حسین گلبخشی

In low ambient temperatures the engine will have poor output torque and power generation at the first cycles of its performance, long start time and huge noxious emissions because of insufficient temperature and quality of air-fuel mixture and failure of flame to burn the charge. It is noted that any attempt to increase the charge temperature leads to better combustion conditions. By applying thin ceramic coating over the piston top surface and increasing the temperature of engine inner walls, more efficient combustion in shorter time can be provided for the engine. In current research, a transient thermal study is developed for simulating the engine performance under cold starting conditions and predicting the effect of insulating layer thickness on the charge temperature before ignition timing. The results demonstrate that the ceramic coating can provide higher temperature for piston crown and effectively improve the temperature of charge during the first cranking cycles of engine cold start stage.

In modern engines with higher compression ratios, severe pressures and non-uniform heating up is occurred for the engine parts. The piston as the most critical part among all automotive components has been the subject of numerous studies on calculation of temperature distribution, but thermal stress analyses are limited. In this study, the piston of gasoline engine XU7 which is widespread in Iran transport section is considered for thermo-mechanical investigation. A 3D linear static stress analysis is constructed in COSMOS Works to determine the stress distribution during the combustion stroke. The maximum normal stress takes place at the middle surface of the piston crown. The von Mises stress is found to be critical on upper parts of the pin-hole area. Effect of ceramic coating layer on stress distributions are evaluated and compared with results obtained for a traditional uncoated piston. According to the results, the maximum stress can be readily controlled by increasing the thickness of ceramic layer.

Among various dissipating mechanisms, the viscoelastic effect of rubber material on creation of rolling resistance is responsible for 10-33% dissipation of supplied power at the tire/road interaction surface. So, evaluating this kind of loss is very essential in any analysis concerned with improving the fuel consumption of vehicles and resultantly energy savage. Hysteretic loss is a fraction of stored strain energy of tire which is dissipated during the rotational motion. In all related references and published articles, the hysteretic loss per revolution of tire is commonly considered as the rolling resistance. However, a reasonable approximation of stresses and strains time histories is very crucial for evaluating the amount of hysteretic loss. Based on the results of 3D static contact analysis, an efficient in-house program coded in Matlab is used for estimating the time variation of stress-strain cycles at different points of a rolling tire. Half the difference between maximum and minimum principal values of six stress and strain components can be used for evaluating the hysteretic loss during one revolution of a rolling tire. The results closely match the related experimental and numerical investigations.