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Abstract : A multi-dimensional CFD study using MTU-KIVA-Geq-CHEMKIN code has been carried out for direct injection compression ignition engine combustion fueled with heavy naphtha, light naphtha, and PRF50 in low-temperature combustion (LTC) regime. At constant fueling, combustion characteristics are investigated as a function of injection timing and injection pressure. Further, operating limits of fuel confined by combustion efficiency, noise level (Maximum Pressure rise rate) and emissions at exhaust valve opening (EVO) are evaluated using parametric variation of initial gas temperature, exhaust gas recirculation fraction, boost pressure. Research conducted focuses on ability of fuel to get good combustion which is combustion efficiency >/= 90%, Pressure rise rate/= 90%, Pressure rise rate
Varying the fuel stratification during gasoline compression ignition (GCI) combustion has been shown to impact important combustion parameters and emissions. The effect of varied injection pressure and injection timing on the fuel stratification and formation of nitric oxide (NOx) emissions was studied at two engine operating conditions. At a 1500 revolutions per minute (rpm) engine condition, a 100 bar increase in injection pressure required a 1.4o crankangle retard of the injection timing to maintain constant NOx emissions. The required injection timing shift to maintain constant NOx emissions at a 1900 rpm condition for a 100 bar increase in injection pressure was 2.5o crankangle. A skip-firing injection strategy illustrated the importance of the second injection in creating fuel stratification and promoting ignition for GCI combustion. The effects of injected fuel mass variability on combustion stability were investigated using a randomized injection strategy. Analysis showed that the injected fuel mass uncertainty required to induce combustion instability was between 3.2-4.8%. Three-dimensional computational fluid dynamics (CFD) and a one-dimensional (1-D) turbulent jet model were used to analyze the fuel-air mixing. A quasi-steady jet timescale was used to non-dimesionalize the time after start of injection. The ability of the timescale to collapse the jet vapor penetration and fuel-mass-weighted PDF of mixture fraction/equivalence ratio were evaluated for a variety of conditions at times significantly after end of injection. The quasi-steady jet timescale reasonably collapsed jet vapor penetration for various injection pressures but did not collapse the fuel-mass-weighted PDFs of equivalence ratio at times of interest during transient changes to the ambient gas density unless changes in spray spreading angle are accounted for. The 1-D jet model was benchmarked to CFD and evaluated at different conditions to analyze the assumptions of the 1-D model. A sensitivity analysis of the 1-D model was conducted. The 3-D CFD results are utilized to analyze the connection between the fuel-air distribution and the engine-out NOx emissions at the constant-NOx engine operating conditions. Computational fluid dynamics results showed similar equivalence ratio distributions resulted in relatively constant NOx emissions.
Fuels in the gasoline auto-ignition range (Research Octane Number (RON)> 60) have been demonstrated to be effective alternatives to diesel fuel in compression ignition engines. Such fuels allow more time for mixing with oxygen before combustion starts, owing to longer ignition delay. Moreover, by controlling fuel injection timing, it can be ensured that the in-cylinder mixture is "premixed enough" before combustion occurs to prevent soot formation while remaining "sufficiently inhomogeneous" in order to avoid excessive heat release rates. Gasoline compression ignition (GCI) has the potential to offer diesel-like efficiency at a lower cost and can be achieved with fuels such as low-octane straight run gasoline which require significantly less processing in the refinery compared to today's fuels. To aid the design and optimization of a compression ignition (CI) combustion system using such fuels, a global sensitivity analysis (GSA) was conducted to understand the relative influence of various design parameters on efficiency, emissions and heat release rate. The design parameters included injection strategies, exhaust gas recirculation (EGR) fraction, temperature and pressure at intake valve closure and injector configuration. These were varied simultaneously to achieve various targets of ignition timing, combustion phasing, overall burn duration, emissions, fuel consumption, peak cylinder pressure and maximum pressure rise rate. The baseline case was a three-dimensional closed-cycle computational fluid dynamics (CFD) simulation with a sector mesh at medium load conditions. Eleven design parameters were considered and ranges of variation were prescribed to each of these. These input variables were perturbed in their respective ranges using the Monte Carlo (MC) method to generate a set of 256 CFD simulations and the targets were calculated from the simulation results. GSA was then applied as a screening tool to identify the input parameters having the most significant impact on each target. The results were further assessed by investigating the impact of individual parameter variations on the targets. Overall, it was demonstrated that GSA can be an effective tool in understanding parameters sensitive to a low temperature combustion concept with novel fuels.
Direct Injection Systems: The Next Decade in Engine Technology explores potentials that have been recognized and successfully applied, including fuel direct injection, fully variable valve control, downsizing, operation within hybrid scenarios, and use of alternative fuels.
Due to increased ignition delay and volatility, low temperature combustion (LTC) research utilizing gasoline fuel has experienced recent interest [1-3]. These characteristics improve air-fuel mixing prior to ignition allowing for reduced emissions of nitrogen oxides (NOx) and soot (or particulate matter, PM). Computational fluid dynamics (CFD) results at the University of Wisconsin-Madison's Engine Research Center (Ra et al. [4, 5]) have validated these attributes and established baseline operating parameters for a gasoline compression ignition (GCI) concept in a light-duty diesel engine over a large load range (3-16 bar net IMEP). In addition to validating these computational results, subsequent experiments at the Engine Research Center utilizing a single cylinder research engine based on a GM 1.9-liter diesel engine have progressed fundamental understanding of gasoline autoignition processes, and established the capability of critical controlling input parameters to better control GCI operation. The focus of this thesis can be divided into three segments: 1) establishment of operating requirements in the low-load operating limit, including operation sensitivities with respect to inlet temperature, and the capabilities of injection strategy to minimize NOx emissions while maintaining good cycle-to-cycle combustion stability; 2) development of novel three-injection strategies to extend the high load limit; and 3) having developed fundamental understanding of gasoline autoignition kinetics, and how changes in physical processes (e.g. engine speed effects, inlet pressure variation, and air-fuel mixture processes) affects operation, develop operating strategies to maintain robust engine operation. Collectively, experimental results have demonstrated the ability of GCI strategies to operate over a large load-speed range (3 bar to 17.8 bar net IMEP and 1300-2500 RPM, respectively) with low emissions (NOx and PM less than 1 g/kg-FI and 0.2 g/kg-FI, respectively), and low fuel consumption (gross indicated fuel consumption
This dissertation discusses the results from three different studies aimed at understanding the importance of fuel chemical structure during low temperature combustion (LTC) strategies, like homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) employed in internal combustion (IC) engines wherein the focus is on high octane fuels. Boosted intake air operation combined with exhaust gas recirculation, internal as well as external, has become a standard path for expanding the load limits of IC engines employing LTC strategies mentioned above as well as conventional diesel and spark ignition (SI) engines. However, the effects of fuel compositional variation have not been fully explored. The first study focusses on three different fuels, where each of them were evaluated using a single cylinder boosted HCCI engine using negative valve overlap. The three fuels investigated were: a regular grade gasoline (RON = 90.2), 30% ethanol-gasoline blend (E30, RON = 100.3), and 24% iso-butanol-gasoline blend (IB24, RON = 96.6). Detailed sweeps of intake manifold pressure (atmospheric to 250 kPaa), EGR (0 -- 25% EGR), and injection timing were conducted to identify fuel-specific effects. While significant fuel compositional differences existed, the results showed that all these fuels achieved comparable operation with minor changes in operational conditions. Further, it was shown that the available enthalpy from the exhaust would not be sufficient to satisfy the boost requirements at higher load operation by doing an analysis of the required turbocharger efficiency. While the first study concentrated on load expansion of HCCI, it is important to mention that controlling LTC strategies is difficult under low load or idle operating conditions. To ensure stable operation, fuel injection in the negative valve overlap (NVO) is used as one of method of achieving combustion control. However the combustion chemistry under high temperature and fuel rich conditions that exist during the NVO have not been previously explored. The second study focused on examining the products of fuel rich chemistry as a result of fuel injection in the NVO. In this study, a unique six stroke cycle was used to segregate the exhaust from the NVO and to study the chemistry of the range of fuels injected during NVO under low oxygen conditions. The fuels investigated were methanol, ethanol, iso-butanol, and iso-octane. It was observed that the products of reactions under NVO conditions were highly dependent on the injected fuel's structure with iso-octane producing less than 1.5% hydrogen and methanol producing more than 8%. However a weak dependence was observed on NVO duration and initial temperature, indicating that NVO reforming was kinetically limited. Finally, the experimental trends were compared with CHEMKIN (single zone, 0-D model) predictions using multiple kinetic mechanism that were readily available through literature. Due to the simplicity of the model and inadequate information on the fuel injection process, the experimental data was not modeled well with the mechanisms tested. Some of the shortcomings of the 0-D model were probably due to the model ignoring temperature and composition spatial inhomogeneities and evaporative cooling from fuel vaporization.Though the results from the NVO injection and boosted NVO-HCCI studies are enlightening, the fundamentals of the autoignition behavior of gasoline, alcohols, and their mixtures are not entirely understood despite the interest in high octane fuels in compression engines from a point of view of better thermal efficiency. The third study focused on higher octane blends consisting of binary and ternary mixtures of n-heptane and/or iso-octane, and a fuel of interest. These fuels of interest were toluene, ethanol, and iso-butanol. In this study, the autoignition of such blends is studied under lean conditions ([phi] = 0.25) with varying intake pressure (atmospheric to 3 bar, abs) and at a constant intake temperature of 155 °C. The blends consisted of varying percentages of fuels of interest and their research octane number (RON) approximately estimated at 100 and 80. For comparison, neat iso-octane was selected as RON 100 fuel and PRF 80 blend was selected as RON 80 fuel. It was observed that the blends with a higher percentage of n-heptane showed a stronger tendency to autoignite at lower intake pressures. However, as the intake pressure was increased, the non-reactive components, in this case, the higher octane blend components (toluene, ethanol, and iso-butanol), reduced this tendency subsequently delaying the critical compression ratio (CCR) of the blends. The heat release analysis revealed that the higher octane components in the blends reduced the low temperature reactivity of n-heptane and iso-octane. GC-MS and GC-FID analysis of the partially compressed fuel also indicated that the higher octane components did affect the conversion of the more reactive components, n-heptane and iso-octane, into their partially oxidized branched hydrocarbons in the binary/ternary blends, and reduced the overall reactivity which resulted in a delayed CCR at higher intake pressures.
Abstract : The gasoline compression ignition (GCI) works on the principle of harnessing the benefits of light distillates in a compression ignition (CI) engine. Recent research has shown that along with air management and after-treatment systems; fuel systems also play a vital role in enabling GCI technology. The injector in the fuel injection system (FIS) is a key component driving the efficiency of the combustion phenomena. Subsequently, injection strategies, characteristics, and overall injection quality influence the combustion process and controls certain metrics like fuel consumption, pollutant emissions, and combustion noise. In this work, a one-dimensional (1-D) model of a heavy-duty diesel injector employed in Cummins ISX15 Engine, built in a commercially available computer software called Gamma Technologies (GT)-SUITE, was studied, and analyzed. This work focuses on developing a generalized methodology from previous work to adapt this injector with gasoline-like fuels by recalibrating the discharge coefficients using in-built GT-SUITE optimization techniques. Post recalibration, the 1-D model closely reproduces experimentally measured injection performance characteristics like rate of injection (ROI) profiles, injected quantities, hydraulic delays, and needle lift profiles for the heavy-duty, high-pressure diesel injector using gasoline-like fuels across engine operating points of interest, thereby enabling GCI. As the previous study has demonstrated the potential of injection rate shaping in the mitigation of oxides of nitrogen (NOx) emissions, this validated 1-D model was further used to investigate various injector geometries to produce custom injection rate shapes. Finally, an optimization methodology was developed to generate rate shape of interest to obtain a single set of the selected dimensional parameters across high-efficiency engine operating points using the in-built GT-SUITE optimization techniques. Furthermore, a full factorial design of experiments (DoE) using the candidate injector geometries, hydraulic components were simulated and post-processed to obtain an optimal rate shape, thereby acting as a validation tool for the optimal rate shape obtained using GT-Suite's optimization methods.