An engine provides mechanical energy from an air/fuel mixture with an efficiency between 20 and 45%. The rest flows in kinetic and heat energy in exhaust gases and in heat energy through metallic bodies due to the frictions. In this context, the cooling system must allow the engine to give its best performance, ensure the durability of this performance and ensure engine reliability by guaranteeing an acceptable level of thermo-mechanical stresses in any point of the engine. This is done thanks to evacuation of the exceeding calories towards outside atmosphere.
Types of cooling systems
It exists different physical principles to evacuate the heat:
- Calories can be extracted by convection, conduction or radiation
- Several intermediate fluids can be used to drive the calories to the absorbing medium (those fluids are called coolant)
- Coolant can be either gaseous, liquid or in phase change
In automotive industry, the main cooling systems are air cooling by natural convection, air cooling by forced convection and liquid water cooling. Natural convection means that cylinder and cylinder heads have fins to ensure efficient convection and conduction, whereas forced convection means that an air turbine and a cooling air housing are installed around the engine. In both cases, the coolant is the air which is the only fluid which evacuates calories. Liquid cooling uses two fluids, air and water. Water evacuates calories from the engine and exchanges them with ambient air in a radiator which is today the most used system in automotive industry.
Because today’s high-performance engines produce more power and thus heat, improved cooling systems are drastically needed to maintain a proper operating temperature. Inside your Chevy engine, water jackets surround the engine, and cylinder heads serve to cool it. These water jackets are supplied coolant by upper and lower hoses connected from the radiator to the engine. The water and/or coolant heated in the engine is continually forced by a water pump to circulate through the engine and out to the radiator. The coolant in the radiator, in turn, is cooled by the fan(s) and also by the air traveling through the radiator as the vehicle is moving. Inside the radiator’s core are rows (typically two to four) that allow the water to flow in one direction, in and out, while outside air passing through the radiator fins cools the water by temperature transfer.
The thermostat is an important component on any engine, especially a high-performance one. It closes when the water temperature is low (cold) to prevent the water from circulating in the engine and opens when water temperature is high (typically above 180 degrees) to allow circulation through the system. Without a thermostat, the water would simply circulate too quickly and possibly even reduce power partly because more horsepower is required to drive the water pump when the restriction of the thermostat is removed. Thermostats help to provide a fast warm-up and also aid the engine to avoid acid formation in the oil, all the while reducing engine wear. Most new GM engines produced until the late ’80s used 195-degree thermostats installed on the outlet side of the engine. GM cars produced since the early ’90s typically use 180-degree thermostats (80- to 82-degree C) that are installed on the water inlet side of the water pump from the lower radiator hose. This is because water traveling from the radiator’s lower hose is generally at a continuously stable and cooler temperature and does not cause the thermostat’s operation to oscillate as frequently.
While originally introduced on high-end imports, one of two types of electronic thermostats will undoubtedly be found on our future commuter vehicles. The first type is basically a conventional thermostat that is opened by electrically heating the surrounding coolant.
The second type is a new design in which the thermostat opening is directly electronically controlled. In either case, the powertrain control module (PCM) will use these types of thermostats to regulate engine temperature to match the demands of part-throttle and wide-open throttle operation.
Cooling System Issues
Automotive engineers are currently faced with increasing the efficiency of the cooling system while reducing cooling system weight. Because many original equipment radiators have marginal cooling capacity, internally or externally clogged radiator core tubes will reduce cooling system performance to the point of overheating.
Electrolysis is, perhaps, the worst problem associated with modern bi-metal engines using aluminum radiators, whereas internal rust corrosion is the worst problem on the older cast-iron engines equipped with brass radiators. So, the additive packages in most coolants contain inhibitors that reduce corrosion caused by rust and electrolysis.
When the coolant’s additive package wears out, rust flakes from the engine’s cast-iron water jackets begin to clog the radiator core tubes. In some rare cases, the water pump impellor and other sheet-steel cooling system components like core plugs will also corrode due to poor metallurgy. In any case, rusty coolant indicates that the cooling system is headed for trouble.
Electrolysis occurs because a very mild electrical current develops between two dissimilar metals exposed to water-based solutions. Unfortunately, electrolysis tends to transfer from one metal to another. This results in the “solder bloom” found on the cores of the old soldered brass radiators. In more modern engines, electrolysis can cause cylinder head gasket failure by severely pitting cylinder head gasket surfaces and eroding the metallic portions of the gaskets themselves.
In the current market, most auto manufacturers supply long-life coolants designed to function with the specific metallurgy and designs of their cooling systems. Most manufacturers address deterioration in their additive packages by recommending scheduled coolant changes.