本研究所建立之引擎數值模擬分析，係使用有限元素軟體與邊界元素軟體分別進行強制振動與聲學響應分析，並探討引擎於2400與2800 rpm之聲功率與聲壓分佈。從聲學分析結果中可知，引擎之板殼類元件對於整體引擎的噪音貢獻量皆較大，因此本研究針對正時蓋板進行結構修改，調整原自然頻率並避開其他模態，以加強剛性進而降低噪音。

本研究利用增加肋條的方式來加強正時蓋板的結構剛性，而肋條之佈建位置初步雖已由本研究中研判得知，稍後亦已藉由ANSYS軟體之最佳化分析可找出肋條之最佳位置。正時蓋板之原自然頻率為1266 Hz，依本研究研判增加肋條後，其自然頻率提高為1300 Hz(上升34 Hz)，增加之肋條重量為110.1 g(佔原重量之3.1 %)；而藉由最佳化分析增加肋條後，其自然頻率為1306 Hz(上升40 Hz)，然肋條重量僅為85.5 g(佔原重量之2.4 %)。故以最佳化分析增加結構剛性，可得到較佳的剛性/質量比值。本研究最後將結構修改前後之正時蓋板代入整體引擎中進行強制振動與聲學分析，求得整體引擎之聲功率。結構修改後之整體引擎之聲功率從修改前的66.9 dB降至66.0 dB(下降0.9 dB)，且於正時蓋板側之最大聲壓值下降了3.3 dB，此結果顯示增加正時蓋板之剛性可有效降低特定頻率之噪音量。

To establish a numerical simulation analysis for engines, this study used finite element software and boundary element software for forced vibration and acoustic response analysis, respectively. The authors also discussed the acoustic power and acoustic pressure distribution of engines at 2400 and 2800 rpm. Results from the acoustic analysis suggest that for the whole engine per se, shell components have a greater noise contribution. Therefore, this study carried out some structural modifications on the front cover, adjusted the original natural frequency, and avoided it from other modes to reinforce its stiffness and reduce the noises.

This study stiffened the front cover structure by including additional ribs. Placement of the ribs was initially determined by the study result, but later, it was improved by the optimization analysis of ANSYS software. The original natural frequency of the front cover was 1266Hz, but after adding more ribs, the natural frequency was increased to 1300 Hz (an increase of 34 Hz). The weight of the added ribs was 110.1g (3.1% of the original weight). Therefore, using the optimization analysis approach to increase the structural stiffness can render a more optimum stiffness/weight ratio. Last but not the least, the study integrated the modified front cover into the whole engine and carried out forced vibration and acoustic response analysis to obtain the acoustic power of the whole engine. After modifying the structure, the sound power of the whole engine was reduced from 66.9 dB to 66.0 dB (a 0.9dB decrease). Furthermore, the maximum sound pressure at the edge of the front cover had a 3.3 dB decrease. These findings have revealed that increasing the stiffness of the front cover can effectively reduce the volume of noises at certain frequency.

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