Cooling System (Automobile)

Cooling System

During the process of converting thermal energy to mechanical energy, high temperatures are produced in the cylinders of the engine as a consequence of the combustion process. A large portion of heat from the gases of the combustion is transferred to the cylinder head and walls, piston, and valves. Unless this excess heat is carried away and these parts are adequately cooled, the engine looses its service life. A cooling system is provided not only to prevent damage to the vital parts of the engine, but also to maintain the temperature of these components within certain limits in order to obtain maximum performance from the engine. An adequate cooling system is a basic requirement of reciprocating IC engines.
A well designed cooling system should provide adequate cooling so that under­cooling is avoided, which is undesirable for several reasons. However under-cooling is not as harmful to the engine as overheating. In the process of cooling, the heat absorbed by the cylinder wall and other components of the engine is dispersed to the atmosphere. Both air cooling and water cooling systems are in use for automobile engines. With air-cooling, the heat passes from the engine directly to the atmosphere. With water-cooling, the water merely serves as a medium for transferring heat from the engine to the radiator, which gives it off to the atmosphere. Satisfactory operation of cooling system depends upon the system’s component design and the operating conditions.
The chapter deals with water-cooling and air-cooling systems, the coolant and antifreeze, various components of cooling systems, cooling system maintenance and service, diagnosis of cooling system troubles, and temperature indicator. Simul­taneous reading of the “Heating System” dealt in the later part of the book in the chapter “Automobile Air-conditioning” may be helpful.
12.1.

Engine Cooling

12.1.1.

Purpose of the Cooling System

The functions of the automobile cooling system are ;
(i) to prevent the development of high temperatures in the combustion chamber thereby saving damage of pistons, cylinders, valve and other ‘engine parts, and the oil which lubricates them,
Hi) to maintain the operating temperature at a safe level over a wide range of speeds, loads and ambient temperatures, and
(Hi) to help in warming up the engine to the required operating temperature as rapidly as possible and then maintain that temperature irrespective of the outside temperature, which may vary from 238 K to 318 K.
Cooling helps proper carburation, provides satisfactory oil viscosity, and helps maintain the correct part clearances within the engine. Peak combustion temperatures in the engine cycle run from 2500 to 3600 K with an average of 925 to 1200 K, throughout the operating cycle. Continued high temperature of this order would weaken engine parts, if heat is not removed to bring the temperature of the components within their physical strength limits.

High Temperature Operation.

At high temperatures oxidation of the engine oil takes place due to which oil is decomposed producing hard carbons and varnish. Continued high temperature may give rise to plugged piston rings and stuck hydraulic valve lifters. At high temperatures oil viscosity is also reduced. This may cause metal-to-metal contact within the engine producing high friction, loss of power, and rapid wear. Reduced oil viscosity also enhances oil consumption. High coolant temperatures may cause detonation and pre-ignition leading to engine damage. The maximum possible temperature on liquid cooled engines is limited by the coolant’s boiling point and the radiator’s capacity. On air cooled engines, it is limited by the air temperature and flow rate.

Low Temperature Operation.

A certain range of operating temperatures is necessary for satisfactory operation of the engine. At too low temperatures, due to improper vaporisation, excess fuel is required for proper engine performance. Cool engine surfaces quench part of the combustion, causing partially burned fuel as soot. It also cools the burnt by products, and condenses water vapour produced during combustion. The unburned fuel soot, and moisture go past the piston rings as blow-by gases which wash the oil from the cylinder wall and dilute the oil in the pan. This causes excessive scuffing and wear of cylinder wall and piston rings.
Each litre of fuel when burnt in the engine produces moisture equivalent of a litre of water. This moisture mixes with unburned fuel and soot at the oil pump and produces sludge. The moisture also combines with unburned hydrocarbons and additive constituents to form carbonic acid, sulphuric acid, nitric acid, hydrobromic acid, and hydrochloric acid. These acids cause corrosion and rusting within the engine. Below 330 K coolant temperature rusting occurs rapidly, and below 320 K temperature water from the combustion process accumulates in the oil. High rate of cylinder wall wear occur if coolant temperature becomes below 340 K.
The minimum normal temperature is controlled by a thermostat and is gradually increased from 345 K to 360 K. Engine operating temperature should be maintained between low temperature and high temperature limits.

Operating Temperatures.

Different parts of the engine operate at different temperatures (Fig. 12.1). Therefore, some regions inside the enclosed cylinder are more prone to overheating than others. Broad mean operating temperatures of the gas charge and the various zones in the cylinder are indicated below :

Fig. 12.1. Engine operating-temperature ranges.

12.1.2.

Modes of Heat Transfer

Transfer of heat takes place due to difference in temperature since heat flows from the hotter to the colder substance, may be solid, liquid, or gas. Three modes of heat transfer are conduction, convection, and radiation.

Conduction.

Conduction is brought about when heat to transferred from particle to particle throughout a body without any visible sign of move­ment. This type of heat flow is most effective in solids, but it can also occur at a much lower rate in liquids.

Convection.

Convection is established when heat is carried bodily by circulating currents of moving particles in liquid or gas. Natural or free convection currents are created entirely by changes in density due to difference in temperature at various levels in the liquid. Heat causes a fluid to expand. This makes the warm fluid less dense than the cooler one, so the lighter particles rise and the heavier one sinks. Consequently, a circulating cur­rent is established. Forced convection is achieved by a pump or fan which creates positive relative move­ment of the fluid over the stationary heated surface.

Radiation.

All substances, whether solid, liquid, or gaseous, emit energy by wave motion which radiates in all directions in straight lines with speed of light. Radiation, unlike conduction and convection, does not require a material medium to transmit heat. The emissive power of a radiating body is directly proportional to the fourth power of its absolute temperature therefore the slightest increase in temperature can considerably increase the heat transfer by radiation.
12.1.3.

Types of Engine Cooling System

Two basic systems of removing the heat from the engine are air-cooled system and liquid-cooled system.

Direct Air-cooled Engine System.

Cool circulating air comes in contact with the exposed and enlarged external surfaces of the cylinder and head. As a result their heat is dissipated to the surrounding air (refer section 12.9 for details).
Advantages.
(a) Air cooled engines operate satisfactorily in both hot and cold climates.
(b) These engines can work at higher operating temperatures than their equivalent liquid-cooled counterparts.
(c) The working temperature in these engines is attained rapidly from cold condition. id) These engines are marginally lighter than liquid-cooled engines of same capacity. (e) These engines do not encounter coolant-leakage or freezing problems.
Disadvantages.
(a) The cooling fans require a relatively large amount of power to run.
(b) Due to the large quantities of intake air passing into the cooling system, the engine may become noisy.
(c) The cooling fins can vibrate and amplify noise under certain conditions.
(d) For proper positioning of the fins between cylinders, the pitch between cylinder centres has to be greater than in liquid-cooled engines.
(e) Each cylinder is required to be cast individually unlike liquid-cooled engines where a rigid mono-block construction is used.
(/) To prevent overheating of the lubricant, the air-cooling is frequently supplemented by an oil heat exchanger.
(g) The presence of the guide cowling and baffles around the cylinders may hinder maintenance.

Indirect Liquid-cooled Engine System.

A liquid coolant transmits the heat from the cylinders and head to a heat exchanger, known as the radiator. Movement of air through this radiator then extracts the unwanted heat and dissipates it to the surroundings.
Advantages.
(a) Greater temperature-uniformity around the cylinders is achieved in liquid-cooled engines causing less distortion compared with air-cooled engines.
(b) The power consumption of the coolant pump and the fan together in liquid-cooled engines is less than that of the fan in air-cooled engines.
(c) The liquid-cooled engine cylinders are much closer, providing a very rigid and compact unit unlike the air-cooled engine.
(d) Both the coolant and the jackets dampen the mechanical noise from the engine.
(e) Liquid-cooled units perform heavy-duty work more reliably than air-cooled engines.
(f) Hot coolant can readily be circulated for interior heating of the vehicle.
Disadvantages.
(a) Liquid-coolant joints may develop leakage.
(b) Care must be taken to avoid freezing of the coolant.
(c) Liquid-cooled units require more time to warm up than the air-cooled engines.
(d) The boiling point of liquid-coolant limits maximum temperature of operation, whereas air-cooled engines can operate at slightly higher temperatures.
(c) Formation of scale takes place in the coolant passages, and the hoses and radiator tubes deteriorate with time.
Once the engine has warmed up, natural convection currents set up between the engine and the radiator due to variation of density and an enclosed circulating loop forms, known as the thermo-syphon cooling system (refer section 12.5 for details). Practically, however, this cooling system has several limitations, some of which are as presented hereunder.
(a) Under certain operating conditions (such as pulling under load at low vehicle speed), unless a very large radiator with very large engine coolant passages are used, the rate of circulation of coolant caused by the convection current cannot match the rate of transfer of heat from the cylinder walls to the coolant.
(b) For satisfactory heat transfer, the radiator header tank is required to be located at a higher level than the cylinder head. This is impractical with modern body styles.
(c) If coolant-circulation control is not provided, the engine has a tendency to be overcooled and seldom attains the optimum operating temperature, even after long running.
(d) Since the large quantity of coolant is used in the cooling system, the engine’s warm-up period is extended.
(e) The large header tank, used to compensate for the low rate of coolant circulation, has a tendency to overheat, causing loss of coolant through evaporation.
To enhance the rate of coolant circulation so that heat removal can be improved in a given time, the basic thermo-syphon system is assisted by installing a centrifugal pump in the engine-coolant lower return-hose. This modified system is called the forced circulation (forced-convection-current) system. Due to increased flow rate the radiator performs more efficiently, so it can be scaled down in size. Also it is not required to install the radiator at a higher level than the engine cylinder head.
In forced circulation system, coolant can be made to flow upwards as well as along the entire cooled passages in the cylinder-block. This permits uniform sharing of the cooled liquid between the in-line cylinders avoiding overheating of the critical zones in the engine.
The engine operates more efficiently when fitted with a pump-assisted cooling system provided overcooling of the system does not take place. Moreover, large volume of liquid circulating round the engine should not hinder the engine and head quickly reaching their working temperatures. Incorporation of a thermostat valve in series with the top hose has solved these problems to a great extent. To avoid excessive pressure build-up in the engine coolant passages, about one tenth of the liquid is directly circulated between the thermostat housing and the inlet side of the pump using a bypass pipe. This also prevents local boiling of trapped coolant due to lack of circulation.