Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI ...engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NO
x
) emissions, while producing lower emissions of carbon dioxide (CO
2), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NO
x
emissions. High NO
x
emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas–hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NO
x
and CO
2 emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.
Analytical expressions for the no-load magnetic field distribution of slotless brushless permanent-magnet (PM) machines with static, dynamic, and mixed rotor eccentricities are presented. The ...proposed analytical expressions can be used for slotless brushless PM machines with any radius-independent magnetization pattern. The analytical expressions and the results for six different magnetization patterns are presented. Based on the analytical magnetic field distribution, the line and phase back-electromotive force waveforms, local traction components and unbalanced magnetic forces are obtained. The analytical results are compared with those from finite-element analyses to validate the derived expressions.
•Triple fuelling of CI engines with hydrogen and natural gas.•Diesel and RME used as pilot fuels.•Triple fuelling presents better trade-off between specific HC and specific NOx.•Reduction in specific ...HC was more prominent at lower loads.
Higher unburned hydrocarbon emissions are attributed to the relatively low temperature combustion of natural gas in compression ignition engines whereas the combustion of hydrogen in compression ignition environment results in higher NOX. These emissions characteristics are explained on the basis of different physio-chemical properties of the two gaseous fuel: higher Cp value in case of natural gas and higher diffusion coefficient, wider flammability limits and shorter quenching gaps for hydrogen are held responsible for these trends. This study assesses the potential of hydrogen being used in combination with natural gas with diesel and rapeseed methyl ester (RME) as pilot fuels. This type of fueling can be referred as ‘triple fueling of the compression ignition engines’ and has the potential to achieve a better trade-off between the higher NOX associated with hydrogen and higher hydrocarbon emissions associated with natural gas based dual fueling of compression ignition engines. The present study has investigated the potential of the triple fueling of the compression ignition engines so far the attainment of a better trade-off between NOX and hydrocarbon emissions are concerned. Comparing the specific NOX and hydrocarbon emissions in different cases reveals that a significant drop in specific hydrocarbon emissions can be achieved at the cost of a small increment in specific NOX. At both speeds (1000 rev/min and 1500 rev/min), the reduction in hydrocarbon emissions is more prominent at relatively lower loads, which can be a potential solution of reducing specific hydrocarbon emissions at lower loads in diesel engine operations. The stoichiometric equation for the triple fueling (Diesel or RME piloted mixture of natural gas and hydrogen) is also presented.
Finite Block Method in elasticity Wen, P.H.; Cao, P.; Korakianitis, T.
Engineering analysis with boundary elements,
09/2014, Letnik:
46
Journal Article
Recenzirano
A new point collocation algorithm named Finite Block Method (FBM), which is based on the one-dimensional differential matrix is developed for 2D and 3D elasticity problems in this paper. The main ...idea is to construct the first order one-dimensional differential matrix for one block by using Lagrange series with uniformly distributed nodes. The higher order derivative matrix for one-dimensional problem is obtained. By introducing the mapping technique, a block of quadratic type is transformed from Cartesian coordinate(xyz) to normalised coordinate (ξης) with 8 seeds or 20 seeds for two or three dimensions. The differential matrices in physical domain are determined from that in the normalised transformed system. Several 2D and 3D examples are given and comparisons have been made with either analytical solutions or the boundary element method to demonstrate the accuracy and convergence of this method.
•Compression ignition engine fueled with manifold injected natural gas.•Engine operated under dual fueling mode with diesel and RME examined as pilot fuels.•Emissions maps presented for specific ...emissions of CO2, HC and NOX.•Novel method of presentation for the comparison of data collected from dual fueled CI engine.•Engine efficiency for a dual fueled engine examined across entire range of operation.
When natural gas is port/manifold injected into a compression ignition engine, the mixture of air and the natural gas is compressed during the compression stroke of the engine. Due to the difference in the values of specific heat capacity ratio between air and natural gas, the temperature and pressure at the time of pilot fuel injection are different when compared to a case where only air is compressed. Also, the presence of natural gas affects the peak in-cylinder (adiabatic flame) temperature. This significantly affects the performance as well as emissions characteristics of natural gas based dual fueling in CI engine. Natural gas has been extensively tested in a single cylinder compression ignition engine to obtain performance and emissions maps.Two pilot fuels, diesel and RME, have been used to pilot natural gas combustion. The performance of the two liquid fuels used as pilots has also been assessed and compared. Tests were conducted at 48 different operating conditions (six different speeds and eight different power output conditions for each speed) for single fueling cases. Both the diesel and RME based single fueling cases were used as baselines to compare the natural gas based dual fueling where data was collected at 36 operating conditions (six different speeds and six different power output conditions for each speed). Performance and emissions characteristics were mapped on speed vs brake power plots. The thermal efficiency values of the natural gas dual fueling were lower when compared to the respective pilot fuel based single fueling apart from the highest powers. The effect of engine speed on volumetric efficiency in case of the natural gas based dual fueling was significantly different from what was observed with the single fueling. Contours of specific NOX for diesel and RME based single fueling differ significantly when these fuels were used to pilot natural gas combustion. For both of the single fueling cases, maximum specific NOX were centered at the intersection of medium speeds and medium powers and they decrease in all directions from this region of maximum values. On the other hand, an opposite trend was observed with dual fueling cases where minimum specific NOX were observed at the center of the map and they increase in all direction from this region of minimum NOX. RME piloted specific NOX at the highest speeds were the only exception to this trend. Higher specific HC and lower specific CO2 emissions were observed in case of natural gas based dual fueling. The emissions were measured in g/MJ of engine power.
► A method for the techno-economic, environmental and risk analysis of marine propulsion systems is presented. ► A tool has been developed, comprising modules for engine and ship performance, engine ...life, economic, emissions and risk. ► The tool enables the evaluation of powerplants in terms of net present cost, through a holistic approach. ► Risk assessment is performed through a stochastic approach, for taking into account uncertainty in future scenario parameters. ► Two different engine configurations have been tested and compared in terms of fuel burn, emissions and overall cost.
A Techno-economic, Environmental and Risk Analysis (TERA) computational method has been developed for marine propulsion systems. The method comprises several numerical models which simulate the life cycle operation of marine gas turbines installed on marine vessels. Using a system-of-systems approach, the effect of operational profile can be taken into consideration in the assessment of a novel prime mover. Stochastic estimates of the powerplant’s life cycle net present cost are generated. The ship performance model plays a central role in the TERA method. This is an integrated virtual marine vessel operating environment that allows the calculation of engine performance and exhaust emissions (nitric oxide (NOx), carbon monoxide CO, carbon dioxide (CO2) and unburned hydrocarbon (UHC)) for a given trip. The life of the gas turbine is assessed through a creep-life prediction method, which plays a significant role on the maintenance cost calculation in the economic model. The economic model predicts net present cost over the operating life of the vessel using stochastic analysis of the earning capacity of the ship powered by the chosen prime mover. The TERA simulation of a 25MW marine gas turbine powering a RoPax fast ferry in an integrated full electric propulsion system is presented as an illustration of the method. The example includes aspects of the systemic analysis of engine and ship performance, accompanied by environmental effect and engine life prediction, coupled with an economic feasibility stochastic study of the selected propulsion system under several journey and economic scenarios.
The purpose of this paper is to illustrate the advantages of the direct surface-curvature distribution blade-design method, originally proposed by Korakianitis, for the leading-edge design of turbine ...blades, and by extension for other types of airfoil shapes. The leading edge shape is critical in the blade design process, and it is quite difficult to completely control with inverse, semi-inverse or other direct-design methods. The blade-design method is briefly reviewed, and then the effort is concentrated on smoothly blending the leading edge shape (circle or ellipse, etc.) with the main part of the blade surface, in a manner that avoids leading-edge flow-disturbance and flow-separation regions. Specifically in the leading edge region we return to the second-order (parabolic) construction line coupled with a revised smoothing equation between the leading-edge shape and the main part of the blade. The Hodson–Dominy blade has been used as an example to show the ability of this blade-design method to remove leading-edge separation bubbles in gas turbine blades and other airfoil shapes that have very sharp changes in curvature near the leading edge. An additional gas turbine blade example has been used to illustrate the ability of this method to design leading edge shapes that avoid leading-edge separation bubbles at off-design conditions. This gas turbine blade example has inlet flow angle 0°, outlet flow angle −64.3°, and tangential lift coefficient 1.045, in a region of parameters where the leading edge shape is critical for the overall blade performance. Computed results at incidences of −10°,
−5°,
+5°,
+10° are used to illustrate the complete removal of leading edge flow-disturbance regions, thus minimizing the possibility of leading-edge separation bubbles, while concurrently minimizing the stagnation pressure drop from inlet to outlet. These results using two difficult example cases of leading edge geometries illustrate the superiority and utility of this blade-design method when compared with other direct or inverse blade-design methods.
Dual-fuel compression ignition (CI) engine operation with hydrogen is a promising method of using hydrogen gas in CI engines via high-cetane pilot fuel ignition. However, hydrogen dual-fuel operation ...with neat pilot fuels typically produce: high NO
x
emissions; and high combustion chamber pressure rise rates (leading to increased “Diesel knock” tendencies). While water-in-fuel emulsions have been used during normal CI engine operation to cool the charge and slow combustion rates in an effort to reduce NO
x
emissions, these water-in-fuel emulsions have not been tested as pilot fuels during hydrogen dual-fuel combustion. In this work two water-in-biodiesel emulsions are tested as pilot fuels during hydrogen dual-fuel operation. Hydrogen dual-fuel operation generally produces at best comparable thermal efficiencies compared with normal CI engine operation, while the emulsified biodiesel pilot fuels generally increase thermal efficiencies when compared with the neat biodiesel pilot fuel during dual-fuel operation. There is also a clear reduction in NO
x
emissions with emulsified pilot fuel use compared with the neat pilot fuel. The thermal efficiency increase is more apparent at higher engine speeds, while the NO
x
reduction is more apparent at lower speeds. This is due to two conflicting effects (exclusive to emulsified pilot fuel) that occur in tandem. The first is the cooling effect of water vapourisation on the charge, while the second is the microexplosion phenomenon which enhances fuel-air mixing. The NO
x
emission reduction is due to the emulsified pilot fuel lowering pressure rise rates compared with the neat pilot fuel, while the efficiency increase is due to a more homogeneous charge resulting from the violent microexplosion of the emulsified pilot fuel. Smoke, CO, HC and CO
2 emissions remain comparable to neat pilot fuel tests. Overall, emulsified pilot fuels can reduce NO
x
emissions and increase thermal efficiencies, however not at the same instance and under different operating conditions. The general trends of reduced power output, reduced CO
2 and increased water vapour emission during hydrogen dual-fuel operation (with neat pilot fuels) are also maintained.
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