Maintenance

In all engines hot metal in contact with coolant causes localized boiling called nucleate boiling at critical metal temperature locations in the engine. Nucleate boiling is a very efficient way to remove heat as the heat of vaporization is so high. This boiling forms vapor which is later recondensed back into liquid when the vapor reaches the appropriate temperature. For ethylene glycol and water (EGW) systems, the recondensation of vapor takes place generally in the radiator. Since vapor by volume from a 50/50 solution of EGW is more than 98% water vapor under one atmosphere of gauge pressure (14.0 PSIG), the water will not recondense until the temperature of the coolant is below the boiling point of water at the system pressure. During moderate loads and ambient temperature conditions, that temperature is normally seen inside the radiator. As the temperature of the coolant rises under stressed conditions, that vapor does not recondense even inside the radiator. Most engine designers and test engineers are unaware that vapor is in fact being generated and recondensed continuously inside the engine cooling system.

As a result of localized boiling, there is a layer of vapor which can build up on the surface of the hot metal within the coolant jackets. That layer keeps the coolant from coming in contact with the hot metal surface. The temperature of the metal covered by the vapor pocket increases, causing a "hot spot". The hotter the spot, the more vapor produced, the larger the vapor pocket becomes, and the higher this critical metal temperature rises. These "hot spots" become so hot that they become secondary "spark plugs" or ignition points and are the cause of engine performance limitations (ignition instability) and emission problems. Thus it has been an important goal of the Evans Cooling System to reduce the vapor build-up on the hot metal surface and reduce or eliminate "hot spots".

Vapor, which is created from localized boiling, actually affects the cooling efficiency of the engine. Large amounts of vapor in the cooling system decrease the amount of liquid to metal contact throughout the cooling system, reducing the ability of the cooling system to remove heat.

In addition as the engine and cooling system is used under stressed conditions or in higher ambient temperature locations, coolant temperatures typically rise above 220° F. As EGW coolant temperatures increase above 220° F, the vapor which is generated cannot be recondensed efficiently inside the system and can be seen as cloudy coolant. Often at about 220° F the pump starts to cavitate and the flow rate of the coolant starts decreasing , increasing further the temperature of the coolant. This results in additional cavitation and the loss of coolant through overflow vents. Evans has developed computerized models of EGW cooling systems which generate vapor tables plotting this phenomenon. These theoretical vapor tables track empirical test data very accurately and are proof that vapor is constantly being generated and recondensed. The vapor tables also allow for accurate design predictions of system components size requirements identified during dynamometer testing.

In examining the vapor generation it became apparent that water is the reason for such a high amount of vapor production within the engine with resultant "hot spots". Water is the cause of cavitation. Water is the reason for requiring pressurized cooling systems to elevate the acceptable operating coolant temperatures above the boiling point of water. Even so the coolant temperatures cannot exceed 224°F for pressurized water. Therefore the use of water as a coolant requires adding poisonous ethylene glycol to raise the pressurized boiling point to 250° and decrease the freezing point. Water has been found to be the reason that additives used for corrosion deplete and "fall out", causing limited coolant life. Water is also the cause of corrosion of parts inside the cooling system and in some systems the resultant accumulation of high concentrations of lead and other heavy metals in the coolant after prolonged use. The solution was to remove the water from the coolant.

In choosing the proper replacement coolant Jack Evans, the inventor, attempted to solve a number of problems: the toxicity/waste stream environmental issue, the cavitation issue, the corrosive coolant issue, the heavy metal deposit issue, the depletion of additives issue, the liquid to metal contact or "hot spot" issue and the overheat issue.

Non-Aqueous Propylene Glycol (NPG) with additives to protect metal surfaces was chosen as the replacement liquid. Because of the specific heat and specific gravity differences between NPG and EGW coolants, it is theoretically necessary to increase NPG's coolant flow approximately 27% over that for EGW to remove equal amounts of heat from the engine. In actual application however, where current cooling systems produce significant amounts of vapor, less flow increase can provide the same, and even increased, heat rejection. Since there is no water in the system to cause cavitation of pumps, the increased speed is easily achieved. The flow can be further increased to provide even better cooling of the engine. The physics of why NPG cooling allows for higher engine performance can be best understood by looking at how the vapor is managed.

Bubble Size: The size of the bubbles formed on the hot metal surface, which then break off into the liquid, directly affect the size of the vapor buildup on the metal surface. Nucleate boiling produces bubbles, the size of which depends on a liquid characteristic known as surface tension. Lower surface tension and directly proportional cohesive characteristics produce smaller surface layer bubble sizes. NPG has lower surface tension and lower cohesive tendencies than EGW.

Another fluid characteristic which works in favor of decreasing bubble size is the difference in vapor pressure. The vapor pressure of water is 100 times that of NPG (vapor by volume from a 50/50 solution of EGW is more than 98% water vapor under one atmosphere of gauge pressure).

The more turbulent flow of the NPG system produces shear forces which tend to shear bubbles into smaller bubbles at the metal surface.

Heat of Vaporization Cal/Mole: Another characteristic, which determines the amount of vapor generated in changing a liquid to a gas when a given weight of liquid changes to a vapor, is called the Heat of Vaporization. When the heat transferred from the hot metal surface vaporizes liquid it does so according to the heat of vaporization. NPG has a heat of vaporization of 12,500 Cal/Mole compared to 9,720 for EGW. Simply stated, each vapor bubble of NPG coolant carries 29% more calories (heat) than a vapor bubble of EGW coolant. Therefore NPG generates less vapor by volume and will displace less coolant from the surface than will EGW for the same amount of heat transferred.

Reduction of "hot spots": Obviously if the vapor bubbles condense back into liquid rapidly there is less vapor traveling through the cooling system. Less vapor means higher metal to liquid contact. The fact that NPG generates less vapor for the same heat transfer helps here also (See Below; "h Molar Heat of Vaporization:").

Compared to NPG, water vapor from the EGW condenses at a lower temperature and hence is not fully condensed until it is in the radiator. However the temperature of NPG in the cooling system is considerably below its saturation temperature (boiling point), readily condensing NPG vapor back into the liquid locally. Evans has been able to ensure that all NPG vapor generated inside the engine rapidly condenses back into liquid before the coolant leaves the engine.

Small bubble sizes assists here also as the smaller the bubble the lower the ratio of vapor volume to bubble surface area (the recondensation occurs at the liquid/gas interface, the surface of the bubble).

Reduction of "hot spots" & turbulent coolant flow: Turbulent flow of the coolant increases coolant scrubbing of the vapor from the surface of the metal, thereby improving the wetting of the metal surface by the coolant.

Other Technical Considerations:

• Boiling Point: 369° F for NPG versus 224° F for 50/50 "EGW" ethylene glycol and water (at atmospheric pressure - 0.0 psig) - benefits include elimination of afterboil and overheating, allowing temperature excursions above those for EGW, faster recondensation of vapor inside the engine, low (2.0 - 4.0 PSIG) or non-pressurized system, no coolant loss operating in high ambient temperatures, and the capability to increase thermostat temperature settings if desired.
• Molar Heat of Vaporization: 12,500 Cals/Mole for NPG versus 9,720 Cals/Mole for EGW - benefits include faster recondensation because less vapor is produced, and a reduction of hot spots because of improved liquid to metal contact. All of which eliminate the occurrence of "Film Boiling" and the accumulation of excessive surface vapor.
• Surface Tension: 35 Dynes/Cm for NPG versus 56 Dynes/Cm for EGW -- benefits include small vapor bubble sizes, allowing for faster recondensation of vapor and increased liquid to metal interface, and decreased area of nucleate boiling centers, again increasing liquid to metal interface.
• Freezing Point: -70° F for NPG versus -38° F for EGW. NPG does not freeze, it crystallizes and supercools (contracts slightly and becomes a viscous slurry).
• Toxicity: EGW is considered a hazardous waste whereas NPG is not as PG is used as a food additive and pharmaceutical base fluid.
• Vapor Pressure: 590 mm of Hg for EGW at 212° F versus 18 mm of Hg for NPG. This is the major reason for the dramatic decrease in cylinder liner and pump cavitation. Although most vehicles overheat at EGW coolant temperatures of approximately 250° F (pressurized to 13.0 psig), the non-aqueous coolant can tolerate temperatures above 350° F. Although using higher coolant temperatures can introduce other problems, (i.e.: increased oil temperatures) the NPG will allow the possibility of increasing coolant temperatures with all the resultant performance improvements as those problems are addressed and resolved. EGW is temperature constrained only by the physics of the liquid.

Over the years engineers have solved many of the problems of using EGW at the limits of its physical properties. The same can be expected to happen with NPG, allowing full use of NPG's high boiling point. Currently, however, most all NPG conversions are operated at traditional thermostat settings (180° - 200°F) with the high temperature capabilities of NPG utilized as a "safety measure".

Important Benefits of NPG Coolant:

For Gasoline Engines:

• Higher Gasoline Efficiency
• Reduces Emissions
• Higher Compression & Power
• Knock Reduction
• Improved Octane Tolerance (lower octane fuel usable).
• Reduction of Hot Spots (Critical Metal Temperatures)

For Diesel Engines:

• Higher Fuel Efficiency
• Lower Particulate Emissions
• Higher Power
• Reduction of Hot Spots (Critical Metal Temperatures)
• Elimanation of overheating and after-boil
• Elimination of Cylinder wall and pump cavitation
• Elimination of corrosion on cooling system parts
• Significant Reduction of Coolant Leaks; NPG operates at a low (i.e.; 4.0 - 7.0 PSIG ) or atmospheric pressure.
• Not a Hazardous or Dangerous Waste.
• Long Life, Stable Coolant. Increased from 40,000 (with EGW) to more than 400,000 miles. The system has been tested to 400,000 miles in a Class 8 Detroit Diesel engine running at North American Van Lines. After 400,000 miles additives have decreased by only 11%, still within initial manufacturing tolerances for the coolant.
• Fleet applications: decreased maintenance requirements and costs.

Secondary Benefits of NPG Coolant:

For Gasoline Engines:

• Non-pressurized: (or low pressure, i.e. 4.0 psig) decreased leaks, lower pressure parts, decrease of thermal flexing or cycling (component life extended), elimination of accidents resulting from accidental removal of radiator caps from hot engines.
• Allows for a totally closed system (Hermetically Sealed) requiring no service checks and is not subject to contamination.
• Improved stability of engine operating temperatures.
• Improved aerodynamic styling. The radiator no longer needs to be higher than the engine and can be placed anywhere.
• Weight reduction possible if higher coolant temperatures are used. Smaller radiators, less coolant, light-weight metals (such as magnesium for engines), small cooling jackets in the engine, smaller fans.
• Decreased duty cycle of coolant fan for the same coolant temperature by allowing for higher temperature excursions for short intervals with no adverse effects on the engine.
• Faster combustion chamber metal surface warm-up, CO reduced in start-up (liners get hot faster) mostly because of lower specific heat of cold NPG.
• Elimination of premature spark plug failure and head cracking by better cooling of head.
• Reduction or elimination of pre-ignition and detonation:
  • Reduce head distortion and cracking at high compression and supercharged / turbocharged boost levels.
  • Reduce head gasket fire ring failure.
  • Reduce piston dome and ring failure.
  • Reduce valve face sinking ("tuliping").
  • Reduce rod bearing failure (caused by cylinder pressure, detonation related, spikes).

For Gasoline Engines:

• Non-pressurized (or low pressure, i.e. 4.0 psig) system provides fewer leaks, lower pressure parts, decrease of thermal flexing or cycling (extended component life) and elimination of accidents resulting from accidental removal of radiator caps from hot engines.
• Elimination of Cylinder Liner Cavitation allowing for reduction of thickness of cylinder liners with the following benefits:
  • Weight and critical engine dimension reduction.
  • Better cooling of piston cylinder wall surfaces.
• Totally closed system requiring no service checks and no contamination.
• Weight reduction if higher coolant temperatures are used with smaller radiators, less coolant, smaller cooling jackets in the engine, and smaller fans.
• Decreased duty cycle of coolant fan for the same coolant temperature by allowing for higher temperature excursions for short intervals with no adverse affects on the engine.
• Faster combustion chamber metal surface warm up of cylinder liners & combustion domes provides lower emissions, improved gas mileage.
• Eliminates frequent maintenance checks of coolant additives and subsequent adjusting of additive levels.

• Reduction of coolant disposal costs as no coolant needs to be replaced (limits of coolant life have not yet been found. Some vehicles have been tested up to 500,000 miles).