As an engineering professor for more than 40 years, my father often told me that the only difference between a difficult problem and an easy one is knowing the correct answer. While that axiom can be applied to a variety of situations, it certainly rings true for our subject of discussion this month: proper cooling of a race engine.
A principle problem in using a liquid to cool internal combustion engines is the fact that coolant temperatures vary within the cooling system. This is generally the result of differences in the locations of heated areas that contribute to unequal temperatures. For example, metal surfaces nearest the combustion space and exhaust passages tend to be higher than elsewhere in an engine's cooling system. While reverse flow systems address certain aspects of temperature extremes and levels, localized "hot spots" can still contribute to detonation, lost power, and potential parts damage.
Of the problems encountered when attempting to provide engine cooling by the use of water or ethylene glycol and water (EGW), a condition can develop called "nucleate boiling." When the localized temperature of a heated internal surface in an engine exceeds the temperature of the coolant at that location, nucleate boiling of the coolant occurs at that spot, creating what we'll call "vapor pockets" at these locations. When these conditions are created, a form of thermal insulation develops at these spots that further contributes to parts damage. Such isolated conditions of boiling can (and frequently do) result in detonation and the parts-damaging effects that result. This is especially true in racing engines that produce high levels of horsepower and, consequently, place higher demands on the cooling system than in production, over-the-road powerplants.
Dennis Wells (Wells Racing Engines) recently commented about this issue on fuel-injected motors. "We frequently run into situations where certain cylinders appear to be in periodic or continuous detonation while others in the same engine are not. We've tried relating this to possible differences among intake paths (intake manifolds and cylinder heads) and have spent considerable time on the airflow bench balancing the flow from port to port. While this has helped, we still experience the detonation problem from time to time. As I understand what it means, nucleate boiling can be a major factor in preventing proper cooling of localized hot spots in an engine. The fact that we run identically calibrated injectors and still see the problem is further evidence that it's a cooling issue."
As I recall, Smokey was once engaged in trying to find ways of addressing localized hot spots by rerouting coolant flow, changing the design of coolant passages and pursuing a path that involved exploration of reverse-flow cooling. With regard to the latter, it was his contention that by introducing cooling fluids initially at a lower temperature around the combustion chamber and exhaust port areas (a benefit of providing coolant to the top portions of an engine first), a higher rate of heat transfer would occur in the areas of localized hot spots, thereby reducing the tendency toward the formation of vapor pockets (nucleate boiling) and subsequent detonation.
While Smokey encountered some coolant pressure- and system-related problems that were not (at the time) resolved, he remained convinced that the reverse flow concept would help reduce the problems of detonation. In fact, he observed this improvement even in the course of not completely solving the other problems. It remained for Jack Evans (Evans Cooling Systems) to resolve the related problems on his own, sometime after Smokey had abandoned his project. Concurrent to solving the problems Smokey encountered, Jack developed his "waterless" (non-aqueous) coolant chemical line of products sold today.
Further to the argument and in addition to conditions leading to detonation, prolonged nucleate boiling can also cause metal fatigue and ultimate parts damage in the form of mechanical failures. In particular, when the temperature gradient across a given section of metal (cylinder heads or block) is both extreme and under rapid change, structural fatigue is accelerated to the point of possible parts failure. The use of non-aqueous coolants addresses these issues in a number of ways.
Notably, this type coolant has a much higher boiling point than pure water or an EGW blend. As a result, it directly impacts an engine (particularly racing engines) in terms of how the cooling system affects nucleate boiling. Stated another way, a higher boiling point enables improved control of localized hot-spots that could otherwise develop without employing this coolant feature. A coolant displaying a higher boiling temperature discourages nucleate boiling by reducing the tendency for hot spots to develop, based on a higher level of heat absorption.
An additional benefit of a non-aqueous coolant is that less system pressure is required to maintain high-temperature control. Since such coolants exhibit higher boiling point temperatures, elevated system pressure (higher than atmospheric) typical of water-based coolants is not necessary. As an example, a version of Evans' coolant products exhibits a boiling temperature of 350 degrees F. Even in a high-output racing engine, based on its increased level of thermal conductivity, the ability of such a coolant to control engine temperature at near atmospheric pressure reduces other system problems associated with water-based coolants. It is said that "locally generated vapor" (at otherwise hot spots) is transformed back into liquid within the coolant, preventing the development of a vapor layer at the locations of high-temperature engine parts.
When you step back and examine the various aspects of high boiling point coolants, other benefits come to mind regarding related engine components. Radiator size and cooling capacity may be downsized. The possibility of dialing in slight increases in spark-ignition timing, if acceptable to the engine, is an option for additional power. The criticality of gasoline octane requirements can be reduced. Oil temperatures and the need for extreme temperature control may be decreased. And these are but a few possibilities worth considering. But the overriding point here trails back to a comment made at the outset of this column; simply knowing the answer to a problem takes it out of the "difficult" category. My father was frequently correct about a number of perspectives.