Abstract: An engine cooling apparatus separately cools a cylinder head and a cylinder block which are positioned around upper and lower portions of an engine combustion chamber. For increased engine output power, the engine cooling system cools the cylinder head to a relatively low temperature and the cylinder block to a relatively high temperature. The cylinder block includes cylinder liners disposed in an oil jacket which is supplied with oil from an oil circulating system via a pressure reducing valve. Oil in the oil jacket returns through a return path into an oil reservoir in the engine, and air bubbles in the oil jacket are returned to the oil reservoir via an air bubble return passage which develops a pressure difference between its opposite open ends.