Keeping Engines Running Cool

The search for an antifreeze/coolant that provides good heat transfer-properties, high boiling protection and low freeze-point protection spans the last 85 years. Len Bartley and Paul Fritz discuss the history and trends of coolants.

Coolant/antifreeze provides year-round protection for your vehicle’s cooling system. While removing heat from an engine, it protects the metals in the engine and cooling system from rust and corrosion. Coolant/antifreeze provides overheat protection in all weather and freeze protection in cold weather. It must be compatible with plastics, rubber and paint finishes. Here is a synopsis of the history, current status and future trends in coolant/ antifreeze technology.

Eighty-five-year search
Besides producing the energy to propel your vehicle, engines produce an enormous amount of heat. One analogy often used is that one heavy-duty diesel engine produces enough heat to warm five average size houses. The search for a heat-transfer medium or antifreeze/ coolant that provides good heat-transfer properties, high boiling protection and low freeze-point protection spans the last 85 years.

Early in the 1900s, people used water in their cooling systems, which has excellent heat-transfer properties but has the tendency to crack engine blocks in freezing weather. Attempts to lower the freeze point of water by adding salt were eventually found to be unacceptable due to the resulting accelerated corrosion problems.

Early in the 1920s, methanol and ethanol water mixtures were used in cooling systems, but evaporation loss made it hard to maintain freeze protection. By the late 1920s, ethylene glycol and water mixtures were used in coolant systems. This ethylene glycol/water mixture had many benefits. It lowered the freeze point better than any other material used up to this point; it was relatively inexpensive; it was chemically stable; and it was compatible with rubber. This made ethylene glycol/water an excellent base fluid for antifreeze/coolant.

By 1950, almost all coolants were ethylene-glycol-based and contained inorganic inhibitors for corrosion protection, including phosphates, borates, silicates, nitrates, nitrites and amines. Even though these inhibitors provided adequate corrosion protection, there were some drawbacks to this technology.

Figure 1: Heat transfer test results for traditional coolant compared to carboxylates

Inorganic inhibitor woes
For the inorganic inhibitors to work, they need to put down a protective barrier, which means that the inhibitors are constantly being depleted from the fluid. This protective barrier can restrict the flow of heat from the metal into the coolant, which can change the performance characteristics of the cooling system over time.

Figure 1 shows the results of dynamic heat transfer tests involving inorganic-based traditional coolant on the one hand, and a newer organic-based extended-life coolant on the other. As discussed later in this article, organic-based extended-life coolants contain inhibitors called carboxylates. In the tests, a constant amount of heat was passed through an aluminum disk, or coupon, and the temperature of the disk was recorded at different points of time for 80 hours. The inorganic oxide layer of the traditional coolant insulates the metal and causes its temperature to rise.

The lack of global acceptance is a problem with some of the inorganic-based inhibitors. Phosphates have been essentially banned in Europe because of hard-water-stability problems. Phosphates are good overall corrosion inhibitors but often need to be balanced with other inhibitors to protect light alloys (primarily aluminum-based alloys) and readily form scale with hard water ions.

Japanese automobile companies refuse to use coolants containing silicates due to their various shortcomings. Silicates are very effective at protecting light alloys. However, they can form abrasive precipitated particles, and they also need to be properly stabilized to stay in solution.

Nitrites, used throughout US industry for enhanced cast-iron protection, also are not globally accepted due to concerns that carcinogenic nitrosoamines may form during use. These concerns have led to many different formulations being developed to address the issues. Not until recently have coolants been developed in which one formulation has global appeal.

Figure 2: Average miles for taxi cabs before water pump failure

Current trends
Cooling-system designs in recent years have been focused on improving fuel economy. Engines and cooling systems are smaller, and the emphasis has been on using light alloys, such as aluminum, whenever possible. Engine designs also now allow for higher temperatures. Higher temperatures and smaller coolant systems cause additional corrosion stress points in cooling systems. These challenges put more emphasis on coolant development, and this has led to improvements in coolant technologies.

Coolant manufacturers work closely with original equipment manufacturers (OEMs) to develop coolants that meet their needs. Globalization is often part of the OEM’s needs. As the OEM market expands and emphasizes the global nature of its products, suppliers to the OEMs must follow the market’s lead and be ready to provide a single technology for use worldwide. These demands in the marketplace led to the introduction of the next generation of coolant technology—extended-life coolants.

Organic inhibitor benefits
Extended-life coolants use carboxylates, which are organic-based inhibitors, for corrosion protection. They do not typically contain traditional inorganic-based inhibitors. These carboxylates provide corrosion protection by interacting chemically at the metallic corrosion sites, rather than laying down a layer of inorganic oxides that provide protection through insulation.

Carboxylate-based coolants are often referred to as Organic Additive Technology (OAT) coolants. As previously shown in Figure 1, carboxylate-based coolants do not impede heat transfer. Other advantages of these coolants: they do not need to be formulated for regional use; they deplete on a limited scale, which means longer service life; and they provide better corrosion protection at higher temperatures than traditional inhibitors.

Still another advantage of organic-based coolant is that it lengthens the life of the vehicle’s water-pump, which leads to reduced warranty costs for the OEM. Figure 2 shows the results of tests in which hundreds of taxi cabs were operated with either traditional coolant or carboxylate coolant in their cooling systems, to see how many miles they could go before their water pumps had to be replaced because of seal failure. As indicated in Figure 2, the taxis with carboxylate-based coolant averaged nearly 27,000 miles farther than the others.

In automotive use, extended-life coolants can last 100,000 to 150,000 miles before flushing and replacing are needed. See, for example, Long Life Performance of Carboxylic Acid Based Coolants, SAE, 1994 by D.A. Washington, et al. This is about three times the length of service that traditional inhibitors afford. Due to the greatly extended drain intervals, about two-thirds less hazardous waste is generated. The inhibitors used in extended-life coolants are essentially nontoxic and extremely biodegradable. However, the ethylene glycol used in traditional coolants is toxic and must be disposed of properly. The smaller volume of waste and the biodegradability of extended-life coolants make this type of antifreeze/coolant very environmentally friendly.

Figure 3: Pitting of a heavy-duty engine cylinder liner caused by insufficient inhibitors

Heavy-duty challenges
Heavy-duty engines run at maximum output and at higher temperatures than automotive engines, presenting different challenges for coolant manufacturers. One of the main concerns in the US heavy-duty engine coolant market is the pitting of the cylinder liner, also known as cavitation corrosion. In some engines, cavitation corrosion occurs during high load conditions when the piston slaps the liner, causing a high-frequency vibration.

The vibration of the liner creates areas of low pressure, causing vapor bubbles to form in the coolant surrounding the liner. These bubbles can implode on the surface of the liner, causing the protective oxide layer to be destroyed. Repeated implosions without a reformation of the protective layer can cause pits to form, as illustrated in Figure 3.

In traditional formulations where the inhibitors deplete rapidly, preventing cavitation requires a supplemental coolant additive after 18,000 to 25,000 miles. The supplemental coolant additive increases the inhibitor level enough to passivate the surface and prevent cavitation damage.

Inhibitors used in extended-life coolants provide excellent protection from cavitation. Because extended-life-coolant inhibitors have limited depletion, they can go 250,000 to 400,000 miles before needing refortification. This makes extended-life coolants very cost effective for heavy-duty applications. Again, in the heavy-duty coolant market we see the same environmental advantages as in the automotive coolant market.

Hybrid technology
There is another class of compounds, called hybrid coolants, that bridges the inorganic and organic inhibitor technologies. Hybrids contain inorganic-oxide-based inhibitors, along with some extended life properties imparted by carboxylates. . These technologies are used in Europe and the US. Care must be taken during formulation to incorporate the advantages rather than the disadvantages of both technologies.

Future trends
Coolant manufacturers face many challenges. Engine designs will continue to change and higher temperature engines may be used to achieve more efficiency and minimize environmental impact. Incorporation of new materials into engine designs will likely create the need for inhibitor technologies to protect these engines. Fuel cells with completely different coolant requirements will begin to gain market share. Environmental regulations worldwide may dictate changes in the base fluid of coolants. Currently, extended-life coolants continue to lead the technology parade and will meet or exceed all requirements for keeping your vehicle protected and running cool.

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