Eran Sher

Currently, compression ignition engine technology is becoming less competitive compared to spark ignition engine technology due to emission standards. Lean combustion strategies are preferable for spark ignition engines, but they require enhanced ignition strategies compared to current stoichiometric spark ignition engines. In this paper, we lay physical foundations for such enhanced ignition strategies that can support lean combustion strategies in spark ignition engines. In this theoretical study, we conduct a preliminary foray into the possibility of controlling the species composition of a cold-plasma kernel as a precursor for an ignition process. We consider a multi-pulse electric source system coupled to a DBD (Dielectric Barrier Discharge) electrode immersed in a dry air medium. The coupled system is designed such that the medium in the electrodes’ gap is subjected to a pulse structure which is composed of a peak voltage followed by a secondary low-voltage pulse. Use is made of a comprehensive mathematical model that includes the electric field, potential and current, with the relevant conservation equations for 24 species and 168 kinetic reactions, while considering different energy modes (rotational, vibrational, electronic excitation, dissociation, and ionization) within the electrodes’ gap. The model was validated against independent published results, and was used to understand the mechanism of energy conversion to species that play a central role in terms of the ultimate ignition dynamics. Various pulse repetition frequencies and different pulse constructions were considered. Special attention was given to the amount of deposited energy, and to the energy channeled to known ignition supportive modes and species. The present results show that the energy deposition can be divided into two main stages which are characterized by high and low voltage levels, respectively. We propose that for more successful ignition, the amount of energy deposition during the second (low voltage) stage should be higher than that during the first (high voltage) stage. During the second stage, the deposition of energy into specific modes can be fine-tuned by setting appropriate voltage levels. Based on these findings, we demonstrate how control of the sequence of voltage pulses can further increase enhancement of ignition and combustion promoting processes.

The growing demand for high-power propulsion systems with high energy density for micro-air vehicles (MAVs) has motivated a worldwide search for alternatives to the present conventional propulsion units which are mainly internal combustion engines (ICEs) and electric motors. Here, we evaluate the advantages of alternative mechanical energy-based storage system having high energy density and high power density characteristics. Additional key requirements for such an alternative propulsion system are high power for taking off and maneuverability, efficient energy conversion during cruise flights, easy speed control, allowing high-altitude operation, low noise signature, low thermal signature, low pollutants emission and easy restarting when needed. The various excellent features of both the internal combustion engine and the electric motor have placed them as the leading means for propulsion systems. However, the breathing difficulties of the ICEs at high-altitude conditions, their relatively low reliability and their high noise signature encouraged the search for alternative solutions. Although electric motors overcome these major drawbacks, the energy density of their energy source is much lower than that of the hydrocarbon fuels (more than an order of a magnitude) and their ability to function in missions requiring long flight durations is significantly inferior. Here, we examine alternative means of energy storage systems. Alternative mechanical energy storages including elastic (different types of springs), compressed air, thermal (phase-change materials and cryogenic reservoirs) and kinetic (flywheels) were analyzed and examined. Promising emerging technologies which are based on exceptional materials like specially designed elastomers and carbon nanotubes (CNTs) have been critically reviewed here. The carbon nanotubes are known for their very high energy density, where the energy can be delivered directly to the aircraft propeller with a remarkably high conversion efficiency.

A wide range of recent applications require high-quality sprays that are characterized by extremely small-sized droplets, a narrow droplet size distribution, and a short breakup length. Fuel injection systems in propulsion units, energy storage, medical implementations, printings, and coatings are just a few examples. Flash-boiling atomization is a unique method that was extensively developed during the past two to three decades and has been proven to generate high-quality demanded sprays. In flash-boiling atomization, the liquid is forced to reach a metastable superheated state by either rapid heating or rapid pressure drop, where vapor bubbles nucleate, become fast-growing, and subsequently break down the liquid into a fine spray in a very short time. This present article focuses on flash-boiling atomization via rapid depressurization, which is presently more relevant to energy systems. The field of flash-boiling atomization has seen rapid growth and popularity in the past two decades. The aim of this article is to quantitatively portray the landscape and evolutionary trajectory of flash-boiling atomization research and applications and to detect new research frontiers and emerging trends in the literature on flash-boiling atomization. We briefly review the basic theories of the flash-boiling atomization mechanism present a comprehensive overview of the field, from its birth in approximately the 1970s to the present, and provide a database comprising 386 articles published on the topic of flash-boiling atomization. We visualize the intellectual structure of flash-boiling atomization research and applications and track its evolvement over the past five decades, thus providing a global overview and a comprehensive understanding of the development of flash-boiling atomization research and emerging applications.

Due to the oxygen shortage at high altitudes, the use of diesel engines in small aircraft is limited to a low ceiling level. Here, we propose to significantly extend the ceiling level by introducing an in-cylinder steam reforming system. In this arrangement, the fuel direct-injection assembly comprises of a two-stage process. In the first stage, a blend of methanol and water is injected into the hot previously compressed cylinder charge onto an in-cylinder catalyst. Residual heat is absorbed due to the blend evaporation and the steam-reforming process to produce hydrogen. In the second stage, diesel fuel with a lower ignition temperature than the hydrogen fuel is injected to initiate combustion, while the absorbed heat (from the first stage) is released through the hydrogen oxidation. Essentially, the absorbed heat is exploited to produce extra hydrogen fuel, which increases the cycle efficiency. In this arrangement, the in-cylinder oxygen content is significantly increased due to the additional oxygen atoms that are included in the methanol and in particular in the water molecules. These are released when the methanol and water are decomposed during the steam-reforming process. We show that owing to the addition of the oxygen content in the cylinder, the flight ceiling level can be extended from 5000 to 9000 ft, and that the indicated efficiency can be increase up to 6%.

Flash boiling atomization is one of the most promising methods to generate unusual fine sprays with a high degree of droplets’ size uniformity. Several mechanisms, while employing different types of atomizers, have been proposed to implement this method. Among them are the household cans where cavitation (the process of nucleation in a liquid when the pressure falls below the vapor pressure), boiling (the process of nucleation in a liquid when the temperature rises above the saturated temperature), and liquid tension under negative absolute pressures.In the present work, we propose and theoretically examine a different method for spray generation employing the flash boiling mechanism, where the liquid is subjected to intense volumetric radiation. We evaluate the bubbles’ nucleation rate, the bubbles’ growth rate, and their size at burst time as a plausible indication of the droplets’ size. Here we consider a mixed regime where both heterogeneous and homogeneous nucleation may occur.Our results suggest that intensive radiation of the liquid might induce homogeneous nucleation, leading to the generation of a huge number of tiny bubbles and subsequently resulting in a fine spray. An increase in the heating power leads to a significant decrease in the bubbles' mean diameter, thus contributing to the refinement of the generated droplets. The atomization process efficiency shows a complex behavior; it is capped to roughly 0.05% where homogeneous nucleation is dominant, whereas for systems where heterogeneous boiling is the main boiling regime it aspires to much smaller values.

While considering the deflagration regime, the thermal theory of combustion proposes that the mechanism of heat transfer from the flame exothermic zone to the front neighborhood reactants layer dominates the flame behavior. The introduction of the Fourier law allows a closed solution of the continuity and energy conservation equations to yield the burning velocity. It is, however, clear that this classical solution does not conform to the momentum equation. In the present work, instead of introducing the Fourier law, we suggest the introduction of a simplified version of the Onsager relationship, which accounts for the entropy increase due to the heat transfer process from the front layer to its successive layer. Solving for the burning velocity yields a closed solution that also definitely conforms to the momentum equation. While it is realized that the pressure difference across the flame front in the deflagration regime is very small, we believe that violating the momentum equation is intolerable. Quite a good fitting, similarly to the classic theory predictions, has been obtained between our predictions and some experimentally observed values for the propagation flame deflagration velocity, while here, the momentum equation is strictly conserved.

We present a new model for the prediction of the explosion limits of the hydrogen-oxygen system. Our model is based on the principle of ignition delay time, postulating that crossing the explosion limit (by increasing the pressure or temperature above it) causes a sharp decrease in the ignition delay time. By using fundamentals of the chain ignition theory, and by employing the Le-Chatelier rule for the explosion limits of fuel mixtures, we develop our model equations. We use numerical analysis to calibrate the constants, and show that our proposed model can accurately capture the unique trend of the peninsula shaped explosion limits. We believe that the relative simplicity of our model will be useful in the analysis of more complex hydrocarbon fuels.

An experimental study on flashing injection of gasoline-dissolved CO2 mixtures through specially designed injectors was performed. The injectors used comprised an expansion chamber, prior to the discharge orifice, in order to promote the CO2 flashing. Malvern Mastersizer-X was used to measure the spray characteristics. High-speed digital video camera was employed to visually register the expansion chamber flow patterns. The geometry of the expansion chamber was optimized to produce minimum SMD. The high-speed camera photographs, have revealed the different flow regimes inside the expansion chamber under different operation conditions, and thus provided a solid ground to explain the relationship between the optimal chamber dimensions, the operation conditions, and the observed characteristics of the spray.

Previous investigations have demonstrated that improvements in gasoline engine performance (engine efficiency, power, exhaust emissions, engine braking, and usable engine speed range) can be accomplished if the valve timing are variable. Because of the difficulties of providing a variable valve timing (VVT) mechanism with acceptable cost, durability and reliability, very few automotive engines in normal production have been equipped till now with variable valve timing. However, owing to recent technology developments associated with electromagnetic and hydraulic valve control, and owing to recent progresses in microprocessor utilization, the application of VVT in the near future is quite feasible. In this work valve timing strategies for maximizing engine torque in terms of the intake closing and exhaust opening timings of a commercial throttled SI engine are studied. The MICE (Modeling Internal Combustion Engines) commercial computer program which simulates an actual SI cycle has been used. Overall performance characteristics such as the cycle efficiency, maximum torque, as well as the maximum amount of work available, are calculated by using the 2nd law of thermodynamics. The model has been calibrated with data obtained from a measured indicator diagram, and validated against the overall performances of the engine. It was concluded that when both valves and spark timings are optimized, the optimal timing of each valve depends linearly (in a good approximation), on the engine speed, while the slope depends apparently weakly on the engine load. Under these operation conditions, the rated power of the engine has been increased by 24.4%, and the engine torque at the rated speed by 9.5%. The maximum torque has been increased by 33.8% from 7.4 Nm at 3,100 rev/min to 9.9 at 1,600 rev/min. The shift of the maximum torque towards a lower engine speed is attributed to the longer expansion stroke (retarding EO), thus reducing the available energy loss to exhaust, and increasing the effective compression ratio while closing earlier the inlet valve, thus increasing the available cylinder charge at IC. The present results are summarized as working maps for the engine designer. These show the influence of the inlet and exhaust valve timing on the engine performance at the entire range of operation conditions (engine load and speed).