There is no doubt that hybrid electric vehicles and fuel cell hybrid electric vehicles contribute greatly to preserving the environment. Both of these types of vehicles use multiple electrical power sources and the power flows between these sources are generally controlled using DC/DC converters. Therefore the DC/DC converter is an extremely important component of both an HEV and FCHV. This paper provides an overview of the Multi-Functional Converter System (MFCS) studies conducted by our laboratory. An MFCS consists of motors, inverters and additional wiring but no DC/DC converters. The MFCS can control the power flow between several AC or DC electrical power sources while, at the same time, controlling the motor torque. There are basically two types of MFCSs, the main difference being in the location of the electrical power sources in the circuits. One group has an electrical power source between the neutral point of the motor and the DC bus line of the inverter. The second group has one electrical power source between the two neutral points of the motors. This paper describes the basic circuit concepts and introduces their characteristic equations, the controller design concepts, and the differences between the circuits of the two groups. Also described are several experiments that prove the validity of the proposed method.

Takahisa Suzuki, Hajime Murata,
Tatsuya Hatanaka, Yu Morimoto

Comprehensive techniques for diagnosing the catalyst layer of a polymer electrolyte fuel cell were developed. The developed techniques consist of an electronic resistance estimation by a four-probe measurement of the catalyst layer with crack compensation by image analysis, a protonic conductivity estimation by ac impedance analysis based on a porous electrode model, and an estimation of gas diffusivity by comparing the limiting current density for the case of a helium-oxygen mixture as oxidant with that for the case of air as oxidant. The techniques were applied to in-house fabricated catalyst layers. It was found that electronic conductivity is sufficiently large to minimize voltage loss, and this is also true of protonic conductivity if the content of the polymer electrolyte is sufficient. Gas diffusivity was smaller than that calculated from the molecular diffusion model. Slow Knudsen diffusion through narrow pores contributes 40% to the total diffusion. From the abovementioned comparison, an increase of power in the fuel cell is attributed to an enlargement of pores in the catalyst layer that reduces the contribution of Knudsen diffusion.

Haruhiko Yamada, Yu Morimoto

A simple simulation method to predict the current distribution in a polymer electrolyte fuel cell is presented. This method combines a model for the material transport along the gas flow and through a membrane electrode gas diffusion layer assembly with experimentally determined IV (current-voltage) characteristics and water transport properties. The IV characteristics and water transport properties were measured under various gas conditions using a small cell having an electrode area of 1 cm². The current distribution and the humidity profile were calculated for a cell with an electrode area of 13 cm² for different flow patterns and stoichiometric flow ratios. The results were compared with experimental data obtained using a segmented cell and chilled-mirror hygrometers. The simulation results agreed well with the experimental data. This method gives us a rough insight into the phenomena of an operating cell and is useful for designing the cell for a membrane electrode gas diffusion layer assembly due to the simplicity of the simulation procedure.