Chapter 1
INTRODUCTION
1.1 BACKGROUND
AND
MOTIVATIONS
Internal combustion (IC) engines have played a fundamental role in the power generation field for over a century. However, in recent years, the high environmental air pollution and the lower fossil fuel reserves urge the development of advanced combustion strategies for achieving significantly reduced brake specific fuel consumption (BSFC) and compliance with future stringent emission regulations.
Homogeneous Charge Compression Ignition (HCCI) combustion, using gasoline or diesel fuels, is being widely studied [1,2,3] as an attractive alternative to conventional spark-ignition (SI) or diesel compression ignition (CI) engine combustion for its reduced emission production and low fuel consumption. In this combustion strategy, fuel is injected and well premixed with air before ignition, thus avoiding fuel-rich regions leading to reduction of particulate matter (PM) emissions. Combustion gradually takes place at the same time in the whole combustion chamber, since lean mixture is used and charge is diluted with exhaust products (EGR). NOx emissions, strongly dependent on the gas temperature, are greatly reduced because the burned gas temperatures are relatively low due to the adopted mixture composition.
However, HCCI combustion has several difficulties that must be overcome for practical use, such as ignition timing controllability and the preparation of homogeneous mixtures. The combustion phasing has to be accurately controlled to avoid too early ignition, which may increase NOx emissions and lead to undesirable high peak pressures. Also late ignition can occur, with consequent incomplete combustion. When direct injection of fuel is used to prepare a premixed mixture, early injection of fuel into the cylinder is required. Early injection at low ambient density may cause spray impingement on the cylinder walls [1], which not only deteriorates the quality of the charge mixture, but also increases the pollutant emissions. Un-homogeneities in the fuel/air mixture can lead to fast heat release and high pressure rise rate, sometimes dangerous for the engine structure. Due to the previous aspects, currently HCCI combustion is not able to cover the whole engine operating field but is feasible only in low/medium load condition.
A few months ago General Motors demonstrated HCCI combustion for the first time in two drivable concept vehicles, the 2007 Saturn Aura and the Opel Vectra. HCCI combustion process is achieved at low vehicle speed and in low/medium load conditions transitioning to spark ignition combustion at higher speeds and during
high engine load. The transition between the combustion processes is still notable in the demonstration prototypes, whereas fuel efficiency is improved by 15%.
Multidimensional computational fluid dynamics (CFD) modeling represents a fundamental resource for engine research and development. This tool provides detailed information of the in-cylinder physics helping visualization and analysis processes. Countless computational studies on air-fuel mixture formation, ignition and combustion processes in engines applications have been performed, as well as on many of other engine internal aspects, showing the usefulness of multidimensional modeling. A good understanding of in-cylinder combustion is one of the basic factors to perform useful modeling. In the present work some focus was turned on implementation and development of advanced ignition and combustion models.
In recent years, with considerable achievements in the fundamental combustion chemical kinetics [4–6] and rapid advances in computer technology, elementary chemical kinetic mechanisms have been coupled with multidimensional CFD codes for IC engine simulations [7]. These advanced models can help to obtain a much clearer picture of the in-cylinder combustion chemistry. Therefore, the coupling between fluid dynamics and chemical kinetics has become an important trend in engine research.
1.2
OBJECTIVE AND APPROACH
HCCI combustion is considered a brilliant strategy to reduce pollutant emissions and fuel consumption. However, as aforesaid, it presents some inconveniences obstacling its applications in large-series production. In the present work some innovative strategies for mixture formation and combustion processes have been studied to overcome some of the HCCI combustion problems while conserving its capabilities.
In the first part of the research gasoline HCCI and PCCI (partially Premixed Charge Compression ignition) combustion have been analyzed to observe the effect of charge stratification and in-cylinder mixture un-homogeneities on ignition phasing controllability. A numerical study of gasoline HCCI/PCCI combustion, with comparisons with experimental data was performed. Accurate modeling of spray characteristics is essential for accurate prediction of the in-cylinder mixture distribution, especially in direct injection PCCI combustion. Thus, the spray model was validated with well-designed experimental measurements. Results in terms of pressure, temperature and heat release rate have been analyzed as well as emission production.
In the second part diesel-gasoline dual-fuel combustion has been proposed as a way to solve the combustion phasing control problem. In this operation, a small amount of diesel fuel is injected as a pilot injection to ignite a pre-mixture of gasoline and air. In this work numerical simulations of dual-fuel combustion processes are presented. The influence of injection timing and mixture composition on emission production, as well as on combustion pressure and on heat release rate has been analyzed with two different injectors. Although dual-fuel combustion is an attractive way to achieve controllable HCCI operation, few studies are available to help the understanding of its in-cylinder combustion behavior.
The third and last part of the research concerns an innovative concept to control HCCI combustion in diesel-fuelled engines. The main purpose of this strategy is the obtaining of diesel HCCI combustion also with high mean effective pressures rendering the combustion behavior more controllable as well. The concept consists in forming a pre-compressed homogenous charge outside the cylinder and in gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the flow rate transfer. High pressure gradients, typical of common HCCI combustion, can be limited as well. A first analysis has been done to test the validity of the concept then two applicative solutions, for two and four stroke cycle operations, have been proposed and analyzed. Results have been compared with a conventional diesel engine application.
For the first and second parts the KIVA-3V code [8–10], developed at the Los Alamos National Laboratory, was used as the basis for the present modeling work. Improvements in its spray, ignition, combustion and emission models have been implemented in a modified version of KIVA-3V by the Engine Research Center (ERC) of the University of Wisconsin-Madison. A combustion model based on the level set method (also called the G-equation method) was implemented by Tan at al. [11] into the ERC version KIVA-3V and coupled with detailed chemistry by Liang [12], better describing premixed and partially premixed turbulent flame propagation conditions [13]. The present work improves and extends the G-equation model by introducing a new combustion evaluation strategy in the burning regions. For the section 3 the AVL Fire 8.51 code [14] was used for the modeling studies.
Numerous available experimental data were collected and compared with the simulation results. The low pressure spray model was first validated in terms of droplet dimension, spatial distribution [15] and spray structure [16] for several ambient and injection pressure. Experiments obtained in a single cylinder, four stroke Caterpillar engine were used to examine gasoline HCCI/PCCI combustion [17,18] as well as numerical simulations. Different injection timings, engine speed and charge equivalence ratios were considered. Due to the absence in the literature of experiments regarding the diesel-gasoline dual fuel combustion strategy, some diesel-natural gas experiments [19] obtained on a single cylinder, four stroke Cummins engine, were used to validate the code for dual fuel combustion. Several tests with different mixture composition and operating conditions were provided. Further on, the validity of the combustion model was confirmed with some diesel liftoff length measurements performed by Sandia National Laboratory [20].
1.3
OUTLINE OF THE THESIS
The thesis is divided in two sections:
SECTION I reports a literature review of current HCCI combustion technologies and a brief description of some of the computational models adopted in this work.
In Chapter 2, the HCCI combustion operating modes are reviewed. The review focuses on the fundamental factors controlling HCCI combustion for both SI and CI applications. Benefits and defections of the current HCCI technology are analyzed.
Chapter 3 gives a literature review of turbulent combustion models. After a brief review of the governing equations, different existing approaches to model turbulent combustion are discussed. The selection of detailed chemical kinetic mechanisms for engine simulations is also discussed.
In SECTION II the research activity is described. Different possible solutions to HCCI combustion limitations have been analyzed.
In Chapter 4 analysis of gasoline HCCI and PCCI combustion is reported, to understand the influence of some operating parameter on combustion behavior and emission production. Initially, the injection models have been validated through available experimental data.
Chapter 5 reports a numerical study of dual fuel combustion through advanced combustion models, to avoid HCCI ignition phasing control uncertainties. This study has been performed to understand the influence of fuel mixture composition and injection timing on combustion behavior. Different injectors, load and engine speed conditions were considered in the simulations.
In Chapter 6 an innovative diesel HCCI combustion solution, capable to control and extend HCCI combustion to the whole engine operating range is analyzed. After testing the validity of the concept, the results related to two constructive solutions have been compared with a conventional diesel application. A final injection optimization has been performed.
A summary of the work and some recommendations for future research are given in Chapter 7.
Further details about the detailed chemistry mechanisms used in this work are given in appendixes A and B. Schematics of preliminary one-dimensional analysis for the innovative diesel HCCI combustion are proposed in appendix C.
