Dynamic modeling of a proton exchange membrane fuel cell system with a shell-and-tube gas-to-gas membrane humidifier

The proton exchange membrane fuel cell (PEMFC) system with a shell-and-tube gas-to-gas membrane humidifier is considered to be a promising PEMFC system because of its energy-efficient operation. However, because the relative humidity of the dry air flowing into the stack depends on the stack exhaust air, this system can be unstable during transients. To investigate the dynamic behavior of the PEMFC system, a system model composed of a lumped dynamic model of an air blower, a two-dimensional dynamic model of a shell-and-tube gas-to-gas membrane humidifier, and a one-dimensional dynamic model of a PEMFC system is developed. Because the water management during transient of the PEMFC system is one of the key challenges, the system model is simulated at the step change of current. The variations in the PEMFC system characteristics are captured. To confirm the superiority of the system model, it is compared with the PEMFC component model during transients.     

Effect of the micro porous layer design on the dynamic performance of a proton exchange membrane fuel cell

The mass transport characteristics of a gas diffusion layer (GDL) predominantly affect the performance of a proton exchange membrane (PEM) fuel cell. However, studies examining the transient response related to the GDL are insufficient, although the dynamic behavior of a PEM fuel cell is an important issue. In this study, the effects of the design of a micro porous layer (MPL) on the transient response of a PEM fuel cell are investigated. The MPL slurry density and multiple functional layers are treated as the variable design parameter. The results show that the transient response is determined by the capillary pressure gradient through the GDL. The trade-off relation for the PEM fuel cell performance under low and high humidity conditions due to the hydrophobic GDL is mitigated by designing a reverse capillary pressure gradient in the MPL.     

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/${\text{cm}}^2$ demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.     

A study on the behavior of evaporating diesel spray using LIEF measurement and kiva code

The effects of change in injection pressure on spray structure in high temperature and pressure field have been investigated. The analysis of liquid and vapor phases of injected fuel is important for emissions control of diesel engines. Therefore, this work examines the evaporating spray structure using a constant volume vessel. The injection pressure is selected as the experimental parameter, is changed from 400 bar to 800 bar by using a common rail injection system. Also, we conducted simulation study by modified KIVA-II code. The results of simulation study are compared with experimental results. The images of liquid and vapor phase for free spray were simultaneously taken by exciplex fluorescence method. As experimental results, the vapor concentration of injected fuel is leaner due to the increase of atomization in the case of the high injection pressure than in that of the low injection pressure. The calculated results obtained by modified KIVA-II code show good agreements with experimental results.     

Study on operating characteristics of fuel cell powered electric vehicle with different air feeding systems

In this paper the modeling of a fuel cell powered electric vehicle is presented. The fuel cell system consisting of a proton exchange membrane (PEM) fuel cell stack and balance of plant (BOP) was co-simulated with a commercial vehicle simulation program. The simulation program calculates the load of the fuel cell depending on the driving mode of the vehicle and also calculates the overall efficiency and each parasitic loss by applying the load in the fuel cell model that is used to estimate the performance of the entire vehicle system by calculating the acceleration performances and fuel economy of the vehicle. Two types of air feeding systems (blower type and compressor type) were modeled by using MATLAB/Simulink environment and the effect of fuel cell stack size (number of cells, cell area) on the fuel economy and performance of the fuel cell powered vehicle was investigated. Using a driving cycle of FTP-75, the required power, BOP component power loss, and system efficiency for two types of fuel cell systems were analyzed. Through this study, we could get a basic insight into the fuel cell powered electric vehicle and its characteristics.     

An experimental investigation of temperature distribution and flooding phenomena of cathode flow fields in a proton exchange membrane (PEM) fuel cell

Water management is considered to be one of the main issues to be addressed for the performance improvement of proton exchange membrane (PEM) fuel cells. In this paper, to investigate cathode flooding and its relationship with temperature distribution, an experimental study was carried out on cathode sides of an operating single PEM fuel cell. For the direct visualization of temperature fields and water transport in cathode flow channels, a transparent cell was designed and manufactured using quartz window. Liquid water transport and distribution in the flow channels were investigated experimentally. Also, the visualization of temperature distributions in the cathode channels was made by using an IR (infra-red) camera. Results indicate that the temperature rise near the exit of cathode flow channels was found. It is expected that this study can effectively contribute to get the detailed data on water transport linked with thermal management during the operation of a PEM fuel cell.     

Comparison of the effects of multiple injection strategy on the emissions between moderate and heavy EGR rate conditions: part 2-post injections

To draw a comparison of the effect of multiple injection strategy on the engine-out emissions under two different EGR rate conditions, the effect of pilot injection on emissions and combustion was evaluated and discussed in part 1. Thus, in the second research as part 2, the effects of post injection on the engine-out emissions were systemically evaluated for two different EGR rate conditions (30 % and 60 %). Since the behavior of diesel combustion is significantly different as EGR rate is changed, the characteristics of post injection was different between two EGR rate conditions. This research was investigated as varying injection parameters such as the timing and quantity of the post injection. The results show that the close post injection with injection interval as 10 degree has the potential to reduce PM emission, regardless of EGR rate. However, the reason of reduction of PM emission is different for each case. For a moderate EGR rate condition, close post injection with interval 10 degree enhances the fuel at bottom of bowl. Thus, the distribution of fuel can be improved. On the other hand, for a heavy EGR rate condition, close post injection with interval 10 degree has the charge cooling effect to prolong the ignition delay, rather than well-matched injection targeting. Especially, there is an effect to oxidize PM emission under moderate EGR rate condition as post injection is applied. However, post injection for late cycle of combustion under heavy EGR rate condition does not oxidize PM emission due to low oxygen concentration (～ 10%).     

Piston crevice hydrocarbon oxidation during expansion process in an SI engine

Combustion chamber crevices in SI engines are identified as the largest contributors to the engine-out hydrocarbon emissions. The largest crevice is the piston ring-pack crevice. A numerical simulation method was developed, which would allow to predict and understand the oxidation process of piston crevice hydrocarbons. A computational mesh with a moving grid to represent the piston motion was built and a 4-step oxidation model involving seven species was used. The sixteen coefficients in the rate expressions of 4-step oxidation model are optimized based on the results from a study on the detailed chemical kinetic mechanism of oxidation in the engine combustion chamber. Propane was used as the fuel in order to eliminate oil layer absorption and the liquid fuel effect. Initial conditions of the burned gas temperature and in-cylinder pressure were obtained from the 2-zone cycle simulation model. And the simulation was carried out from the end of combustion to the exhaust valve opening for various engine speeds, loads, equivalence ratios and crevice volumes. The total hydrocarbon (THC) oxidation in the crevice during the expansion stroke was 54.9% at 1500 rpm and 0.4 bar (warmed-up condition). The oxidation rate increased at high loads, high swirl ratios, and near stoichiometric conditions. As the crevice volume increased, the amount of unburned HC left at EVO (Exhaust Valve Opening) increased slightly.     

Study on the performance of a proton exchange membrane fuel cell related to the structure design of a gas diffusion layer substrate

Although characteristics of the gas diffusion layer (GDL) affect the performance of a proton exchange membrane fuel cell (PEMFC), mass transfer mechanisms inside the GDL and the performance of the PEMFC have not been directly correlated. To determine the design parameters of the GDL, the effects of substrate design of the GDL on performance of a PEMFC are investigated. By adding an active carbon fiber (ACF), which has a high surface area, the substrate is designed to have a different pore size structure. The results show that steady-state and transient responses are determined by capillary pressure gradient characteristics of the GDL made by pore size distribution of the substrate. The small macro-pore functions as water-retaining passage and the large macro-pore functions as water-removal passage. It is concluded that both small and large macro-pore must be present on the substrate to facilitate its function in a wide range of operating conditions.     

Investigation of autoignition of propane and<Emphasis Type="Italic">n</Emphasis>-butane blends using a rapid compression machine

The effects of pressure and temperature on the autoignition of propane andn-butane blends were investigated using a rapid compression machine (RCM), which is widely used to examine the autoignition characteristics. The RCM was designed to be capable of varying the compression ratio between 5 and 20 and minimize the vortex formation on the cylinder wall using a wedge-shaped crevice. The initial temperature and pressure of the compressed gas were varied in range of 720-900 K and 1.6-1.8 MPa, respectively, by adjusting the ratio of the specific heat of the mixture by altering the ratio of the non-reactive components (N₂, Ar) under a constant effective equivalence ratio (ø_f=l.0). The gas temperature after the compression stroke could be obtained from the measured time-pressure record. The results showed a two-stage ignition delay and a Negative Temperature Coefficient (NTC) behavior which were the unique characteristic of the alkane series fuels. As the propane concentration in the blend were increased from 20% and 40% propane, the autoignition delay time increased by approximately 41% and 55% at 750 K. Numerical reduced kinetic modeling was performed using the Shell model, which introduced some important chemical ideas, represented by the generic species. Several rate coefficients were calibrated based on the experimental results to establish an autoignition model of the propane andn-butane blends. These coefficients can be used to predict the autoignition characteristics in LPG fueled SI engines.