Engine cooling system: circuits and fluid path

Engine temperature affects engine power, fuel consumption and exhaust toxicity. (from engine temperature - power, consumption and toxicity)

Until the engine has warmed up, the thermostat keeps the coolant in the small circuit — inside the engine, bypassing the radiator. (cold engine: small circuit — past the radiator)

The system is closed: under a pressure of one to one and a half bar, the coolant boils not at one hundred but at one hundred and fifteen to one hundred and thirty degrees. (closed system: 1.0–1.5 bar → boiling at 115–130 °C)

The expansion tank is only half full, and the coolant is a mixture of water and concentrate: it does not freeze, boils later and protects light alloys from corrosion. (tank half full · coolant: water + concentrate — frost, boiling, corrosion)

Many modern engines have two circuits: the coolant flows separately through the cylinder head and the cylinder block and can have different temperatures. (two circuits: head and block - separately, different temperature)

Two flows are controlled by two thermostats in a common housing: one through the head, the other through the block. (two thermostats in a common housing: head · block)

One third of the coolant goes to the cylinders and two thirds to the combustion chambers in the cylinder head, through which it flows crosswise. (⅓ fluid to the cylinders · ⅔ to the combustion chambers · cross flow in the cylinder head)

In one engine, below eighty-seven degrees both thermostats are closed: the coolant flows through the pump, cylinder head, thermostat housing, heater core and expansion tank, and the engine warms up faster. (below 87 °C: both closed — pump, head, housing, heater, tank)

From eighty-seven to one hundred and five degrees, the cylinder head thermostat is open: the head is kept at eighty-seven, the block keeps warming up, and the radiator is switched in. (87–105 °C: head thermostat open — head 87 °C, block warming up · radiator)

Above one hundred and five degrees, both are open: the block settles at one hundred and five, the head at eighty-seven. (above 105 °C: both open — block 105 °C, head 87 °C)

A hot block means less friction in the crank mechanism. (hot block → less friction in the crank mechanism)

A cool head cools the combustion chambers better: the cylinders are better filled and the engine is less prone to knock. (cool head → better filling, less prone to knock)

The pressure in the cylinder head circuit is higher, so the thermostat there can be two-stage: first a small disc opens, and then it pulls the large one with it. (two-stage thermostat: small disc first, then it pulls the large one)

In one engine series, the pump is built right into the thermostat housing and is driven by a toothed belt from the exhaust camshaft. (pump built into the thermostat housing · driven by toothed belt from the exhaust camshaft)

In engines with multi-point injection, the cylinder head thermostat opens as early as eighty degrees. (MPI engines: head thermostat — from 80 °C)

With electronic control, a heating resistor is built into the thermostat's wax filling: the control unit heats it and adapts the temperature to the load. (electronic control: heating resistor in the wax filling)

Without current, such a thermostat operates at one hundred and ten degrees, and with heating - earlier; at partial load it reduces fuel consumption and harmful emissions. (no current - 110 °C · heated - earlier → lower fuel consumption and emissions)