The word “ occasionally” means that “sometimes” the car cannot be started and driven because of a mechanical problem.  But, the problem does not happen all of the time.

偶爾，我的車子無法發動。

句中 occasionally = on rare occasions = on occasion = once in a while，意思與sometimes, at times, from time to time 等相近。總之，這車子引擎故障（發動不了）的問題，並非經常發生。