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Predicting turbulence in a zero-dimensional (0D) simulation is an undeniably challenging task due to the complexity of in-cylinder charge motion. In that respect, a physics-based understanding of in-cylinder flow phenomena is of vital importance for establishing high-fidelity 0D turbulence models under diverse engine conditions. The proposed 0D model is hence built upon the kinetic energy analyses of tumble, evidenced by three-dimensional computational fluid dynamics. Specifically, the overall 0D turbulence model consists of an intake model, a spinning up model, and a turbulence production model. The major difference between this model and the existing 0D turbulence models is that this model is grounded on a kinetic energy perspective of tumble, as opposed to an angular momentum perspective of tumble. That is, the behaviors of tumble, such as spinning up and vortex breakdown, are interpreted and modeled based on the changes in the kinetic energy of tumble. This enables the proposed model to secure wider applicability at various engine conditions, which were otherwise difficult to achieve. Therefore, the 0D simulation in this study was able to predict turbulent intensities at the conditions, differing in valve strategy, engine geometry, and engine operation, without changing any validation constants. Furthermore, along with the validation points used in this study, better predictions of turbulent intensity were achieved using the proposed model compared to the existing state-of-the-art model.

This study explored the prediction of the effect of dilution on the ignition delay of syngas and hydrocarbon fuels from dimensionless parameters. In this regard, the dilution effect was categorized into two sub-effects. The first is a non-chemical effect, which mainly represents the effect of decreasing the fuel-air concentration and the effect of heat-release damping. The second is the chemical effect induced by participation of the diluent in the reaction pathway. Then, we derived the determining parameters of each sub-effect based on chemical kinetics. The non-chemical effect was found to be related to the normalized sensitivity of the ignition delay to the total concentration, n ˜ ; the ratio of the molar constant volume specific heat of the diluent and the fuel-air mixture, c v , D * ; and the average Zel'dovich number during the ignition delay, Z ˜ e m , which represents the effect of temperature rise during the ignition delay on the reaction rate. Meanwhile, the extent of the chemical effect was found to be related to the sensitivity of the ignition delay to the reaction rate coefficient of the three-body reactions, S τ 0 − k j , multiplied by the sensitivity of the reaction rate coefficients to the third body concentration, S k j − [ M ] j , and the ratio of the collision efficiency of the diluent and the fuel-air mixture, η D , j * . Based on this relationship, we investigated the correlation between the extent of dilution effect and the aforementioned dimensionless parameters, n ˜ , c v , D * , Z ˜ e m , S τ 0 − k j , S k j − [ M ] j , and η D , j * , under the temperature range of 600–1400 K, pressure of 1–80 bar, and equivalence ratio of 0.5–2 with 13 different syngas or hydrocarbon fuels. Consequently, a strong correlation was found between the dilution effect and the dimensionless parameters, with the deterministic coefficient over 0.95. Furthermore, we suggested a proper form of the empirical correlation for the ignition delay that can cover a wide range of diluent concentrations, considering the extent of the chemical effect appropriately. This study presents the first dimensionless analysis for the dilution effect on ignition delay, developing a general framework for prediction of the effect based on the thermochemical characteristics of a fuel-air mixture.

Notwithstanding the continuous technological development of spark-ignition engines, the physics of in-cylinder phenomena has not been explicitly clarified to date. Furthermore, the advent of diverse engine technologies and in-cylinder compositions has increased the difficulty of understanding these phenomena using existing analysis methods. Under those circumstances, the modeling based on the fundamental combustion behaviors is becoming critical for analyzing combustion. Built upon the zero-dimensional flame surface density model, therefore, the models proposed herein attempt to reproduce a vast majority of combustion behaviors revealed to date, such as differential diffusion, thermo-diffusive instability, cutoff scales, distributed reaction, and flame quenching. As a result, we successfully predicted the combustion of numerous operating points of four different SI engines, including the extremely lean operating points of gasoline/air mixture up to [Formula: see text]. Furthermore, the extensive thermodynamic ranges are explored for the validation of the models with the aid of continuously variable valve timing modules; the pressure and the calculated temperature of the unburned mixture at ignition timing are varied from 4.5 to 28.6 bar and 559–794 K, respectively. In this study, we suggest an expression for a fully-developed turbulent flame speed well-fitted with the explored ranges of chemical and thermodynamic states by inspecting the influencing factors for flame wrinkling. Additionally, we analyze combustion using the developed models and identify several distinctive features of both stoichiometric and lean-burn points. Lastly, this study reveals the importance of flame thickness (or inner cutoff), which has a comparable or more severe effect on early flame propagation compared to planar laminar flame speed.

Natural gas has been suggested as an alternative fuel for compression ignition (CI) engines because it generates significantly less pollutants and has a smaller carbon footprint, while maintaining the cycle efficiency. However, the high auto-ignition temperature of natural gas typically requires the use of a quasi-compression ignition method such as homogenous charge compression ignition (HCCI) or a dual-fuel mode. Yet, methods such as these are unable to realize the full potential of high efficiency and low emissions. This motivated us to evaluate the applicability of a high-pressure direct-injected natural gas jet for a pure CI engine under various engine load, injection timing, and injection pressure conditions using a single cylinder CI engine with a compression ratio of 15.5. The intake temperature was raised to 400 °C to achieve the auto-ignition temperature of methane after compression and a multi-hole GDI injector capable of operating at a pressure of 400 bar was used for fuel injection. As a result, CI operation was possible, but ignition occurred after the end of the injection in the form of a partially premixed flame rather than a diffusion flame for all conditions under which the engine was operated. A sufficient equivalence ratio of 0.8 was achieved at an injection pressure of 300 bar and injection duration of 1.6 ms with the multi-hole GDI injector, and the GMEP value varied from 2 bar to 5.5 bar according to the equivalence ratio. However, at an equivalence ratio of 0.7 or higher, the combustion efficiency decreased drastically because of the limit of the volume fraction of the piston bowl at TDC and the narrow nozzle distribution of the GDI injector. Measurement of the pollutants indicated that the total number of particles from methane CI operation exceeded that of diesel at an equivalence ratio of 0.74 in the same engine setup because of incomplete combustion; however, the total mass of the particulate matter (PM) was approximately 15% of the diesel because the average particle size from methane CI was approximately one third smaller than that of diesel. However, the high post-compression temperature for the auto-ignition of methane gas resulted in a high peak temperature, which resulted in the production of approximately 2000 ppm NOx under all test conditions. The results of the parametric study showed that, as the injection timing retarded, the ignition timing was almost equally retarded, thus the combustion was still observed in the form of a diffusion flame. As the injection pressure increased, the ignition delay became shorter and the emission of PM could be further reduced, but the NOx emission increased.

We investigated the effect of inhomogeneous mixing of a fuel–air mixture in a spark-ignition engine on knocking characteristics and the dependency of the effect on the fuel, especially for various ethanol reference fuels with a fixed RON of 100. We assumed that a locally lean spot and rich spots exist in the end gas owing to inhomogeneous mixing and calculated their thermodynamic states with a multizone spark-ignition engine simulation. Subsequently, the ignition delay around the state was measured using a rapid compression machine at varying temperatures and equivalence ratios. The obtained results were processed to calculate ξ, which is the ratio of sound speed to auto-ignition propagation speed, and ϵ , defined as the time required for acoustic front to exit the hot spot divided by the excitation time. Then, we analyzed the knocking occurrence and intensity from the locally lean spot and rich spots based on Zel'dovich and Bradley's ξ– ϵ theory. Our results show that the lean spot has a shorter ignition delay than the stoichiometric mixture (ξ > 0) regardless of the ethanol content, whereas the rich spot does not (ξ ξ − 2 value. Further analysis was conducted by dividing ξ into the effect of the temperature gradient, ξT, and that of the equivalence ratio gradient, ξϕ. Consequently, we found that the magnitude of ξT is related to the activation energy of the fuel, while that of ξϕ is determined by the dependency of the pre-heat release characteristics of the fuel on the equivalence ratio.

A solid oxide fuel cell (SOFC) hybrid system is a system that combines an SOFC with an additional power generation device to increase the efficiency of the system. The SOFC–gas turbine hybrid system has been primarily investigated for SOFC hybrid systems. However, the current power generation capacity of an SOFC is less than several MWs; for this generation capacity, an internal combustion engine is generally more efficient and economical than a gas turbine. Focusing on this point, recently, the concept of an SOFC–internal combustion engine hybrid system was proposed. However, the operation of this system has not been experimentally studied yet. In this paper, as the first step in an experimental investigation of the hybrid system, an experimental study on the operation of an internal combustion engine fuelled by SOFC anode off-gas was conducted. To successfully combust the SOFC anode off-gas, which includes a large amount of diluents (H2O and CO2), the homogeneous charge compression ignition (HCCI) method was selected instead of spark ignition as the combustion strategy of the internal combustion engine in the hybrid system. For the HCCI engine experiments, a single-cylinder HCCI engine and experimental equipment for emulating SOFC anode off-gas were constructed. Various HCCI engine experiments were performed while varying several system control parameters, e.g., the fuel utilization factor of an SOFC, which primarily affects the composition and flow rate of the engine intake gas. The experiments indicated that, in general system operating condition, HCCI engine operation yields a significant amount of power (w/25–30% gross indicated efficiency) and produces significantly low NOx emissions (

Well-to-wheel (WTW) analysis of battery electric vehicles (BEVs) has been mainly performed in the U.S., China, and Europe, which are countries that can produce sizable amounts of fuels or import additionally required fuels through land transportation. However, the situation characterizing these countries is far different from that of countries dependent on the import of fuels through maritime transportation, such as Japan, South Korea, and Taiwan, because the dependence on fuel imports through maritime transportation affects not only the energy mix but also the complexity and results of WTW analysis. In addition, determining the WTW greenhouse gas (GHG) emissions of driving BEVs in these countries is very important since these countries have large amounts of GHG emissions and strong interests in the widespread adoption of BEVs. Therefore, unlike previous studies, this study evaluates the WTW GHG emissions of BEVs in South Korea as an example of such countries. To perform WTW analysis of BEVs, comprehensive life cycle analyses of five power generation fuels (coal, natural gas, petroleum-based fuel, uranium, and bio heavy oil) are conducted. In addition, ten different power generation technologies and two different electric grids (mainland and the Jeju Island) in South Korea are analysed. The fuel economies of BEVs and conventional vehicles are also considered for evaluating the WTW GHG emissions. The result of this study shows that import processes commonly increase the life cycle GHG emissions of power generation fuels. The GHG emissions of natural gas from the upstream process are ∼40% higher than those of the U.S due to the liquefaction and regasification required for importing natural gas. However, although natural gas produces large amounts of GHG emissions from the upstream process, the electricity generated from natural gas still produces the lowest GHG emissions among the fossil fuels like other countries due to the high efficiency of combined cycle systems. The life cycle GHG emissions of electricity of the Korean mainland and Jeju are calculated to be 578 g CO2 eq/kWh and 544 g CO2 eq/kWh, respectively, which are higher than that of the EU and lower than that of the U.S. and China. Driving BEVs in South Korea was found to have advantages of 90–110 g CO2 eq/km and 50–60 g CO2 eq/km on average over driving internal combustion engine vehicles (ICEVs) and hybrid electric vehicles (HEVs), respectively, not only on the Korean mainland but also on Jeju Island. Because the GHG emissions from the upstream process of major power generation fuels and the life cycle GHG emissions of the electricity from major power generation technologies are determined in this study, these results are expected to be informative for other countries, which may have different detailed generation mixes, in similar situations.

This study provides methodologies, data collection and results of well-to-wheel greenhouse gas analysis of various H2 production pathways for fuel-cell electric vehicle (FCEV) in Korea; naphtha cracking, steam methane reforming, electrolysis and coke oven gas purification. The well-to-wheel (WTW) greenhouse gas emissions of FCEV are calculated as 32,571 to 249,332 g-CO2 eq./GJ or 50.7 to 388.0 g-CO2 eq./km depending on the H2 production pathway. The landfill gas (on-site) pathway has the lowest GHG emissions because the carbon credit owing to use landfill gas. The electrolysis with Korean grid mix (on-site) pathway has the highest GHG emissions due to its high emission factor of the power generation process. Furthermore, the results are compared with other powertrain vehicles in Korea such as internal combustion engine vehicle (ICEV), hybrid electric vehicle (HEV) and electric vehicle (EV). The averaged WTW result of FCEV is 35% of ICEV, is 47% of HEV, and is 63% of EV.

Focusing on the possibility of using ammonia as a hydrogen carrier, a combustion strategy for pure ammonia as an engine fuel to convert the stored energy into a usable form has been proposed, which uses ammonia itself as a combustion promotor. Under this strategy, the conditions of sufficiently high temperature and pressure to cause the spray combustion of ammonia are obtained by the auto-ignition of a mixture of pilot-injected ammonia and air. To confirm the feasibility and the operating characteristics of an engine with the proposed ammonia combustion strategy, engine modeling has been conducted, the combustion strategy has been simulated, and a parametric study has been performed. To analyze the NO production mechanisms in an engine with the proposed combustion strategy, the NO production process has been classified into four phases, and the NO production in each phase has been analyzed under various start of injection timing and fuel amounts.

We developed a plant model and model-based controller for a heat recovery system with a swirling flow incinerator located in South Korea. First, a plant model was designed to understand the dynamic characteristics of the system. The system was divided into three parts: incinerator, boiler, and steam drum. Then, each part was modeled with a multi-zone, zero-dimensional modeling scheme. Consequently, we found that the plant model followed the dynamic behavior of the target system, particularly for the seven important variables: boiler inlet temperature, superheater inlet temperature, boiler outlet temperature, oxygen mole fraction, steam production rate, steam pressure, and steam temperature. Then, we designed a multi-input multi-output controller for the system with a nonlinear control model. In the controller design, input variables were selected as air flow rate, fuel supply rate and steam valve opening area; while output variables were oxygen mole fraction of exhaust gas, steam production rate and steam pressure. A nonlinear control model was constructed by reducing the plant model. Then a linear quadratic regulator was applied while evaluating control gain at each time step. Finally, we validated the controller on the plant model, which adequately suppressed the disturbance on fuel composition and adjusted the steam production rate.

To meet the tightened fuel economy or greenhouse gas emission standards in the transportation sector, many studies focus on the vehicle powertrain. Because hydrocarbon-based fuel is mainly used, an increase in the internal combustion engine efficiency is essential to meet the future regulations. This study discusses the improvement of spark-ignition engine efficiency through intracycle speed profile modulation. An intracycle speed profile modulating strategy could be adopted in the series hybrid electric vehicle powertrain architecture because the internal combustion engine shaft is not directly connected to the vehicle driveshaft. Under the wide-open throttle condition, the net indicated efficiency with the optimized profile increases by 1.3 %p compared to the peak efficiency achievable in conventional operation over 1,000–5,000 rpm and increases by 5 %p compared to the conventional operation with the same cycle average speed. This leads to the conclusion that the engine speed modulation in a spark-ignition engine would be largely effective near knock-limited operating conditions of a conventional engine. The efficiency gain is achieved by improving various performance parameters, conventionally in a trade-off relation. Combustion phasing is improved to the level of knock-free operating condition. In the optimal speed profile, the intake and compression process speed profiles are modulated to suppress knocking. The expansion process speed profile is mainly influenced by heat loss, and the gas exchange process speed profile is related to the pumping work. These propositions lay the foundational knowledge for further investigation into the topic in the future, including the hardware implementation.

Based on the recent improvements in high-temperature fuel cells, distributed power generation fuel cell system of small scale (∼hundreds kilowatts) has been widely investigated. To improve the system efficiency, most developments focused on the fuel cell stack, but little was paid attention to the intrinsic exergy destructions at the other parts of a typical configuration. The main objective of this study is to investigate a feasibility of reducing the exergy destruction in the reforming process of fuel cell system, by using a homogeneous charge compression ignition (HCCI) engine as a replacement of existing reforming subsystems, i.e. steam methane reforming (SMR), partial oxidation (POX), or autothermal reforming (ATR), in a solid oxide fuel cell (SOFC) system. To do this, parametric studies with exergy analysis were conducted by using in-house 1-D SOFC and 0-D HCCI simulation models. In results, due to the work production from HCCI reforming engine in addition to the work of the fuel stack, it is demonstrated that HCCI-SOFC system has higher system efficiency than partial oxidation (POX) and autothermal reforming (ATR) systems, which use similar partial oxidation reaction for reformer operation. Furthermore, because of no requirement for catalyst, the HCCI system demonstrates wider operating range than that of POX and ATR systems. When compared to the steam methane reforming (SMR)-SOFC system, the HCCI-SOFC system has the lower total work but slightly higher exergetic system efficiency, mainly caused by large amount of heat exergy needed to operate endothermic reforming process in the SMR process. Based on our simulation data, the exergetic efficiency of the HCCI-SOFC system shows 6.0%, 2.1% and 0.4% higher than POX, ATR and SMR systems at the highest efficiency points of each strategy, while 5.5%, 5.8% and 3.8% higher than POX, ATR and SMR systems at 99% methane conversion points in each reformer, respectively.

In this study, the effect of external-exhaust-gas-recirculation (EGR) on anti-knock characteristics of ethanol reference fuels (ERFs) with research octane number (RON) 100 was analyzed by measuring the ignition delay of the simulated end gas from a representative spark ignition (SI) engine operation, i.e. ASTM RON test condition. An in-house SI engine model was used to derive temperature and pressure profiles of the end gas with and without external-EGR for various ERFs, and then the ignition delay was measured by using a rapid compression machine along the derived temperature-pressure paths. The effect of external-EGR on the ignition delay of the simulated end gas was divided into two effects affecting the auto-ignition behavior: composition effect and temperature effect, then each effect was evaluated separately. As a result, the composition effect by adding external-EGR was maximized when the fuel is ERF10. With a regression analysis, it was found that there is the correlation between the amount of composition effect and the amount of pre-heat release in the end gas during a flame propagation; therefore, it is understood that ERF10 shows the most sensitive composition effect due to its pre-heat release characteristic. On the other hand, ERF with higher ethanol content was more sensitive to temperature effect by external-EGR on the ignition delay. It is found out that the amount of temperature effect depends mainly on latent heat of fuel; therefore, high latent heat of ethanol in ERF leads to its being influenced more by temperature effect. Consequently, ERF10 has the highest external-EGR sensitivity in anti-knock behavior at RON test condition, and it is further discussed that the optimum ERF for external-EGR strategy could vary from ERF0 to ERF10 according to different engine operating conditions.

Numerical study on the performance of the polymer electrolyte membrane (PEM) fuel cell vehicle with variable operating pressure was conducted to investigate the effect of blower and backpressure control valve. We conducted system level modeling for PEM fuel cell vehicle to predict its performance, where the model is based on the commercial fuel cell vehicle manufactured by Hyundai-Kia Motors and thus is validated against the real driving data. We considered most parts of PEM fuel cell vehicle powertrain, i.e. blower, membrane humidifier, and backpressure control valve at cathode, hydrogen recirculation system using ejector at anode, and the system for refrigeration and air conditioning. In building such a large system model, most of the sub system models were developed based on either zero- or one-dimensional approach, but still capturing critical physical phenomena in each device. Among these components, we focused on the effect of blower and backpressure control valve in cathode side, since these are the main control parameters in the aforementioned vehicle to influence the operating characteristics of the fuel cell stack and thus the performance of the vehicle. From the system modeling results, as the rotation speed of the blower increases, the stoichiometric number for cathodic air becomes higher and the relative humidity becomes lower. When pressure is higher, power generation from stack is proportional to stoichiometric number, however, it has greatly influence on relative humidity, and the variation of system performance is similar to that of stack. In case of the backpressure control valve, the stoichiometric number is higher and the relative humidity is lower, when back pressure is low. This change begins to reduce system power. In the end, we developed the performance curve based on the blower RPM and the valve angle, at various vehicle speeds. The proposed performance curve could provide a useful means to understand overall operating characteristics of the fuel cell vehicle.

A thermally stable and self-sustainable portable energy conversion system is designed using low temperature thin film SOFCs (solid oxide fuel cells). Hydrogen and air gases are fed into this system, which is successfully operated at 319 °C. Each cell, comprised of Pt (anode)/YSZ-GDC-YSZ/Pt (cathode), is manufactured using sputtering and atomic layer deposition, and the area of a single cell is 2.56 cm2. The maximum absolute power and power densities at 500 °C are measured to be 44 mW and 17 mW cm−2, respectively. To the best of our knowledge, this represents the highest absolute power reported at such a low operating temperature regime. To increase the system's temperature, a catalytic burner using Al2O3 and a Pt catalyst is manufactured by dip-coating. After hydrogen gas and air pass through the anode and cathode sides, respectively, the mixed fuel gases are supplied to the catalytic burner, which undergoes an exothermic reaction. We successfully demonstrate that this system is heated up to 319 °C (from room temperature) without any other initial heat source; we also measure the electrical power simultaneously.

An over-expected amount of heat losses occurred in the internal combustion engine (ICE) of a solid oxide fuel cell (SOFC)–ICE hybrid system and achieving the target efficiency of the hybrid system was seriously impeded by these heat losses. The objectives of this study are to understand the causes of these over-expected heat losses and to propose a new engine heat transfer correlation applicable to engine operation with unconventional fuels, including but not limited to SOFC anode off-gas. First, the effects of composition on the gas convection coefficient were analysed; the engine intake gas in the hybrid system was analysed to have a 1.3–1.4 times larger convention coefficient than general engine intake gas because of its unusual composition. Second, considering the effects of composition, we proposed a new engine heat transfer correlation, named ‘composition-considered Woschni’. Third, to validate this new correlation, experiments and simulations of engine operation were conducted while varying the composition of intake gas. The simulation results using the composition-considered Woschni correlation showed much better predictivity than those using the original Woschni correlation. The total root mean square values of the error rates of the main performance indices for all operating conditions decreased from 12.4% to 5.6% by applying this new correlation.

This paper compares the well-to-wheel (WTW) greenhouse gas (GHG) emissions of representative vehicle types–internal combustion engine vehicle (ICEV), hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), battery electric vehicle (BEV), and fuel cell electric vehicle (FCEV)–in the future (2030) based on a WTW analysis for the present (2017) and an analysis of various energy policies that could affect future emissions. South Korea was selected as the target region because it has detailed energy policies related to alternative vehicles. The WTW analysis for the present was performed based on three sets of subordinate analyses: (1) life cycle analyses of eight base fuels; (2) life cycle analyses of electricity and hydrogen; and (3) analyses of the fuel economies of seven vehicle types. From the WTW analysis for the present, the national average WTW GHG emissions of ICEV-gasoline, ICEV-diesel, ICEV-liquefied petroleum gas, HEV, PHEV, BEV, and FCEV were calculated as 225, 233, 201, 159, 133, 109, and 55 g-CO2-eq./km, respectively. For calculating the WTW GHG emissions in the future, two policies regarding electricity production and three policies regarding hydrogen production were analysed. Three cases with varying the degrees of improvements in fuel economies were considered. Six future scenarios were constructed, and each scenario represented the case in which each energy policy is enacted. In the reference scenario, for compact car, the WTW GHG emissions of ICEVs-gasoline, HEV, PHEV, BEV-200 mile, FCEV were analysed as 161, 110, 97, 86, and 91 g-CO2-eq./km, respectively. The differences between ICEV/HEV and BEV were predicted to decrease in the future mainly due to larger improvements of ICEV/HEV in fuel economies compared to that of BEV. The future life cycle GHG emissions of electricity and hydrogen were calculated according to energy policy. Both two policies regarding power generation were confirmed to increase the benefits of utilizing BEVs, but current energy policy regarding hydrogen production were confirmed to decrease the benefits of utilizing FCEVs. Based on the comprehensive results of this study, a framework was proposed to evaluate the impacts of an energy policy regarding electricity and hydrogen production on the benefits of using BEVs and FCEVs compared to using HEVs and ICEVs. This framework can also be utilized in other countries when they assess and establish their energy policies.

We propose a novel modeling, estimation, and control framework for homogeneous charge compression ignition (HCCI) engines, which, by utilizing direct in-cylinder pressure sensing, can detect, and react to, the wide spectrum of combustion, thereby allowing for the prevention or even recovery from partial burn or misfire, while significantly improving the stability of transition control. For this, we first develop a discrete-time cyclic control-oriented model of the HCCI process, for which we completely replace the Arrhenius integral by quantities based on the in-cylinder pressure sensing. We then propose a nonlinear state feedback control based on the exact feedback linearization and the switching linear quadratic regulators (LQRs), and also present how the state and other quantities necessary for this control can be estimated by using the in-cylinder pressure sensing. We also provide a new modeling approach for heat transfer, which, through principal component analysis (PCA), can systematically allow us to choose most significant variables, thereby substantially improving control and estimation precision. Simulation studies using a continuous-time detailed HCCI engine model built on matlab/simulink and Cantera Toolbox are also performed to demonstrate the efficacy of our proposed framework for the scenarios of engine load transition and partial burn recovery with the enlarged regions-of-attraction with less stringent actuation limitation also shown.

A novel model explaining the water transport mechanism through Nafion ® 117 is developed to understand the effects of parameters such as phase difference, pressure, mass flow rate, and temperature for applying polymer electrolyte membrane (PEM) fuel cell application. The model considers water uptake characteristics based on the polymer membrane structure and adopts four different processes to explain water permeability considering interfacial transport across the boundary. To understand the effect of phase difference, we change the relative humidity at each inlet of the experimental apparatus from 10% to 100%. The results suggest that capillary pressure is the most dominant factor in water transport across the membrane. When we change the pressure at each side of the apparatus from 1 to 3 bar, the water transport amount is maximized at 1 bar for each inlet. The mass flow rate varies from 1 to 5 Lpm. Owing to changes in molar concentration, the amount of water transport is increased when the mass flow rate at dry side inlet is minimized. Moreover, the temperature effects on the water transport amount is analyzed. As the temperature increases, the amount of water transport at the dry and wet side outlet is increases and decreases, respectively, because of the interaction between diffusivity and surface tension. The above results are verified using measured data and compared with calculated results obtained by a conventional method. In the end, this study developed the empirical equation which can be used in the fuel cell model.

In this paper, we present our experiment and simulation study of a free piston linear engine based on operating conditions and structure of the linear engine for generating electric power. The free piston linear engine includes a two-stroke free piston engine, linear generators, and compressors. In the experimental study, the effects of key parameters such as input caloric value, equivalence ratio, spark timing delay, electrical resistance, and air gap length on the piston dynamics and electric power output are investigated. Propane is used as a fuel in the free piston linear engine, and it is premixed with the air to make a homogeneous charge before go into the cylinder. The air and fuel mass flow rate are varied by a mass flow controller. The experimental results show that the maximum generating power is found with the value of 111 W at the input caloric value of 5.88 kJ/s, spark timing delay of 1.5 ms, equivalence ratio of 1.0, electric resistance of 30 Ω, and air gap length of 1.0 mm. In order to check the durability of the linear engine, a durable test is conducted during 100 h. The experimental results show that there are no problems for the linear engine after about one hundred hours of the durable test. Beside experimental study, a simulation study is conducted to predict operating behavior of the linear engine. In the simulation study, the two-stroke free piston linear engine is modeled and simulated through a combination of three mathematical models including a dynamic model, a linear alternator model and a thermodynamic model. These mathematical models are combined and solved by a program written in Fortran. Besides, the effects of key parameters such as reciprocating mass, spark timing and spring stiffness on the piston dynamics and electric power output of the linear engine are also investigated. The simulation results show that the simulation and experimental data are nearly similar at the same initial conditions. In addition, a highest generating power of the linear engine can be easily found by optimizing the key parameters.

In the present study, large eddy simulations (LES) of a turbulent diffusion flame with a hybrid fuel of methane and hydrogen are carried out to investigate the effects of operating conditions on the flame shape and the pollutant formation. A parametric study is performed by changing the values of hydrogen concentration and the background pressure and temperature. A flamelet-progress variable approach (FPVA) is used for a tabulated chemistry and a dynamic subgrid scale (SGS) model is used as the LES model. The result shows a good agreement with a previous experiment in the statistics of flow fields and composition. Also, the results affected by each operating parameter show consistent tendency with previous studies. Then, we investigate how the effect of added hydrogen changes for different background pressure and temperature conditions including those of a direct injection internal combustion engine. It is found that both the high compression ratio and hydrogen addition can lead to more compact and focused reaction zone by changes in the reaction and mixing rates. This also affects the pollutant formation, and the reduction of CO and NO is observed with hydrogen addition.

Experimental and computational study on the operating characteristics of the fuel cell with changing back pressure at the cathode and anode outlets was performed for both steady and transient operations. In the experiments, the back pressure was controlled by a pressure regulator in the fuel cell outlet, and the effects of the operating pressure under various load conditions were observed in steady-state operations. The transient responses of the fuel cell to the load changes in different operating pressures were also examined. Additionally, in order to fully understand the effects of the major parameters in various operating pressures on the fuel cell performance, a dynamic simulation model of the cell stack was developed. The performance analysis revealed that a higher voltage was produced by the cell when a high pressure was maintained because the high pressure facilitated high concentration of the reactant gas near the membrane. The mass flow rate of the liquid water also increased with increasing pressure in a given humidity, which helped to maintain higher voltage than otherwise. However, the overshoot and undershoot of the output voltage during load transition also increased with increasing back pressure, primarily because of the difference between the rates of gas supply to the membrane and gas consumption near the membrane. Whereas the consumption of the gas near the membrane was rapid, the fresh reactant gas required some time to reach the membrane. The inherent time lag resulted in undershoot and overshoot with changing load. It was observed that the remaining liquid water was also an important factor which affected the level of undershoot or overshoot during the load change. Finally, an operating strategy using a back-pressure regulator was suggested and applied in a transient operation of irregular multi-step load variation. The back pressure should be decreased before the load change and restored back to the pre-set level for the steady-operation. The former was shown to reduce the size of undershoot or overshoot during transition, and the latter could lead to the efficient operation by reducing parasitic losses in the fuel cell system.

The objective of the present study is to understand the effects of added hydrogen and operating conditions on turbulent fluctuation and dynamic flame stability in a turbulent diffusion flame with methane/hydrogen hybrid fuel. A large eddy simulation (LES) technique is used to predict turbulent statistics accurately. A parametric study is performed by changing hydrogen concentration, background pressure and temperature, which include the conditions similar as in a direct injection internal combustion (IC) engine. Turbulent statistics such as RMS flow fields, flame stability index and combustion noise are analyzed to evaluate the flame stability. A normalized Rayleigh index proves to be a useful indicator for the stability of turbulent diffusion flame, as compared to the RMS of flow variables. From the combustion noise spectra, hydrogen addition stabilizes mainly the primary vortex shedding in the standard condition but reduces the noise level in the entire frequency range as the compression ratio increases. Also, it is concluded that stabilization of unsteady fluctuation is highly correlated to the reduced flame length.

This study aims at performing the first comprehensive well-to-wheel (WTW) analysis on greenhouse gas (GHG) emissions and energy uses of petroleum-based automotive fuels, i.e., gasoline and diesel, in Korea. Although the Korean Renewable Fuel Standard is supposed to take effect in 2015, there are very few WTW results available in Korea. In this study, all relevant processes in the whole fuel cycle are covered, which will provide Korea-specific results to policy makers and stakeholders in Korea. Input raw data were collected with the help of Korean petroleum industries and related association, as well as governmental institutions. Literature survey was carried out, especially for overseas processes in the crude oil recovery fields. The GREET model, developed by the U.S. Argonne National Laboratory, was adopted as a tool for WTW calculation, and most of the data were replaced by using the Korean specific information. Additional analysis was also performed for the refining process which was the most energy-intensive in the fuel life cycle. A process-level allocation method was used in calculating the refining energy use of individual petroleum products, which could reflect the detailed refining processes. The well-to-pump (WTP) GHG emissions of Korean gasoline and diesel are calculated as 12,047–12,677 and 11,025–11,643 g CO2 eq./GJFinal_fuel, respectively. The main difference comes from the higher GHG emission in the refining process of gasoline than in diesel. As compared to other countries, the WTP results of Korean fuels are smaller than those of the USA and Europe mainly due to higher refining efficiency, while larger than those of Japan with a significant difference in GHG emissions regarding crude oil recovery. In the WTW results with all the Korean vehicle models in 2014 considered, similar weight of diesel models demonstrates overall lower WTW emissions than gasoline models, since the former has both lower WTP GHG emissions and better fuel economy than the latter. In this study, a comprehensive WTW GHG analysis on Korean gasoline and diesel was performed to provide Korea-specific results to policy makers and stakeholders. The analysis was based on the official data provided by Korean petroleum associations and companies as well as literature survey and our own analysis. There is a relatively large uncertainty from the refining process, which should be further investigated to improve the accuracy of the results.

For past decades, substantial developments have been accomplished in internal combustion (IC) engine technology, but there still remain some possible improvements. The combustion in an IC engine is a highly intricate phenomenon, thus, numerous factors correlated with different forms of loss decides the efficiency of an engine. In spark-ignition (SI) engines, the combustion duration is considered important because it plays a key role in determining the combustion phasing for best possible energy conversion. The geometry of engine components may directly change the burning rate of air-fuel mixture, therefore, it should also be considered as significant as other aspects like exhaust gas recirculation (EGR) rate or boosting in investigation of the engine performance. This is the reason the development engineers are putting their effort to design an engine with optimized flow motion. Tweaking the flow dynamics via design modification or use of auxiliary device influences the turbulence level inside the combustion chamber, thus, the burning rate as well. Intake port orientation, masking, and piston shape are one of the typical design parameters manipulated for such purpose, and profound understanding on the effect of these design parameters on burning rate is encouraged in order to assist the optimization process. The design optimization process should be based on a fundamental understanding of how the design parameters affect the flow motion and combustion characteristics. This study aims for a simpler and faster method to investigate the consequences of design modifications. As a base model, a physics-based quasi-dimensional (QD) engine model is developed for simulation of SI combustion phenomenon. It is modeled to consider the change in flow motion and turbulence properties via simplified modeling. The advantages of such QD model is that it requires much less computational resource compared to 3D CFD model, and allows a greater degree of freedom within the simulation process which facilitates parametric studies. A zero-dimensional (0D) turbulence submodel is used to describe energy cascade mechanism, and turbulence intensity is calculated reflecting the effect cause by design modification. According to the sensitivities drawn from parametric study, the results of each effect on burning rate and other engine performance properties are compared individually and collectively. A qualitative analysis suggests how sensitive each effect are at given operating conditions. The result infers that the flow concentration by port design modification boosts the burning rate, but it is advantageous in terms of fuel economy to enhance the breathing ability by valve masking. The product of this comparative study assists an intuitive understanding on how the design modification would affect the engine operations, and it is encouraged to develop the model further via validation with experiment data to provide more reliable output. It is believed that it can be utilized as a good reference in engine design process.

The objective of the proposed hybrid system is to increase the system efficiency by using the residual fuel of the anode off-gas from a solid oxide fuel cell in an internal combustion engine (ICE). In this study, a novel hybrid system using spark-assisted ignition (SAI) in an ICE operation is proposed. Since this is the first attempt of using a new combustion concept in a hybrid system, feasibility of SAI and its effect on the system operation are investigated in this study. To analyze the effect of the combustion on the system, the engine experiments on the SAI were conducted by changing the engine operating parameters, such as the intake temperature, equivalence ratio, and spark timing. For the system-level analysis, a zero-dimensional model for the fuel cell and the balance of plants was developed and validated. The results of the engine experiments were integrated directly with the system model. The performance of the hybrid system using SAI was analyzed from the energy and exergy perspectives. Under the operating conditions of this study, the anode off-gas combustion can be controlled stably (COV: 5–7%) through the SAI, even though the intake temperature is decreased to ~280 °C at the low compression ratio of 8.2. It leads to an increase in exergy efficiency of the engine to ~37%. Consequently, thermal self-sustainability is improved and the indicated efficiency of ~61.6% is achieved in the hybrid system. The SAI engine is responsible for ~14% of the system power and produces considerably low NOX emissions (

In this study, an alternative scheme improving the Livengood-Wu model-based knock predictability for downsized and boosted spark-ignition (SI) engines with two-stage ignition fuels is proposed by introducing a newly corrected ignition delay correlation applicable to two-stage ignition fuels. While there is little chance of cool-flame heat release realized within the in-cylinder mixture under certain knock-prone operations, the effect of unnecessarily accounted cool-flame exothermicity to the Livengood-Wu integration process and the eventual overestimation error is found to be as large as 2-4 crank angle degrees (CADs) on the knock timing, which could limit the potential workability and the benefits of the downsizing with boost strategy. This correlation is also useful for mapping the ignition delay domain of two-stage ignition fuels without their cool-flame heat release that is subject to the relevant pressure-temperature trajectories of the engine unburned gas, and rapid compression machine (RCM) experiments are conducted to reproduce the unburned gas behavior of a boosted SI engine.

A novel proton exchange membrane fuel cell (PEMFC) system that employs a turbocharger to reuse waste energy is proposed. This study is focused on optimal placement of the PEMFC turbocharger, using either the hydrogen feed stream or the stack outlet as the motive force. A one-dimensional isothermal two-phase model for a PEMFC and an isenthalpic model for the turbocharger are developed for system analysis. To find a better energy source, the system efficiency, power generation, and interaction with the balance of plant (BOP) are considered. To evaluate the feasibility of both systems, an exergy analysis is also performed. The results show that the system efficiency is higher when the turbocharger is installed after the hydrogen tank and that the amount of power generation is also larger when the turbocharger is located in the stack outlet. For the exergy analysis, the case for the turbocharger installed after the hydrogen tank shows higher performance. Therefore, it is preferable to install the turbocharger after the hydrogen tank.

A solid oxide fuel cell (SOFC)–internal combustion engine (ICE) hybrid system is a recently-proposed distributed electric power generation system to achieave extremely high efficiency beyond that of current technologies. The objectives of this study are to find the operation characteristics of the SOFC in the hybrid system and determine the operational design point of the hybrid system. To accomplish these objectives, operation of a 5 kW-class SOFC–ICE hybrid system was analysed by integrating the experimental results of the internal combustion engine with the simulation models of other system components. Two unique characteristics of SOFC operation were found and analysed. First, the SOFC in the hybrid system should utilize anode inlet gas with low temperature (e.g., 750–800 K) and low external reforming rate (e.g., 30–40%), which decreases the SOFC temperature, especially at the entrance, where the current density becomes very low, decreasing SOFC performance. Second, the overall effects of pressure pulsation caused by the engine on SOFC operation are insignificant since the flow path between the SOFC and the engine acts as a damper, reducing the pressure pulsation amplitude. The estimated variations were 0.2 mV in SOFC cell voltage. To determine the operational design point of the hybrid system, parametric analyses of system operation were conducted while varying several control parameters, e.g., SOFC fuel utilization factor. The design point of system operation was determined by considering system performance and operational stability. Near-zero pollutant emissions and 59.0% system efficiency were achieved at the determined design point, a 7.9% absolute and 15.5% relative improvement compared to that of an SOFC stand-alone system.

In Korea, natural gas is widely used as city gas, fuel for electricity generation, and fuel for transportation (e.g., city bus). However, the environmental impact associated with the use of natural gas in Korea has not been paid much attention to. In this study, well-to-wheel (WTW) analysis on the greenhouse gas (GHG) emissions and energy uses associated with natural gas in Korea was performed by considering every step from feedstock recovery to final use in the vehicle operation. The raw data used in the analysis were mainly provided by Korean natural gas industry and related associations. The additional information, especially for the processes in foreign countries, was also collected by literature survey. We adopted the GREET (Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation) model as a base WTW analysis tool, which was developed by the U.S. Argonne National Laboratory. However, the WTW analysis on natural gas in Korea is far different from that of the U.S, because ~99 % of natural gas used in Korea is imported from the oversea countries in the form of liquefied natural gas (LNG). For this reason, detailed parameters in GREET were changed for Korean situation, and especially, significant modifications were made on liquefaction, LNG transportation and storage, and re-gasification processes. As a result of the analysis, the well-to-pump GHG emissions of city gas and compressed natural gas are calculated as 25,717–30,178 and 28,903–33,422 g CO2 eq./GJFianl_fuel, respectively. The WTW GHG emission of compressed-natural-gas-fueled city bus is calculated as 1,348–1,417 g CO2 eq./km. These values are relatively larger than those of the U.S., because most of the natural gas used in the U.S. is transported by pipeline in a gaseous state, which typically takes less energy and associated GHG emissions, as compared to the import of LNG in Korea. Finally, sensitivity analysis is performed on the parameters, which have either range of values among various sources or uncertainties due to lack of accurate information. The results show that further investigation on three parameters, i.e., CO2 venting during natural gas processing, CH4 leakage in Korea, and CH4 leakage during recovery process, would be helpful to further improve overall accuracy of the analysis.

In a previous study, a new hybrid system of molten carbonate fuel cell (MCFC) and homogeneous charge compression ignition (HCCI) engine was developed, where the HCCI engine replaces the catalytic burner and produces additional power by using the left-over heating values from the fuel cell stack. In the present study, to reduce the additional cost and footprint of the engine system in a hybrid configuration, the possibility of engine downsizing is investigated by using two strategies, i.e. the use of a turbocharger and the use of high geometric compression ratio for the engine design, both of which are to increase the density of the intake charge and thus the volumetric efficiency of the engine. Combining these two strategies, we suggest a new engine design with ∼60% of displacement volume of the original engine. In addition, operating strategies are developed to run the new hybrid system under part load conditions. It is successfully demonstrated that the system can operate down to 65% of the power level of the design point, while the system efficiency remains almost unchanged near 63%.

The effects of anisotropic bending stiffness of a gas diffusion layer (GDL) on membrane electrode assembly (MEA) degradation were investigated. We prepared GDLs with a fiber direction perpendicular to the major flow (i.e., “90° GDL”) and with a fiber direction parallel to the major flow (i.e., “0° GDL”). To analyze the mechanical durability as a function of GDL anisotropy, we examined cell performances such as the I – V characteristics and impedances and the hydrogen crossover characteristics during wet/dry cycles. The results showed that the 90° GDL fuel cell is superior to the 0° GDL fuel cell in terms of higher I – V performance, lower resistance at high frequency, and lower hydrogen crossover through the MEA. Mechanical degradation of the 0° GDL was investigated using scanning electron microscopy (SEM).

Recently, precise analysis of energy flow in engines has become necessary to improve fuel economy. An integrated engine thermal management model, which is introduced in this paper, is suitable for that process. The model consists of six sub-models for thermal mass, coolant, lubricant, heat transfer, friction, and exhaust. The sub-models are coupled to each other and they exchange heat and signals. Combustion energy flow analysis and temperature estimation of the engine components and working fluids were simulated under various conditions. Simulation results were compared with experimental data and they showed good agreement. Then, a variable-speed water pump (VSWP) to control coolant flow was applied in place of a conventional water pump. Engine warm-up time decreased with proper coolant flow control, and fuel economy could be improved by 2.5%.

The 21st century has brought with it a strong push in the automotive industry to develop energy technologies that can better address the global challenges posed by the depletion of finite fossil fuel resources and the pollution caused by using these fossil fuels as an energy resource. The two key words dominating automotive powertrain research today, therefore, are efficiency and emissions-There is a strong push for cars that burn less fuel and emit fewer pollutants. Some of the technologies in the spotlight include fuel cells, batteries, hybrid powertrains, and advanced IC engine strategies. Though fuel cells and battery technologies are very promising in the long-term future, they are currently more expensive and less practical than hybrid and advanced combustion engine strategies and are expected to remain so in the near future. In addition, technological advances that have made electronic control systems ubiquitous in todays automobiles have enabled combustion strategies that would not have been possible even a decade ago. One such advanced engine strategy that presents significant opportunities for reduced emissions and higher efficiencies is homogeneous charge compression ignition (HCCI).

In-cylinder pre-processing (or recompression reaction) of pilot-injected fuel during negative value overlap (NVO) has been investigated as a method to extend the low-load limit of residual-effected homogeneous charge compression ignition (HCCI). In an effort to elucidate the chemical and thermal effects involved, model calculations have been performed on the recompression reaction and ignition delay of the recompression products using a reduced n-heptane mechanism (160 reactions, 1424 reactions) and a zero-dimensional kinetics model. Parametric studies were performed to cover possible operating choices for HCCI and to understand their effects on the recompression reaction and mixture ignitability. From the study it is demonstrated that the extent of recompression reaction is limited by chemical kinetics, not thermodynamics, and that residual oxygen during NVO is a determining species for the extent and speciation of the recompression reaction. The recompression product mixture exhibits an overall shorter ignition delay than those of the base fuel, except under lean conditions when significant oxidation during NVO leaves only a small amount of fuel available for main ignition. The thermal consequence of the recompression reaction is also largely dependent on oxygen: at near-stoichiometric conditions, the recompression reaction is endothermic from fuel pyrolysis, whereas at lean conditions, the exothermic recompression reaction becomes dominant. Therefore, the chemical and thermal consequences of the recompression reaction exhibit competing effects on mixture ignitability, which leads to an optimum oxygen concentration (equivalence ratio) for reducing ignition delay and extending HCCI operability.

Experiments have been reported in the literature in which the low-load limit of a retention-mode HCCI engine operating on gasoline has been significantly extended by pre-processing of the fuel during negative valve overlap. This paper presents experimental studies in which this low-load-limit extension is demonstrated and characterized using simple, single-component hydrocarbon fuels with relatively well-known chemical kinetics. The model fuels were n-heptane and i-octane and this choice was made both because of the extensive work that has been undertaken to develop their chemical kinetic mechanisms and because these fuels span the range of ignitability that is likely to be of interest for HCCI engines. The experimental results reported here show that both fuels exhibit load extension to as low as 1 bar net indicated mean effective pressure when operated at high residual mass fractions, low equivalence ratios, and an appropriate choice of compression ratio (13 for n-heptane, 18 for i-octane). Near the low-load limit, combustion is stable, exhibits slightly advanced timing, has relatively low unburned hydrocarbon emissions, negligible (< 5 ppm) NO emissions, and slightly increased CO emissions (compared to higher load conditions). The indicated efficiency of low-load operation with recompression reaction is somewhat reduced, mainly due to increased heat transfer and decreased combustion efficiency.