New papers published
online (abstracts)

In order to provide fuel cell durability, it is important to reduce iron (Fe) contamination to the polymer electrolyte membrane of the fuel cell stack and to control the humidity to an appropriate level. The new fuel cell system developed jointly with General Motors Company adopts a double-layer coating of titanium and carbon on the stainless steel bipolar plates of the fuel cell stack that reduces Fe elution to 1/10 the previous amount. In addition, the humidification level is increased by estimating the amount of water vapor with a highly accurate humidification model and using a high-efficiency water vapor transfer unit and an electric coolant mixing valve to perform active humidity control. The amount of water contained in the membrane was increased by lowering the coolant temperature and supplying air controlled to high humidity with a high degree of accuracy. As a result, the durability is more than double that of the previous system.
In addition, the start-up time in a low-temperature environment of -20°C was shortened to 1/9 that of the previous system by performing humidity conditioning control when the system is stopped and rapid warm-up control at start-up.
The fuel cell system cost was also reduced to 1/3 that of the previous system by reducing the cost of the fuel cell stack and simplifying the system.

Hiroyasu OZAKI、Kaoru YAMAZAKI、Takayuki OGAWA、Shuto MANIWA、Taneaki MIURA、Todd W. HUSTON

A new structure fuel cell stack developed jointly with General Motors realized twice the durability at 1/3 of the cost while maintaining the output and safety performance of the previous fuel cell stack.
In order to increase durability, control-based temperature and humidity management was adopted and the stack structure was enhanced to achieve less iron elution from the bipolar plates, better cooling performance, and more stable power generation due to the stack end unit parts, etc.
To achieve lower cost, first the current density of the Membrane Electrode Assembly was increased and the power consumption of auxiliary devices that compensate for stack power generation was lowered, enabling maintenance of the same net power as the previous system with fewer cell layers. Next, the structure was simplified while enhancing the cooling performance of the bipolar plates. In addition, less platinum is used in the electrode catalyst of the Unitized Electrode Assembly, the polymer electrolyte membrane was thinned, and a fully automated line was applied from assembly of the Unitized Electrode Assembly to the stacking process.

Choichi ISHIKAWA、Manabu IWAIDA、Hideharu NAITO、Shigeru INAI、Hiromichi YOSHIDA、Nobuyoshi MUROMOTO

A bipolar plate for a new structure fuel cell stack was developed jointly with General Motors Company. In order to achieve both lower cost and higher productivity while enhancing the functionality of the bipolar plate in this study, a bipolar plate structure that assumes high-cycle molding was developed. A coating consisting of carbon and titanium is applied to the stainless steel base material before stamping. The seal structure was changed from a multilayer rubber seal structure to a metal bead seal structure formed by stamping after coating and a welded seal structure. In addition, a structure without a rubber coating was adopted for high-cycle molding.

Kazuo NUNOKAWA、Hiroshi SHINKAI、Satoshi OYAMA、Kentaro ISHIDA、Shuhei GOTO

Honda’s newly structured fuel cell stack features a Unitized Electrode Assembly, which consists of a Membrane Electrode Assembly that generates electricity and an outer resin frame structure. Specifications of the Unitized Electrode Assembly were developed that have greater durability and lower cost than the previous fuel cell stack, while maintaining its output performance. Specifically, the amount of Pt was reduced to 1/5 that of the previous specifications due to increased activity by using a Pt alloy catalyst for the electrode catalyst, and the thickness of the electrolyte membrane was reduced to 3/5. For the outer resin structure, the cost was reduced to less than half that of the previous injection-molded product by using a general-purpose resin film.
As for the durability associated with the above-mentioned changes, the researchers developed degradation models for the various components of the Unitized Electrode Assembly and worked to increase the accuracy of those models. These efforts achieved both increased durability and lower costs by reflecting the material specification limit ranges in operation control based on the models obtained.

Takashi KATO、Akira NAKAHARA、Takuya OHKURA、Masakuni YAMAMOTO、Satoru TERADA、Akihiro NODA、Nagayuki KANAOKA

New fuel cell stacks are being developed with the aim of improving productivity while maintaining the same quality for mass production. In particular, the time it takes to perform shorting inspection of the membrane electrode assembly is an issue that limits productivity, so development has been conducted to help reduce this time. Conventional shorting inspection on flat bases detects the electrical characteristic values of the entire membrane electrode assembly. In this approach, signals of local short circuits are small relative to the change in detected value due to the charge, and thus they could not be judged until the change was complete. In the development discussed in this paper, the detection sensitivity for signals in each local area was increased by using a split-terminal base. Shorting signals greater than the charge change were successfully detected locally. This lowered the impact on the inspection judgment, and the measurement waiting time caused by charging was lowered by 87% compared with the conventional method. In addition, the researchers developed an algorithm for synthesizing peripheral currents to deal with the misjudgment factor of a short circuit in the terminal gap, which presented an issue in the use of split bases. As a result, inspection total time was reduced to 10 seconds, thus contributing greatly to increasing cell productivity.

Hiroaki KAWANISHI、Hiroyuki YAMAGISHI、Kensuke NANBA

The phenomena inside a porous hollow fiber membrane humidifier were analyzed and a one-dimensional humidifier model that can estimate the water transfer rate was constructed to enable control of the water content in the fuel cell stack of a fuel cell system to the appropriate range. The calculation accuracy of the humidifier model was enhanced by investigating and considering the impact of the dry-up phenomenon that may lower the water transfer rate due to partial drying of the porous membrane. In addition, verification was performed by comparing the calculation results of the humidifier model with the results of humidifier evaluation tests and measurements of the water transfer rate in bench operation of the fuel cell system. The estimation accuracy of the humidifier model was confirmed to satisfy the target accuracy of ±15% or less that is needed for use in performance design and control optimization of fuel cell systems.

Shuto MANIWA、Yohei HIDAKA、Koichi KIMURA、Nobuki KOIWA

A new hydrogen ventilation structure for fuel cell systems has been developed that uses a waterproof and dustproof filter for simplification towards multipurpose development of fuel cell powertrains. A 78% reduction in terms of volume was achieved compared to conventional model hydrogen ventilation structures for fuel cell systems with ducts installed. In this paper, under the assumption of normal operation and the possible occurrence of a hydrogen leak, it was confirmed that the hydrogen concentration in the hydrogen system and under the front hood was equal to or less than the unusual detection for hydrogen concentration through the hydrogen ventilation structure that was adopted. Furthermore, this structure newly introduces what is known as a “forced ventilation system,” where part of the air supplied to the cathode electrodes is taken into the hydrogen ventilation structure, which has enabled reducing the concentration of hydrogen inside the hydrogen ventilation structure of the fuel cell system during normal operation compared to its concentration in conventional models.

Sena TAKEKOSHI

In order to shorten the development period and help reduce development costs for new vehicle models, a method for automated scheduling of actual vehicle tests was devised that helps to minimize the number of test vehicles and calculate the shortest test schedule. Use of mathematical optimization methodology for collision test scheduling and use of the vehicle routing problem solution in particular in mathematical optimization methodology for function test scheduling enabled incorporation of the constraints into the automated calculations to be considered. Simple applications were created and verified using the applications with a spreadsheet software. As a result, a scheduling task that previously took an expert 90 hours could be accomplished in 6 hours.

Astushi ARISAKA、Takeshi MIYAZAKI、Kazuki FUKUI

The less-expensive and more versatile Power Exporter e: 6000 was developed based on the Power Exporter 9000 external power output device. A lower price was achieved while maintaining high quality by optimizing power output in response to market needs, reducing the number of parts in line with the reduced power output, and addressing change points with minimal cost. In addition, the first known function to provide three-phase 200 V AC output with three linked units has been added to realize Vehicle to Load and Vehicle to Home as an electrical facility for general use, thereby expanding use applications to large electrical equipment such as industrial equipment. The power supply cable length was extended and the control was modified to conform to the “Electric Vehicle V2L DC Charge/Discharge System Guidelines” to enhance connectivity with even more EVs, including those of other automakers.

Yuuna YAHAGI、Tomoyuki KOZU、Takuya ODAKA、Kenta YAGASAKI、Takahiro FUKUI、Kosuke IHARA

In order to provide a natural feeling in turn for general drivers in daily driving, a method was examined for adjusting the pitching behavior produced by steering in control of acceleration and deceleration linked with lateral motion. In consideration of the characteristic that the pitch angle response is twice the steering frequency, a control law was devised to reduce unnatural feeling in turn. The effect of the control law and the controlled variable was verified by simulation, and it was confirmed that the timing at which the pitching behavior occurs can be adjusted without impacting the rolling behavior. Next, actual vehicle tests were conducted, and the vehicle behavior was confirmed to be similar to that in the simulation. In addition, sensory evaluation showed that the method is effective in enhancing the feeling in turn, and that there is an appropriate value for the control strength.

Naoto OHKUBO

My main activities so far have been primarily in fields related to pressure vessels and piping, with very little crossover into the automotive industry. Recently, I participated in a NEDO project(1) related to on-vehicle hydrogen tanks for automobiles, which gave me the opportunity to learn about this type of activity. This exposure to activities in fields that I’d had little experience in beforehand helped me to become aware of issues and complications in this field. I feel that reliability design is a subject that cannot be avoided when it comes to considering the development of this field in Japan. Therefore, I have decided to summarize this topic in this paper.
The “Reliability Design” mentioned in the title is the key phrase here. I’d imagine that few people have considered the design of hydrogen tanks from the perspective of reliability design in the past. However, G.W. Mair from BAM in Germany has really emphasized the importance of introducing the concept of “reliability design” for pressure tanks such as hydrogen tanks(2).
In the past, safety in terms of standards in the machinery field has often been secured through the safety factor. In the mechanical engineering field, there are opportunities to learn about the safety factor through lectures on the core academic field of “Material Mechanics.” The definition of the safety factor as written in the textbook(3) on material mechanics that I studied at the time was “Allowable Stress = Material Strength/Safety Factor,” and I was taught that the safety factor is determined empirically. Safety factors based on this concept are called empirical safety factors or deterministic safety factors. While not all of the safety factors listed in today’s standards in the machinery field may be determined empirically, it seems that the basic concept has not changed greatly since then. Design using safety factors is extremely simple and can be said to be a convenient method for both designers and regulators.
If the impact from the damage to a mechanical structure is not large, it is considered reasonable to perform safety management by using a simple deterministic safety factor like this. On the other hand, since enhancement in the performance of mechanical products has been sought after in recent years, there are limits on performing safety management with just a deterministic safety factor. For example, damage in the event of an accident may become more severe when enhancement in performance is pursued. The concept of risk-based design, or reliability design, is important for rational design in these situations.
In this paper, I shall provide an outline on trends in related standards, applicable examples, current issues, and more, starting with introductory content so that even readers unfamiliar with reliability engineering will be able to understand. In addition, since the English expression for “Safety Factor” is “Safety Factor,” it should really be called “Safety Coefficient,” but since the term “Safety Factor” is also widely used, I’ve decided to use both terms in this paper without differentiating between them.

Shinsuke SAKAI

A method was developed that can estimate the corrosion resistance of anodized aluminum regardless of the aluminum material or treatment conditions. It was confirmed that the constant current anodic polarization method can be used to evaluate the state of voids and foreign matter in anodized aluminum and the corrosion resistance by measuring the initial potential increase rate when a constant current is applied to anodized aluminum.
In addition, the change in the initial potential increase rate showed that the factor lowering the corrosion resistance of anodized aluminum due to an increase in the amount of added contents in aluminum alloys is rooted in the change in the anodized aluminum structure.
This method can estimate the corrosion resistance of anodized aluminum in a short time, enabling determination of the specifications of anodized aluminum at an early stage.

Hideaki MIYASHITA、Kohei TOMITA、Yuki YAMAGUCHI、Takahiro KUWABARA

We have developed a new steel strip for gaskets that can seal exhaust gas in 900°C environments. The concept for the material specified the seemingly conflicting requirements of high performance through the active use of γ’-Ni3 (Ti, Al) as a strengthening phase for high-temperature stability, together with a higher melting point for manufacturability. To enhance the relaxation properties, alloying elements that raised the amount and stability of strengthening phase γ’ and suppressed the precipitation of the harmful phase η-Ni3Ti were investigated using thermodynamic calculations, and 5.9 at% of Ti+Al and a ratio of 1.0 of Al/Ti were selected. In addition, to enhance hot rollability, the melting point was increased by reducing Nb and Cr contents as compared to Alloy 718. To maintain initial springiness and the stability of γ’ at high temperatures, the cold rolling reduction ratio was set at 30% to 40%. As a result, in heat load tests at 900°C, gaskets made of the developed alloy performed at the same level as Alloy 718 gaskets at 750°C.

Hiroki YAMAMOTO、Isamu SAITO、Yoshiki KUMAGAI、Yoshinori SUMI

Evaluation tests are conducted in order to verify the design accuracy of vehicle-mounted high-pressure containers that use carbon fiber-reinforced plastic layers. We have developed a method to identify the state of damage generated in carbon fiber-reinforced plastic layers that eventually leads to bursting in these tests, in order to enhance the design accuracy of such containers.
For this method, we used acoustic emission technology and processed the measured results into dimensionless data. This enabled us to set threshold values for the appearance of major damage that is the prelude to bursting. For the first time in the world, this makes it possible to terminate the test directly before the bursting takes place.
Applying this technology will enhance the design of high-pressure containers by enabling a precise analysis of the state of damage in the carbon fiber-reinforced plastic layer of a high-pressure container.

Takanori SUZUKI、Takashi KUWAYAMA、Masamichi KAWABE、Ichiro HARADA、Toshihiko KANEZAKI