How Does The Engine Cooling System Work?

Unlike an air-cooled engine, a liquid-cooled engine does not run hot in summer and cold in winter

If you have internal combustion in a machine, heat from that combustion is going to be lost into the metal parts of the machine. If heat accumulates beyond a desired level in certain critical parts, those parts will fail. To control the temperature of those parts and enable them to survive their duty, heat has to be continually taken out of them as fast as combustion and friction are putting it into them.

This four-cylinder engine from a Honda CBR600RR is water-cooled. Earlier engines were air-cooled. Cooling fins were put on the cylinders and the cylinder heads, and as the motorcycle went down the road, air drifted between the fins, was heated, and carried away the heat that would otherwise have accumulated in the cylinder and head, leading to abnormal combustion and failure.

Air-cooled engines run hot in the summer and cold in the winter. If we cool an engine with pumped, circulated water or other liquid coolants, such as a 50/50 mix of antifreeze and water, and if we put a thermostatic valve in the system, we can choose the temperature at which the engine will operate. The typical operating temperature for liquid-cooled engines is 195 degrees or so Fahrenheit.

A pump delivers water that has been cooled by passing through the air-to-water heat exchanger, which we call the radiator. It is then sent into the cylinder block. Water rapidly flows from the water pipe, around the cylinders, and upward to enter the really hot part of the engine, the cylinder head. Water circulates over the top of each of the four combustion chambers. It then goes back to the radiator, and the circuit is completed.

The thermostatic valve sees to it that a liquid-cooled engine does not run hot in summer and cold in winter by opening and closing automatically. If the water gets too hot, the thermostat opens more, allowing more water to circulate through the radiator. This cools the water back down to the set point. If it’s wintertime, the valve doesn’t let any coolant go to the radiator to be cooled until the engine itself has reached operating temperature.

So the nice thing about water-cooling is that the conditions for the lubricant and for the moving parts are thermostatic; they stay at the same temperature. That is the purpose of the cooling system.