Experimental Investigation of Engine Valve Train Friction Considering Effects of Operating Conditions and WPC Surface Treatment oleh Muhammad Usman Bhutta, Muhammad Huzaifa Najeeb, Muhammad Usman Abdullah, Samiur Rahman Shah, Muhammad Khurram, Riaz Ahmad Mufti, Kiyotaka Ogawa, Jawad Aslam, Rehan Zahid, Mian Ashfaq Ali, Muazzam Arshad