The Bubble Battle

Few things seem more robust than on-highway diesel engines. The cast iron hulks are built to withstand pressures that would tear their automotive counterparts to pieces. They boast life spans that can exceed 1.6 million kilometres, and in some cases can generate in the neighbourhood of 600 hp and 1,850 lb-ft of torque.

If only they could withstand tiny bubbles in the coolant.

The fight against these bubbles–and a process known as cavitation corrosion–represents one of the many reasons heavy-duty engines need to be protected by a specialized mixture of chemicals.

The cavitation challenge can be traced to the combustion process. The pounding motion of a piston causes its respective cylinder to vibrate, generating bubbles in the coolant that flows around the cylinder liner. These bubbles can implode against the liner with enough force to pit the metal, eventually chewing their way into the combustion chamber.

But effective chemistry can make a difference. The addition of nitrites and nitrite-molybdates, for example, can coat the liner walls with a protective film.

“You might see some likenesses,” says Chevron-Texaco market development specialist Carmen Ulabarro, referring to several of the chemicals that are found in coolants used in car and truck engines alike. After all, Chrysler’s automotive engines still require nitrites to protect water pumps.

“But the level of inhibitors [is] generally not the same.”

Jobbers can tell the difference between an automotive coolant and its heavy-duty counterpart with a quick look at the container. A truck coolant should meet the ASTM D-6210 standard, while automotive versions will meet ASTM D-3306. There may also be a reference to a truck formula passing standards outlined in the American Trucking Association’s Technology and Maintenance Council RP-329.

The need for heavy-duty coolant formulas may seem surprising to those who sell automotive coolants to their fleet customers. But even when the traditional green formulas are used, truck owners still need to add a mixture of supplemental coolant additives (SCAs) that will meet heavy-duty needs.

Fully formulated coolants include corrosion inhibitors such as borate that buffer pH levels, phosphates to fight corrosion, silicate to protect aluminum, and nitrate to protect light alloys and steel, says Fred Alverson, a Shell Global Solutions coolants advisor. The engines also need to be protected from the scale that wants to form in the presence of high operating temperatures.

“Hard water scale can block a cooling system’s ability to transfer heat, resulting in overheating. It has been reported that only 1/16-inch of scale will reduce cooling system heat transfer efficiency by 40 percent. Scale formation can also cause localized hot spots, which in turn can cause distortion and damage the engine,” he explains.

When the all-important SCAs are not pre-mixed into a coolant, or begin to wear out, they can be topped up in the form of a liquid or through a filter that delivers small amounts of the additives over time.

A proper balance of SCAs needs to be maintained regardless of the way the chemicals are delivered. Sodium silicate may fight corrosion on aluminum surfaces, for example, but an excessive amount will form a gel that can plug various passages in the cooling systems. A heavy dose of nitrites will corrode solder. And a high level of dissolved solids will damage water pump seals.

Extended Life Coolants

The trucking industry’s Extended Life Coolants emerged in 1996, Ulabarro says, referring to the latest generation of formulas. A year after General Motors introduced its “Dexcool” Extended Life Coolants for automotive engines–marking the beginning of the end for annual flush-and-fills–the engine makers at Caterpillar released standards for extended-life formulas that could be used in the diesel domain.

“That brought the heavy-duty industry into the Organic Additive Technology (OAT) area,” she says.

Granted, the additives themselves have needed to evolve in the years since then. Strict emission-related rules required most engine makers to introduce Exhaust Gas Recirculation systems, which increased temperatures under the hood. Surprisingly, the coolant temperature itself rose by as little as five to ten degrees Fahrenheit, partly because the manufacturers also increased the overall size of the cooling systems, Ulabarro says. “But there are hot spots, and those hot spots are tremendously hot.”

The high temperatures lead to a number of challenges. The glycol itself can oxidize and generate acidic and corrosive glycolates, formats, and acetates, Alverson explains. Coupled with high flow rates and pressure drops in the cooling system, nitrites can even transform into ammonia that can corrode the soft metal in aluminum air coolers and radiators.

So far, Extended Service Interval and Extended Life Coolants have been up to the challenge. Some formulas now promise to last a million kilometres or more, and in selected cases don’t even need the addition of “extenders” midway through their life.

But the promise of universal Extended Life Coolants came to an end in 2006, when Detroit Diesel and Mercedes-Benz engineers announced they didn’t want nitrites in their formulas. Regulators in Europe were balking at the use of nitrites in automotive applications for environmental and safety reasons and these German-owned companies were following suit.

Nitrites were always a chemical fix to a mechanical problem, Alverson says. That’s why some engine makers began to address cavitation with new metallurgy, seals, liner support and cylinder block clearances. (Another fact may have been forgotten by North American groups that continue to support the use of nitrites: early OAT coolants were shown to fight cavitation without them.)

Meanwhile, the different types of solutions have presented a challenge for those who maintain the engines. Mechanics order test strips to measure the level of nitrites in Caterpillar, International, Mack, Volvo and PACCAR equipment. But these tests show nothing in Detroit Diesel and Mercedes-Benz engines. Field tests for these cooling systems are largely limited to measuring the coolant’s freeze point, which simply reflects the ratio of glycol and water.

Further confusing matters, Cummins engine makers announced that they were willing to accept coolants with or without nitrites, as long as the formulas passed a standard known as Cummins 14603. And Hybrid Organic Acid Technology formulas (HOATs) emerged with some of the benefits of OAT coolants, but still requiring the addition of SCAs.

New demands

The biggest coolant change of all may be coming within a few years, as manufacturers prepare to meet another round of emission-related standards to take effect in 2010. Most manufacturers have suggested that at least part of their solutions will involve increasing the rate of Exhaust Gas Recirculation, and that is expected to lead to even higher heats under the hood.

Today’s inhibitors may be able to handle higher temperatures, but remember that additional heat can cause glycol to oxidize–generating damaging acids in the process.

“Any OEM that understands coolant and coolant technology is already looking at different inhibitor technology or is evaluating different base fluids,” Ulabarro says.

Put another way, the manufacturers may need to consider a coolant based on something other than glycol if they want to build trucks with smaller cooling systems.

The smaller sizes could represent a significant difference for fleet owners. At the very least, the equipment would be lighter, and designers could consider new aerodynamic improvements at the front of the truck. Both of these changes could enhance fuel economy.

An increased use of water may be one of the most surprising options of all. It is quite effective at transferring heat, even if it tends to expose engines to more cavitation-related issues, Ulabarro says.

Alverson be
lieves that glycol will still be around for several years, even if other ideas are being tested in lab settings. Ionic fluids, which consist of engineered molecules, may offer potential as heat transfer fluids, but they cost $1 to $10 per gram. Nano technology has also been able to better transfer heat.

Then again, the biggest issue of all may have nothing to do with the features and benefits of the chemistry.

“If it costs more,” Ulabarro says, “it’s going to be like pulling teeth.”