• Energy Research

Abstract This experimental investigation evaluates the combustion and exhaust emission characteristics of cottonseed oil and its (methyl ester) bio-diesel in blends with 20% by vol. of either n -butanol or diethyl ether (DEE), fueling a standard, experimental, single-cylinder, four-stroke, high-speed direct injection (HSDI), ‘Hydra’ diesel engine. The tests are conducted using each of the above fuel blends or neat cottonseed oil or its neat bio-diesel, with the engine operating at three different loads. Fuel consumption, exhaust smoke, nitrogen oxides (NOx), carbon monoxide (CO) and total unburned hydrocarbons (HCs) are measured. The differences in the performance and exhaust emissions of these fuel blends from the baseline operation of the diesel engine, i.e. when working with neat cottonseed oil or its neat bio-diesel, are compared. Fuel injection and combustion chamber pressure diagrams are obtained, and heat release rate analysis of the latter ones is performed revealing some interesting features of the combustion mechanisms. These results and the widely differing physical and chemical properties of n -butanol and DEE against those for the cottonseed oil and its bio-diesel are used to aid the correct interpretation of the observed engine behavior. It is revealed that n -butanol and DEE, which can be produced from biomass (bio-butanol and bio-DEE), when added to the vegetable oil or its bio-diesel improve the behavior of diesel engine.

Dielectric microspacers (DMS) are a novel micro-technology that can be used to achieve a fixed micron/sub-micron gap distance between two separated surfaces, such as the emitter (cathode) and the PV cell (anode) of a near-field thermophotovoltaic converter (TPV). One of the system’s challenges is the flow of undesirable excess thermal energy from the cathode to the anode that might cause the PV cell to overheat. This work investigates the possibility of integrating this technology into a hybrid thermionic-photovoltaic (TIPV) converter operating at ultra-high temperatures (>1000 °C) without any risk of collector’s overheating, which might lead to its mechanical failure. A steady-state 3-D CFD model was developed in Fluent v17.1 solver to assess the system’s thermal behavior when the two electrodes were separated by a distance of 8–10 μm. The heat transfer through conduction across the system components and the net photon/electron flux between the two electrodes were simulated. Different cathode temperatures within the range of 1500–2500 K and various DMS shapes (capillary, cylindrical), patterns (e.g., ring-shaped) and sizes were studied. Results show that thermal performance is not affected by the DMS pattern, even for thermal conductivities of 80 W/(m·K), whereas the possibility of mechanical failure is considerable for Tcathode > 2000 K.

Developed explicit simulation program is used to study the effect of the cylinder wall temperature on the transient performance of a turbocharged Diesel engine. The simulation, based on the filling and emptying approach, provides several innovations, such as detailed analysis of the thermodynamic and dynamic differential equations on a degree crank angle basis, analysis of transient mechanical friction and also a detailed mathematical simulation of the fuel pump. Each equation in the model is solved separately for every cylinder of the six cylinder Diesel engine considered. The model is validated against experimental data for various load changes. The effect of the cylinder wall temperature on the transient response of the engine is studied and depicted in detailed, multiple diagrams. Special reference is made to the interesting limited cooled (adiabatic) case. It is shown from the analysis that after a ramp increase in load, the speed response as well the other important properties of the engine and turbocharger are marginally impacted by the cylinder wall temperature, affected only slightly when an adiabatic wall configuration is chosen.

Abstract The cyclic variability in a spark-ignition (SI) engine is examined fueled with methane/hydrogen blends with the use of an in-house computational fluid dynamics (CFD) code. A recent methodology is followed, which has been developed with the main aim at providing accurate predictions of the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) in a fraction of time. Instead of simulating several tens of engine cycles, the methodology is based on the numerical results obtained from just 5 cycles, which are then processed for developing suitable fitted correlations of the main parameters as a function of a normalized distance. The latter expresses the distance of the spheres of the initial flame within the computational cell at the spark-plug region with the local turbulent eddy, and provides a smooth transition from the laminar burning regime to the fully turbulent one. This sub-model is included in the ignition numerical approach and is applied here in a SI engine with 3 different hydrogen contents, 10%, 30% and 50%, and three equivalence ratios, 1, 0.8 and 0.7, showing that the COV of IMEP is well predicted compared to the available measured data. Other parameters of engine cycle variations are also examined, such as the distribution of the IMEP. The variability of NO (nitric oxide) emissions is also examined, showing that for the stoichiometric cases it follows a distribution similar to a normal (Gaussian) one, while for lower ratios it is positively skewed. Overall, the methodology seems to provide reliable results for the whole range of the operating conditions examined, while the next steps of this activity will focus on similar cases for engine with variable speed and load, with the final goal to include additional mechanisms that contribute to the engine cycle variations.

Abstract The present two zone model of a direct injection (DI) Diesel engine divides the cylinder contents into a non-burning zone of air and another homogeneous zone in which fuel is continuously supplied from the injector and burned with entrained air from the air zone. The growth of the fuel spray zone, which comprises a number of fuel-air conical jets equal to the injector nozzle holes, is carefully modelled by incorporating jet mixing, thus determining the amount of oxygen available for combustion. The mass, energy and state equations are applied in each of the two zones to yield local temperatures and cylinder pressure histories. The concentration of the various constituents in the exhaust gases are calculated by adopting a chemical equilibrium scheme for the C–H–O system of the 11 species considered, together with chemical rate equations for the calculation of nitric oxide (NO). A model for evaluation of the soot formation and oxidation rates is included. The theoretical results from the relevant computer program are compared very favourably with the measurements from an experimental investigation conducted on a fully automated test bed, standard “Hydra”, DI Diesel engine installed at the authors’ laboratory. In-cylinder pressure and temperature histories, nitric oxide concentration and soot density are among the interesting quantities tested for various loads and injection timings. As revealed, the model is sensitive to the selection of the constants of the fuel preparation and reaction sub-models, so that a relevant sensitivity analysis is undertaken. This leads to a better understanding of the physical mechanisms governed by these constants and also paves the way for construction of a reliable and relatively simple multi-zone model, which incorporates in each zone (packet) the philosophy of the present two zone model.

The vital issue of exhaust emissions during transient operation of diesel engines has been studied so far mainly on an experimental rather than simulation basis, owing to the very high computational times required for the analysis of each transient cycle. The study of transient emissions, however, is extremely important to manufacturers, since newly produced engines must meet the stringent regulations concerning exhaust emissions levels. In the present work, a comprehensive two-zone transient diesel combustion model is used for a preliminary evaluation of the effect of various parameters on nitric oxide (NO) and soot emissions during transient operation after load changes. The parameters are divided into three categories according to the specific sub-system examined, i.e. engine, load and turbocharger. Demonstrative diagrams are provided for the development of NO and soot emissions during the transient event, which depict the effect of each parameter considered. Moreover, the peculiarities of each case are discussed mainly in relation to turbocharger lag effects. For the current engine-load configuration, it is found that exhaust valve opening timing and cylinder wall insulation affect considerably NO and soot emissions. Additionally, load characteristics as well as turbocharger (T/C) mass moment of inertia play an important role on the development of transient NO and soot emissions.

This paper introduces a Model Predictive Control (MPC) strategy for the optimal energy management of a district whose buildings are equipped with vertically placed Building Integrated Photovoltaic (BIPV) systems and Battery Energy Storage Systems (BESS). The vertically placed BIPV systems are able to cover larger areas of buildings’ surfaces, as compared with conventional rooftop PV systems, and reach their peak of production during winter and spring, which renders them suitable for energy harvesting especially in urban areas. Driven by both these relative advantages, the proposed strategy aims to maximize the district’s autonomy from the external grid, which is achieved through the cooperation of interactive buildings. Therefore, the major contribution of this study is the management and optimal cooperation of a group of buildings, each of which is equipped with its own system of vertical BIPV panels and BESS, carried out by an MPC strategy. The proposed control scheme consists of three main components, i.e., the forecaster, the optimizer and the district, which interact periodically with each other. In order to quantitatively evaluate the benefits of the proposed MPC strategy and the implementation of vertical BIPV and BESS, a hypothetical five-node distribution network located in Greece for four representative days of the year was examined, followed by a sensitivity analysis to examine the effect of the system configuration on its performance.

An experimental study has been conducted to evaluate the use of various blends of cottonseed oil or its methyl ester (bio-diesel) with diesel fuel, in blend ratios from 10/90 up to 100/0, in a fully instrumented, four-stroke, High Speed Direct Injection (HSDI), Ricardo/Cussons 'Hydra' diesel engine. The tests were conducted using each of the above fuel blends or neat fuels, with the engine working at a medium and a high load. Volumetric fuel consumption, exhaust smokiness and exhaust-regulated gas emissions such as nitrogen oxides, carbon monoxide and unburnt hydrocarbons were measured. The differences in the performance and exhaust emissions from the baseline operation of the engine, that is, when working with neat diesel fuel, were determined and compared, as well as the differences between cottonseed oil or its methyl ester and their blends. Theoretical aspects of diesel engine combustion were used to aid the correct interpretation of the engine behaviour.

Abstract The present work reviews the literature concerning the effects of alcohol/diesel blends on the exhaust emissions of diesel engines operating under transient conditions, i.e., acceleration, load increase, starting and transient/driving cycles. Two very promising alcohols are covered in this survey, namely ethanol and n-butanol. The analysis focuses on all regulated exhaust pollutants, i.e., particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), with results for unregulated emissions, carbon dioxide and combustion noise radiation also included. The main mechanisms of exhaust emissions during transients are identified and discussed, with respect to the fundamental aspects of transient operation and the differing properties of alcohols relative to the reference diesel oil. Based on the published studies up today, summarization of emissions data and cumulative trends are presented, for the purpose of quantifying the alcohol blends benefits or penalties on the regulated emissions during various driving cycles. Particularly for the emitted PM and smoke, a statistically significant correlation with the oxygen content exists (R2=0.85 and 0.95, respectively). A similar correlation holds true for the heavy-duty, engine-dynamometer data of engine-out CO. Finally, a detailed list is provided that summarizes the main data from all studies published so far.