Robert R. Raine

Modeling of nitric oxide formation in spark ignition engines with a multizone burned gas

A review of the literature on nitrogen oxide concludes that the thermal nitric oxide (NO) mechanism is the most relevant to the nitrogen oxide emissions from spark ignition engines. The review has also shown that the use of multiple burned gas zones is likely to be important for the accurate prediction of NO emissions. The multizone formulation developed here results in a particularly efficient formulation, as burning can occur within a zone. Previous multizone spark ignition engine simulations all appear to have created a new zone for each crank-angle increment during combustion. The present studies indicate that 5 or 10 burned zones are likely to be sufficient for most purposes. In reality there will be some mixing between the zones, and it could well be that with discrete burned zones, the most accurate simulation will be with a finite number of zones. The investigations with the multizone model showed lower predictions of NO than the single burned zone model; a result consistent with that of earlier workers. The outputs from the simulation have been compared with measurements from a gas engine, that encompass particularly wide variations in ignition timing (2–30 deg btde) and nondimensional air fuel ratio (0.8 < λ < 1.8). The earlier multizone NOx models had been compared with a limited range of mixtures close to stoichiometric, and in most cases the Minimum ignition advance for the Best Torque (MBT ignition timing). The opportunity was also taken to compare the NOx predictions, resulting from using different recommendations for the reaction rates in the extended Zeldovich mechanism. Finally, mean cycle modeling and cycle-by-cycle modeling were compared. There was close agreement in NO predictions only in the region of the MBT ignition timing. The significance of cycle-by-cycle modeling should be investigated further, with carefully calculated burn rate data.

Analysis of Combustion in a Small Homogeneous Charge Compression Assisted Ignition Engine

Abstract Combustion analysis has been conducted on a small two-stroke glow ignition engine, which has similar combustion characteristics to homogeneous charge compression ignition (HCCI) engines. Difficulties such as unknown ignition timing and the polytropic index have been addressed by combining both heat release and mass fraction burn analyses. Results for all operating conditions have shown good correlations between the two methods. The engine has been fuelled with a mixture of methanol, nitromethane, and lubrication oil. The effect of nitromethane on combustion is difficult to determine, since altering nitromethane content also changes the air-fuel ratio under the current experiment set-up. However, it is still possible to show that nitromethane shortens the combustion periods beyond the uncertainty created by the mixture strength and cycle-by-cycle variations. The results further show that a faster combustion does not necessarily give a higher indicative mean effective pressure (i.m.e.p.) in this engine. This is because the start of combustion can shift away from its optimum value when nitromethane is added. The initial combustion period is found to be between 10 and 30° CA (crank angle); the main combustion period is between 25 and 50° CA. These combustion periods are comparable to a traditional spark ignition engine. In a very rich mixture, the ‘hot’ glow plug has been found to change significantly the combustion characteristics. Further study would be recommended to elucidate the effect of glow plugs. Lastly, in the case of poor combustion, cycle-by-cycle analysis shows that a misfire or partial burn cycles are always followed by high i.m.e.p. and fast burn cycles.

Characterization of Exhaust Particulates from a Dual Fuel Engine by TGA, XPS, and Raman Techniques

Particulate matter (PM) emitted from a dual fuel engine is characterized using thermogravimetry, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Thermogravimetric analysis (TGA) provides the mass fractions of elemental carbon and volatile materials in PM; XPS provides the possible chemical compositions in the topmost layer of PM surface and Raman analysis provides the possible structure of the carbon presented in PM. Dual fuel engine uses both liquid (diesel) and gaseous fuels simultaneously to produce mechanical power and can be switched to only diesel fueling under load. The dual fuel engine is operated with natural gas and simulated biogases (laboratory prepared) and results are compared between the dual fueling and diesel fueling under the same engine operating conditions. Significantly higher volatile fractions in PM are obtained for dual fueling compared to diesel fueling complementing the gravimetric results. The maximum contribution of the graphitic carbon or aliphatic carbon such as hydrocarbons and paraffins (C═C or C─C) are found in the topmost atomic layers of both the diesel and dual fuel PM samples. The other chemical states are found to be the carbon-oxygen functional groups indicating significant oxidation behavior in the PM surface. Lesser aromatic content is noticed in the case of dual fuel PM than diesel PM. The carbon in dual fuel PM is found to be more amorphous compared to diesel PM. These characterizations provide us new information how the PM from a diesel engine can be different from that from a dual fuel engine.

The Use of Biogas in Internal Combustion Engines: A Review

Biogas derived from organic waste materials is a promising alternative and renewable gaseous fuel for internal combustion (IC) engines and could substitute for conventional fossil fuels. The aims of this study are to review the past researches on biogas fuelled IC engines and from this review, to identify current research needs. A detailed analysis of the previous results of biogas fueling on the emissions and performance of spark ignition (SI) and dual fuel compression ignition (CI) engines is presented. The literature review reveals that the published research on biogas fueled IC engines are not rich in number and the scenario of biogas-diesel dual fuel engines is even worse. According to the analysis, biogas fueling in IC engines causes lower power output compared to natural gas, irrespective of the engine operating conditions. However, the use of biogas allows exhaust nitrogen oxides (NOx) emissions to be reduced substantially. Both experimental and computational analyses have been done in the case of SI engines. However, there are needs to investigate the exhaust emissions for the biogas-diesel dual fuel engines both experimentally and computationally. Also the effect of H2 S on engine emissions and life/durability, which is neglected very often in the literature, needs to be investigated.Copyright

Electron Microscopy Investigation of Particulate Matter from a Dual Fuel Engine

The particulate matter (PM) of a dual fuel engine was characterized in size, morphology and fractal geometry by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Particulate samples were collected from the diluted exhaust of the engine operated on diesel fuel, natural gas (NG) and synthetic biogas. The engine operating condition was kept the same to compare the results between diesel and dual fuel PM. SEM images yielded agglomerate number, size distributions and a shape descriptor. TEM was used to investigate the primary particle size distribution in agglomerates and the fractal dimensions of the sampled PM. Long chainlike PM agglomerates appeared for the diesel high load condition, whereas PM agglomerates for dual fueling were found to be smaller in size and with more spherical shapes. All of the measured PM appeared to have a bi-modal number size distribution irrespective of engine fueling condition. The average primary particle diameter increased for dual fuel PM (ranging from 26.9 to 29.5 nm) compared to diesel PM (26.4 nm). The average diameter tended to increase with the introduction of CO2 in the gaseous fuel. PM fractal dimensions were in the range from 1.69 to 1.88 for different PM samples. Higher fractal dimensions (from 1.73 to 1.88) were obtained for dual fuel PM compared to diesel (high load) PM (1.69). This finding also implies that diesel PM are more chainlike and elongated compared to the PM measured for dual fueling conditions. The very different engine fueling conditions used here give valuable understanding of the formation processes of PM.

Study of a Small Size Cogeneration Gas Engine in Stoichiometric and Lean Burn Modes: Experimentation and Simulation.

A medium size diesel engine converted to natural gas operation on the Otto principle has been studied under stoichiometric and lean burn operation in order to evaluate the potential to reduce the NOX exhaust gas emissions below the stringent limit prescribed by the Swiss Federal Clean Air Act - 250 mg/mN3, 5 % O2 (at normal (N) conditions, 5 % residual oxygen and dry). While both operational modes fulfill the prescribed NOX limit, lean burn operation, combined with turbocharging, provides a higher brake power and a better fuel conversion efficiency.

Investigation of the Effects of Flame Holder on the Combustion Characteristics of a Flat Flame Micro Combustor

Different flame holder materials and geometric parameters (pore size, thickness etc.) were experimentally tested in terms of pressure loss and combustion characteristics for the micro combustor of the ultra micro gas turbine (UMGT). Alumina ceramic porous plates, sintered stainless steel and perforated plates were used as flame holders within a 46 mm quartz walled micro combustor. The range of the average pore size of the porous plates and the sintered stainless steel plates are between 100 μm to 580 μm with an average porosity of 43∼54% and thickness of 1 mm to 4 mm. The perforated plate had a hole diameter of 1 mm and the distance between the centres of the holes was 2 mm. It was observed that the pore size and the thickness affect the pressure loss. However, at low incoming flow velocities all the flame holders had pressure losses of less than 5%. Flat flame combustion was achieved with all the different flame holders, but it stabilized at different incoming velocities and air to fuel ratios, according to different flame holder material and geometric parameters.Copyright

Identification of Time-Varying Time Constants of Thermocouple Sensors and its Application to Temperature Measurement

The present study addresses the problem of estimating time-varying time constants associated with thermocouple sensors by a set of basis functions. By expanding each time-varying time constant onto a finite set of basis sequences, the time-varying identification problem reduces to a parameter estimation problem of a time-invariant system. The proposed algorithm, to be called as orthogonal least-squares with basis function expansion algorithm, combines the orthogonal least-squares algorithm with an error reduction ratio test to include significant basis functions into the model, which results in a parsimonious model structure. The performance of the method was compared with a linear Kalman filter. Simulations on engine data have demonstrated that the proposed method performs satisfactorily and is better than the Kalman filter. The new technique has been applied in a Stirling cycle compressor. The sinusoidal variations in time constant are tracked properly using the new technique, but the linear Kalman filter fails to do so. Both model validation and thermodynamic laws confirm that the new technique gives unbiased estimates and that the assumed thermocouple model is adequate.

Experimental Study on Flat Flame Combustion for Ultra Micro Gas Turbine Applications

ABSTRACT The requirement for efficient power sources for portable electronics and miniature mechanical devices, such as laptops, micro robots, or micro aerial vehicles, has led to research on the ultra micro gas turbine (UMGT). The ultra micro gas turbine is one of the most promising power sources for small-scale applications due to its higher power and energy densities compared to currently used batteries. In order to realize UMGT as a viable power source, its individual components have to be developed, since downscaling introduces new problems for each component. Since the micro combustor is one of the key components of UMGT, it has to be improved. Until now, it has been very difficult for micro combustors to achieve wide flame stability, high combustion efficiency, and clean combustion with low pressure loss, due to the associated downscaling problems, such as high heat loss and small residence time. In order to achieve wide flame stability, the effect of preheating the reactants on flat flame stabilization was investigated experimentally on a 46-mm inner diameter quartz-walled flat flame combustor. The experimental results showed that preheating improves flat flame stabilization. Since preheating increases the burning velocity, flat flame can stabilize at higher incoming flow velocities as long as they are lower than the burning velocities. From the relationship between the incoming flow velocities and burning velocities, a correlation was obtained for flat flame micro combustors for micro power generation applications. With this correlation, it is possible to determine the minimum combustor diameter required for stable flat flame combustion. Also, in order to have a stable flat flame at higher mass flow rates, the correlation enables the calculation of the required reactants temperature, and is a major contribution to the design of micro combustors.

Thermal Performances of a Small-Scale Regenerative Combustion Chamber for Ultra-Micro Gas Turbine

ABSTRACT New manufacturing techniques and technologies have led to the development of a new research topic focusing on fluid dynamics and combustion at small scale, in particular in the last 20 years. One of the most promising technologies is represented by the ultra-micro gas turbines, which were developed with the aim of providing a portable and clean power source for devices, such as unmanned aerial vehicles and drones, global positioning system, exoskeletons for military applications, and backup emergency power supply. Currently, not many prototypes have been built because of the issues posed by scaling down the system. Among these there are excessive fluid dynamic and thermal losses decreasing the overall efficiency, elevated combined thermal and mechanical stress of components, and the necessity of developing high speed bearings. This study focuses on investigating experimentally a possible solution to effectively recover heat contained in the combustion products to preheat the unburned mixture, increasing the overall efficiency. Thus, an 18-mm internal-diameter regenerative combustion chamber surrounded by two sets of helicoidal channels was developed. The combustion chamber included a sintered steel porous medium to allow the flame to stabilize. The combustion products were recirculated in order to maximize the heat transferred to the cold mixture, with benefits in terms of flammability limits and fuel consumption. Mass flow rate and equivalence ratio were varied in the tests and the gas temperatures at different locations within the combustion chamber were measured, along with the composition of the combustion products. The heat released was included in the range 65 W and 343 W. Tests were run in both a non-insulated and insulated configuration and comparisons were made. Resulted showed the achievement of clean combustion of liquefied petroleum gas with combustion efficiency higher than 99% for lean mixtures. Also, a good level of heat recovery was achieved, reaching 45% and 23% of heat transferred to the reactants from the exhaust gases in the insulated and non-insulated combustion chamber, respectively.