Cooling System Components

The following components are part of the engine cooling system:

• Engine block, with integral water jacket

• Cylinder head, with integral water jacket

• Bypass system

• Water pump

• Thermostat

• Thermostat housing

• Radiator

• Expansion tank

• Pressure relief cap

• Cooling fan

• Pipe-work

• Coolant

Engine Block

When an engine is to be liquid cooled, the designer ensures that the engine block has sufficient and efficient flow path for the cooling liquid. This allows coolant to circulate around the areas of the engine where combustion heat is found, which is mainly in the region of the cylinder liners and the combustion chamber in the cylinder head assembly. This flow path is called the water jacket and normally is cast into the engine block in the production process. As stated, the main considerations are that the coolant must surround the cylinder liners to absorb heat that is conducted through the cylinder walls from within the cylinders. There must be provision for a coolant pump to be housed within the flow path or water jacket. At the top surface of the block, where the cylinder head is to be mounted, there must be sufficient paths to allow the coolant to pass to, and to return from, the cylinder head.

Cylinder Head

The main function of the coolant within the cylinder head is to remove heat from around the cylinder head combustion chamber area. A position for the engine thermostat also may be included in the design of the cylinder head.

Bypass System

The bypass system allows rapid warm-up of the coolant to operating temperature. While the thermostat is closed, the bypass circuit (some manufacturers utilize the heater matrix for this circuit) allows coolant to circulate continually around the engine block and the cylinder head, thereby retaining the heat from the engine within the engine assembly until the engine has reached the optimum temperature for the most efficient running.

Water Pump

The water pump is used to pump water for purposes such as engine cooling, to promote high volumetric efficiency, to ensure proper combustion, and to ensure mechanical operation and reliability.

Thermostat

The thermostat acts as a controller for the engine coolant temperature. Therefore, the opening temperature set point of the thermostat that is fitted is determined by the most efficient operating temperature of the engine. The thermostat valve can remain closed until a set temperature is reached (i. e., the amount of heat in the engine needed to maintain its required efficient running temperature). Once this temperature is attained, opening the thermostat valve allows heat to be carried away to the cooling system, preventing overheating due to retention of excess heat energy.

In a vehicle, the thermostat helps to get the engine warmed to its optimum heat range quickly because it is manufactured to be in a normally closed state. However, in engine testing, the thermostat usually is fitted but is set in a normally open state by jacking it open. This is necessary to retain the flow-restricting characteristics of the thermostat within the engine cooling system, while eliminating the interference of the thermostat in the engine temperature as test conditions are altered during the test procedure.

The choice of thermostat, with regard to the ambient temperature at which the engine is to be operated, is the manufacturer’s consideration. If the ambient temperature were low, then a high-temperature-opening thermostat would be fitted so that the engine operates as near to optimum temperature as possible, and vice versa for high-ambient — temperature areas. Test technicians may need to be aware of this if testing an engine for use in extreme-temperature environments.

Thermostat Housing

This is a housing in which the thermostat will be held, and it can be positioned to the front top of the cylinder head or located remotely from the head, as long as it is situ­ated within the flow of the main water flow around the engine. The housing normally is connected to the upper section of the radiator.

Radiator

The radiator usually is located to the front of the vehicle within the direct flow of air. It may hold most of the coolant; the remainder is held in the engine and associated pipe-work. The construction of the radiator is such that it has two tanks connected by small-diameter pipes. These pipes are spaced apart, and the resultant gaps are fitted with corrugated cooling vanes. The normal layout is one tank at the top of the radia­tor and the other at the bottom. These tanks are connected to rubber hoses, which in turn are fitted to the thermostat housing (top tank) and the coolant pump (lower tank). In engine test cells, the radiator seldom is used because the cooling and temperature controlling of the engine under test conditions are managed more effectively by the use of heat exchangers and a raw water supply, with temperature and flow rate controlled more closely.

Expansion Tank

This is a separate tank that is fitted to the system and serves three purposes. First, as the engine warms up, the coolant expands. This means that extra room within the system is required to accommodate this expansion, hence the title “expansion tank.” The second purpose of the expansion tank is to allow any aeration of the coolant to separate out within the expansion tank. Thus, to allow both functions, the tank is never filled to its maximum but only to its halfway point. Third, the vapor pressure suppresses boiling of the coolant in the system. The expansion tank often is used in test-cell engine cooling to allow both functions to be retained.

Pressure Relief Cap

This simple device serves two purposes. First, it allows the engine to run at higher tem­peratures without boiling by pressurizing the coolant system. Second, it relieves excess pressure buildup within the cooling system. The cap is made so that the cooling system operating pressure is maintained to the cap blow-off pressure. The range of tempera­tures at which pressure relief caps are operated depends on the engine manufacturer’s requirements. Thus, great care must be exercised because an incorrect pressure cap can affect the results of tests conducted on the engine, which will alter the characteristics of the cooling system—in some cases, quite dramatically.

Cooling Fan

This fan draws air across the cooling surfaces of the radiator. Because tests seldom are run with radiators fitted to the test bed, the cooling fans normally do not remain fitted to the test engine.

Pipe-Work

Pipe-work around the engine can be made of steel, aluminum, or rubber and is used to link various components of the system to the engine. Rubber pipe serves best because it acts as an isolator, as the engine is rubber mounted to the bedplate and the heat exchang­ers are solid mounted. Any engine vibrations will be absorbed by the rubber hoses and will not be transmitted to the heat exchangers, which would cause failure.

Coolant

Engines require protection against both corrosion and, in low-ambient-temperature climate testing, freezing. Modem antifreeze solutions also improve coolant efficiency in high temperature ranges. Therefore, the engine coolant is made up of water and antifreeze. The main component of antifreeze is ethylene glycol, and this reduces the risk of freezing, depending on the mixture strength (commonly 50% water and 50% eth­ylene glycol). At this strength, the boiling point of the coolant mixture rises to 109°C. Ethylene glycol also has a higher boiling point than water, so it allows engines to be run at higher operating temperatures without the risk of boiling. Of primary concern to the technician is that with so many variants available, the correct product and solution strength must be chosen for the right engine or engine test requirement.

Fire warning—Care must be taken because ethylene glycol has an ignition point of 125°C; therefore, the mixture strength must be kept to the absolute minimum required with a small safety margin. Because of this, the most common mixture strength used is 50-50 water/ethylene glycol. If a sufficiently strong mixture of water and ethylene glycol were to be spilled onto a hot exhaust system, then there would be a high risk of fire.

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