Shop Class: A Beginner’s Guide to Cooling Systems
Be Cool, Keep Warm
In the classic air-cooled VW boxer engines—and even current motorcycle, ATV, and aircraft configurations—engine cooling is achieved through airflow alone. Cast cooling fins, surrounding cylinder blocks and heads, are able to adequately disperse engine heat into the air traveling across. The source of ambient airflow is directed from vehicle speed and/or cooling fans.
Air-cooled engines work great for certain applications but are not the way to go with modern-day cars and trucks. The likelihood of an overheat condition and the lack of heat volume for larger passenger compartments are two big reasons behind the advantage of liquid-cooling.
Liquid-cooled engines are configured with a maze of internal passages allowing the liquid to pass through the engine and collect engine heat. The high-temperature liquid is then routed to a radiator, where the engine’s heat is dispersed into the air traveling past the fine tubes of the radiator core.
The initial drawback to liquid-cooling was the potential to freeze up in winter conditions. Water inside a cast iron or aluminum engine, once frozen, expands with substantial force – enough to crack the metal castings. So antifreeze came about.
Antifreeze is a liquid additive that decreases water’s freezing temperature, while increasing its boiling point. Originally, methanol (methyl alcohol) was mixed with water for engine coolant, but there were problems. Yeah, it sounds like ethanol, and it has similar corrosive properties, which quickly plugged-up or damaged radiators and engine blocks. Methanol would also evaporate quickly on vented systems and require frequent refills.
Ethylene glycol was the fix, typically as a 50/50 mix with water. Keep in mind it’s a poison, but an excellent automotive coolant. The mix freezes at lower temps and boils higher, with anti-corrosive additives and lubricants to prolong water pump life.
Basic System Flow
We’ll start at the water pump: a simple centrifugal pump that uses an impeller to move liquid.
Cool liquid is taken in to the engine from the radiator. The water pump transfers it through all the engine’s internal coolant passages where heat is absorbed. The coolant then travels out of the engine to the intake side of the radiator. Engine heat is dispersed into the ambient air through the tubes and fins making up the radiator core. The lower temperature coolant goes back to the engine, and the cycle continues.
This is the basic flow of heat transfer, but there’s a lot more to it.
Basically a centrifugal pump with an impeller, a water pump can also be driven by a number of different sources, including a drive belt, timing belt, gear, or chain, depending on engine design. Some models use electrically powered auxiliary pumps to aid in flow under certain conditions.
Engine Block/Cylinder Head
The coolant passages typically travel through both the block and head to absorb as much heat as possible. The source of engine heat comes predominantly from the combustion chamber portion of the cylinder head.
The engine’s thermostat is a spring-loaded open/close valve that blocks coolant flow to the radiator. At a specified coolant temperature (180-220 degree range) wax inside a chamber liquefies and expands, opening the valve. With its expansion and contraction, the desired temperature is maintained. This event serves two purposes.
Engines operate most efficiently at their normal operating temperature, so we want warm-up to occur as quickly as possible. During warm-up, the thermostat remains closed, limiting coolant to the engine so the block warms up quickly. The warming engine coolant can also be routed to the heater core (we’ll get to that). This raises passenger compartment temperature quickly on cold mornings. That said, an engine will warm up more quickly (and thus operate more efficiently) if we keep the heater “OFF” until the engine is at an appropriate operating temperature.
This is the heart and soul of the cooling system, where engine heat is dispersed. The radiator is comprised of an inlet tank where hot coolant enters from the engine, passes through the core, then an outlet tank, before the lower-temperature coolant exits en route back to the engine.
The core is made up of rows of narrow tubes routed from the inlet to outlet tanks. Between the tubes, thin metallic fins are attached to increase the surface area and enhance heat transfer as air passes through.
Radiators vary in design and have been manufactured using brass, copper, plastic and aluminum. The majority of today’s motor vehicle applications use plastic tanks and aluminum cores.
There are different styles, but the radiator cap typically seals the top of the radiator at a specified pressure. As coolant temperature and pressure increases within the radiator, coolant passes by the cap and into an expansion tank. As coolant cools off and pressure decreases, coolant is drawn back into the radiator. That’s why you’ll see expansion tank levels vary with coolant temperature.
A heater core is virtually a miniature radiator within the HVAC system under the dash. Coolant is bypassed from the engine with hoses through the firewall. But here the dispersed engine heat has a useful purpose. The blower motor passes inside or outside air through the core and past the hot engine coolant, warming it and routing it into the passenger compartment to keep occupants cozy.
Certain applications use a “hot water valve” to reduce or stop heater core coolant flow in accordance with A/C operation.
Rubber hoses are the thruway of a cooling system. Large diameter upper and lower hoses connect the engine to the radiator. Smaller diameter hoses supply coolant to the heater core. And a variety of size and shape hoses route coolant to components for other purposes. An example is coolant flow to a throttle body to warm the air/fuel mixture during cold conditions.
While traveling at 75 mph on the interstate, there’s plenty of air going through the grill and radiator to disperse engine heat, but not the case at low speeds or idle, so cooling fans are used as a secondary source of airflow. This is needed to control engine temperature during all driving conditions and maintain A/C performance. The air conditioning condenser, usually mounted in front of the radiator, uses the same airflow to disperse heat from the passenger compartment.
Originally, the old standby steel coolant fan mounted to the water pump between the engine and radiator did the trick. But this method put a load on the engine, affecting fuel economy and performance. Temperature-controlled clutches were used with the fan to reduce load when max airflow wasn’t needed, but that wasn’t a complete fix.
Electrically operated cooling fans are now the standby, eliminating engine load and allowing more precise control of airflow. There are all sorts of configurations, but two fans mounted behind the radiator are very common. A shroud to concentrate the flow of air surrounds the plastic fan blades.
The powertrain control module controls current electric fan setups. Fan run time and speed are determined by coolant temperature, A/C demands, and additional inputs.