The carbon dioxide emission prevention advantage of generating power with high hydrogen content fuels using gas turbines motivates an improved understanding of the ignition behavior of hydrogen in premixed and partially premixed environments. Hydrogen rich fueled flame stability is sensitive to operating conditions, including environment pressure, temperature, and jet velocity. Furthermore, when premixed or partially premixed operation is desired for nitric oxide emissions reduction, a diluent, such as nitrogen, is often added in allowing fuel/air mixing prior to combustion. Thus, the concentration of the diluent added is an additional independent variable on which flame stability dependence understanding is needed. The focus of this research is on characterizing the dependence of hydrogen jet flame stability on environment temperature, jet velocity, diluent concentration, and pressure by determining the dependence of the liftoff height of lifted flames on these 4 independent parameters. Nitrogen is used as the diluent due to its availability and effectiveness in promoting liftoff.

Experiments are first conducted at atmospheric pressure in scoping subsequent research where the additional parameter of pressure is added. The stability and liftoff characteristics of a nitrogen diluted hydrogen jet flame at atmospheric pressure in a vitiated co-flow are investigated experimentally and numerically with particular attention focused on regimes where multiple stabilization mechanisms are active. Information gleaned from this research is instrumental for informing modeling approaches in flame transition situations when both autoignition and flame propagation influence combustion characteristics. Stability regime diagrams which outline the conditions under which the flame is attached, lifted, blown-out, and unsteady are experimentally developed and explored. The stability of the flame is investigated with a 1D Reynolds Averaged Navier Stokes parabolic numerical model which shows that under certain conditions, local turbulent flame speeds exceed the local velocity for the production of stable lifted hydrogen flames. These modeling results suggest that the dominant flame stabilization mechanism is flame propagation, and likely tribrachial flame propagation, consistent with the conclusions of prior studies for jet flames issuing into ambient environments such as the research of Muñiz and Mungal (1997). The lifted regime is further characterized at atmospheric pressure in determining liftoff height dependence on co-flow temperature, jet velocity, and nitrogen dilution. A strong sensitivity of liftoff height to co-flow temperature, jet velocity, and nitrogen dilution is observed. The numerical model results trend well with the experimentally developed stability regime diagrams. Liftoff heights predicted by Kalghatgi's correlation are unable to capture the effects of nitrogen dilution on liftoff height for the heated co-flow cases. A uniquely formulated Damköhler number was therefore developed which acceptably captures the effects of jet velocity, nitrogen dilution and environment temperature on liftoff height. Satisfactory agreement between the correlation results which relies on propagation parameters in its formulation further indicates that stabilization is indeed dominated by propagation.

The unsteady regime is also investigated experimentally at atmospheric pressure. The unsteady regime is characterized by rapid ignition events of an initially unburned jet of fuel, and these events are always followed by subsequent blowout events. The frequency by which these ignition events occur are measured and insights are drawn regarding the impact of nitrogen dilution, jet velocity, and co-flow equivalence ratio on ignition frequency. Nitrogen addition to the fuel increases autoignition delay times which reduces ignition frequency, though it also reduces the speed of flame propagation which increases the frequency of blowoff. Consequently, when the level of nitrogen dilution added to the fuel is moderate, increases in dilution increase ignition frequency, and when high levels of nitrogen are added, further increases reduce ignition frequency because each ignition event is preceded by a blowoff event. Jet velocity increases lead to broader ranges of nitrogen dilution where unsteady behavior is observed. Finally, increases in co-flow equivalence ratio result in unsteady behavior for greater levels of nitrogen dilution

Experiments are also conducted at elevated pressure with co-flow temperature, jet velocity, and nitrogen dilution still parameterized. Strong sensitivity of liftoff height on co-flow temperature and pressure is observed both when jet velocity and jet Reynolds number are held constant as pressure is varied. With confinement, which is required in achieving elevated pressure, liftoff height sensitivity on jet velocity is diminished. The Damköhler number is again utilized in assessing its utility in incorporating the pressure effect, and satisfactory correlation results are demonstrated. Elevated pressure results and atmospheric pressure results (without confinement) indicate that the Damköhler number can be used in scoping experimental lifted flame research at elevated pressures and temperatures and in informing numerical modeling approaches for research as well as in industry.

Homogeneous Charge Compression Ignition (HCCI) engines are one of the most promising engine technologies for the future of energy conversion from clean, efficient combustion. HCCI engines allow high efficiency and lower CO2 emission through the use of high compression ratios and the removal of intake throttle valves (like Diesel), and allow very low levels of urban pollutants like nitric oxide and soot (like Otto). These engines, however, are not without their challenges, such as low power density compared with other engine technologies, and a difficulty in controlling combustion timing.

This dissertation first addresses the power output limits. The particular strategies for enabling high power output investigated in this dissertation focus on avoiding five critical limits that either damage an engine, drastically reduce efficiency, or drastically increase emissions: 1) ringing limits, 2) peak in-cylinder pressure limits, 3) misfire limits, 4) low intake temperature limits, and 5) excessive emissions limits. The research shows that the key factors that enable high power output, sufficient for passenger vehicles, while simultaneously avoiding the five limits defined above are the use of: 1) high intake air pressures allowing improved power output, 2) highly delayed combustion timing to avoid ringing limits, and 3) using the highest possible equivalence ratio before encountering ringing limits. These results are revealed by conducting extensive experiments spanning a wide range of operating conditions on a multi-cylinder HCCI engine.

Second, this dissertation discusses strategies for effectively sensing combustion characteristics on a HCCI engine. For effective feedback control of HCCI combustion timing, a sensor is required to quantify when combustion occurs. Many laboratory engines use in-cylinder pressure sensors but these sensors are currently prohibitively expensive for wide-scale commercialization. Instead, ion sensors made from inexpensive sparkplugs are proposed for sensing combustion timing. Ion sensing, however, is unreliable under certain HCCI conditions. The dissertation presents two strategies for improving the usefulness of ion sensors in HCCI engines: 1) the use of tiny fractions of metal-acetate fuel additives that expand the useful range of ion sensors, and 2) the use of ion sensors for detecting excessive ringing that must be avoided in HCCI engines. These two innovative research efforts make ion sensors viable for sensing combustion characteristics across the full range of HCCI operation, making them effective for use in engine control systems.

In summary, this Ph.D dissertation addresses two important technical challenges facing HCCI engines: power output limits, and difficulty in sensing combustion characteristics for control applications. The strategies proposed in this dissertation research bring HCCI engines closer to widespread commercialization allowing vehicles to operate with significantly higher efficiency and with cleaner emissions.

Carbon capture has been deemed crucial by the Intergovernmental Panel on Climate Change if the world is to achieve the ambitious goals stated in the Paris agreement. A deeper integration of renewable energy sources is also needed if we are to mitigate the large amount of green house gas emitted as a result of increasing world fossil fuel energy consumption. These new power technologies bring an increased need for distributed fast dispatch power and energy storage that counteract their intermittent nature. A novel technological approach to provide fast dispatch emission free power is the use of the Argon Power Cycle, a technology that makes carbon capture an integral part of its functioning principle. The core concept behind this technology is a closed loop internal combustion engine cycle working with a monoatomic gas in concert with a membrane gas separation unit. By replacing the working fluid of internal combustion engines with a synthetic mixture of monoatomic gases and oxygen, the theoretical thermal efficiency can be increased up to 80%, more than 20% over conventional air cycles. Furthermore, the absence of nitrogen in the system prevents formation of nitrogen oxides, eliminating the need for expensive exhaust gas after-treatment and allowing for efficient use of renewable generated hydrogen fuel. In the case of hydrocarbon fuels, the closed loop nature of the cycle affords to boost the pressure and concentration of gases in the exhaust stream at no penalty to the cycle, providing the driving force to cost effective gas membrane separation of carbon dioxide.

This dissertation investigates the potential benefits of the Argon Power Cycle to improve upon current stationary power generation systems regarding efficiency, air pollutants, and greenhouse gas emissions.

The internal combustion engine has vastly improved over the past 100 years. With global warming and pollution being a rising concern, engineers are working towards improving efficiency and emissions of engines. The spark-ignited engine (or gasoline engine) offers improvement in emissions with a sacrifice in thermal efficiency. The compression ignition engine (Diesel engine) increases the thermal efficiency, due to operation at higher compression ratios, but emits high amounts of particulate matter and oxides of nitrogen (NOx). Although improvements in fuel refinement have decreased the amount of engine pollutants, the output of pollutants for both spark-ignited and Diesel engines is still too great.

This dissertation explores two advanced engine concepts with alternative fuels for improving thermal efficiency and reducing emissions in automobiles. The first concept investigated is a spark-ignited internal combustion engine operating using hydrogen, oxygen, and argon. Basic engine theory predicts such an engine will see a considerable improvement in engine efficiency (theoretically ~75%, and in practice ~50% including heat transfer and friction losses) over standard engines. These gains in thermal efficiency are due to argon's monatomic structure, which yields a high specific heat ratio (γ = 1.67 compared to γ < 1.4 for air). The water produced by the combustion of hydrogen can be extracted in the exhaust by a condenser, allowing the recycling of nearly pure argon in a closed loop system. Therefore, argon re-fueling is not required.

Achieving efficiencies above 50% with a hydrogen-oxygen-argon engine, however, is difficult due to engine knock limiting spark advance. In an effort to obtain the highest efficiency of this engine concept, experiments were conducted using single and dual spark-ignition for high argon concentrations. Results showed dual spark-ignition slightly increased indicated thermal efficiency, but was still limited by engine-knock. A three-zone model showed that argon as a working fluid increases in-cylinder temperatures, unburned gas temperatures, and laminar flame speed. The model suggests that specific heat ratio affects end gas temperatures more than increasing flame speed.

The second engine concept investigates variables and fuel trends for predicting ignition in homogenous charge compression ignition (HCCI) engines. Octane number, a metric for fuel performance in gasoline engines, and cetane number, a metric for fuel performance in Diesel engines, do not accurately predict fuel performance in HCCI engines. To develop a metric for predicting fuel performance in HCCI engines, correlations between ignition of fuels in an HCCI engine and varying engine parameters are investigated. A relationship between fuel chemistry and ignition in HCCI engines is also explored. Results show that previous methods for predicting ignition do not correlate well with experimental data and auto-ignition is highly sensitive to fuel chemistry.

A single-zone well-mixed-reactor model is used to investigate three different mechanisms for predicting auto-ignition in the HCCI engine. All three mechanisms accurately predicted the auto-ignition order of fuels containing isooctane and n-heptane, but did not predict auto-ignition of blends containing toluene and ethanol. Blends of toluene and n-heptane were further investigated using the model to identify potential problems with the toluene mechanisms. The model results showed increasing the amount of toluene linearly by volume did not result in a linear advance in auto-ignition.

This work investigates oxycombustion in a spark-ignited (SI), piston engine. Oxycombustion is the process in which nearly pure oxygen and fuel are burned in a diluent of exhaust gas which is recirculated from the exhaust to the intake, referred to as exhaust gas recirculation (EGR). In typical air burning engines, EGR is used to reduce combustion temperatures, thereby reducing the formation of the pollutant nitric oxides (NO_x). Conversely, the motivation for power generation using oxycombustion with EGR is the ease with which carbon dioxide (CO₂) can be isolated in the exhaust for sequestration through a carbon capture and sequestration (CCS) process. At similar operating points the negative effect of EGR on the thermal efficiency of traditional air engines is magnified in oxycombustion due to the very high EGR levels. Therefore, it is the objective of this study to determine where the operating limits can be extended beyond the normal limits of air systems, regaining lost thermal efficiency.

Two main EGR diluents are considered: dry EGR which is primarily CO₂, and wet EGR, which includes both CO₂ and water. Dry EGR has been found to have significantly higher knock resistance than air, allowing operation at higher compression ratios (CR) which can ultimately produce higher thermal efficiencies than air operation when combusting a low octane fuel. In fact, dry EGR produced higher thermal efficiencies when operating with low octane fuels (which have very poor knock resistance) than it did with high octane fuels (which have very high knock resistance). The primary reason for this anomaly was determined to be minor auto-ignition events, which significantly decrease combustion duration but do not produce significant knock, occurring with the low octane fuel. When operation is limited by engine configuration rather than knocking, as with methane, the performance of dry EGR is very low relative to air, roughly proportional to the decrease in theoretical thermal efficiency due to the decrease in the ratio of specific heats. In order to take advantage of the improved knock resistance it is necessary to operate in a region with a high enough CR and low enough oxygen concentration that minor auto-ignition events are common. Wet EGR produces similar trends as dry EGR, but cannot attain the high CRs which dry EGR operated at due to the elevated intake temperature required to prevent water condensation. Ultimately, despite the chemical and thermal advantages of water compared to CO₂ in regards to laminar flame speeds, wet EGR with high water content is not a reasonable choice when operating in an IC engine. Despite its limitations in an IC engine, wet EGR has several advantages which should be carefully considered in applications where high initial temperatures are not propagated negatively.

Stratified combustion, combustion of fuel/air mixtures with temperature and/or mixture-composition stratification, is present in many combustion-related phenomena and applications such as forest wildfires, mining explosions, vessel ruptures, gas turbines, and reciprocating engines to name a few. A new generation of highly efficient internal combustion (IC) engines capable of satisfying stringent emission requirements, including modern direct-injection gasoline engines and gas turbines with lean premixed pre-vaporized (LPP) combustors, requires more comprehensive understanding and control of stratified combustion. Fundamentally, stratification of temperature or mixture composition affects a wide range of combustion characteristics such as flame speed, flammability, mode of combustion, instability, and others.

This dissertation aims to identify, analyze and evaluate fundamental processes in the combustion of stratified mixtures, using theoretical analysis and advanced numerical simulation tools. ASURF-Parallel, a transient numerical solver of compressible reacting flow, is developed on the basis of the original A-SURF and exploited for stratified combustion simulations. A domain-decomposition parallelization scheme using Message Passing Interface (MPI) is developed and implemented in ASURF-Parallel to speed up the otherwise time-consuming numerical simulations. A significant speedup with the speed-up factor up to 10 is achieved on lab-scale servers.

Effects of stratification on flame speeds, lean flammability limit, and modes of combustion are numerically investigated and studied. For flame speeds, laminar flame speeds of stratified flames propagating from rich mixtures to lean mixtures are generally faster than those of the corresponding homogeneous flames, primarily due to the preferential diffusion of lighter species and radicals such as H2, H and OH, i.e., the chemical effect. The degree of enhancement in flame speeds can be correlated to the degree of stratification, leading to the development of a transient local stratification level (LSL) model which is able to determine the stratified flame speeds incorporating both chemical effect and memory effect. For lean flammability limits, the extension introduced by stratification is very weak due to reduced overall reactivity and reduced degree of stratification. For modes of combustion, different modes can be realized by specific reactivity gradients, regardless of the sources of such gradients. Pressure waves introduced by ignition in a closed chamber can also lead to different modes of reaction front propagation and end-gas combustion. A transient reactivity gradient method is proposed to identify the onset of detonation.

Biodiesel is a notable alternative to petroleum derived diesel fuel because it comes from natural domestic sources and thus reduces dependence on diminishing petroleum fuel from foreign sources, it likely lowers lifecycle greenhouse gas emissions, and it lowers an engine’s emission of most pollutants as compared to petroleum derived diesel. However, the use of biodiesel often slightly increases a diesel engine’s emission of smog forming nitrogen oxides (NOx) relative to petroleum diesel. In this paper, previously proposed theories for this slight NOx increase are reviewed, including theories based on biodiesel’s cetane number, which leads to differing amounts of charge preheating, and theories based on the fuel’s bulk modulus, which affects injection timing. This paper proposes an additional theory for the slight NOx increase of biodiesel. Biodiesel typically contains more double bonded molecules than petroleum derived diesel. These double bonded molecules have a slightly higher adiabatic flame temperature, which leads to the increase in NOx production for biodiesel. Our theory was verified using numerical simulations to show a NOx increase, due to the double bonded molecules, that is consistent with observation. Further, the details of these numerical simulations show that NOx is predominantly due to the Zeldovich mechanism.

Homogeneous charge compression ignition (HCCI) engines lack direct in-cylinder mechanisms, such as spark plugs or direct fuel injection, for controlling the combustion timing. Many indirect methods have been used to control the combustion timing in an HCCI engine. With any indirect method, it is important to have a measure of the combustion timing so the control inputs can be adjusted for the next cycle. In this paper, it is shown that microphones and knock sensors can be used to detect combustion in HCCI engines. The output from various microphones and a knock sensor on an HCCI engine are measured at light and high loads. The combustion timing data obtained from the sensors are compared to the combustion timing data obtained from a piezoelectric cylinder pressure transducer. One of these sensors is selected and used for closed-loop control of the combustion timing in a single cylinder HCCI engine.

This demonstration system is intended to meet the California Energy Commission’s primary goal of improving California’s electric energy cost/value by providing a low-cost high-efficiency distributed power generation engine that runs on landfill gas. The project team led by Makel Engineering, Inc. includes UC Berkeley, CSU Chico and the Butte County Public Works Department.

The team has developed a reliable, multi-cylinder Homogeneous Charge Compression Ignition (HCCI) engine by converting a Caterpillar 3116, 6.6 liter diesel engine to operate in HCCI mode. This engine utilizes a simple and robust thermal control system. Typically, HCCI engines are based on standard diesel engine designs with reduced complexity and cost based on the well known principles of engine dynamics. Coupled to an induction generator, this HCCI genset allows for simplified power grid connection.

Testing with this HCCI genset allowed for the development of a control system to maintain optimal the inlet temperature and equivalence ratio. A brake thermal efficiency of 35.0% was achieved while producing less than 10.0 ppm of NOx and 30 kW of electrical power. Less than 5.0 ppm of NOx was recorded with a slightly lower brake thermal efficiency. Tests were conducted with both natural gas and simulated landfill gas as a fuel source. This demonstration system has shown that landfill gas fueled Homogeneous Charge Compression Ignition engine technology is a viable technology for distributed power generation.