Before we begin, it's important to note that what follows is simply a collection of thoughts and information intended to further your thinking about the importance of creating a balance between engine heat and power. There is no intent to advocate one type of cooling system over another. However, we think some interesting notions will come to the surface. Probably the best place to start is by concentrating on an engine's combustion space. As is the case with a majority of engine modifications, this is the spot where changes to peripheral components will be reflected. The influence of cooling system efficiency can also be measured in this space. So with that in mind, let's examine a few essentials.
Last month, we touched on how an inefficient cooling system can allow (or promote) both pre-ignition and detonation. We included the fact that volumetric efficiency (essentially torque output) can also be affected, both positively and negatively, by the control of pre-combustion, air/fuel charge mixture temperatures. Specifically, hotter such charges tend to create combustion-related problems, cooler ones can enhance torque output. But in addition to these influences from the cooling system, problems can arise from overheated lubricants. Excessive heat leads to the physical and chemical breakdown of oil and a corresponding decrease in film strength. In turn, this condition can promote increased losses from friction horsepower, over-stressed parts such as pistons, and subsequent failure of other components.
This plot of heat rejection as a function of air/fuel ratio demonstrates how enrichment af
Overall, and as you might expect, there are certain engine conditions that create equally certain cooling system requirements. One is mechanical compression ratio. At or near TDC during the power stroke, we know that a greater expansion rate of combustion gases occurs. Any increase in mechanical compression ratio will thus lead to higher cylinder wall temperature, during this time. However, near BDC on the same power stroke when additional cylinder wall area (surface area) is exposed to the expanding combustion gases, there is generally a reduction in exhaust gas temperature, as the exhaust cycle begins. What's critical is that any increase in coolant temperature early in the power stroke correspondingly increases engine sensitivity to spark timing and detonation, thereby placing additional demands on cooling system efficiency in this particular area of the engine. So, if there was a way to increase cooling efficiency or reduce coolant temperature in this location, a chance to raise compression ratio and/or spark timing (without a higher risk of power loss or parts damage) could result. You may want to remember that for later.
In addition, we also know that overall combustion temperatures and burn rate are a function of air/fuel charge ratios. As these ratios fluctuate, even within a given combustion space or any combustion cycle, they affect the temperature differential between the combustion space and surrounding coolant system, again in that area. Keep in mind that at any given rpm, piston speed (cycle rate) is unchanged from cycle to cycle. But as air/fuel ratio changes, so does flame rate, resulting in differing amounts of surface area over which to "spread the flame," so to speak. At least in theory, such changes place different demands on the cooling system, in that particular area. Don't worry, we're coming back to this.
Under these conditions, especially when elevated combustion temperatures are involved, spark timing can be used as a crutch to prevent damage. But this can be problematic. As an example, if air/fuel charges are poorly mixed, an increase in spark timing to burn the effectively "leaner" mixture will add negative (pre-TDC) work to the process while increasing cooling requirements. Less spark timing adds heat to the cooling system because of cylinder pressure and gas expansion further beyond when TDC occurs. Finally, as power or engine speed is increased (all else being equal), we can expect additional heat to be rejected to the cooling system. Of course, if we increase piston displacement, additional combustion surface area also increases, thus more surface area over which to spread combustion temperatures. From this we can conclude that large displacement engines will lose less heat per unit of combustion surface area than smaller displacement engines.
For purposes of this discussion and generally true, about 50 percent of an engine's total combustion goes to the cylinder heads and valve seats, 30 percent to the cylinder walls and 20 percent to the exhaust ports. And it's been determined that about 50 percent of all engine heat is linked to the compression, combustion, and expansion cycles in the total combustion process. The point here is that just keeping an engine operating in the proper range of temperature in a number of locations is not only critical to performance, but overall durability. Given the fact that operational temperatures vary depending upon a particular location in the engine, the circulation, direction, and duration coolant is allowed to reside in such areas becomes almost a science unto itself. To better understand how this is reduced to practice, let's examine the three basic ways coolant can be introduced and circulated through an engine.
Suppose we define the "normal" coolant flow path as follows: coolant enters the lower portion of the cylinder block from the lower portion of the radiator or heat exchanger, passes upward to the cylinder heads and out into the upper radiator tank (or heat exchanger). Either a thermostat or (restrictor orifice) is used to retain coolant in the engine until some desirable temperature is reached, after which flow through the entire system is completed, as driven by a coolant pump. Further, in this type system or any other, bubbles will form in the coolant (as it is exposed to heated metal surfaces), depending upon the boiling point of the coolant and pressure in the system. This condition is called "nucleate boiling" as evidence that the coolant is absorbing heat. If this condition continues to higher temperatures, steam "pockets" that create separation between the heated surfaces and coolant, leading to localized loss of coolant efficiency and the potential for damaged parts. Space doesn't permit further discussion of this condition, but suffice to say that coolant boiling point, flow rate, and system pressure combine to effectively absorb, remove, and control engine heat. Note that this direction of coolant flow provides "pre-heated" coolant to the hottest part of the engine (the cylinder heads), having first passed through the heated cylinder block. This condition tends to place limits on mechanical compression ratio and spark timing, as you will see in a moment.
The second method of flow introduces coolant first into the cylinder heads, after which it passes downward through the cylinder block and out into the lower portion of the radiator. Straightaway, you can see that this approach should allow more freedom in setting limits on compression ratio and spark timing. Combustion efficiency and the range of combustion temperatures allowed integrate right into this method. Early experimenters with reverse flow systems experienced problems determining how to deal with elevated pressures and temperatures in the upper portion of the cooling system. However, through proper venting techniques, reverse flow systems have been used in both production and racing engines. Last, a version of the reverse flow method whereby coolant enters the lower portion of the cylinder block, passes upward through the cylinder heads, travels back into the upper portion of the block and exits the engine from the upper part of the cylinder block has been labeled a "modified" reverse flow system. While the approach doesn't appear to provide all the benefits of a true reverse flow path, it does improve upon some of the drawbacks inherent with a normal flow system.
Admittedly, there is far more that could be discussed regarding these three approaches to coolant flow direction and its attending benefits and drawbacks. But we wanted to at least include a quick review of them as part of the importance in striking a balance between power and the ability to properly control the heat that's unavoidably associated with making good horsepower.