Water has excellent heat transfer characteristics but water boils at temperatures that are easily reachable in an engine. Once the water boils, its heat transfer characteristics are poor. Cooling system failure is the point at which the water in the cooling system boils. All coolants in common use (except Evans waterless coolants) are water, glycol, and additives.

The water in conventional coolants dominates their characteristics. The boiling point is raised only slightly by the glycol. Localized boiling of coolant creates volume-occupying water vapor that cannot condense except below the boiling point of water. Locations of high heat intensity boil locally, producing vapor that is nearly entirely water vapor (the ethylene glycol fraction of the vapor condenses readily, leaving the water vapor by itself). The water vapor condenses at a lower temperature than where the coolant boils. If the conditions don't exist whereby the water vapor can condense (i.e., the coolant being cooler than the boiling point of water for the pressure at that location), the water vapor remains within the system. To the extent that water vapor exists, liquid coolant is displaced, reducing heat transfer. Even though there is glycol in the mixture, the failure temperature for the cooling system is still the boiling point of water (for the pressure of the system).

Pressurizing the cooling system raises the boiling point. At sea level, a 15 psig cap raises the boiling point of water from 212° F to 249° F. At sea level, a 15 psig cap raises the boiling point of 50% ethylene glycol and 50% water from 225° F to 263° F. Increases in elevation reduce these boiling points. The water-based coolant in a typical heavy-duty engine cooling system operates very close to its boiling point.

THE CONSEQUENCES OF OPERATING A COOLANT NEAR ITS BOILING POINT:

There is reduced heat transfer at critical locations in the cylinder head because the presence of water vapor insulates the hot spot. The coolant pump may be operating near the area of incipient cavitation. The pressure drop at the vortex of the coolant pump may cause flash vaporization of coolant at that location. Some of the vapor condenses violently within the impeller chamber, leaving the evidence of cavitation erosion. (The vibrations of pump cavitation also damage pump seals.) Some of the vapor, entrained in the coolant, makes it through the pump, reducing the efficiency of the pump and reducing the density and the heat transfer capability of the coolant.

Vibration at the cylinder liners creates an instantaneous low pressure followed by an instantaneous high pressure at each cycle of the vibration. When the coolant is near its boiling point, the low-pressure part of the cycle causes the instantaneous creation of vapor that collapses in the high-pressure part of the cycle. Cavitation erosion is evidence of this phenomenon.

Recently the cooling of exhaust gas for recirculation (EGR) has become an additional load to heavy-duty cooling systems that are already operating close to the boiling point of the coolant. Additional stress from high ambient temperatures and mountainous terrain can cause a cooling system failure. Today’s solution from the manufacturers is to de-rate the engine power before the cooling system fails. Power de-ration causes delay and inconvenience to the operator.

Evans NPG+ waterless coolant is a blend of glycols, soluble corrosion inhibitors, and no water. It boils at 375° F and the freezing point is -40° F. In contrast to water-based coolants, the operating temperature of NPG+ is substantially below its boiling point. The following chart of the boiling points of NPG+, 50/50 EG/water, and water vs. elevation illustrate that difference:

Note that normal coolant temperatures are close to the boiling point of water but are much colder than the boiling point of NPG+.

ADVANTAGES OF THE LARGE SEPARATION BETWEEN COOLANT OPERATING TEMPERATURE AND THE COOLANT BOILING POINT AFFORDED BY THE USE OF NPG+ COOLANT.

There is no vapor in the cylinder head. Any localized vapor condenses immediately into surrounding fluid, preventing vapor from insulating hot metal from liquid coolant. Control of metal temperatures is maintained at all times.

There is no pump cavitation because the lower pressure at the inlet to the pump won’t reduce the boiling point enough to cause flash vaporization (unless the inlet path to the coolant pump is unduly restricted).

There is no cylinder liner cavitation erosion because the vibration of the cylinder liners doesn't make vapor during the low-pressure part of the cycle and hence there is no collapse of vapor during the high-pressure part of the cycle.

A RESERVE CAPACITY is created. Under conditions of particularly high thermal stress, the coolant will run hotter but the cooling system will not fail and metal temperatures will remain under control.

On shut-down there is no afterboil, regardless of the thermal stress on the engine before shut-down.

Changes in elevation have negligible effect on the operation of the cooling system.

The cooling system will not fail regardless of load or ambient temperatures. There is no reason to de-rate engine power for reasonably higher coolant temperatures because metal temperatures will remain under control regardless of the coolant temperature.

With greater heat loads than ever before, truck manufacturers struggle with the daunting task of trying to keep the cooling system coolant below the boiling point of water to prevent cooling system failure. The fans move more air than ever before at the cost of using more horsepower (and fuel) than ever before.

The way to save money is to install Evans NPG+ coolant and then keep the fan turned off at temperatures below, say, 230° F.

The way to maintain performance is not to de-rate power unless the coolant exceeds a reasonable temperature, say, 260° F or so.

Buying your new truck with NPG+ installed by the factory is recommended. Along with the NPG+ we recommend that you specify the 230° F fan turn-on and also the increased de-rate temperature. In that way you will have the best package for fuel savings and performance. NPG+ can also, of course, be retrofitted.

Go to NPG+ overview to read about other features of Evans NPG+.

For coolant purchases, order toll free at (888) 990 2665 or online, using our secure ordering page.

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Copyright © 2008 Evans Cooling Systems, Inc. Page last revised 06/04/2008