Thermodynamics of water cooled engine

I have no education or expertise on internal combustion engines, but it seems to me that the basic theories of thermodynamics, particularly heat transfer theory, should apply to our water cooled engines.

I was trained by the US Navy as a nuclear reactor operator. The intensive training included 6 months of physics classroom instruction, with a heavy emphasis on fluid dynamics, thermodynamics and heat transfer theory. Nuclear powered ships differ only from conventional ships in the source of heat to generate the superheated steam to drive the propulsion turbines. The rest of the engine room is pretty much the same regardless of the heat generation source.

With a conventional ship, an oil-fired boiler creates the steam. With a nuclear ship, the nuclear reactor heats water flowing through the reactor in a sealed 'primary' system, which then heats the water in the secondary (steam generating) conventional system via 'heat transfer'. In other words, the sealed primary heat generating system is physically isolated from the secondary system and depends entirely on heat transfer to create steam for the turbines.

This is the same principle, albeit on a much simpler application, that is behind the cooling system of water-cooled internal combustion engines. The heat created by the internal combustion process transfers thru the engine block to the water flowing through the water jacket. This water continues outside the engine through hoses to a radiator which then releases this stored heat energy to the ambient air via another heat transfer process. The water is moved through the system by a pump, which is typically driven by belt or gearing off the engine.

If the water flow stops, or slows, less heat is moved from the engine to the coolant water, causing overheating of the engine. If the air flow through the radiator is slowed or stopped, then overheating of the coolant water results, since it cannot release its stored heat energy fast enough. To create air flow over the radiator fins when the vehicle is not moving quickly enough down the road, modern engines typically employ one or more electric fans to create air flow. If these fans don't work, or are not adequate for moving sufficient quantities of cool air through the radiators when the vehicle is moving slowly or stopped, heat buildup in the coolant system can cause an overheating situation.

So, the coolant system's effectiveness depends on both adequate coolant flow AND adequate air flow for the amount of heat being generated by the engine. If the heat energy created by the engine increases beyond the amount the cooling system was designed for, then overheating will result. The effectiveness of heat transfer is also affected by the difference in temperatures between the surfaces of the heat transfer medium. So, when ambient temperatures are high, the radiator fins are less effective at transferring heat and the coolant water heats up. Typically, this will cause a thermostat to open, increasing water flow through the system. Once the thermostat is fully open, the system has reached its maximum cooling capacity and further increases in engine temperature or ambient temperature will cause overheating.

Heat buildup in a closed water cooled system will cause an increase in water pressure until the pressure relief valve (radiator cap) opens, releasing superheated water which instantly flashes to steam upon encountering the much lower atmospheric pressure. Atmospheric pressure is lower with increased elevation, so the relief valve is more likely to blow at higher elevation, all other factors being constant.

If the temperature gauge indicates an engine overheating situation (and the temperature gauge is reading accurately), then at some point, one of several failsafes will actuate to protect the engine. There is the ECU computer, which can reduce engine power to reduce heat generation to a level that can be handled by the cooling system. The thermostat will open to max coolant flow. The fans will kick onto high speed (even though the vehicle may be moving quickly down the road). Finally, the relief valve will blow, releasing all the coolant water. I'm pretty confident the ECU would shut down the engine before that happens.

So if the coolant water flow is impaired, overheating should occur at ANY vehicle speed, because the hot water can't release its energy via the radiator, regardless of how much air is moving past the radiator fins. This can occur with pump failure or clogging of the water jacket or clogging of the radiator coolant passages.

If the air flowing past the radiator fins is insufficient for effective heat transfer from the coolant water to the ambient air, overheating will result, although depending on the cause of inadequate air flow, this condition will self-correct with increased vehicle speed. Air flow that is inadequate for conditions can result from one or a combination of; dirty radiator fins; excessive ambient temperature; malfunctioning fans.