Engine Cooling Without The Water

New nonaqueous propylene glycol coolant formulated for improved engine performance, better cooling; "not just a better antifreeze"

Although Evans Cooling Systems Inc. has been producing performance engine components for years - company founder, chairman and CEO Jack Evans has deep roots in auto racing as an engine builder and tuner the Conn. -headquartered firm believes its most innovative contribution in the heavy-duty engine arena is Evans NPG coolant.

NPG coolant is a patented nonaqueous propylene glycol coolant with a proprietary additive package formulated to make it especially suitable as an engine coolant. It's said to be free of most of the shortcomings inherent in traditional coolants such as ethylene glycol and water (EGW).

"In some cases, one of the first misconceptions we have to dispel is that our product is just a better brand of antifreeze," said Steve Pressley vice president of Evans Cooling Systems. "Evans NPG coolant doesn't help water do its job-it eliminates the water altogether."

The nonaqueous coolant's properties are significantly different from those of other coolants or additives in popular use today, said Pressley. For example, its boiling point far exceeds a water-based coolant's vaporization or boiling points, so Evans NPG coolant never boils over, even under the most demanding conditions. It boasts a higher molar heat of vaporization and condenses any vaporized coolant sooner and within the coolant jackets, allowing it to absorb even more heat as it circulates on its way to the radiator.

While the Evans coolant is more expensive initially, Pressley noted that savings over the engine-powered equipment's service life can be significant. Evans NPG coolant is essentially a "lifetime" product - it has been tested by Evans in heavy-duty on-highway truck diesel engines to well over 500,000 mi. and 10,000 hrs. of operation, yet evidenced no chemical deterioration when it was independently analyzed. Further, Pressley said there's no need to add costly supplemental coolant additives (SCAs) or boosters required by most long-life coolants. And coolant system component life is increased, free from any additional scaling, corrosion, cavitation and electrolysis after installation.

"As an engine approaches operating temperature, the critical area hot metal surfaces of the cooling jackets start to boil the coolant that's in direct contact with them," Pressley explained. "This phenomenon is called nucleate boiling, and the coolant is operating at peak efficiency at this stage. This boiling action absorbs considerable amounts of heat as the liquid turns to vapor, most of which (in water - cooled systems) circulates out of the cooling jacket. The vapor later recondenses back into liquid in the radiator when it cools down to the appropriate temperature.

As this engine thermal cutaway shows, inside the water jacket, heat-stressed metal exceeds the thermal capacity of conventional coolant Area A shows the coolant boils, forming a vapor barrier at the metal's surface. This barrier acts as an insulator to prevent efficient heat transfer from the metal to the coolant, causing localized overheating and coolant vaporization. At Area B, Evans' nonaqueous propylene glycol NPG coolant bathes the entire cooling jacket and significantly improves coolant surface effectiveness. Heat transfers more efficiently from the metal to the coolant.

"But as the engine temperature continues to increase, especially in reaction to conditions like heavy vehicle loads, constant high-throttle settings or high ambient temperatures, the performance of water-based coolants drops off sharply," Pressley said. "A 50/50 mixture of ethylene glycol and water both at approximately 224°F in normal atmosphere, or around 255°F in the avenge pressurized cooling system.

"When EGW coolant temperatures exceed this boiling point (even though it is only at the cooling jacket surfaces of a hot engine), the vaporization rate of EGW rapidly increases. When that occurs, the vapor bubbles begin to merge and collect, forming a vapor barrier between the cooling jacket surface and the coolant. This barrier, called film blanketing, keeps a water-based coolant from directly contacting the metal surface and absorbing more heat. The result is extreme localized overheating at critical cooling locations within the engine.

"The hotter the metal, the more vapor produced, the larger the vapor barrier becomes and the higher the metal temperature rises. These 'hot spots' become so hot that they become secondary ignition points and are the causes of engine performance limitations (ignition instability) and emission problems."

This graph shows the heat flux curve. The bottom of the curve is convection or liquid to metal heat transfer. The heat transfer increases as nucleate boiling begins (Area B on engine thermal cut-away). As film boiling begins heat transfer is reduced (Area A oncut-away).

The Evans NPG coolant has a boiling point of 369°F in normal atmosphere. "The coolant reaches the nucleate boiling stage, then maintains that state indefinitely - therefore, it never film boils," said Pressley. "Increases in vehicle load, sustained high throttle-setting operation and high ambient temperatures won't elevate engine coolant temperatures to the boiling point of NPG coolant, so performance is maintained. Damaging localized overheating is prevented."

Pressley added that the coolant has lower surface tension and lower cohesive tendencies than EGW, which qualities minimizes the 'clinging' of vapor and reduces the size of the bubbles that form on the hot metal surfaces during nucleate boiling. This minimizes the vapor buildup on the metal surfaces and reduces the size of the vapor bubbles that subsequently circulate through the cooling system.

"Significantly reduced vapor bubbles also mean decreased cavitation, which, of course, is one of the better-known causes of erosion damage to components like coolant pump impellers and wet cylinder liners" he said. "The bubble volume is reduced due to the lower vapor pressure of NPG (1/35th of EGW). The smaller NPG bubbles recondense in the coolant stream sooner, turning back into a cooling liquid instead of an aerated stream of vapor and liquid, like EGW on its way to recondense in the radiator."

When water is removed from the cooling system, electrolysis is dramatically reduced, as well as corrosion and scale buildup, Pressley said. As is well-known, EGW coolant additives tend to fall out of suspension when EGW overheats or is subjected to long periods of inactivity. Evans NPG coolant is a permanent soluble blend formulated to never separate or suffer additive dropout or degradation.

Pressley also emphasized that due to its high boiling point, NPG coolant doesn't need a pressurized cooling system to suppress boiling. A closed system is recommended, however, since the propylene glycol component of the coolant attracts moisture from the atmosphere, and a closed system prevents that and also helps coolant containment. But the recommended pressure is very low, typically between 4 and 5 psi, and many reportedly use Evans NPG coolant successfully in unpressurized systems. The low, or no system pressure, results in reduced coolant component failures and longer service lives for coolant seals, gaskets and hoses, according to Pressley.

Another important advantage of NPGs propylene glycol base, noted Evans, is that it is essentially nonpoisonous, and indeed its fluid base and additives are used in such products as pharmaceuticals, cosmetics and even foods. The EPA PG rating has determined that Evans NPG coolant is essentially nonhazardous, the company pointed out. LD50 testing in laboratory rats (an effective measure of toxicity) has also shown Evans NPG to be safe.

Because the boiling point of NPG coolant is so far above that of EGW, some equipment owners attempt to use that quality to wring additional performance from their diesel engines. "Evans sells higher temperature fan switches and thermostats for certain diesel applications, which increase running temperature," Pressley said. "While the temperature increases must be moderate to hold oil temperatures to reasonable limits, the higher operating temperatures result in a more efficient-running engine, which improves fuel economy."

NPG Coolant has been tested by Evans in heavy-duty on-highway truck diesel engines to well over 500,000 miles and 10,000 hours of operation, yet evidenced no chemical deterioration.

The company also added that the higher operating temperatures result in a quieter running engine both because of the increased efficiency and the fact that the radiator cooling fan operates less often.

While the NPG coolant has been tested to 500,000 mi./10,000 hrs. primarily in Class 8 on-highway truck engines, Evans Cooling Systems said the product is equally applicable to off-highway mobile equipment, as well as stationary machinery such as diesel generating sets.

Evans said it warrants NPG Coolant in liquid-cooled diesel engines, and cooling system and related components affected by the cooling system, for 500,000 mi. or five years of operation, whichever comes first.

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