Bottom Line
Cooling Systems
Nothing could be further from the truth

by Paul Abelson, technical editor

Of the major systems in our trucks (power train, air and braking, electrical and cooling systems) the cooling system is the most misunderstood and most often abused. A great number of truckers, along with almost all motorists, think all they have to do is check the level of coolant, add water when it is low, and they’ve done their maintenance.

The Technology and Maintenance Council, or TMC (it used to be just The Maintenance Council) has 12 “Recommended Practices” on coolant, cooling systems and related accessories, including shutters, winter fronts and fan drives. All parts of the system require regular inspection, if not actual maintenance. Here’s why.

Your engine burns diesel fuel in a precise, controlled manner, in order to convert the latent heat energy in the fuel into the mechanical energy that moves your truck and its load. The engine, being a relatively inefficient machine, can capture and use only about 40 percent of the heat energy created. To put this in perspective, an engine going 70 mph, getting 6 mpg, will create 1,552,000 Btu (British thermal unit) of total heat energy per hour. Of this, about 621,000 Btu will go toward overcoming internal friction, rolling resistance and aerodynamic drag to move the rig. The remainder, more than 930,000 Btu, is either lost out the exhaust stacks or absorbed into the cooling system. About half the heat is lost each way, or between 460,000 and 490,000 Btu. Now these are pretty big numbers, but not very meaningful if you’re not an engineer. To give you an idea of what kind of heat that translates to, 1 Btu is the energy required to raise a pound of water 1 degree Fahrenheit. The typical big bore Class 8 cooling system holds up to 11.5 gallons of coolant, or about 92 pounds. There’s enough heat absorbed into the cooling system to raise the temperature of all the coolant in the system by 5,200 F in just one hour.

That gives us a new respect for the system. Exactly what is this “system” that manages so much heat? It involves everything that has a part in transferring surplus heat energy from the combustion chamber to be dissipated to the outside air. In its travels, the heat flows through the cast iron cylinder heads and liners, into the coolant surrounding the liners and flowing through passageways in the head. The water pump, driven by the engine through a fan belt, circulates the coolant at flow rates varying from about 50 gpm at 1,000 rpm to 100 gpm at 2,100 rpm. These vary depending on engine make and model, and the ratio of the diameters of the crankshaft (driving) pulley and the water pump (driven) pulley.

The coolant itself is formulated to absorb heat, resist freezing in winter and boiling in summer, protect cylinder liners, prevent scale build-up and lubricate the water pump. We’ll look more closely at coolants later.

The coolant is pumped through hoses, held in place by hose clamps, to the radiator. Radiator is technically a misnomer, because it does very little radiating. Heat is transferred to the air by convection, so perhaps the radiator should be called the “convector.” As the hot coolant flows through the tubes of the radiator, heat, which travels from hot to cold, is given up through the copperbrass or aluminum tubesto fins on the tubes. The fins increase the surface area exposed to the airflow, which makes the transfer of heat, from coolant to tube to fin, to air, more efficient. The system is kept closed and under pressure by the radiator cap. The thermostat is a valve that controls coolant flow from the engine to the radiator.

Since air is what absorbs engine heat from the finned radiator tubes, an air shroud ducts air through the radiator. When the truck is standing still or moving slowly, a fan pulls air through the radiator. The fan is driven by the fan belt. It consumes engine power, so it should be turned off when not needed. The fanclutch disengages the fan to prevent any power drain, along with the thermostat, and to keep the engine from super cooling when idling.

Some trucks have shutters to block airflow when the engine is cold.

All these components and devices, in bold type above, are part of a truck’s cooling system. In-cab heaters and defrosters that use hot engine coolant as their heat source are technically part of the HVAC (Heating Ventilating and Air Conditioning) system, but they rely on coolant for their operation, and share maintenance issues with the cooling system: hose and hose clamp condition, heat exchanger condition and air flow through the heat exchanger.

All the components that make up the cooling system have one function – to move heat from the engine and dissipate it into the air. The principles are similar to what we find in a refrigeration or air conditioning unit, but without the compressor and the liquefied gas. Instead of the compressor, we have the water pump. Instead of a refrigerant gas, we have a liquid coolant. And instead of letting cooled air bring heat back to the evaporator, we draw heat directly from the engine. Heat flows into the coolant, which carries it to the radiator where, like the condenser of the reefer unit, the heat is absorbed by the air. Then the cooled liquid flows back into the engine to repeat the process.

The amount of heat that the coolant can absorb is a function of the rate at which it is pumped through the system. In an engine cruising at about 1,600 to 1,800 rpm, all the coolant travels from engine to radiator and back about eight or nine times a minute. If a water pump starts to fail, that number will drop. Dwell time will increase. The temperature of the coolant will rise and the engine starts to overheat. If it does overheat, heads can warp, altering compression of the gaskets. That could lead to leaking or blown head gaskets, which could result in coolant entering the cylinders and working its way into the oil. Glycol and oil form a thick sludge that can block oil passages, starving bearings of lubrication. The sequence of events shows why water pump life is important to engine health. It also shows why it is important to look for signs of coolant every time you check oil. Be alert for sludge on the dipstick or the sweet aroma of glycol when you pull the stick out. Also, look for oil floating in the coolant.

Glycol is in the coolant to prevent water from freezing. Water has a unique property not found in other materials in nature. It expands when it freezes.

All other substances, except for a few highly engineered ceramics and plastics, contract when they get colder. Ice, however, is less dense than the water surrounding it. That’s why icebergs and ice cubes float. Because water (ice) expands when it solidifies, it generates tremendous forces. In nature, it splits rocks. On the roads, it seeps under pavement, freezes, expands and heaves concrete or blacktop pavement to create potholes. That same force can crack a cast iron engine block.

Early antifreeze was made from alcohol. You would open the radiator cap when you were done for the day, siphon or drain a few gallons of water and replace it with alcohol. When you next ran the engine, the alcohol would evaporate and you’d have to keep topping off with water. You’d repeat the process the next time you planned to turn the engine off when it was below freezing. One way to avoid adding alcohol was to keep the engine running continuously. To keep the radiator from freezing and cracking, many operators ran without thermostats. This may have led to the belief that truck engines had to idle continuously. When the industry switched from gasoline to diesel, cold weather starting problems reinforced this belief.

“Permanent” antifreeze was developed before World War II for use in aircraft with liquid-cooled engines. It has many of the best properties of alcohol-based antifreeze, but it doesn’t cook off at normal engine operations. Thus, the term “permanent.”

Water is a better coolant than glycol, but most can’t be used alone. A 50/50 mixture of ethylene glycol (EG) will protect an engine to minus 34 F. If you’ll be in colder temperatures, 60 percent EG protects to minus 63 F. Higher concentrations become more and more viscous at low temperatures, actually reducing cooling system performance. The greatest protection (minus 92 F) occurs with a 68 percent EG concentration. If concentration increases beyond that, protection is reduced. Also, 100 percent EG will freeze at 4 F.

Propylene Glycol (PG) is similar, better in some ways and worse in others. Unlike EG, PG is non-toxic. It is even used as a sweetener in children’s cough medicine. A 50/50 PG/water mix will protect to minus 27 F. PG can be used in greater concentrations, and some operators use 100 percent PG. Without water, PG protects to about minus 75 F. Sellers of PG coolant claim many benefits to not having water in the system, but PG is far more costly than EG to begin with, and most operators are hard pressed to justify going to PG at 50 percent, let alone 100 percent.

Because there is water in most coolant, problems can arise. Water promotes rust. It can boil under certain circumstances, such as when cylinder liners flex rapidly. That causes steam bubbles to form next to the liner. The force of coolant rushing in to fill the bubble impinges on the liner. Water can actually punch holes through unprotected cast iron cylinder liners, allowing coolant to flow into the cylinders and the oil. PG manufacturers claim that without water, 100 percent PG will not cause these cavitation holes.

Any water used in coolant should have as low a mineral content as possible. If you are mixing coolant, use only de-ionized or distilled water. Avoid “hard water” like the plague. Minerals in the water will precipitate inside cooling passages. They form scale that insulates the engine, making it retain heat, but just under the scale. This results in uneven cooling, creating hot spots that can warp engine parts.

To fully protect the engine and cooling system from the ravages of water, supplemental coolant additives (SCAs) were created. These chemicals do several things. They inhibit rust, lubricate the water pump, combat scale formation, and put a protective coating on metal parts. SCAs cannot prevent water from cavitating on cylinder liners, but the layer of protection they put on the liners absorbs the impact and prevents holes. More SCAs adhere to the liners to replace any that were removed, so protection is continual until the SCAs are consumed. They must be replenished from time to time. (More on that later.)

To avoid having to test SCA levels and replenish the chemicals, Texaco developed coolant based on EG with organic acid additives. It is marketed as Extended Life Coolant (ELC) by Texaco and a number of engine manufacturers under their own brand names. Organic acid coolants need not be changed for 500,000 miles, and require only a recharge of additives at about the halfway mark.

There are some problems with extended life coolants, so if you decide to switch over, here’s what you need to know. ELCs are not compatible with standard SCAs. If you fail to flush the cooling system completely, including all hoses and in-cab heaters, or if you (or unknowing truckstop personnel) add more than a few quarts of ordinary anti-freeze, you’ll dilute the organic acid package enough to negate its benefits. Also, while you can test for anti-freeze protection, there is no way to test for additive concentration as you can with conventional SCAs. Tests are involved and tell you only if the coolant is still good. They don’t indicate how much life is left. They could become unacceptable tomorrow.

Two years ago, Cummins engines began experiencing failures in gaskets and seals containing silicones. Head gaskets in N-14 and M-11 engines started crumbling, due to “drying out.” Texaco’s ELC formulation has no silicates that keep the gaskets protected, so silicon actually leached out. Some fleets at TMC have reported heater hose failures where the insides of hoses have been exposed to air. The failures did not occur on the “wet” side.

My opinion is that it’s still too early to know how ELCs will hold-up long term, or if any additional engine problems will arise. Conventional coolants that require SCAs are tried and true. We all know, or should know, how to maintain them.

Now that you understand how involved the entire cooling system is, you can appreciate the importance of proper maintenance. Next month, we’ll look at some recommended maintenance procedures to keep your cooling system managing all that heat. We also will examine the greater demands on the system that exhaust gas recirculation and other strategies to meet emissions requirements will create, and we’ll preview some exciting developments to help in the battle to keep our engines cool.