Abstract: A four-stroke internal combustion engine includes a crankcase with a bottom, an oil pan and an oil spray generating assembly. The oil spray generating assembly having a closed sidewall, a valve, a bottom cap and nozzles is attached to the bottom of the crankcase and is enclosed by the oil pan. The closed sidewall is formed at the bottom of the crankcase to define a mist chamber enclosed by the bottom cap. The valve is mounted in the mist chamber with a valve port defined through the crankcase and a resilient valve flap corresponding to the valve port attached to the bottom of the crankcase. The nozzles are respectively mounted in the bottom cap out of the mist chamber inside the closed sidewall. The operation of the engine will produce an oil vapor for lubrication with the lubricating oil flowing out of the nozzles.