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With the total amount of air pollution caused by vehicle emissions on the increase, the problem has now became a global concern, and various regulatory measures have been put into effect in each region of the world. This is especially true in California, U.S.A, where countermeasures have been adopted early. There, the ULEV (Ultra Low Emission Vehicle) standard, which was ones deemed impossible for gasoline engines to meet, is now in effect. In response to these developments, Honda announced the ULEV system for a 2.2 liter gasoline engine with a closed-coupled catalytic converter (CC) and an under-floor catalytic converter (UF) at the beginning of 1995, and reported on the system's emission characteristics. 1) A new ULEV system has been developed based on the previous system but using only UF, aiming for marketable improvements in product characteristics such as higher output. The new system features the ultra low heat capacity and high heat insulating (ULOC) exhaust manifold.

The objective of the study was to confirm whether or not it is possible to use the restraint ratio as an evaluation parameter for the thermal fatigue durability of the exhaust manifold. The study revealed that there was a conformity between the thermal fatigue observed in the material experiment and the restraint ratio. The ratio, under certain temperatures, is an effective tool in estimating the thermal fatigue durability of the manifold. An analysis was carried out by regarding the local restraint condition of the exhaust manifold as a spring system. This provided a method where the restraint ratio was reproduced for the manifold under engine operation temperatures of up to 300°C. As a result, the actual restraint ratio can be obtained by performing a relatively simple process. Since the restraint ratios that were calculated correlate with those that were actually measured, quantitative evaluation of thermal fatigue durability using the restraint ratio was found to be possible.