. The prechamber spark-ignited engines are knock-tolerant and provide robust ignition. An active prechamber (which involves the introduction of fuel/ fuel and air directly into the prechamber) is more effective in providing the required prechamber jets compared to a Passive prechamber (premixed prechamber charge introduced from the main chamber). With proper optimization of prechamber volume, air–fuel mixing, and strength (Air–fuel ratio) in the prechamber and the pressure rise in the prechamber (thereby the ΔP across the prechamber nozzle), it is possible to achieve a significant reduction in emissions and improvement in thermal efficiency. The current scope is to review the state-of-the-art active prechamber spark-ignited engines, analyse the parameters studied, and present key observations on the parameters of importance and their range for designing prechamber engines which need to be investigated through Modelling and experiments.

This chapter will discuss recent research efforts in the multidimensional modelling of gaseous-fuelled engines carried out by using Computational Fluid Dynamics (CFD). First, the chemical and physical properties of the two most relevant gaseous fuels for engines, namely compressed natural gas (CNG) and hydrogen (H₂), will be discussed with specific reference to the engine operation and performance. Secondly, CFD modelling efforts on conventionally port fuel injected (PFI) and conventionally spark-ignited CNG/H₂ engines will be discussed and analyzed. A detailed discussion will also be provided on CFD modelling efforts for advanced direct injection (DI) gaseous engines. Finally, the chapter will focus on modelling advanced ignition technologies for gaseous engines, and in particular, passive and active pre-chamber ignition engine operation.

This chapter summarizes the modelling effort to unravel pre-chamber combustion developed under the FUELCOM 3 project at KAUST. CONVERGE^™ CFD solver was utilized to understand combustion in a narrow-throat pre-chamber. Extensive assessment of geometries (pre-chamber, piston, and their correlation), combustion models, and semi-empirical flame speed correlations were performed. In most cases, the pre-chamber was fuelled (active operation mode), which allows to regulate the pre-chamber composition, while the main chamber was operated with excess air–fuel ratio (lean conditions). The model was successfully validated against engine experiments (both metal and optical engines). Aiming at a drop-in design in existing diesel engines, the pre-chamber features a narrow and long channel (called throat). Two combustion models, G-equation and the multi-zone well-stirred reactor (MZ-WSR) were assessed and later compared; when using the former model, both laminar and turbulent flame speeds were from a look-up table and Peters’ correlation, respectively. The modelling results are well-aligned with the experiments and reveal that the narrow-throat and the jet-piston interaction have a significant influence on combustion and flow development. Towards fundamental combustion aspects, the accurate prediction of the laminar flame speed of lean charges was found to be critical and more relevant than empirical turbulence corrections for high Karlovitz regimes. To connect the fundamentals of combustion and practically relevant engine metrics, the Borghi-Peters diagram provided additional insights into the turbulent combustion regimes observed in pre-chamber engines. Finally, the pre-chamber optimization using machine learning is discussed.

Boosting solutions such as turbochargers are one of the popular technologies in the development of IC engines. Turbochargers date back to their usage on aircraft engines in the early nineteenth century and started usage in heavy-duty Caterpillar engines in 1954. Later they were introduced in passenger car applications in the early 1960s and became more popular after the 1990s due to the usage of high-power density engines in passenger cars. Popular technologies in engine air management systems such as variable valve timing (VVT), supercharging, turbocharging and intercooling have significantly improved engine power-to-weight and power-to-displacement ratios. Turbocharging internal combustion engine is widely recognized as one of the best and most cost-effective technologies for the enhanced benefits in power density, low-end torque, fuel consumption (CO₂ emissions), transient performance etc. Basis the benefits of fuel efficiency from turbocharging, it is considered as one of the key technologies to adopt for CAFE standards (Corporate Average Fuel Economy) with downsized engines. Globally turbocharging has gained popularity and higher penetration year on year in diesel, gasoline, and bi-fuel /Natural gas engine types. The boosting system has been undergoing upgradation with various add-on features to meet performance and emission needs. The introduction of emission norms like RDE (Real Driving Emission) drives engines to optimise for a reduction in scavenging, maintain λ = 1 in full operations and usage of the gas particulate filter to meet the Particulate number (PN) limit. In gasoline engines, there is a need to incorporate advanced combustion technologies like Miller/Atkinson and Low-Pressure Exhaust Gas Recirculation (LPEGR), with lean burn combustion for improved fuel economy. In this chapter, the benefits and challenges of the turbocharger boosting system and its integration into the gasoline engine are described. Additionally, the simpler ways of modelling the engine air management system to help the selection of the right boosting solutions are described.

Higher levels of air pollutants and greenhouse gas emissions from fossil fuel-based internal combustion engines have raised environmental concerns and it has resulted in stringent emission regulations. Recent emission norms have made it a mandatory requirement to adopt after-treatment emission control systems for controlling and reducing emission levels to meet the required emission standards. This chapter will discuss the recent research efforts in modelling engine emissions, and after-treatment systems like oxidation catalyst filters, particulate filters, and selective catalytic reduction systems. Detailed discussions on the mechanisms for soot emission modelling and NOx formation are covered in this chapter. Literature studies on one-dimensional, two-dimensional, and three-dimensional modelling of filters and catalysts coupled with CFD studies for application in three-way catalytic filters, NO_x reduction catalysts, electrically heated catalysts, and particulate filters (soot accumulation and regeneration) are also discussed. Future directions for modelling advanced after-treatment systems to reduce regulated and unregulated emissions are also explored.

Catalytic converters are essential automotive devices that transform harmful products of combustion generated by internal combustion engines into less toxic gases in compliance with environmental regulations. Chemical reactions through catalysts enable the conversion of toxic products of combustion. However, the materials required for catalytic converters are expensive, making it necessary to minimize cost while maximizing performance, reliability, and robustness. In this chapter, we begin with a general description of the types of catalytic converters, enumerate the chemistry behind catalytic converters, highlight analytical tools available to evaluate the performance of materials and catalysts, detail the computational fluid dynamics (CFD) tools, and describe how these can be used to evaluate the performance of catalytic converters both at the component level and under installed conditions. Therefore, the available analytical tools that model a catalytic converter at different scales from the molecular to the vehicular level can be used to reduce the time and costs of the design process for the catalytic converter.