What Do the Large Circulation and Small Circulation Refer to?

[StJayla]

Large circulation and small circulation refer to: Large circulation refers to the process where the coolant flows out of the engine, passes entirely through the radiator for cooling, and then re-enters the engine to cool it; Small circulation refers to the process where most of the coolant flows out of the engine without passing through the radiator for cooling and directly re-enters the engine to cool it. When the car engine is just started, the coolant first undergoes small circulation, which helps the engine quickly reach its normal operating temperature. Once the normal operating temperature is achieved, the coolant then switches to large circulation, thereby maintaining the engine within a reasonable operating temperature range. The large circulation route includes the water pump, distribution pipe, cylinder block water jacket, cylinder head water jacket, thermostat, radiator, and water pump.

[KeeganLee]

I see this every day when repairing cars. Simply put, it's about two cooling paths for the engine. When the engine starts cold, the thermostat stays closed, preventing coolant from taking the long route. Instead, it circulates only within the engine block—this is called the small cycle, which heats up the engine in under three minutes. Once the coolant reaches about 90 degrees, the thermostat opens up, allowing coolant to flow through the radiator. The fan kicks in to dissipate heat—this is the large cycle. If the temperature still spikes after the large cycle activates, it's likely due to a stuck thermostat or a failed water pump. Last time, my colleague's car overheated in traffic, and upon inspection, sure enough, the thermostat was rusted shut. Regular maintenance is key—keep an eye on the coolant color; if it turns black, replace it immediately.

[OKylie]

You're asking a very technical question. Car engine cooling is divided into small and large cycles, all controlled by the thermostat switch. When starting a cold engine, it warms up first, with coolant circulating only within the engine block and cylinder head, bypassing the radiator entirely, allowing water temperature to rise quickly. Once the temperature gauge needle passes the midpoint, the wax in the thermostat melts and pushes the valve open, suddenly allowing the coolant to circulate fully: from the engine to the radiator at the front grille, where the wind carries away the heat before returning to the engine. If your heater isn't warm in winter, it's likely because the small cycle hasn't switched to the large cycle, often due to a faulty thermostat in older cars. Replacing this valve isn't expensive, but you'll need to drain all the coolant, which is quite a hassle.

[LeZoey]

To put it simply, it's like the difference between a sprint and a marathon. The small circulation is like running laps in a boiler room: the water pump circulates coolant back and forth within the engine block, not even reaching the front radiator, solely to raise the coolant temperature rapidly. Once the blue temperature light on the dashboard goes off, the thermostat clicks open, and the coolant suddenly switches to off-road mode: flowing from the engine → upper hose → radiator → cooled by the fan → lower hose → water pump → back to the engine, making a big loop around the entire car body. I've seen an old Buick's temperature skyrocket because the main circulation loop was blocked by a plastic bag. Regularly using a high-pressure air gun to blow out the radiator fins can prevent this.