An open-looped cooling system is disclosed. The cooling system typically includes a cylinder head including a first water jacket through which cooling water flows, a cooling water inlet through which the first water jacket communicates with outside, and a first cooling water outlet through which the first water jacket communicates with outside, a cylinder block including a second water jacket through which the cooling water within the first water jacket flows, the second water jacket being configured to communicate with the first water jacket, and a second cooling water outlet through which the second water jacket communicates with outside, and a thermostat configured to control a flow rate of the cooling water flowing out from the second cooling water outlet of the cylinder block based on a temperature of the cooling water.