The Boiling Cycle

The coolant used in an engine undergoes a boiling phase that is required to remove heat from the cylinder head. Up until very recently the premise of most cooling system designs was to eliminate or not allow boiling to occur. In simple terms the idea of having boiling coolant in an engine was thought of as the result of either poor engineering or a component failure. Most readers are familiar with the dreaded "boil over" that would occur and leave motorists stranded on the side of the road waiting for the engine to cool and figuring out how they would transport water from a nearby stream or lake into their vehicle's radiator.

When it comes to boiling it is true that it is not desirable to have this occur in the radiator. The radiator's job is to cool the liquid while the liquids task it to cool the engine. If the radiator of an engine is boiling over it means the coolant is becoming superheated and turning to a vapor and when it re-condensed it contained too much heat for the radiator to dissipate. So as you can see we need to identify when it is good and sound engineering to have the coolant boil.

Before this topic can be explored it needs to be recognized that the load on the cooling system and the coolant itself is not the same under all driving conditions. At idle and light load such as cruising down a highway, the engine is not required to produce much power. The amount of power required to move the vehicle at 100km/h is only a fraction of what the engine can produce at maximum output. On the average a typical older engine only requires 25 to 30 horsepower to cruise on level ground at 100km/hr. Since an internal combustion engine is nothing more than a heat pump, the thermal load the liquid coolant and the radiator is exposed to be proportional to the heat of the power produced. If the same vehicle is asked to climb a long grade at a steep angle, pull a trailer, or produce maximum power for passing or racing, then the load on the coolant and radiator increases.

A coolant must work to meet the transient needs of the engine. Many falsely believe that water is the best coolant since it has a specific heat of 1. That would be true if the engine that it was used in was going to do nothing but idle or never produce anywhere near maximum power during its lifetime. With a specific heat of 1 and good thermal conductivity qualities, water does an excellent job of cooling an engine. That is until the load is increased and the coolant starts to boil in the cylinder head. Never mind the fact that water creates rust and corrosion that insulates the liquid from the water passages, diminishing thermal transfer in the engine and radiator. When this happens in the engine the water would stay cooler since there is less heat rejected into it but the metal surface temperature of the cylinder head would be much higher. In addition, water offers no lubrication properties for the pumping mechanism (water pump) and will freeze when cooled.

Traditional anti-freeze that is a mix of EG and water solves some of the problems of pure water but brings along its own set of issues. Water by itself has a very high surface tension and when it does boil it is harder for it to release and re-condense back to a liquid. Thus is has limited ability to remove heat from the boiling point area and follow the laws of physics.

When water is employed as a coolant, a use that is common with competition and especially drag race engines, the flow is usually restricted. The theory being, "slow the water down so that it has time to cool off in the radiator." This is an inaccurate response to the use of a poor coolant. To limit boil-over the higher pressure is required to raise the vapor point of the water to try to keep the cylinder head cool. When a restrictor is placed in the flow path the system pressure increases and thus, the boiling point. In tests conducted by independent sources some race engines had pressures as high as 60 psi or greater in the rear of the cylinder head because of the use of a flow restrictor. The engine builder thought they were controlling heat by reducing flow but in actuality were creating such a high pressure to make up for the deficiencies of water as a coolant, even when treated with chemicals that are suppose to enhance its ability. What is usually not taken into consideration is the added horsepower required to drive the water against 60 psi of pressure that the restrictor created. These are all Band-Aid efforts to make up for the poor qualities of water as a coolant in anything other than an engine that is going to idle and produce only a minimum amount of its rated maximum power.

It is advantageous for the coolant to boil in the cylinder head but it needs to refrain from undergoing a phase change for as long as possible for it to be the most effective. Then it must release easily so it can move from that spot and take heat with it. Then it is required to re-condense quickly and be pumped with ease. Once it enters the radiator is must possess enough thermal dissipation to drop in temperature and be ready for the entire event again.

As the coolant is asked to work harder boiling is occurring in the cylinder head water jacket. Thus, the anti-freeze can actually wear out. This is a result of the additive package becoming neutralized and consumed from the high temperatures and constant phase change that is occurring.

Evans Waterless Engine Coolant has a lower surface tension than EG/water, boils at 190 degrees C at atmospheric pressure allowing the coolant to remove more heat from the cylinder head and eliminates all water and the possibility of corrosion, along with cylinder liner cavitation in wet sleeved race engines such as the big bore LS series.

Even with a 15 psi pressure cap water will boil at 124 degrees C (water/anti-freeze will boil at around 128 degrees C at 15 psi), Evans has an advantage of over 60 degrees C.

When choosing a coolant for a high performance engine you need to consider the worst-case conditions, not idle or light load. Traditional coolants are inferior when the load/horsepower increases.

For more information visit www.evanscoolants.com.au