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Engine Cooling - Radiator

Overview

Heat, identifiable by temperature changes, represents a form of energy. Within an engine, chemical energy from fuel undergoes conversion into heat, subsequently transforming into motion. However, this conversion isn't highly efficient; roughly 30% of the energy translates into movement, while about 50% exits through the exhaust, leaving the remainder to heat the engine. Managing excessive heat is crucial to prevent damage. Components expand when heated, and at elevated temperatures, this expansion can lead to seizing, piston and valve seat burning, and accelerated engine oil deterioration.

The role of cooling systems is to regulate engines at an optimal temperature. This allows for the design of components that tightly fit during expansion, maintaining precise running tolerances. Aligning ignition and fuel settings with the required temperature ensures clean and efficient fuel combustion. A swift warm-up is necessary to minimize unwanted exhaust emissions, as a cold engine tends to produce higher emission levels.

The 'normal' operating temperature for engine coolant is typically around the boiling point of water, facilitating efficient combustion. Minimizing warm-up time further reduces harmful exhaust emissions. Two primary types of cooling systems exist:

1. Air-cooled systems involve the direct passage of air over cylinder heads and cylinders . Fins integrated into these components increase surface area, aiding in heat dissipation.

2. Liquid-cooling systems utilize coolant to extract heat from the engine, releasing it into the passing air stream . This liquid coolant circulates continuously through a closed system, driven by the water pump's impeller. Heat is gathered within the engine and dissipated via the radiator into the passing air stream. The majority of modern cars and light vehicles utilize liquid cooling systems.

System Operations

The coolant within an engine consists of a blend of water, antifreeze, and inhibitors. Typically, ethylene glycol serves as the antifreeze, necessitating inhibitors to prevent corrosion and foaming. These inhibitors have a lifespan of approximately two years, requiring coolant changes at these intervals. It's crucial to select the appropriate coolant mixture to align with the manufacturer's specifications. Notably, aluminum-alloy engines are more susceptible to corrosion than cast-iron engines.

The antifreeze is mixed with water in a specified ratio, often a 50/50 mixture, allowing for higher engine temperatures before boiling and preventing freezing. Ethylene glycol antifreeze offers an advantage by forming a semi-solid wax solution before solidification, facilitating movement of expanding ice crystals within the water passages.

A 50/50 coolant mix elevates the boiling point to about 105°C (222°F) and guards against freezing down to -34°C (-30°F). For colder temperatures reaching -65°C (-90°F), a maximum mixture of 65% ethylene glycol is suitable. Concentrations higher than this begin to freeze at higher temperatures, hence the recommendation not to exceed 65% ethylene glycol.

Areas with 'hard' water containing calcium or chalk may cause deposits that block water passages in the radiator or water jacket when heated. It's advisable to avoid frequent topping up with tap water in such regions, considering the use of distilled water or water sourced from elsewhere.

Heat transfer within the cooling system employs three methods:

  • Convection occurs in the water jacket, facilitating internal coolant flows from the cylinder block to the cylinder head .
  • Conduction takes place through cylinder and combustion chamber surfaces, transferring heat to the coolant .
  • Radiation emanates from the radiator and cooling fins, dissipating heat to the atmosphere .

Four primary factors influence heat transfer:

  • Temperature variance between the engine and coolant
  • Temperature discrepancy between the coolant and air stream through the radiator
  • Surface area of radiator tubes and fins
  • Rate of air and coolant flow through the radiator

Modern engines favor water cooling due to its precise temperature control necessary for exhaust emission regulations. Rapid warm-up significantly reduces exhaust emissions and prevents water particle formation in the combustion chamber and exhaust during cold engine phases.

The water jacket, integral to the cylinder block and head , utilizes casting sand to shape internal water passages. Post-casting, the sand is removed through holes in the cylinder block and head surfaces.

To ensure even coolant flow to all engine areas, variations exist in the internal designs of the head and block. Consistent cooling around combustion chambers and upper cylinder walls remains vital, as these areas are susceptible to cracking and deterioration due to overheating or the use of incorrect antifreeze solutions.

Pressurization is essential in liquid cooling systems to counteract the formation of bubbles as coolant heats up. Most modern systems operate with an operating pressure equivalent to about one atmosphere, obtained by restricting air loss above the coolant in a radiator header tank or an expansion tank.

The pressure cap, traditionally fitted to the radiator but now commonly found on the expansion tank, includes key components such as the sealing ring, pressure valve, vacuum valve, and a bayonet or screw fitting. This cap maintains operating pressure, allowing excess pressure to escape while retaining the desired pressure level.

In liquid-cooling systems, the coolant carries heat to the radiator, where air flow dissipates it into the atmosphere. The fan, either electrically driven or connected to the crankshaft by a belt, assists in forcing air through the radiator. Various fan designs, including variable-pitch and viscous-hub types, have been utilized.

Advancements in coolant circulation aim for improved temperature control by mixing cold and hot water upon entry into the engine, contrasting earlier systems' cold-fill approach.

 


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