The Washington Times - May 16, 2008, 12:56PM

 

Cooling Systems

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Automobile engines are known in engineering circles as “heat” engines. That is, they burn fuel and air inside a combustion chamber to create work. Unfortunately, they aren’t very efficient at it, and even the best of them only convert about 25 percent of the fuel’s energy into power. The rest is converted to heat, half of which goes into the cooling system and the rest into the exhaust (okay, and a little into the oil.)

 

We’re talking about a lot of heat, whether it’s coming out of your Stovebolt 6 or 427 V8! Typical modern (post 1950) engines traveling at 60 mph have combustion temperatures that reach 4000 degrees F and will generate 150,000 BTU’s (enough heat to comfortably warm a 4000 square foot house at zero degrees outside).

 

The cooling system of the car has to keep just enough heat to run the engine at its most efficient temperature and get rid of the rest, regardless of the speed the car is moving. Too low a temperature robs the engine of power and contributes to “cold sludge,” the enemy of engine life. Too high a temperature and the oil boils, causing catastrophic failure of the moving parts.

 

The Components - Why Are They There?

  

Automotive cooling systems consist of the radiator, water pump, engine cooling passages, radiator cap, fan, hoses, thermostat, expansion tank and the coolant itself. Usually included in these are the coolant bypass and heater core. It’s worth taking a little time to give a short description of each and why they are needed. If they weren’t needed the manufacturers wouldn’t put them there!

  

Radiator

 

The radiator is the heart of the cooling system because it’s job is to transfer excess engine heat to the air. It does so by providing a huge surface area to the air so that heat can be dissipated. Warm coolant enters the top of the radiator through the upper hose, then travels down through the radiator through - or outside of - its metal tubes that are surrounded by metal fins. As this happens, air is drawn past the fins by the engine fan and forward movement of the car.

 

Radiator designs over the years have seen many variations. Down-flow radiators were largely replaced by cross-flow models and eventually automakers moved from copper to aluminum for radiator construction.

 

Failure modes of radiators include: clogging from rust or grease; fractures in tubes or top/bottom tanks; fins clogged with leaves or bugs; bent fins; restricted air flow due to a/c condensers.

 

Cap

  

The radiator cap is a vital component, most often overlooked or taken for granted. Its job is to pressurize the cooling system, since the laws of physics dictate that for each pound-per-square-inch pressure over atmospheric the boiling point of water is raised three degrees.

 

Since most caps pressurize to 15 psi, the boiling point of coolant is raised from 212 to 257 degrees. Over time the springs in caps fatigue, reducing the pressure in the system and allowing expanded coolant to flow out of the radiator’s overflow hose.

 

Expansion Tank

 

The expansion tank, if equipped, functions as a collection device for the radiator. Its function is to collect heat-expanded coolant from the radiator. When the system cools down an induced vacuum draws the coolant from the tank back into the radiator.

 

 

Water Pump

 

The water pump is there to circulate coolant through the system at a calculated rate. It is driven by the engine through a belt and typically pumps upwards of 2,500 gallons of coolant through the system each hour. Without it most engines would overheat because they would suffer from “localized” boiling due to the coolant not moving fast enough to exchange heat.

 

Water pumps are amazing devices. They sit mounted to the engine for years without routine maintenance. They have a central shaft that is carried in sealed bearings that have pressurized water on one side and a belt-driven pulley on the other. Great forces are always acting upon them in the form of belt tension and engine revs, yet they last many years without leaks or failure.

 

Inside the pump is an impeller-type blade that spins with the belt-driven shaft. That blade picks up water in the cooling system and pumps it through the engine in an endless cycle. Failure modes of pumps include bearings, seals and the impeller itself. Over time, impeller blades corrode and dissolve due to acids in the coolant. It is not uncommon to disassemble old pumps and find “bare” impellers showing no blades.

 

Pump speed is designed by the engine maker to provide efficient coolant flow. Increasing pump speed through the use of a larger drive pulley is more likely to create coolant foaming and cavitation, causing the engine to run hotter.

 

Thermostat

 

The thermostatis an automatically adjustable valve that controls the flow of coolant through the system to maintain a constant temperature. It does so through the use of a spring-loaded “valve plate” assembly that moves when a sealed wax-like material expands and contracts. Thermostats are designed to fully open at a specific temperature.

 

When the engine is cool the thermostat’s piston closes the valve plate. This allows the coolant only to circulate through the engine block, with the exception of the small amount pumped through the bypass. Engine blocks reach operating temperature more quickly when the thermostat is closed.

 

As the engine warms up the thermostat’s valve plate opens, allowing circulation through the radiator. The valve plate opens and closes down during engine operation to maintain as closely as possible the designed-in temperature (e.g.: 195 degrees F). Thermostats can fail in the open or closed position, leading to overheating, cold sludge and other problems.

 

Caution: Never remove the thermostat in an attempt to run the engine cooler. In most cases this will cause the engine to overheat due to the coolant moving through the block passages too fast. Some older engines might run too cool, leading to sludge and acid buildup in the oil.

 

Fan

 

The fan is placed in front of or behind the radiator to force air through, particularly when the car is stationary or moving slowly. Fans can be engine-driven or electric and may be accompanied by a shroud that directs the flow of air through the radiator. 

 

Hoses

 

Hoses connect all coolant-moving components together. They provide the flexibility to absorb engine movement and thermal expansion and are strong enough to contain the pressurized coolant. Typical arrangements include an upper and lower radiator hose, heater hoses and expansion tank hoses, if equipped.

 

Cooling system hoses are the arteries and veins of the engine’s “blood” supply. Their ease of installation and flexibility belie their importance, not to mention the way most people take them for granted. Typically, hoses are replaced only when they have failed or in situations where they have been damaged during removal.

 

The most important thing to consider about cooling system hoses is that they aren’t permanent. Hoses, because they are made of synthetic rubber, deteriorate from the effects of heat, oil, abrasion and ozone in the air, not to mention electrochemical degradation (above).

 

Heat damage occurs due to underhood temperatures that frequently exceed 300 degrees F, excessive pressure and repetitive heating cycles of the coolant. Eventually the interior fibers get damaged, causing the hose to feel soft or creating bulges.

 

Oil leakage or accumulation on hose surfaces acts as a solvent to the synthetic rubber. Over time the hose can disintegrate, and all hoses are susceptible to damage from punctures, rubbing against metal components and incorrect or stretched installations.

 

Ozone in the air attacks rubber compounds. Your weatherstripping, hoses and tires fade and crack because ozone breaks down the molecular bonds in rubber over time, particularly at stress points. Ozone is a major part of urban air pollution and is generated in automobile exhausts. It is generally in high concentration under the car’s hood.

 

Tip: Never pull or twist a hose to take it off. You can easily break or damage radiator fittings or other fragile connections. Cut the end of the hose with a sharp blade and peel it off its fitting.

 

Coolant Bypass

 

Coolant bypass systems are in place to provide minimal coolant flow when the thermostat is closed. This prevents the water pump from creating a vacuum or pumping air and keeps a small amount of coolant moving through the system during warm up to prevent localized boiling. Some engines incorporate a bypass in the thermostat housing or water pump and others utilize an external bypass hose. Closing off a bypass will normally cause the engine to run hot because the original design of the engine requires the extra flow through the bypass to run efficiently.

 

Heater Core

 

The heater core is nothing more than a small radiator placed near or under the dash to provide heat for passengers. The heater hoses bring warm coolant to the core where a fan blows cabin air through for thermal transfer. Heater cores also provide a little more total cooling system capacity and slightly aid overall engine cooling.

 

Engine Cooling Passages

 

These are narrow, complicated voids surrounding the heat-producing areas of the engine block and cylinder heads. As coolant moves through these voids heat is transferred to it. That heat is dissipated in the radiator as lower temperature coolant is pumped through the passages.

 

If coolant isn’t changed often enough or the vehicle isn’t driven often, rust and scale form on the surfaces of the coolant passages. Eventually, localized boiling occurs inside the engine, resulting in overheating.

 

 

What About The Coolant?

  

Here’s where we begin a detailed look at the cooling system. In the old days cars were cooled only with water. It worked just fine, but tended to freeze during winter and helped to rust out the engine blocks. Early concoctions used to keep water from freezing included methanol, glycerin, kerosene, cooking oil and even booze. None worked well and all carried their own particular dangers, not to mention the waste of good drinkin’ hooch!

 

Good old H2O was - and still is - the best heat-transfer medium around, but it needs to be mixed with something that will keep it from freezing, rusting metals, evaporating and boiling over. The Prestone Company introduced the first modern “permanent antifreeze solution” in the late 1920s, based on ethylene glycol. Variations of it are still in use today because it provides a high boiling point (223 degrees F) and will keep water from freezing down to minus 20 degrees F.

 

Technically, the mixture of water and antifreeze is called “coolant.” That’s because the mixture has other stuff suspended in it (additives) that help to prevent electrochemical degradation, commonly called Galvanic Action. It is caused by the pH balance of the cooling fluid having a higher than normal acid balance. Then the stuff becomes acidic it starts to “eat” metallic parts such as the radiator, water pump, head gaskets, hoses and, well, everything.

 

All coolants have additives that neutralize the acidity of the water and ethylene glycol (which is actually acidic itself). These additives “sacrifice” themselves over time, eventually allowing the coolant mixture to return to its acidic state. That’s why coolants are supposed to be replaced every three years or so. Another reason to replace coolant periodically is to prevent clogging due to rust, scale and grease accumulation in the radiator passages. Fine particles of crud in the engine are constantly circulated and over time build up a layer of hard scale inside the tubes of the radiator.

 

Important Point! - A coolant mixture’s anti-freezing capability never goes away. Its anti-corrosion capability deteriorates, requiring flushing and replacement periodically.

 

Many people wonder why they should use antifreeze and water in a 50-50 mixture, even though they live in warm climates. The reason is that the 50-50 mixture brings the system’s pH to above 8.0. When it comes to automotive cooling systems the higher the pH the better. Since water alone barely hits a pH of 7.0, the addition of antifreeze (with a pH of 12.0 or better) makes it easy to achieve desired levels.

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