Cooling System Basics for Spark Ignition Engines
SuperFlow Advanced Engine Technology Conference, December 7, 1992
Common Misconceptions
Coolant temperatures are not an accurate indicator of metal temperatures. The coolant's maximum temperature is it's pressure corrected vapor point. The metal can be several hundred degrees hotter than the adjacent coolant.
Temperatures of critical areas must be determined by checking the metal at a controlled distance from the combustion chamber surface. This eliminates discrepancies caused by the variances in metal thicknesses.
Higher coolant flow will ALWAYS result in higher heat transfer. Coolant cannot absorb heat after it reaches it's pressure corrected vapor point. Furthermore, coolant absorbs heat at a progressively slower rate as it approaches this point.
Energy Loss
Spark ignition engines loose almost 33% of their energy input through the cooling system.
Energy loss is very simple to calculate on the dyno or the vehicle. All you need is the inlet coolant temperature, outlet coolant temperature, coolant flow and the specific heat of the coolant.
Following is a typical engine:
Inlet temperature = 180 F
Outlet temperature = 190 F
Coolant flow = 100 GPM
Specific heat of coolant = 1.0
1 HP = 5.2769885 GPM 1 F
{ (Outlet-Inlet)CS} / 5.2769885 = HP loss
{(190-180) 100*1.0} / 5.2769885 = 189.5 H
Basic Functions of the Cooling System
Peak temperature in the combustion chamber is in excess of 5000 F. Aluminum melts at 1220 F, Iron at 1990-2300 F. Therefore, the obvious primary function of the cooling system is the prevention of component damage.
However, spark ignition (SI) engines experience pre-ignition and subsequent detonation at temperatures much lower than those resulting in component failure.
Poor cooling system performance results in component damage in SI engines but, this damage is a result of pre-ignition/detonation. Not the temperature alone.
This secondary function of controlling pre-ignition/detonation is actually the most important in the SI engine.
Engines
On traditional flow configurations the block is pressurized by the water pump and functions as a manifold. The head gasket distributes the coolant through it's orifices. Block pressure must be consistent from front to rear to insure uniform coolant distribution. Low pressure will results in less flow around the rear cylinders.
Reverse flow systems pressurize the cylinder heads and bleed off through the block. Coolant gains only 1-2 F as it goes through the block. Reverse flow decreases the temperature of the coolant through the cylinder heads by this amount. The fact that steam rises complicates reverse flow systems and generally makes the 1-2 F reduction in coolant temperature insignificant at best.
The flow through each orifice in the head gasket can be determined by measuring the pressure drop across each orifice while coolant is being forced through the engine.
Coolant flow has a direct relationship to area and an exponential relationship to pressure. Meaning that when you double the area of an orifice and maintain pressure the flow doubles, but when you double the pressure and maintain area the flow is only increased by 1.414 (the square root of 2).
Strategic Flow systems take advantage of the knowledge gained through flow mapping. 100% of the coolant flow crosses the critical exhaust seat area first and is then distributed according to need to the other areas of the engine. Coolant is taken from the highest point thus eliminating the pitfalls of reverse flow systems.
Radiators
The most important criteria for any radiator is it's surface area. The thickness of the core is increased only after the surface area is maximized. Adding thickness to a radiator does not increase it's efficiency the same extent as surface area, but in no case will additional thickness alone decrease the efficiency.
The radiator becomes less efficient as the coolant outlet temperature approaches ambient. Therefore, a low flow rate keeps the coolant in the radiator longer. The longer the coolant stays in the radiator the lower the efficiency of the radiator.
Non-laminar or turbulent coolant flow must be maintained within the radiator core.
When baffles are inserted in the tanks to force the water to go through the radiator twice, the water spends the same amount of time in the radiator but must go twice the distance. Thus doubling the sped of the water.
Crossflow radiators with a fill cap always have the cap on the outlet side. Upright radiators have the cap in the inlet side and thus subject the filler cap to the pressure drop of the radiator's core in addition to the system pressure. This can lower the effective pressure of a 22 PSI cap to as low as 10 PSI.
Thermostat housing restrictors were useful when upright radiators were used with 7 lb. caps. The restrictor slowed the flow and kept the pressure in the radiator down. This prevented the cap from expelling water and causing the car to overheat. Most people wrongly assumed the car ran hot and expelled water. The cars actually expelled water and ran hot.
Hoses
Large diameter hoses with large radius bends should be used. Never use braided hoses, they will always result in higher metal temperatures.
