Design of a viable homogeneous-charge compression-ignition (HCCI) engine : a computational study with detailed chemical kinetics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, February 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references."September 2004."The homogeneous-charge compression-ignition (HCCI) engine is a novel engine technology with the potential to substantially lower emissions from automotive sources. HCCI engines use lean-premixed combustion to achieve good fuel economy and low emissions of nitrogen-oxides and particulate matter. However, experimentally these engines have demonstrated a viable operating range that is too narrow for vehicular applications. Incomplete combustion or misfire can occur under fuel-lean conditions imposing a minimum load at which the engine can operate. At high loads, HCCI engines are often extremely loud and measured cylinder pressures show strong acoustic oscillations resembling those for a knocking sparkignited engine. The goal of this research was to understand the factors limiting the HCCI range of operability and propose ways of broadening that range. An engine simulation tool was developed to model the combustion process in the engine and predict HCCI knock and incomplete combustion. Predicting HCCI engine knock is particularly important because knock limits the maximum engine torque, and this limitation is a major obstacle to commercialization. A fundamentally-based criterion was developed and shown to give good predictions of the experimental knock limit. Our engine simulation tool was then used to explore the effect of various engine design parameters and operating conditions on the HCCI viable operating range. Performance maps, which show the response of the engine during a normal driving cycle, were constructed to compare these engine designs. The simulations showed that an acceptably broad operating range can be achieved by using a low compression ratio, low octane fuel, and moderate boost pressure. An explanation of why this choice of parameters gives a broad operating window is discussed. Our prediction of the HCCI knock limit is based on the autoignition theory of knock, which asserts that local overpressures in the engine are caused by extremely rapid chemical energy release. A competing theory asserts that knock is caused by the formation of detonation waves initiated at autoignition centers ('hot-spots') in the engine. No conclusive experimental evidence exists for the detonation theory, but many numerical simulations in the literature show that detonation formation is possible; however, some of the assumptions made in these simulations warrant re-examination. In particular, the effect of curvature on small (quasispherical) hot-spots has often been overlooked. We first examined the well-studied case of gasoline spark-ignited engine knock and observed that the size of the hot-spot needed to initiate a detonation is larger than the end-gas region where knock occurs. Subsequent studies of HCCI engine knock predicted that detonations would not form regardless of the hot-spot size because of the low energy content of fuel-lean mixtures typically used in these engines. Our predictions of the HCCI viable operating range were shown to be quite sensitive to details of the ignition chemistry. Therefore, an attempt was made to build an improved chemistry model for HCCI combustion using automatic mechanism-generation software developed in our research group. Extensions to the software were made to allow chemistry model construction for engine conditions. Model predictions for n-heptane/air combustion were compared to literature data from a jet-stirred reactor and rapid-compression machine. We conclude that automatic mechanism generation gives fair predictions without the tuning of rate parameters or other efforts to improve agreement. However, some tuning of the automatically-generated chemistry models is necessary to give the accurate predictions of HCCI combustion needed for our design calculations.by Paul E. Yelvington.Ph.D

Combustion of Synthetic Jet Fuel: Chemical Kinetic Modeling and Uncertainty Analysis

Reaction mechanisms for jet-fuel combustion were built with the aim of providing a better description of the chemistry to reacting flow simulations used to design future aircraft engines. This research effort focused on combustion of Fischer–Tropsch synthetic jet fuel (S-8) in vitiated air at conditions relevant to jet engines, augmentors, and interturbine burners (T=650–1700  K P=1–20  atm, and Φ=0.5–2 in air). The complex S-8 fuel mixture was approximated with a two-component surrogate mixture of n-decane and iso-octane. A wholly new, elementary-step reaction mechanism for the surrogate consisting of 291 species and 6900 reactions was constructed using automatic mechanism generation software. Statistical analyses were conducted to determine reaction rate-constant sensitivity, model prediction uncertainty, and consistency of the model with published ignition delay time data. As a test application, the S-8 reaction model was used to estimate augmentor static stability using a simple Damköhler number analysis that showed increased stability with temperature from 800 to 1400 K and NO concentration from 0 to 1000 ppm (v/v). The ability to quickly generate accurate mechanisms for simple surrogates allows for new synthetic fuels to be quickly modeled and their behavior predicted for an array of experimental conditions and practical applications.United States. Air Force (contract number FA8650-13-M-2401

On piston engines as hydrocarbon gas reformers for modular, distributed chemical production

Hydrocarbon gases, and especially stranded gases such as those produced as a byproduct of tight oil production, are an underutilized feedstock that is often flared, vented, or reinjected. Improved utilization of stranded gas faces two primary technical/economic challenges. First, the gas is inherently distributed geographically and the supply varies temporally, which requires rethinking the characteristics of the chemical plant used to process the gas. Second, methane—the primary constituent of hydrocarbon gases—is relatively inert and requires activation through a synthesis gas intermediate prior to conversion to useful products. This methane activation step requires reforming chemistry which adds to the cost and complexity of the plant. This paper explores the potential of piston engines as non-catalytic, partial-oxidation reformers, providing an enabling technology for distributed production of chemicals from stranded gas resources. The motivation for using an engine in this process is to take advantage of the modular, scalable, fast-responding characteristics of engines, leveraging this highly developed and mass-manufactured device for purposes beyond power generation or transportation. This paper provides a perspective on technical, economic, and environmental aspects of engine reforming for production of low-carbon fuels and chemicals from stranded gas

Catalytic Hydrothermal Liquefaction of Food Waste Using CeZrOx

Approximately 15 million dry tons of food waste is produced annually in the United States (USA), and 92% of this waste is disposed of in landfills where it decomposes to produce greenhouse gases and water pollution. Hydrothermal liquefaction (HTL) is an attractive technology capable of converting a broad range of organic compounds, especially those with substantial water content, into energy products. The HTL process produces a bio-oil precursor that can be further upgraded to transportation fuels and an aqueous phase containing water-soluble organic impurities. Converting small oxygenated compounds that partition into the water phase into larger, hydrophobic compounds can reduce aqueous phase remediation costs and improve energy yields. HTL was investigated at 300 °C and a reaction time of 1 h for conversion of an institutional food waste to bio-oil, using either homogeneous Na2CO3 or heterogeneous CeZrOx to promote in situ conversion of water-soluble organic compounds into less oxygenated, oil-soluble products. Results with food waste indicate that CeZrOx improves both bio-oil higher heating value (HHV) and energy recovery when compared both to non-catalytic and Na2CO3-catalyzed HTL. The aqueous phase obtained using CeZrOx as an HTL catalyst contained approximately half the total organic carbon compared to that obtained using Na2CO3—suggesting reduced water treatment costs using the heterogeneous catalyst. Experiments with model compounds indicated that the primary mechanism of action was condensation of aldehydes, a reaction which simultaneously increases molecular weight and oxygen-to-carbon ratio—consistent with the improvements in bio-oil yield and HHV observed with institutional food waste. The catalyst was stable under hydrothermal conditions (≥16 h at 300 °C) and could be reused at least three times for conversion of model aldehydes to water insoluble products. Energy and economic analysis suggested favorable performance for the heterogeneous catalyst compared either to non-catalytic HTL or Na2CO3-catalyzed HTL, especially once catalyst lifetime differences were considered. The results of this study establish the potential of heterogeneous catalysts to improve HTL economics and energetics

Emission Performance and User Acceptance of a Catalytic Biomass Cookstove in Rural Guatemala

A catalytic rocket stove was developed to reduce emissions and improve efficiency compared to open cooking fires or traditional semienclosed cookstoves, called poyos, typical of rural Guatemala. Traditional stoves often emit particulate matter and carbon monoxide at sufficient levels to cause respiratory illnesses and other health problems. Using focus group results, the stove was tailored to the needs of Guatemalan cooks. Field trial participants were provided with stove training to ensure that stoves were operated correctly. Somewhat surprisingly, the field trial demonstrated a high level of user acceptance in rural Guatemala, where users cooked 93% of the time with the catalytic stove despite having to change some cooking practices. In the field trial, the stove reduced emissions by as much as 68% and improved fuel efficiency by as much as 61% during real-world cooking events relative to the traditional poyo. An additional qualitative portion of the field study identified strengths and weaknesses of the stove that are being addressed as part of an iterative design process